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Kelompok I






Ancient Astronomy

Early peoples used the sky as a compass, a clock, and a calendar. Sunrise and sunset marked east and west respectively, as well as dividing time into day and night. The phases of the Moon defined a month, and the month – together with the annual motion of the Sun in the sky – was used to devise calendars. As cultures began to trade with each other, the positions of the stars were used as a navigational aid.

Prehistoric Astronomy

Evidence of celestial observations survive even from prehistoric times – that is, before written records began. The standing stones found in some parts of Europe are thought to be early observatories, with the stones arranged to mark the position of the Sun or Moon on certain dates, such as the equinoxes. Religious ceremonies were probably held on these days, although the exact function of the stones is unknown.

The First Science 

Astronomy is often regarded as the first science. Almost all the earliest civilizations studied the motions of celestial objects and applied the knowledge they gained to timekeeping, trade, and the organization of everyday life. This was the first attempt to understand natural forces, and to harness those forces for the benefit of humankind.


The earliest humans survived by hunting animals and gathering plants. Although few traces of these ancient societies remain, there can be little doubt that they would have observed the periodic return of the seasons, and the unchanging patterns of the stars. Bones unearthed in Africa, for example, show incisions that appear to group the phases of the Moon into a calendar.

The start of agriculture

Humans began settling into farming communities in about 9000 bc. In Western Asia, two main cultures – the Egyptians and the Babylonians – appeared. For both, timekeeping was essential for organizing religious and economic life, and the movements of the Sun and Moon were used to devise calendars. These were based either on the solar year – that is, the 365!/4 days it takes for the Earth to orbit the Sun – or on the lunar year.

The lunar year

The Babylonians devised their calendar from observations of the Moon, which takes 29!/2 days to complete a cycle. Babylonian calendars, which probably first appeared in around 3000 bc, were based on 12 lunar months, with a period of 29 days alternating with a period of 30 days to give a lunar year of 354 days. To bring the lunar year in line with the solar year, which governed the seasons, extra months were inserted every few years.

Babylonian beliefs

The Babylonians believed that the Sun, Moon, planets, and stars had been placed in the sky by the gods, and they observed them closely. They found that the planets travelled in a certain band of the sky – now known as the zodiac – and they divided the patterns of stars in this band into constellations. They also recorded observations of eclipses, meteors, and comets.

The Egyptians

The ancient Egyptians paid little attention to planetary movements and used astronomy only for the purpose of timekeeping. They were the first people to develop a calendar based on the solar year; it began with the flooding of the River Nile, an annual event of vital importance to the communities farming along its banks. The flooding coincided with the dawn rising of Sirius, the brightest star in the sky.

The solar year

The Egyptian solar calendar measured each month as 30 days. After 12 months, 5 extra days were added to give a year of 365 days. Because a solar year is actually one quarter of a day longer than this, the calendar eventually got out of step with the seasons. When it was adopted by the Romans in about 46 bc, the missing !/4 day was taken into account by adding an extra day – a leap day – every 4 years. This system formed the basis for the calendar used today in most parts of the world.

The Chinese

Chinese astronomy dates back to around 4000 bc, and the Chinese too developed a calendar that is still used today. Convinced that the movements of the Sun, Moon, and planets were a guide to foretelling the future, the Chinese made detailed observations of the sky, and their recordings of comets and supernovae are of considerable use to astronomers today. They paid special attention to predicting solar eclipses, which they thought brought bad luck. Believing it to be caused by a dragon eating the Sun, people would take to the streets during an eclipse, banging pots and pans in an attempt to frighten the beast away.

The Mayan civilization

The Mayans, who inhabited Central America between around 2000 bc and ad 900, believed that planets and stars were gods. They built pyramids from which to observe the changing motions of celestial objects, and developed an accurate solar calendar. They also made calculations that predicted solar and lunar eclipses.


Since ancient times, people have seen imaginary shapes among groups of stars in the night sky. Using lines, they have joined the stars in these groups together to form figures called constellations. Today, an internationally agreed system divides the sky around the Earth into 88 areas, each containing a constellation. Each of the constellation patterns is meant to represent a creature or object, and a number of them are named after mythological figures. From the Earth, stars in a constellation appear to be near each other, but in reality they lie at different distances from us.

The Zodiac

As the Earth orbits the Sun, the Sun seems to move against a changing background of stars. The Sun’s annual “path” is known as the ecliptic and the band of sky extending about 9° either  side of it is called the zodiac. Ancient civilizations divided the zodiac into 12 constellations as a way of measuring time, but a 13th, Ophiuchus, now also falls within this band of sky.

Passage of time

The Sun is said to “pass through” Taurus in May/June.


Ancient civilizations used the Sun’s journey across the zodiac as a way of marking the passage of time. Gradually, people became convinced that the regular motions of celestial objects could be used to predict the future, and all kinds of superstitions became associated with the zodiacal constellations. Astrology – the belief that the position of the stars and planets at the moment of birth determines the characteristics of a person’s life – grew out of these superstitions.

Astrological dates

According to astrology, the dates that the Sun passes through each zodiacal constellation are as follows:

Aries    21 March-20 April

Taurus 21 April-21 May

Gemini            22 May-22 June

Cancer 23 June-23 July

Leo      24 July-23 August

Virgo   24 August-23 September

Libra    24 September-23 October

Scorpius          24 October-22 November

Sagittarius       23 November-21 December

Capricornus     22 December-20 January

Aquarius          21 January-19 February

Pisces  20 February-20 March

Astronomical dates

However, those dates were established over 2,000 years ago. Because of the change in position of the Earth’s axis against the celestial sphere and changes in the boundaries between constellations when they were officially defined, the actual times are as follows:

Aries    19 April-13 May

Taurus 14 May-20 June

Gemini            21 June-20 July

Cancer 21 July-10 August

Leo      11 August-16 September

Virgo   17 September-30 October

Libra    31 October-22 November

Scorpius          23 November-29 November

Ophiuchus       30 November-17 December

Sagittarius       18 December-18 January

Capricornus     19 January-15 February

Aquarius          16 February-11 March

Pisces  12 March-18 April

Ancient Observations

In ancient times, before there were telescopes, stars and planets could be viewed only with the naked eye. The earliest record of a structure built for making celestial observations is in 750 BC, in Babylon (in modern Iraq). Early astronomers used sighting aids such as sticks to help them identify stars, and simple instruments such as dividers were used to measure angles between one star or planet and another. Sundials and water clocks were used to keep time during observations.

Fact File

Between about AD 800 and the 15th century, the Arabs built a number of observatories in Western and Central Asia. A particularly well-known example was built at Samarkand, in Central Asia, by a Mongolian prince named Ulugh Beg (1394-1449). It was equipped with the best astronomical instruments available at the time, including a wall-mounted, 40-m (131-ft) sextant – a tool used to measure the passage and height of celestial objects.

Observational Techniques

The instruments and working techniques of astronomers today are radically different from those of the 1950s. The telescope, for example, has changed dramatically in appearance, and in addition to being used on Earth, can  now also be found in orbit around the Earth, or journeying to other planets. And although telescopes still collect light from stars, other forms of radiation, such as radio waves, infrared, and X-rays, are now routinely recorded and analysed.



Astronomers conduct most of their detailed studies of space from observatories. The site of an observatory is one of its most important features, as telescopes must be situated far from city lights, which overwhelm faint starlight. They are often constructed near oceans as the air there is steadier, so the stars “twinkle” less, providing clearer images.

Mauna Kea

Located on an extinct volcano in Hawaii, Mauna Kea is a modern observatory used by astronomers from many different countries. It is situated 4,200 m (13,800 ft) above sea level, higher than most clouds, on an island surrounded by the Pacific Ocean.

These conditions make Mauna Kea one of the best space-observation sites in the world, providing extremely clear images of celestial objects. Because the observatory is situated at such a high altitude, its telescopes are able to collect infrared and microwave radiation, which are blocked by the lower layers of the atmosphere.

Fact File

Stars appear to twinkle in the sky because the light they emit travels through the Earth’s atmosphere.

This blanket of shifting gases bends the starlight in different directions, causing the “twinkling”. Seen from above the atmosphere, the stars shine steadily.

The Ancient Greeks

The Greek civilization began emerging in around 900 BC and for the next 1,000 years it made enormous contributions to astronomy. As early as the 6th century BC, the Greeks realized that the Earth was a sphere. They made the first accurate measurements of the circumference of the Earth, the size of the Moon, and the distance between the two. Although they mistakenly believed the Earth to be a fixed sphere around which the rest of the Universe rotated, some of their ideas and methods of observation were still in use in the late-17th century.

The Geocentric Universe

Most early civilizations believed that the Universe was geocentric, but it is the Greek astronomer Ptolemy (c. AD 90-168) who is best known for the idea. His book, the Almagest, set out rules for calculating the motions of celestial objects. In order to explain the retrograde motion of some planets, Ptolemy proposed that the planetary orbits followed a complex series of epicycles.

Arabic Astronomy

Between about AD 180 and the 16th century, the Arabs dominated the field of astronomy. Their cultural centre was Baghdad (in modern Iraq), and it was here that Ptolemy’s Almagest was translated into Arabic in the 8th century. The book was greatly extended by Al Battani (c. AD 850-929), the most famous of the Arab astronomers. Instruments such as the astrolabe – an ancient Greek invention – were developed into precision tools, and the Arabs made more accurate observations of star positions than the Greeks had managed.

The Heliocentric Universe

Since the dawn of astronomy, the Universe had been thought of as geocentric, with the Earth, stationary at the centre, circled by the moving Sun, planets, and stars. Then in 1543, the Polish astronomer Nicolaus Copernicus (1473-1543) proposed a heliocentric universe in his book De revolutionibus orbium coelestium (“On the Revolutions of the Celestial Spheres”). This radical theory put the Sun at the centre of the Universe, with the Earth and the other five known planets following circular orbits around it. This Copernican model formed the basis for our modern view of the Solar System.

Observational Proof

Copernicus arrived at his radical conclusion after a close study of the geocentric system that was accepted in his day. He became convinced that the epicycles used to explain the retrograde motion of the planets were too complicated to be plausible, and proposed that planetary movements could be explained more elegantly by putting the Sun at the centre of the Universe. Although it still incorrectly assumed that planetary orbits were circular, and epicycles were still used to explain planetary motion, the Copernican model provided a totally new way of looking at the Universe.

Fact File

Copernicus wrote De revolutionibus orbium coelestium a full 10 years before allowing it to be published. Many people think he hesitated because his theory was contrary to the teachings of the Roman Catholic Church, and he did not want to be condemned as a heretic. However, it is just as likely that he held back from publication because his ideas were so challenging for the time – especially the suggestion that the Earth moved – that he feared ridicule.

Early Telescopes

For centuries, astronomers had no optical aids through which to observe the sky. Then, during the closing years of the 16th century, simple telescopes began appearing on European battlefields. In around 1609, the Italian astronomer Galileo Galilei built a refracting telescope after hearing a description of the instrument’s effects. Since that time, the telescope has been the astronomer’s most fundamental tool.

Early Astrophysics

In the mid-19th century, the focus of astronomy changed. Instead of concentrating on the position of celestial objects in the sky, astronomers became curious about the nature of those objects. Stars were no longer simply moving pinpoints of light – their mass, size, and physical composition assumed great importance. The invention of the spectroscope – an instrument for analysing light – linked physics to astronomy. Astrophysics had begun.

Kelompok 2

Analysing Starlight

Using a spectroscope, astronomers split the light from a star to form a spectrum. Each star has a unique spectrum consisting of a pattern of lines that reveal the star’s chemical composition. In the late-19th century, a team at Harvard Observatory in the USA studied the spectra of hundreds of thousands of stars and then classified them into seven main spectral types. Their system is still used today.

The power of stars

Spectroscopy marked a turning point in understanding the nature of stars. By classifying stars according to their spectral characteristics, astronomers came to understand how they are born, how they die, and the source of their energy.

The Harvard system

The Harvard Observatory team began by classifying stars according to the strength of the hydrogen lines in their spectra. Stars with strong hydrogen lines were classed as type A; type B showed slightly weaker hydrogen lines, and so on. Later, team member Annie Jump Cannon (1863-1941) realized that hydrogen lines were strongest at a certain temperature, and that hotter and cooler stars had fewer hydrogen lines. She reordered the classification system according to temperature, and this system – O, B, A, F, G, K, M – is still used today.

Hertzsprung and Russell

In the early-20th century, Danish astronomer Ejnar Hertzsprung (1873-1967) and American astronomer Henry Norris Russell (1877-1957) independently plotted the spectral class of stars against their brightness. Both quickly realized that the results showed that stars fell into groups that represented different stages

in a stellar life cycle. The Hertzsprung-Russell diagram, or H-R diagram, remains central to modern astronomy.

Understanding stellar energy

From about 1920, a number of astrophysicists tried to pinpoint the source of a star’s energy. British astronomer Cecilia Payne-Gaposchkin (1900-79) proved that stars consist largely of hydrogen, and that the make-up of most stars is the same. In 1929, English astronomer Arthur Eddington (1882-1944) showed that a star’s energy came from the conversion of hydrogen to helium. This, and his work on the relationship between a star’s mass and its luminosity, was built upon by others in the decades following.

Birth of Cosmology

Cosmology is the study of the form, birth, life, and death of the Universe. At the beginning of the 20th century, cosmologists thought that everything in the Universe was part of the Milky Way Galaxy. In 1923, the US astronomer Edwin Hubble proved the existence of other galaxies by showing that stars in the Andromeda Nebula (later called the Andromeda Galaxy) were outside the Milky Way Galaxy. He also found that these galaxies were moving away from ours. Over the next decade, numerous theories regarding the origin of the Universe were developed.

The Advance of Astronomy

Astronomers are always finding new objects in the sky. Discoveries often result from new inventions or improvements in technology – the invention of the telescope, for example, led to a leap in celestial knowledge, and a steady increase in the power of these instruments continues to expand our knowledge of space. Today’s telescopes can detect radiation outside the range of visible light, and these instruments, along with high-speed data collection, have caused more discoveries to be made during the 20th century than during all previous centuries put together.



What is Cosmology?

Cosmology is the branch of astronomy that deals with the origin, large-scale structure, and evolution of the Universe. Astronomers construct imaginary “model universes” using mathematics, and compare the properties of these models with those of the known Universe. Two well-known models are the Steady-State universe (in which new matter is continuously created in order to maintain a constant density) and the Big-Bang model (in which the Universe is expanding from a single explosion).

History of Cosmology

The Greek mathematician Euclid (fl. c. 300 BC) defined space by the three dimensions of length, breadth, and height. When the English physicist and mathematician Isaac Newton (1643-1727) described the Universe, he did so in terms that Euclid would have understood – an infinite space defined by the three dimensions of length, breadth, and height. There is, however, a difficulty with the idea of infinite space. Olbers’ Paradox, named after the German astronomer Wilhelm Olbers (1758-1840), states that, if stars are scattered uniformly throughout infinite space, there will be a star in any direction we care to look. In the absence of anything that might obscure the light coming from distant stars, the whole sky should have the brightness of the Sun, which it clearly does not.


General relativity

This difficulty with Newton’s theory was finally removed in 1915, when the German-born American scientist Albert Einstein (1879-1955) introduced his General Theory

of Relativity. Einstein showed that space,and the matter within it, is finite but unbounded. (Imagine a two-dimensional universe in the shape of the surface of a sphere. It would be finite but would have no edges or boundaries.) Einstein’s finite but unbounded universe

was static but could quite easily have been expanding or contracting.

Expanding universe

The idea of an expanding universe gathered support from a discovery made in 1929 by the American astronomer Edwin Hubble (1889-1953). He found that the observable galaxies in the Universe (including our own) are moving. He also noticed that these galaxies move faster the further they are from us. The Belgian scientist Georges Lema”tre (1894-1966) claimed, in 1931, that this expansion was triggered by the spontaneous disintegration of what he called the “primal atom” (a single entity containing all of the matter and energy in the Universe).

Steady state

The English astronomer Fred Hoyle (1915- ) was unwilling to accept what he mockingly termed the “big-bang” theory. He spoke instead of the perfect cosmological principle when, in 1948, he said that the Universe should look the same from whatever place, and at whatever time, it is examined. In short, that the Universe is in a steady state. He claimed that this steady state was maintained by the continuous creation of matter throughout space to balance the expansion of the Universe. (The required rate of creation, about one hydrogen atom per litre every 20 years, is too slow to observe in a laboratory.) There are a number of fundamental differences between the Steady-State Theory and the Big-Bang Theory. For example, the proportion and density of old and new galaxies should be the same throughout the Universe according to the Steady-State Theory. The proportion and density of what appear to us to be young objects should, however, increase with distance according to the Big-Bang Theory.


Although the nature of quasars (quasi-stellar objects) is not yet fully understood, they do appear to be young, densely packed objects found only at the limit of the visible Universe. The very existence of quasars caused many people to question the validity of the Steady-State Theory. Support for the Steady-State Theory mostly collapsed with the discovery of microwave background radiation, which was first identified by Arno Penzias (1933- ) and Robert Wilson (1936- ) in 1965. This radiation had been predicted by the Ukrainian-born American astronomer George Gamow (1904-68), and can be viewed as an echo of the Big Bang.

Birth of the Universe

The big-bang model proposes that the Universe exploded into existence about 15 billion years ago. It started out unimaginably small, bright, hot, and dense, but has been expanding ever since. (It now has a radius of about 15 billion light years.) During the course of this expansion some of the mass of the Universe has condensed to form countless billions of stars. These stars are concentrated in galaxies, of which there are about 10 billion in the known Universe. These galaxies are grouped into clusters, which are themselves grouped into superclusters, separated by vast distances in empty space.

Big-Bang Theory

The big-bang model is currently the only widely accepted explanation for the origin of the Universe. The Big Bang itself was extremely hot and energetic, and in the first few seconds after the explosion all that existed in the Universe was radiation and various subatomic particles. The radiation left over after the explosion is still detectable from Earth as faint microwaves emanating from all directions of the sky. This is called the cosmic microwave background radiation.

Steady-State Theory

The Steady-State theory suggests that the Universe had no beginning and will have no end. It also suggests that the density of the Universe remains constant, requiring the continuous creation of new material at a rate that exactly compensates for the expansion of the Universe. When the microwave background (radiation from the Big Bang) was identified by Arno Penzias (1933- ) and Robert Wilson (1936- ) in 1965, the Steady-State Theory was greatly undermined. Some astronomers still adhere to the theory, especially its originator, Fred Hoyle (1915- ).

Birth of the Solar System

The Solar System began to form about 5 billion years ago from a cloud of interstellar gas and dust. Gravity caused the cloud to begin contracting and produced a dense sphere of gas at the centre of the cloud; it also caused the cloud to spin faster and faster. As it spun, the cloud material flattened out to form a disc that surrounded the central region. This dense region eventually became hot enough to undergo nuclear reactions, at which point it became our local star, the Sun. Meanwhile, the smaller members of the Solar System formed from material in the disc. These objects included the planets, asteroids, and comets.

In the Beginning

Astronomers of the 20th century have proposed differing explanations of the Universe’s origins. Georges Lema”tre (1894-1966), a Belgian mathematician, suggested that the Universe began as a single hot, dense, point that exploded.  The British astronomer Fred Hoyle (1915- ) developed the Steady-State Theory, which rejected the idea that the Universe had ever had a beginning. The discovery of the microwave background in 1965 offered proof of the Big-Bang Theory.

Scale of the Universe

After the Big Bang the material from which galaxies later formed was sent rushing off in all directions. These same galaxies are still rushing away from each other. Measuring the size of the Universe depends on our  ability to measure the distance between us and the furthest galaxies. Astronomers measure and analyse the light from galaxies to calculate how far away these are. They estimate that the furthest are about 15 billion light years away.

Distances in Space

Units of measurement such as the kilometre and the mile are inadequate for measuring the huge distances that exist in space. Astronomers measure the distances to stars or galaxies in light years or parsecs (1 parsec = 3.26 light years). The nearest star to the Sun is about four light years away. Within the Solar System, astronomers use the astronomical unit (AU), which is equal to 150 million kilometres (about 93 million miles) – the average distance from the Sun to the Earth.

Speed of Light

Visible Light from a distant galaxy may have left that galaxy before the Earth had even formed. Nothing moves faster than light, which travels through a vacuum at a constant speed of 299,792.49 km (186,282.04 miles) per second, but the vast distances of  space mean that light from even the nearest star (after the Sun) takes 4.2 years to reach us. These distances

are so great that they are measured in light years – 946,000 billion km (588,000 billion miles) – the distance light travels through space in one year.

Edge of the Visible Universe

With the most modern telescopes we can see objects up to 15 billion light years away, which is close to the edge of the known Universe.

The light that we can see from these objects started its journey around 15 billion light years ago, so their appearance can give us an idea of what the early Universe looked like.

Fact File

Although the Universe is finite in size (according to the Big-Bang Theory), it has no true boundary. If we were able to travel to the most distant galaxies, on arriving there, we would still see distant galaxies in all directions. As the German-born US physicist Albert Einstein (1874-1955) explained, this is because space-time is curved and has no edges.

Density of the universe

The Universe is expanding, but gravitational forces between all of the matter in the Universe is slowing this expansion. The more concentrated the mass of the Universe, the higher is its density, and the more the expansion is slowed. A high-density Universe may contract eventually, because of the strength of the gravitational attraction between its constituent parts. If, the density is not high enough, the Universe will expand forever.

Dark matter

The ultimate fate of the Universe is likely to depend on how much “dark matter” it contains. Dark matter is matter that is invisible to us, detectable only by the effects of its gravity. The exact nature and distribution of dark matter is unknown, but astronomers have predicted that it is likely to make up around 90 per cent of the mass of the Universe. Some is thought to

exist in black holes, some as dim stars, and some in dark haloes surrounding galaxies.

Structure of the Universe

The Universe consists of huge superclusters of galaxies surrounded by unimaginably large volumes of empty space. Each galaxy consists of billions of stars. These stars are made of matter, which is composed  of particles that are far too small to be visible. Protons, neutrons, and electrons are the most common particles, and these are usually grouped together as atoms. Protons and neutrons are made up of even smaller particles called quarks.


Birth of the Solar System

The Solar System began to form about 5 billion years ago from a cloud of interstellar gas and dust. Gravity caused the cloud to begin contracting and produced a dense sphere of gas at the centre of the cloud; it also caused the cloud to spin faster and faster. As it spun, the cloud material flattened out to form a disc that surrounded the central region. This dense region eventually became hot enough to undergo nuclear reactions, at which point it became our local star, the Sun. Meanwhile, the smaller members of the Solar System formed from material in the disc. These objects included the planets, asteroids, and comets.

Our Place in the Universe

The explosion that created the Universe flung matter in every direction. This material evolved into the Universe we have today. Astronomers put the size of the Universe at about 15 billion light years in radius, although the  force of the Big Bang is still causing it to expand. The Universe is populated by galaxies, each with billions of stars. We know for certain that one of these stars has a system of planets and that one of these planets, the Earth, supports life.

The Solar System

The Solar System consists of the Sun and various celestial objects held by the Sun’s gravitational field. These objects include the planets and their moons, interplanetary gas and dust, and vast numbers of asteroids, comets, and meteoroids. The Sun contains 99.86 per cent of the System’s mass. The planet Jupiter accounts for most of the remainder. The orbits of the planets cover a volume of space about 80 astronomical units (AU) wide; the orbits of the comets make the whole system around 200,000 AU wide.

Kelompok 3


Titius Body Law

In 1766, the prussian astronomer Johann Titius (1729-1796) devised a formula for gauging the distances of the planets from the Sun in astronomical units (AU; 1 AU is equal to the distance of the Earth from the Sun, which is 149,597,870 km or 92,955,730 miles). The formula was published by the German astronomer Johann Bode (1747-1826) in 1772. It uses the symbol D for a planet’s distance from the Sun and the symbol N for one of the numbers in the following sequence: 0, 3, 6, 12, 24, 48, 96, 192. Use the first number in the sequence to find the distance of Mercury, the closest planet to the Sun; use the second number to find the distance of Venus, the next planet out, and so on. To do this, replace the N in the formula with the appropriate number from the sequence, add four to the number, and then

divide by 10:

D = (N + 4) / 10

Object and      Predicted         Actual

ÔN’-value        distance ÔD’   distance

Mercury 0            0.4                0.39

Venus 3               0.7                0.72

Earth 6                            1                   1

Mars 12               1.6                1.52

Asteroid belt 24   2.8               1.7-4.0 (midpoint = 2.85)

Jupiter 48             5.2                5.2

Saturn 96             10                  10

Uranus 192          19.6               19.6

Neptune —          —                 30.05

Pluto 384             38.8               39.44


At the time the formula was devised, only Mercury, Venus, Mars, Jupiter, and Saturn had been discovered, leaving spare the values for D of 2.8, 19.6, and 38.8. The discovery of Uranus in 1781 corresponded with a value for D of 19.6. Astronomers then began searching for a missing planet at a distance of 2.8 AU, between Mars and Jupiter. This led to the discovery in 1801 of the first-known member of the asteroid belt, Ceres, by the Italian astronomer Giuseppe Piazzi (1749-1826). Pluto, which was discovered in 1929, has a less close correspondence with its “D” value than the other planets have with theirs. And for Neptune, the formula fails to work at all.

How the Solar System Works

The Solar System is held together by the Sun’s gravity. Each object in the System orbits the Sun at its own speed in an elliptical path. While the planets orbit the Sun, the moons orbit their  planets. Except for the comets, the objects in the System all move around the Sun in the same direction as the Earth does (anticlockwise when viewed from above the North pole).

The Sun

The Sun is a 5-billion-year-old main-sequence star. It is a sphere of mainly hydrogen and helium gases, about 1.4 million km (870,000 miles) wide. It contains 750 times the mass of all its planets put together and has seven times the mass of the average star. In its core, nuclear reactions convert mass into electromagnetic radiation, a form of energy. This energy radiates outwards, making the Sun shine. It also warms the other objects of the Solar System, which are held in orbit by the Sun’s gravity.


The Sun’s Energy

The Sun’s core is a nuclear “furnace” with a temperature of 15 million °C (27 million °F) and a density 160 times that of water. Under these conditions, hydrogen nuclei in the core fuse to form helium nuclei. In the process, 0.7 per cent of the mass being fused is converted into energy. Of the 600 million tonnes (590 million tons) of hydrogen that fuse in the Sun’s core each second, 4 million tonnes (3.9 million tons) are converted to energy. The Sun’s hydrogen “fuel” will last for another 5 billion years.

Fact File

The sun loses about 1 million tonnes (1 million tons) of hydrogen into the solar wind every second. It would take 100,000 billion years for the solar wind to disperse the entire mass of the Sun into interplanetary space. However, the Sun’s natural lifespan is only 10 billion years.

Solar Wind

The Sun’s corona (outer atmosphere) contains particles energetic enough to escape the Sun’s gravity. These particles spiral away from the Sun at speeds of up to 900 km (560 miles)  per second and form the solar wind.  They follow the lines of the Sun’s magnetic field, and, because they carry electric charges, they fill the Solar System with electric currents. The region filled by the solar wind is called the heliosphere.


Solar Cycles and Sunspots

The Sun’s rotation creates a magnetic field. The Sun’s equatorial regions spin faster than its polar regions, which causes the magnetic field lines to become wound up inside the Sun. If they break through the surface, the lines cause solar activity such as sunspots, flares, and prominences. This activity, particularly the sunspots, follows an 11-year cycle.

Prominences and Flares

Violent solar activity often occurs near sunspots. Flares are “flashes” of energy that

may last for several hours; they occur when a massive build-up of magnetic energy is

suddenly released. Prominences are eruptions of flaming gas that may shoot hundreds of thousands of kilometres into space. Looped prominences may remain suspended by the solar magnetic field for weeks at a time.

Death of the Sun

In about 5 billion years’ time, most of the hydrogen in the Sun’s core will have fused to form helium. Gravity will then make the core contract, which will increase its pressure and temperature. Hydrogen will start to “burn” in a shell around the core. The new energy generated by nuclear fusion in the shell will cause the Sun’s outer layers to swell until the star becomes a huge red giant. Its outer layers will then drift off into space to form a planetary nebula. The core will remain behind as a fading white-dwarf star.

Fact File

When the sun swells to become a red giant, its diameter will increase by at least 150 times. The expanded gases will fade from hot yellow to cooler red. The Sun’s brightness will increase by about 1,000 times, however, because its greater surface area will radiate more light.

Astronomical Unit

One astronomical unit is defined as the average distance between the Earth and the Sun, which is equal to 149,597,870 km (92,955,778 miles). It is the unit of measurement for distances inside the Solar System.  There are 63,240 astronomical units in a light year.


Introducing the Universe

The Universe contains everything that exists, from the largest galaxy to tiny subatomic particles. Most of the mass of the observable Universe comes from stars, which we see by their light. Objects we cannot see with the eye may  be detected if they emit other types of radiation than light. Despite the huge number of objects in it, the Universe is a very empty place with vast volumes of cold, dark space. Only one tiny part of it is known to have life – planet Earth.

Fact File

Some scientists think there could be more than one universe. After all, an explosion like the Big Bang that created our universe may have happened more than once, and our independent

universe could exist along with one or more others. While no other universe has ever been detected, some theories say that a space traveller might get to one if it were possible to pass safely through a black hole.

Astronomical Unit

One astronomical unit is defined as the average distance between the Earth and the Sun, which is equal to 149,597,870 km (92,955,778 miles). It is the unit of measurement for distances inside the Solar System.  There are 63,240 astronomical units in a light year.

Introducing the Universe

The Universe contains everything that exists, from the largest galaxy to tiny subatomic particles. Most of the mass of the observable Universe comes from stars, which we see by their light. Objects we cannot see with the eye may  be detected if they emit other types of radiation than light. Despite the huge number of objects in it, the Universe is a very empty place with vast volumes of cold, dark space. Only one tiny part of it is known to have life – planet Earth.

Fact File

Some scientists think there could be more than one universe. After all, an explosion like the Big Bang that created our universe may have happened more than once, and our independent

universe could exist along with one or more others. While no other universe has ever been detected, some theories say that a space traveller might get to one if it were possible to pass safely through a black hole.

Nicolaus Copernicus

Nicolaus Copernicus was a Polish medical doctor and astronomer who changed our interpretation of the Earth’s place in the Universe. He proposed, in his famous work De revolutionibus orbium coelestium (“On the Revolutions of the Celestial Spheres”), that the Earth rotated daily on its own axis and that it moved around the Sun in a year-long orbit, rather than the Universe rotating around the Earth. He also set out methods for calculating the size of the Solar System and the motions of the planets. It was more than a century, however, before his ideas were proved correct and accepted by science.

The Fabric of the Universe

Everything in the Universe is made of matter. All matter was probably created about 15 billion years ago in a huge explosion called the Big Bang. Within minutes, subatomic particles called electrons and protons joined to form hydrogen and helium. Over the course of billions of years, stars began to form. Nuclear reactions within the stars created – and continue to create – heavier elements. When stars explode, these elements are flung through space. Apart from subatomic particles, everything in the Universe is made of elements and combinations of elements.

Structure of the Universe

The Universe consists of huge superclusters of galaxies surrounded by unimaginably large volumes of empty space. Each galaxy consists of billions of stars. These stars are made of matter, which is composed  of particles that are far too small to be visible. Protons, neutrons, and electrons are the most common particles, and these are usually grouped together as atoms. Protons and neutrons are made up of even smaller particles called quarks.


What is a Galaxy?

A galaxy is a collection of stars, gas, and dust held together by gravity. The smallest galaxies are a few hundred light years across and contain about 100,000 stars. The largest are up to 3 million light years across and contain more than 1,000 billion stars. Galaxy shapes are classified according to a system based on that introduced by the American astronomer Edwin Hubble (1889-1953). Little is known with any certainty about the evolution of galaxies, beyond the fact that they all began life billions of years ago as huge, spinning clouds of gas and dust.

Irregular Galaxies

Irregular galaxies have no regular shape or structure. They are typically less massive than other galaxies, and most of their stars are bright and young. Although many irregular galaxies contain regions of luminous gas in which stars are being born, much of their interstellar gas and dust has yet to condense to form new stars. Irregular galaxies make up only about 5 per cent of the 1,000 brightest galaxies, but about a quarter of all known galaxies are irregular.

Spiral Galaxies

Spiral galaxies have arms that form a spiral pattern around a central bulge, or nucleus. The arms form a disc around the nucleus; as the nucleus spins, the arms follow behind it. The youngest stars in spiral galaxies are found in the loosely packed arms; older stars lie mainly in the dense nucleus. The oldest stars of all reside in a sparsely populated spherical halo that surrounds the galactic disc. The arms also contain much gas and dust that have yet to form stars.

Barred-Spiral Galaxies

A barred-spiral galaxy has an elongated, bar-shaped central bulge, or nucleus. While the nucleus rotates, an arm seems to follow at each end. Some astronomers think that our own galaxy may be a barred spiral. Spiral and barred-spiral galaxies range in shape from those with large, central bulges circled by tightly wound arms, to those with small bulges and loose arms. Although in the past, barred-spirals and spirals have been classified as two distinct types of galaxy, astronomers today recognize them to be similar.

Elliptical Galaxies

Elliptical galaxies range in shape from ellipsoidal (roughly the shape of an American football) to spherical, with intermediate shapes in-between. Unlike other galaxies, in which we can see dust reflecting blue light from hot young stars, elliptical galaxies appear yellow. This is because the process of star formation in them has stopped, so that nearly all of their light comes from old red-giant stars. Both the least and the most massive galaxies observed so far are elliptical.

Naming Deep-Space Objects

some nonstellar objects, such as galaxies and nebulae, are known by a popular name, but many are known only by a number. In 1774, the French astronomer Charles Messier (1730-1817) published a catalogue of 45 celestial objects and added to it over the following decade. Each object in the catalogue is designated the letter “M” (for Messier”) followed by a number. Thousands of other celestial objects are known by their NGC number, as listed in the New General Catalogue prepared by Danish astronomer John Ludwig Emil Dreyer (1852-1926).

Fact File

The estimated 100 billion galaxies that make up the Universe are separated by vast distances. Even those galaxies that belong to a relatively close grouping, or cluster, have huge, empty voids between them. The Large Magellanic Cloud (LMC), for example, is part of the Local Group along with our own galaxy, the Milky Way Galaxy. The LMC is the galaxy nearest to our own, yet it lies 170,000 light years away from us.

Active and Unusual Galaxies

all galaxies emit a certain amount of electromagnetic radiation. Some galaxies radiate unusually large amounts, and these are called active galaxies.

The energy comes from a very massive but compact source at an active galaxy’s centre. The energy is often in the form of X-rays or radio waves as well as light, and the amount of energy radiated is too great to be produced by stars. Astronomers think that the only type of object capable of releasing so much energy is a supermassive black hole. It may be that galaxies with a relatively low output of energy, such as our own, contain a small, central black hole.

The Milky Way Galaxy

our galaxy, the Milky Way Galaxy, is a spiral galaxy containing about 500 billion stars. It formed from a huge cloud of gas and dust about 10 billion years ago. At the centre is a dense, spherical nucleus of stars that may also contain a black hole. The nucleus is encircled by a disc of spiral arms containing hot young stars; the nucleus and disc are surrounded by a sparsely populated halo of very old stars.

Kelompok 4

Supermassive Black Holes

Black holes are thought to be the “powerhouses” at the centre of quasars and other active galaxies. Such black holes are called supermassive because they may contain as much mass as one hundred billion suns, equivalent to the mass of an entire galaxy. The gravitational attraction of this mass is enormous and pulls surrounding gas and stars towards the hole. As this matter spirals into the hole, it emits huge amounts of electromagnetic radiation. Galactic cannibal

As a black hole attracts more material, it becomes increasingly massive. Everything that is sucked in by a black hole disappears forever.


quasars, or quasi-stellar (star-like) objects, are thought to be the active cores of distant galaxies. They are the brightest, the fastest, and the most distant-known objects in the Universe. Like a star, a quasar appears from the Earth as a tiny point of light. Although  quasars are only about the size of the Solar System, the light from some has travelled more than 10 billion light years to reach us. For us to be able to detect such a distant object, it must be very bright: some quasars radiate as much energy as 100 giant galaxies.


Radio Galaxies

all galaxies emit radio waves, visible light, and other types of radiation.

The radio energy from a radio galaxy is much more intense than that of

ordinary galaxies. It is emitted from two huge lobes, or clouds, of particles travelling away from the visible galaxy. The lobes are formed by jets ejected at  high speeds (up to one fifth the speed of light) from the galaxy’s centre. These huge outpourings of energy are thought to be given off by an accretion disc that surrounds a supermassive black hole spinning at the heart of the galaxy. Only about one galaxy in a million is a radio galaxy.


Colliding Galaxies

Most galaxies are separated from their neighbours by distances of hundreds of thousands of light years. Some galaxies, however, approach one another closely enough for their mutual gravitational attraction to drag material from one to the other; this produces strands of material called tidal tails that form bridges between galaxies. Closer encounters may involve galaxies passing through each other or merging, producing a massive distortion in their shapes.

Fact File

Lying some 200,000 light-years from our galaxy are two small galaxies called the Magellanic Clouds. They are joined to one another by a stream of gas, and each is also linked to our galaxy by a long line of gas called the Magellanic Stream. These streams may have been formed by a near collision between the three galaxies about 200 million years ago.

Galaxy Clusters

Most galaxies belong to a cluster or group of galaxies that is held together by gravitational attraction. Our galaxy belongs to a small, irregularly shaped cluster called the Local Group. Irregular clusters contain from a few up to several thousand galaxies of all types. A regular cluster is a roughly spherical group of at least 1,000 closely packed galaxies, most of which are elliptical. Even such closely packed galaxies are separated by distances of hundreds of thousands of light years. Neighbouring clusters make up yet larger structures called superclusters.


Superclusters are groups of galaxy clusters and are among the largest structures in the Universe. A typical supercluster contains perhaps 10 heavily populated clusters arranged in the shape of a winding filament or string that may be a hundred million light years long. Our own cluster, the Local Group, is part of the Local Supercluster, which contains several hundred galaxy clusters. The supercluster strings form the boundaries of huge voids that separate one supercluster from another.


Fact File

There is often a giant elliptical galaxy at the centre of a richly populated cluster of galaxies. The most massive galaxies known are found at the centres of such clusters. Observations indicate that the most massive galaxies in such a cluster merge with the central giant galaxy in a process known as “galactic cannibalism”. The central cannibal galaxy may contain more than one nucleus.


Star Birth

stars are born together inside huge clouds of gas and dust. The process begins when a region of a cloud increases in density. This change in density could be triggered by the passage through the cloud of a supernova shock wave, for example. Under gravity, the dense region contracts, becoming denser and hotter, and eventually forms one or more stars undergoing nuclear reactions. The gas and dust in the original cloud had a temperature a few degrees above absolute zero (-273.15 °C or -459.67 °F). When condensed at the centre of stars, this material has a temperature of at least 10 million °C (18 million °F).

Fact File

Protostars give out some heat and light before they begin shining brightly as fully fledged stars. Much of this radiation is absorbed by the dust in the nebula that surrounds a protostar. The dust gives the radiation back out, mainly in the infrared region of the spectrum. Although astronomers may not be able to see protostars shining directly, they can detect their presence in a nebula using infrared telescopes.

Maturing Stars

A protostar becomes a star when nuclear reactions begin in its core, causing the star to shine for millions or billions of years. During these reactions, hydrogen in the core fuses to form helium, a heavier element. As the star matures towards old age, the hydrogen runs out and different reactions occur, with progressively heavier elements forming at each stage. This process, called nucleosynthesis, keeps a star alive by radiating sufficient electromagnetic energy from its core to prevent its outer layers collapsing inwards. As the process continues, the star’s properties change.

Star generations

The oldest stars in a spiral galaxy such as our own are found in a huge, sparsely populated halo that surrounds the central nucleus.

Main-Sequence Stars

in the core of a main-sequence star, nuclear reactions fuse hydrogen to form helium. Stars spend most of their lives in this stable state. When these reactions cease, the helium core contracts, heating the surrounding shell of hydrogen gas until fusion starts in this shell. The energy radiated by this new reaction applies pressure to the star’s outer layers, which expand and cool. The star then evolves into a red giant or a supergiant, depending on its initial mass.

A star in its primeWhen their properties are plotted on a Hertzsprung-Russell diagram, about 90 per cent of stars fall into a band on the diagram called the Main Sequence.

Nuclear Fusion

Stars shine by a process of nuclear fusion. For fusion to occur, a star’s core temperature must be at least 10 million °C (18 million °F). Under these conditions, protons (hydrogen nuclei) fuse to form helium nuclei.

In the process, 0.7 per cent of the mass of the hydrogen is converted to energy. In low-mass stars, such as the Sun, the process of fusion is fairly simple and is known as the proton-proton chain. More massive stars form progressively heavier elements in more complex fusion reactions as they reach old age.

Proton-proton chain

Protons collide and fuse together to form more complex helium atoms. In the process, they release energetic particles such as neutrinos, positrons, and photons.

Selected Stars

Most of the stars visible in the night sky are main-sequence stars. Some prominent ones are listed below:

Star                                                    Constellation

The Sun                                                           —

Regulus                                               Leo

Fomalhaut                                           Piscis Austrinus

Procyon                                               Canis Minor

Spica                                                   Virgo

Fact File

Main-sequence stars have core temperatures of at least 10 million °C (27 million °F). This temperature is required for nuclear fusion to take place. Their surface temperatures range from about 3,000 to 40,000 °C (5,400 to 70,000 °F).

Red Giants

An elderly main-sequence star of up to 3 solar masses evolves into a red giant. This ageing star has a contracting helium core surrounded by a hydrogen fusion shell. Radiation from this shell causes the star’s outer layers to expand and cool, and the star becomes a giant. Meanwhile, the core becomes hot enough for helium to fuse to form carbon. When all the helium has fused, the outer layers drift away to form a planetary nebula, while the core collapses to form a dying white dwarf.

Middle-age spread

The outer layers of a red giant are pushed outwards by radiation from internal nuclear reactions. As they expand, the outer layers cool and change colour from yellow to red.

Selected Stars

some prominent red giants are listed below:

Star                      Constellation

Mirach                  Andromeda

Arcturus               Boötes

Menkar                 Cetus

Scheat                  Pegasus

Beta Gruis            Grus

Cool Giant

while a main-sequence star such as the Sun has a core temperature of about 15 million °C (27 million °F), the core temperature of a red giant eventually reaches about 100 million °C (180 million °F). The surface temperature of the red giant is lower than that of the main-sequence star, however, because the giant’s surface lies at a greater distance from its core. When the Sun becomes a red giant in about 5 billion years’ time, its diameter will increase from about 1.4 million km (870,000 miles) to at least 200 million km (125 million miles).


a main-sequence star of at least 10 solar masses evolves into a supergiant. Inside a supergiant’s core, a nuclear furnace fuses a series of successively heavier elements at temperatures of billions of degrees. The surface temperatures of these stars range from about 3,500 to 50,000 °C (6,300 to 90,000 °F), which produces a range of colours from cool red to hot blue. Even those with relatively cool surfaces are as luminous as many thousands of suns, however, because their huge surface areas give off so much radiation.

Celestial alchemist

Most of the elements we know today, such as carbon, oxygen, nitrogen, iron, gold, silver, and platinum, were created by nuclear reactions inside massive stars.

Selected Stars

some prominent supergiants are listed below:

Star                 Constellation

Antares            Scorpius

Betelgeuse       Orion

Polaris             Ursa Minor

Rigel                Orion

Deneb              Cygnus

Schedar           Cassiopeia

Fact File . The iron core of a supergiant reaches temperatures between 3 and 5 billion °C (5.4 and 9 billion °F). The largest-known supergiant, Betelgeuse (which lies in the constellation of Orion), has a diameter about 400 times that of the Sun and a volume that could encompass 64 million suns. The most massive supergiants – those with about 100 times the Sun’s mass – are about a million times more luminous than the Sun.

Variable Stars

Variable stars vary in brightness, whether regularly or irregularly. Cataclysmic variables, such as novae or supernovae, suddenly become many thousands of times brighter because of a violent outburst of energy. Pulsating variables, many of which are huge red giants, swell and contract regularly, dimming and brightening in time with their pulses. Cataclysmic and pulsating variables vary in both true brightness and apparent brightness. Eclipsing binary stars, however, vary in only apparent brightness; this occurs when one star passes in front of the other and blocks light travelling from it to Earth.

Stellar pulsePulsating stars have an unstable structure that causes them regularly to pulsate.

Chemical factories.

Nucleosynthesis is the process by which stars produce progressively more complex chemical elements. These elements are eventually released into space, where they combine to form the matter that makes up everything in the Universe – including ourselves. The rate at which this occurs, and the elements produced, depend on the mass of the star.

Low-mass stars

All stars begin the process of nucleosynthesis by fusing hydrogen to form helium in their cores. Hydrogen is the simplest and most abundant element in existence. When a star

of less than about 10 solar masses has a core made almost entirely of helium, the core contracts. As it contracts, the core heats up and ignites a shell of hydrogen surrounding the core. While hydrogen in this shell fuses to form helium, the core becomes hot enough to begin fusing helium to form carbon. When the helium in the core is used up, nuclear reactions in the star cease.

High-mass stars

In stars of more than about 10 solar masses, the initial stages of nucleosynthesis are similar to those in stars of lower mass. Massive stars use their nuclear fuel more quickly than lower-mass stars do because the temperature and pressure in their cores are higher than those in stars of lower mass. Whereas a low-mass star such as the Sun spends about10 billion years burning hydrogen, a high-mass star takes less than 10 million years – a tenth as long. Once a high-mass star finishes the helium-burning stage, which lasts for about half a million years, the increasing temperatures in its core allow it to carry on fusion reactions to produce increasingly heavier elements.

To begin with, the carbon core contracts and heats up a second fusion shell around itself but inside the first fusion shell. This new shell consists of helium fusing to form carbon. Meanwhile, carbon burns in the core for less than a thousand years to produce a core of oxygen. When the core contracts, a third shell is created around the core – this one burning carbon – while the oxygen core burns for perhaps 6 months to produce silicon. Silicon burns in the core for only a day and produces a core of iron with a temperature between 3 and 5 billion °C (5.4 and 9 billion °F). The iron core, which is surrounded by five shells undergoing fusion, collapses in on itself when it tries to fuse; this is because iron fusion does not give out energy as the other fusion reactions do, but takes energy in. This collapse explodes the star, whose material then spreads through space to become part of other stars and planets.

Dying Stars

A star enters its dying phase when nuclear reactions in its core finally cease, causing the star’s structure to become unstable. A star of relatively low mass “burns” its nuclear fuel slowly over billions of years then evolves into a red giant. It then disintegrates to form a planetary nebula (an expanding shell of gas) surrounding a white dwarf. A star of high mass uses up its fuel more quickly, over only millions of years, and then evolves into a supergiant. It then erupts in a huge explosion called a supernova. The remaining core forms a neutron star or a black hole.

White Dwarfs

When a red giant dies, it sheds up to 90 per cent of its mass, which then forms a planetary nebula around the collapsing core. As the core shrinks, its matter becomes compressed far more than matter could ever be compressed on Earth. At a certain point, the core’s  matter resists further compression: the core has become a white dwarf with, at most, 1.4 solar masses and a volume similar to that of the Earth. White dwarfs are so dense that a single teaspoonful of their matter would weigh as much as 1.4 tonnes (1.4 tons).

Kelompok 5

Selected Stars

White-dwarf stars are so small and faint that very few have been detected by even the most powerful telescopes. Some of those that are known are listed below:

Star                   Constellation

Sirius B             Canis Major

Procyon B         Canis Minor

Van Maanen      Pisces

40 Eridani B      Eridanus


A supergiant star of more than 10 solar masses ends its life in a huge explosion called a supernova. The explosion is so energetic that it may outshine an entire galaxy of billions of stars. For a time, the supernova may appear from Earth to be a new, very bright star. If the explosion leaves behind a core of between 1.4 and 3 solar masses, the core shrinks to form a neutron star. If the core is greater than 3 solar masses, gravity forces it to contract further to form a black hole.

Fact File

The energy released in a supernova explosion could destroy tens of thousands of planets such as the Earth. Supernovae are not entirely destructive, however; these explosions distribute the elements created inside stars through interstellar space, where they are incorporated into new stars and planets. The carbon atoms that form parts of the molecules that make up much of our food and our bodies were originally created inside stars.

Selected Supernovae

Despite their intense brightness, only two or three supernovae are observed in our galaxy each century, as they are often hidden by interstellar dust. These are some that are known:

Supernova                  Constellation

Tycho’s Star                Cassiopeia

Kepler’s Star                Ophiuchus

The Crab Nebula         Taurus

SN 1987A                   Large Magellanic Cloud

SN 1993J                    Galaxy M 81 in Ursa Major

Neutron Stars and Pulsars

When a massive star explodes as a supernova, its core may survive. If the core has between 1.4 and 3 solar masses, gravity compresses it beyond the white-dwarf stage, until its protons and electrons have been squeezed together to form neutrons. This type of object is called a neutron star. When the star is about 10 km (6 miles) across, its contraction halts. Some neutron stars are detected from Earth as pulsars, which, as they rotate, emit two beams of radiation.

Fact File

Pulsars are rotating neutron stars that emit beams of radio waves, which may be detected on Earth as a pulse. The pulse rate matches the pulsar’s rate of spin. Slow-spinning pulsars spin about once every 4 seconds. Fast-spinning pulsars spin about 30 times a second. Binary pulsars, each of which exist in an orbital system with a companion star, spin at up to 1,000 times a second. Some pulsars emit intense X-rays and visible light as well as radio waves.

Selected Stars

Listed below are some of the many neutron stars that have been detected as pulsars:

Star                              Constellation

PSR 1919+21              Vulpecula

PSR 1913+16              Aquila

PSR 0531+21              Taurus (The Crab Pulsar)

PSR 0833-45               Vela (The Vela Pulsar)

Centaurus X-3             Centaurus

Black Holes

When a star explodes as a supernova, it may leave behind a collapsing core. In cores of more than three solar masses, gravity completely overcomes any outwards pressure from the core’s compressed material. In theory, the core contracts into a point of zero volume but infinite density, called a singularity. The gravitational field of the singularity is so intense that it creates a region of space called a black hole around the singularity: once inside a black hole, nothing, not even light, can get out.

Prime Suspects

Astronomers have not identified any black holes for certain. They think, however, that the stellar objects listed below are likely to be black holes:

Object                                                Constellation

Cygnus X-1                             Cygnus

LMC X-3                                Dorado

A 0620-00                               Monoceros

Interstellar Matter

The regions between stars in a galaxy are filled by the interstellar medium, or ISM. It consists mainly of hydrogen and helium gases, with traces of other gases, and a tiny amount of dust. This material is uneven in distribution and temperature, and its density is many billions of times less than that of air. Much of the ISM consists of clouds, some of which we can detect as nebulae if they emit or reflect light from stars in or around the cloud, or if they block light from more distant objects. The ISM is enriched by particles from stellar winds and by matter ejected from dying stars.


Cosmic-Ray Particles

cosmic rays are highly energetic atomic particles travelling through space at almost the speed of light. Primary cosmic rays are high-energy particles outside the Earth’s atmosphere that may enter our atmosphere and collide with other particles to produce secondary cosmic rays. Cosmic rays with the highest energy originate outside our galaxy and come from active galaxies and quasars. Lower-energy cosmic rays come from supernova explosions, supernova remnants, and pulsars inside our galaxy. The lowest-energy cosmic rays originate in the Solar System and come from solar flares.


A Nebula is a cloud, or any other object, made of interstellar gas and dust. Bright nebulae are clouds or wisps of gas that glow or scatter starlight. Dark nebulae do not glow and can be seen  only if they obscure light coming from starfields or bright nebulae behind them. Many objects that were once called nebulae have been reclassified. These objects appeared to astronomers in earlier centuries to be fuzzy, cloud-like structures, but later astronomers with improved telescopes were able to identify these “nebulae” as galaxies or star clusters.



Mercury is the closest planet to the Sun and the second smallest in the Solar System. It has a high density, with about 80 per cent of its mass in its huge, iron core. The surface is scarred by thousands of craters and by scarps (cliffs) that formed when the planet’s young core cooled and shrank, contracting the surface crust. Mercury has almost no atmosphere because its surface gravity is too weak to hold on to one. Being so close to the Sun, with no atmosphere to keep heat in during the night, Mercury has a surface temperature that ranges from -180 to +430 °C (-290 to +800 °F).

Layer                                                     Thickness                              Composition

Crust                                                        –                                          Silicate rock

Mantle                                                 600 km (370 miles)                 Silicate rock

Core (radius)                                       1,800 km (1,100 miles)           Iron and nickel Atmosphere

Mercury’s surface gravity is too weak to hold on to gas particles near the surface. As a result, Mercury’s atmosphere is virtually non-existent, being about 1,000 billion times less dense than the Earth’s atmosphere.


Average distance from Sun      57.93 million km  (36.00 million miles)

Diameter at equator                  4,879 km  (3,032 miles)

Rotation period                         58.65 Earth days

Orbital period                            87.97 Earth days

Orbital velocity                         47.89 km/s     (29.75 mps)

Surface temperature                 -180 to +430 °C   (-290 to +810 °F)

Mass (Earth = 1)                        0.06

Average density                        5.43  (water = 1)

Surface gravity                           0.38  (Earth = 1)

Number of Moons                      0

The Earth’s History

The newly born Earth was a semi-molten ball of very hot material. While the heavier elements sank to the centre to form a metal core, the lighter elements rose up to form the rocky mantle and crust. Over billions of years the planet cooled, the surface solidified, an atmosphere developed, and oceans formed. Today, the planet continues to evolve: the crust is renewed by volcanic eruptions on the ocean floors and is constantly changed by earthquakes and continental drift. The proportions of different gases in the atmosphere are also slowly changing due to human influence.

Continental Drift

The surface of the Earth is a rocky crust composed of over a dozen separate plates. Each continent is embedded in one or more plates. These plates move relative to one another at about the same rate that a fingernail grows. The rigid plates “float” on semi-molten rock, which is churned slowly by hot currents emanating from the Earth’s iron core. As the rock churns, the plates above it move slowly over the surface.

The Magnetosphere

As the Earth spins, eddies in its outer core of molten iron generate electric currents. These currents generate a magnetic field that extends into space, forming a protective “blanket” around the planet. This field, known as the magnetosphere, protects the Earth from the high-speed streams of charged particles that flow from the Sun in the solar wind. Some of the particles become trapped in the field in two regions called the Van Allen Belts.

The Inhabited Planet

The Earth is unique in the Solar System, having abundant supplies of water, oxygen, and nitrogen, all of which are essential to life as we know it. Living organisms first appeared on the Earth about 3.8 billion years ago, and the dinosaurs appeared around 150 million years ago. One explanation for their extinction (around 65 million years ago) is that a meteorite hit the Earth, filling the atmosphere with dust. This would have blocked light and heat from the Sun and caused a brief ice age, during which the dinosaurs would have died of cold and hunger.

The Moon

The Moon is the Earth’s only natural satellite. It is a rocky sphere with a diameter about a quarter that of the Earth. The Moon emits no light of its own, but we can see it when it reflects sunlight towards us. It is a lifeless and dusty place with no water. It also has no atmosphere because its surface gravity is too weak to hold on to gas particles. The lunar surface is scarred by thousands of craters. Volcanic lava has seeped through the lunar crust into some of the largest craters, forming maria, or “seas”.


Layer            Thickness                                   Composition

Crust            60-100 km (40-60 miles)            Rock and dust

Mantle          1,000 km (620 miles)                 Solid rock

Outer core       –                                               Semi-solid rock

Inner core (radius)       _                                 Rock or iron


Average distance from Earth  384,500 km (238,900 miles)

Diameter at equator                3,476 km  (2,160 miles)

Rotation period                       27.32 Earth days

Orbital period                          27.32 Earth days

Orbital velocity                       1.02 km/s (0.63 mps)

Surface temperature                -155 to +105 °C (-250 to +220 °C)

Mass (Earth = 1)                     0.01

Average density                      3.34     (water = 1)

Gravity (Earth = 1)                 0.16

Number of Moons                   0

Formation of the Moon

The moon and the Earth formed at around the same time, 4.5 billion years ago. The exact origins of the Moon, however, are still a mystery. It might have formed alongside the Earth in the young Solar System, or it might have been captured at a later date by the Earth’s gravitational field. The most popular theory is that it was formed when the Earth was hit by a Mars-sized asteroid.

Craters and Seas

The lunar surface was heavily cratered by meteorite bombardment more than 3.5 billion years ago. The craters are up to 300 km (185 miles) across and are rimmed by walls of

rocky mountains blown out by the impact. Some craters have terraced walls or concentric rings of mountains, and many have central mountainous peaks. The most spectacular craters have huge, bright rays of ejecta. Some of the largest are filled with solidified volcanic lava, forming maria (seas).


Mars, the red planet, is the fourth planet from the Sun. It is the planet most similar to our own: its day is only slightly longer than ours; its pattern of seasons is similar to ours, although twice as long; and Mars has clouds, volcanoes, canyons, mountains, deserts, and white polar caps that shrink and grow with the seasons. Mars is an arid, cold, and lifeless planet: its surface is littered with rocks and is mainly covered in red dust, while its atmosphere is thin and poisonous to human beings.


Layer                Thickness                              Composition

Crust                40-50 km (25-30 miles)          Solid rock and water ice

Mantle              2,000 km (1,250 miles)          Silicate rock

Core (radius)   1,250 km (780 miles)              Solid rock and iron


CO2 = 95 %, Nitrogen = 2.7 %  , Argon =1.6 %    , oxygen, CO, water vapor= 0.7 %


Average distance from Sun    227.94 million km  (141.64 million miles)

Diameter at equator                6,786 km (4,217 miles)

Rotation period                      24.62 hours

Orbital period                         686.98 Earth days

Orbital velocity                      24.14 km/s (15.00 mps)

Surface temperature               -120 to +25 °C            (-180 °F to +80 °F)

Mass (Earth = 1)                    0.11

Average density(water = 1)   3.95

Surface gravity (Earth = 1)     0.38

Number of moons                   2

The Moons of Mars

Mars has two tiny moons, Phobos and Deimos. Their irregular, potato-like shapes suggest that they are asteroids that have been caught by the gravitational field of Mars and pulled into orbit around the planet. Both moons have cratered surfaces.

The Martian landscape

The southern hemisphere of Mars is scarred by impact craters that formed at least 3.5 billion years ago. The surface of the northern hemisphere is younger because much of it has been formed by more recent volcanic activity. Mars has two of the most  spectacular features in the Solar System: Olympus Mons, the largest-known volcano, and Valles Marineris, a canyon up to 7 km (4.5 miles) deep and 600 km (360 miles) wide. It also has many smaller channels, probably formed in the past by running water.

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Jupiter is the fifth planet from the Sun and the first of the four gas giants. It is the largest and the most massive of the planets: its volume is 1,300 times that of the Earth, and its mass is two and a half times that of all the other planets combined. Jupiter’s banded clouds consist mainly of hydrogen and helium gases. The planetary interior begins about 1,000 km (600 miles) below, where the hydrogen becomes liquid. Deeper still the hydrogen is metallic. At the heart of Jupiter is a very hot rocky core – about 35,000 °C (63,000 °F).


Layer               Thickness                               Composition

Atmosphere     1,000 km (620 miles)              Mainly hydrogen and helium gases

Outer mantle   22,500 km (14,600 miles)       Liquid hydrogen

Inner mantle    33,000 km (20,500 miles)       Liquid metallic hydrogen

Core (radius)   14,000 km (8,700 miles)         Rock


Hydrogen = 86 % , Helium = 13 % , ammonia, methane, water vapor = 1 %


Average distance from Sun    778.33 million km (483.63 million miles)

Diameter at equator                142,984 km     (88,846 miles)

Rotation period                       9.84 hours

Orbital period                          11.86 Earth years

Orbital velocity                       13.06 km/s (8.11 mps)

Cloud-top temperature            -150 oC (-240 °F)

Mass (Earth = 1)                     317.93

Average density (water = 1)   1.33

Surface gravity (Earth = 1)     2.54

Number of moons                   16

Belts and Zones

Jupiter spins very fast, as do all the gas giants, taking less than 10 hours to rotate once. This fast spin produces bands of clouds in the upper atmosphere, called zones and belts, that run parallel to the equator. Weather disturbances, apparent as spots, ovals, and streaks, may last for months or years.

The Great Red Spot was first observed more than 300 years ago.

The Great Red Spot

The Great Red spot is a huge anticyclonic region – a storm – in Jupiter’s upper clouds. Since the spot was first observed, its diameter has been seen to reach three times that of the Earth. Its red and pink colouring may be caused by phosphorus sucked into the Spot’s eddying gas currents from the lower atmosphere. The Spot is higher and colder than its surroundings and completes one anticlockwise rotation in about 12 Earth days.

Jupiter’s Rings

Jupiter’s ring system was discovered in 1979 by the Voyager I probe. There are three named rings: the doughnut-shaped Halo Ring, which is 22,800 km (14,170 miles) wide; the narrow, bright Main Ring, which is only 6,400 km (3,980 miles) wide; and finally the Gossamer Ring, the least dense and the widest at 85,000 km (53,000 miles).

The Moons of Jupiter

Jupiter’s 16 moons fall into four groups of four. The first group starts about 130,000 km (80,000 miles) from the planet; the second is about 200,000 km (125,000 miles) beyond that. The third group begins 9 million km (5.7 million miles) further away, and the fourth lies a similar distance beyond that. All the groups, except the fourth, orbit in the same direction as Jupiter spins. All the moons are small, except for those in the second group, the Galileans, which are similar in size to our moon.


Uranus is the seventh planet from the Sun and the third of the four gas giants. Its rocky core is wrapped in a mantle of gases and ices. Surrounding the mantle is an atmosphere containing methane, which gives Uranus its distinctive blue-green colour. Uranus lies in the cold outer reaches of the Solar System, and its cloud tops have a temperature of -210 °C (-346 °F). Although it has 15 moons and a ring system, Uranus itself is almost featureless. The only features observed by the space probe Voyager 2 during its 1986 visit were a few methane clouds.


Layer               Thickness                               Composition

Atmosphere     1,000 km (620 miles)              Mainly hydrogen and helium gases

Mantle             10,000 km (6,000 miles)         Ice, liquid water, ammonia and methane

Core (radius)   8,000 km (5,000 miles)           Solid Rock


Hydrogen = 85 %, Helium = 12 %, methane = 3 %


Average distance from Sun    2.87 billion km (1.78 billion miles)

Diameter at equator                51,118 km (31,763 miles)

Rotation period                       17.90 hours

Orbital period                          84.01 Earth years

Orbital velocity                       6.81 km/s (4.23 mps)

Cloud-top temperature            -210 oC  (-350 oF)

Mass (Earth = 1)                    14.53

Average density(water = 1)    1.29

Surface gravity (Earth = 1)     0.79

Number of moons                   15

The Sideways-Spinning Planet

The rotational axis of Uranus is tilted by about 98° to the plane of the planet’s orbit around the Sun. So, unlike the other planets, Uranus spins on its side. The extreme tilt of Uranus results in its poles each spending 42 Earth years in continuous sunlight and 42 Earth years in darkness during a single orbital period of 84 Earth years. Uranus is so far away from the Sun, however, that the temperature difference between summer and winter at the poles is only 2 °C (3.6 °F).

The Rings of Uranus

The rings surrounding Uranus are difficult to see because they are made of some of the darkest material in the Solar System. They were first seen from the Earth in 1977, when they obscured light from a star. Voyager 2 looked closely at the system of 11 narrow rings in1986. They consist of rocks about 1 m (1 yd) across. The Epsilon Ring varies in width from 20 to 100 km (12 to 60 miles).

The Rings of Uranus

The rings surrounding Uranus are difficult to see because they are made of some of the darkest material in the Solar System. They were first seen from the Earth in 1977, when they obscured light from a star. Voyager 2 looked closely at the system of 11 narrow rings in1986. They consist of rocks about 1 m (1 yd) across. The Epsilon Ring varies in width from 20 to 100 km (12 to 60 miles).


Neptune is the eighth planet from the Sun and the fourth gas giant. It is similar in size and structure to its neighbour Uranus. The bright blue colour of its atmosphere is caused by methane. There are several cloud features, the most prominent being the Great Dark Spot, a huge storm system as big as the Earth. The cloud features are blown around the planet by the fastest winds in the Solar System, at speeds of up to 2,200 km/h (1,370 mph). Beneath the clouds is a mantle of ices and gases and a small rocky core.


Layer               Thickness                                Composition

Atmosphere                 –                                   Hydrogen, helium, and methane gases

Mantle             10,000-15,000 km                   Icy and liquid water, ammonia, and methane Core (radius)   6,000 km (3,700 miles)               Solid rock


Hydrogen = 85 %, Helium = 13 %, methane = 2 %


Average distance from Sun      4.49 billion km) (2.79 billion miles)

Diameter at equator                49,528 km      (30,775 miles)

Rotation period                       19.20 hours

Orbital period                          164.79 Earth years

Orbital velocity                      5.47 km/s         (3.40 mps)

Cloud-top emperature             -220 °C            (-360 °F)

Mass (Earth = 1)                                             17.14

Average density(water = 1)    1.64

Surface gravity(Earth = 1)      1.20

Number of moons                   8

The Great Dark Spot

The Great Dark Spot and Small Dark Spot are oval-shaped anticyclones in Neptune’s atmosphere. The fastest winds in the Solar System sweep them “backwards” around

the planet (in a direction opposite to Neptune’s direction of spin).

A small cirrus cloud called the Scooter lies at a different altitude to the spots, where it is less windy; this cloud remains in the same position relative to Neptune’s core and is

carried by the planet’s rotation in the opposite direction to the spots.

Fact File

The Great Dark Spot is a huge mass of gases as wide as the Earth, which is swept around Neptune at speeds of around 1,000 km/h (620 mph) – almost as fast as the speed of sound. The winds blow at twice this speed, around 10 times faster than wind speeds in a hurricane on Earth.

The Moons of Neptune

Of Neptune’s eight moons, only Triton and Nereid were known of before Voyager 2 investigated Neptune in 1989. Triton is the coldest-known object in the Solar System, with a surface temperature of -235 °C (-391 °F ). It is surrounded by a thin atmosphere of nitrogen.

Neptune’s Rings

Neptune’s rings stretch from about 40,000 to 63,000 km (25,000 to 39,000 miles) from the planet. There are one broad and three narrow rings, all very dark. The Adams and Le Verrier Rings are named after astronomers who predicted Neptune’s existence and position; the Galle Ring is named after Neptune’s discoverer, the German astronomer Johann Galle (1812-1910). The Voyager 2 probe discovered clumps of ring material in the Adams Ring, whose existence astronomers cannot explain.


Pluto, the ninth planet from the Sun, is a cold, dark world in which the Sun appears as only a bright star in the sky. Pluto is smaller than the Earth’s Moon, and consists of rock and ices. It has a thin atmosphere that forms when the planet is close to the Sun but freezes as it moves away. Pluto’s orbit is tilted further from the ecliptic than that of any other planet, and is also very elongated. One orbit lasts 248.5 years and, for about 20 of those years, Pluto is closer to the Sun than Neptune is. These unusual features lead some astronomers to propose that Pluto may, in fact, be a huge asteroid.


Layer              Thickness                     Composition

Crust              50 km (30 miles)          Icy water and methane

Mantle           150 km (90 miles)         Water ices

Core (radius) 900 km (560 miles)       Rock and, possibly, ice

Atmosphere: methane mixed with nitrogen.


Average distance from Sun    5.91 billion km  (3.67 billion miles)

Diameter at equator                2,290 km  (1,423 miles)

Rotation period                       6.39 Earth days

Orbital period                           248.54 Earth years

Orbital velocity                        4.74 km/s        (2.94 mps)

Surface temperature                -230 oC (-380 °F)

Mass (Earth = 1)                     0.01

Average density(water = 1)    2.03

Surface gravity (Earth = 1)     0.04

Number of moons                   1


Charon, Pluto’s only moon, probably once had a similar composition to Pluto. Today, however, Charon is covered mainly by dark water ice, and Pluto mainly by bright methane ice. It may be that methane molecules have been gradually attracted away from Charon towards Pluto by the planet’s stronger gravitational field. Like any objects in an orbital system, Pluto and Charon orbit around a common centre of mass. A large moon, Charon’s diameter is about half that of Pluto and accounts for 12 per cent of the system’s mass. The system’s centre of mass lies outside Pluto’s surface.



The Cosmos Series

Cosmos is the name given to a series of Soviet satellites and spacecraft. Some are military satellites, others prototypes for new craft. In the early days of space flight, when launches were less reliable, a new probe or satellite would be given a Cosmos number at launch, and a name, such as Luna or Venera, once the mission was under way. The first Cosmos satellite was launched on 16 March 1962. Over 2,300 have been launched to date.

Cosmos 1

Launch date  16 March 1962

Cosmos 1 was launched, carrying equipment to measure the Earth’s upper atmosphere.

Cosmos 4

Launch date  26 April 1962

Cosmos 4 was a first in a long series of Soviet reconnaissance missions. The spacecraft, an unpiloted Vostok with camera equipment installed, was recovered after 3 days in orbit.

Cosmos 7

Launch date  28 July 1962

The seventh Cosmos vehicle undertook an orbital mission. Although press releases at the time emphasized the mission’s scientific role, it was subsequently revealed that the primary aim of the mission was to photograph American military bases.

Cosmos 954

Launch date  18 September 1978

Remains of this nuclear-powered satellite fell on Canada in January 1978. At the time, the incident caused a major row between the governments of the USSR and Canada, because the multi-million dollar bill for the clean-up operation was presented to the Soviet ambassador for Canada.

Cosmos 186 and 188

Launch date  30 October 1967

Cosmos 186 and 188 achieved the first automatic rendezvous and docking during Cosmos 186’s 49th Earth orbit. The spacecraft were linked together for 3 1/2 hours.

Cosmos 1074

Launch date  31 January 1979

Cosmos 1074 was an extended test flight for the new generation of a Soyuz piloted spacecraft. The purpose of the 60-day unpiloted mission was to test the spacecraft’s modified systems and durability for extended missions to the Salyut space stations.

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Cosmos 1267

Launch date  25 April 1981

Cosmos 1267 was the second flight of the Star module – also known as a “Heavy” Cosmos module. The spacecraft was almost as large as its Salyut target, and had the capacity to carry

a wide variety of scientific and military payloads into orbit. After manoeuvres in low-Earth orbit, a re-entry capsule was released from the module on 24 May, prior to the successful docking with Salyut 6 on June 19.

Cosmos 1374

Launch date  3 June 1982

The launch of Cosmos 1374 signalled the beginning of development of the Soviet space shuttle. This particular model had a wingspan of only 2.5 m (8 ft). However, it provided Soviet scientists with useful information regarding the durability of reusable heatshield materials.

Cosmos 1686

Launch date  27 September 1985

Cosmos 1686 saw the further development of the Star series of spacecraft. This model differed greatly from its predecessors in that it did not have the capacity to return materials to Earth via a re-entry module. It was stocked with a wide range of scientific equipment and telescopes to be used for astral observation and also provided a testbed for many of the electrical and mechanical systems that were to be fitted to the Mir space station. It docked with Salyut 7, and, with the Star module attached, the volume of the space station was almost doubled.

The Vanguard Programme

Set up to launch the first US satellite, the Vanguard programme was not a success. The aim of the project was to build a rocket capable of launching a 9-kg (20-lb) satellite into orbit. The first two launch attempts were public failures, and by the time Vanguard 1 went into orbit on17 March 1958, Explorer 1 had become the first US satellite and the USSR had already launched two Sputnik satellites. There were two more successful Vanguard launches, but eight of the 11 attempts failed, so the programme was terminated.


A payload is the cargo that a rocket or shuttle carries into orbit. This may be a satellite, a spacecraft, or a scientific instrument. The size of payload that a rocket can carry depends on the size of the rocket and its payload bay, and on the amount of thrust produced by the rocket’s engines. The US Space Shuttle, for example, can deliver 29,500 kg (65,000 lb) of cargo into orbit.

The Vostok Programme

Between April 1961 and June 1963, there were six Vostok missions, each sending a cosmonaut into Earth orbit. The first of these missions carried the first person into space, the cosmonaut Yury Gagarin. Vostok craft had two parts, a spherical cabin for the cosmonaut and an equipment module.

Strelka and Belka

The first animals to go into orbit and return to Earth were two dogs named Strelka and Belka. Their mission, in August 1960, was a rehearsal for Vostok and proved that a living creature could travel into space and return safely to Earth. The dogs spent a day in space, orbiting the planet 18 times, before their capsule fired its motor to re-enter the atmosphere and return to Earth. There were two more successful flights with dogs prior to Yury Gagarin’s historic journey in Vostok 1.

First Person in Space

On 12 April 1961, Yury Gagarin became world famous as the first person to travel into space. In his spacecraft, Vostok 1, he travelled once around the Earth on a flight lasting only 108 minutes. Because scientists did not know what the effects of weightlessness

on him might be, Gagarin’s spacecraft was controlled entirely from Earth.

First Woman in Space

On 16 June 1963, the final Vostok spacecraft, Vostok 6, was launched to join Vostok 5, which had been in orbit for 2 days. The cosmonaut on board Vostok 6, Valentina Tereshkova, became the first woman in space. The two spacecraft spent another 3 days in orbit before returning to Earth. It was nearly 20 years before another woman, Svetlana Savitskaya, went into space, on Salyut 7 in 1982.

Vostok 2

The second person to travel into space, Gherman Titov, was the first to spend a whole day there. His Vostok 2 craft was launched on 6 August 1961. Titov was able to do little except take  photographs, however, as he was the first to suffer from space sickness. This is similar to travel sickness on Earth and many astronauts experience it, but it was unexpected before this mission.

Two Craft in Orbit

The third and fourth Vostok spacecraft orbited the Earth together. Vostok 3, carrying Andryan Nikolayev, was launched on 11 August 1962, and Pavel Popovitch, in Vostok 4, followed on 12 August. They stayed in orbit for 3 days and came as close as 6 km (3 1/2 miles) to each other. However, they were unable to manoeuvre, so any attempts to dock or rendezvous the craft were not possible. They transmitted the first TV signals from space, which showed the public what weightlessness was like.

The Mercury Programme

After several test flights, the first of the USA’s six successful piloted Mercury space flights took place on 5 May 1961. The first two missions were suborbital, but the remaining four took their astronauts into Earth orbit. The Mercury capsules were just large enough to hold one astronaut and his equipment.

A small escape rocket attached to the capsule could pull it to safety if there was an emergency during a launch. At the end of a mission, the Mercury craft splashed down into the ocean. The shock of impact was softened by an air cushion, which inflated at the bottom of the capsule.

The Mercury Seven

ON APRIL 9, 1959, NASA announced the names of the men chosen to become America’s first astronauts. Selected from over 500 applicants, the Mercury Seven were all experienced test pilots from the US Air Force, Navy, or Marine Corps. Each was required to be less than 1.8 m (5 ft 11 in) tall, under 40, and in peak physical condition. The future astronauts underwent rigorous training to prepare for their missions.

First American in Space

On 5 may 1961, Alan Shepard became the second person to travel in space, and the first American, 4 weeks after Yury Gagarin’s historic first space flight. Shepard’s Mercury capsule did not go into orbit. It did a “hop”, taking  the astronaut 187 km (116 miles) high, and landing in the sea 485 km (303 miles) from the launch site. Virgil Grissom repeated the feat 11 weeks later, but had to be rescued when his capsule started to sink on splashdown.

Americans in Orbit

On 20 February 1962, John Glenn became the first American to go into orbit. His Mercury flight, in Friendship 7, took him around the Earth three times and lasted almost 5 hours. There were two more Mercury orbital flights before the final mission, when Gordon Cooper completed 22 orbits on a 34-hour flight. This made him the first American to spend more than a day in space.

Key Date

Mercury test flights

Launch dates  July 1960 to April 1961

Seven Mercury test flights were undertaken during this period, including, on 31 January 1961, a suborbital mission, which was survived by Ham, a chimpanzee.

Mercury 3

Launch date  5 May 1961

Alan Shepard became the first US citizen to travel into space when his Mercury spacecraft Freedom 7 was launched from Cape Canaveral. Shepard’s suborbital flight lasted 15 minutes and 22 seconds.

Mercury 4

Launch date  21 July 1961

Virgil Grissom became the second American astronaut to reach space. His capsule Liberty Bell 7 was launched by a modified Redstone ballistic missile. After a successful flight lasting 15 minutes and 37 seconds, the capsule splashed down in the Atlantic, sinking before it could be recovered. Grissom managed to swim clear and was subsequently picked up from the water.

Mercury 5

Launch date  29 November 1961

A chimpanzee called Enos returned to Earth after completing two orbits in a Mercury craft. The mission paved the way for orbital flights involving human beings.

Mercury 6

Launch date  20 February 1962

John Glenn became the first American to orbit the Earth in Friendship 7. Glenn completed three orbits of the Earth.

Mercury 7

Launch date  24 May 1962

Scott Carpenter completed three orbits of the Earth before landing his Aurora 7 capsule

400 km (250 miles) off course.

Mercury 8

Launch date  3 October 1962

Nine hours and 13 minutes after it was launched from Cape Canaveral, Walter Schirra’s Sigma 7 splashed down in the Pacific Ocean only 7.2 km (4!/2 miles) from recovery ship the US aircraft carrier Kearsarge.

Mercury 9

Launch date  15 May 1963

Gordon Cooper became the first American to spend more than 24 hours in space, orbiting the Earth for 34 hours and 20 minutes in his craft, Faith 7. Cooper’s mission was the final flight in the Mercury programme.

Early Rockets

Resembling fireworks, the first rockets were made about a thousand years ago in China. Their design hardly changed until the 19th century, when rockets such as the Congreve rocket were used in battle. In 1903, a Russian scientist, Konstantin Tsiolkovsky, proposed that rockets using liquid propellants could reach space. Enthusiasts started to build such rockets during the 1920s. In 1926, an American scientist, Robert Goddard, achieved the first successful launch. During the 1930s, the German military took control of rocket groups and developed the V2 rocket.

Balloons in the Atmosphere

In addition to rockets and satellites, astronomers use balloons to take their instruments high into the atmosphere. This means that they can detect the types of radiation, such as X-rays, that cannot reach telescopes on the ground. Balloons also allow scientists to study the upper reaches of the atmosphere and monitor the ozone layer, which protects us from the Sun’s harmful ultraviolet radiation.

The V2 Rocket

In 1944, towards the end of World War II, a powerful new weapon was used by the Germans to attack Paris, London, and Antwerp – the V2 rocket. Developed by Wernher von Braun’s team at PeenemŸnde in Germany, the rocket was able to carry a warhead weighing 1,000 kg (2,200 lb), a distance of 320 km (200 miles). The maximum height it attained was 160 km (100 miles), making the V2 the first rocket to reach space.

From Missiles to Space Rockets

Following World War II, a race began between the USSR and the USA to build rockets capable of transporting nuclear weapons. These missiles were so powerful that they could be modified to carry a payload, such as a small satellite, into space and put it into Earth orbit. Among these were modified Jupiter C and Sapwood missiles, which launched Explorer 1 and Sputnik 1 respectively.

Sputnik 1

On 4 October 1957, the USSR astonished the world by launching the first ever artificial satellite, Sputnik 1. It was a metal sphere the size of a large beach ball with four antennae to send radio signals back to Earth. To announce that it was in space, Sputnik transmitted a beeping sound. The satellite was equipped with instruments to measure the temperature and density of the top of the atmosphere, and it sent the results down to Earth for 21 days until its batteries ran out. After 96 days in orbit Sputnik 1 fell back into the atmosphere, burning up on re-entry.

Sending Animals into Space

After the first successful satellites had been launched, scientists wanted to send human beings into space. However, they did not yet know if people could survive space flight. The first space traveller was, therefore, a Soviet dog named Laika. Apes soon followed, and, since then, many species of animals have been into space, yielding much information on the effects of space travel.

The Explorer Programme

A series of over 50 satellites, the Explorer programme started with the first US satellite, Explorer 1, in 1958, and continues to provide useful information to scientists today. Explorer satellites have investigated phenomena  such as radiation from objects in space, radiation around the Earth, the upper layers of the atmosphere, and the Sun’s radiation. Recent Explorer craft have flown through a comet’s tail and mapped cosmic microwave background radiation.

Explorer 1

Launched on 1 February 1958, Explorer 1 was the first US satellite to reach Earth orbit. Being only 16.5 cm (6 1/2 in) in diameter, it contained miniature instruments to measure atmospheric temperature, detect radiation levels, and observe meteoroids. Explorer 1 is famous for discovering one of the Van Allen Belts, regions of strong radiation around the Earth. The satellite reached a height of 2,540 km (1,575 miles) above the surface of the Earth. Explorer 1, the USA’s first satellite was launched from Cape Canaveral. The Van Allen Belts -Êbelts of radiation in the Earth’s magnetosphere – were discovered by this mission. Explorer 1 continued to orbit the Earth until 1970.

Explorer 2

Launch date  5 March 1958. Disaster struck at the launch of Explorer 2 when its rocket booster exploded on the launch pad.

Explorer 3

Launch date  26 March 1958

The third Explorer satellite was placed into orbit to continue the discoveries made by Explore

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Explorer 23

Launch date  6 November 1964

Explorer 23 was launched to investigate the potentially destructive effects of meteoroids on space probes.

Explorer 33

Launch date  1 July 1966

The fourth in a series of 10 Interplanetary Monitoring Platforms, Explorer 33, investigated the Earth’s magnetic field and radiation near the Earth. The first in the series was Explorer 18, which was launched on November 26 1963, and the last was Explorer 50, launched on 26 October 1973.

Explorer 42

Launch date  12 December 1970

The first of three Small Astronomy Satellites (the others were Explorers 48 and 53), Explorer 42 carried out surveys of X-ray and gamma-ray sources in space. The Small Astronomy Satellites also found the first evidence of a black hole.

Explorer 49

Launch date  10 June 1973. The second of two Explorer radio astronomy satellites (the first was Explorer 38), Explorer 49 was actually placed in lunar orbit to shield its instruments from interference from Earth. Both Explorer 38 and Explorer 49 listened to radio emissions from the Milky Way.

Explorer 55

Launch date  20 November 1975

The last of three Explorers to study the Earth’s atmosphere, Explorer 55 conducted research into pollution in the Earth’s upper atmosphere, including the ozone layer.

ISEE 1 and 2 (International Sun-Earth Explorers)

Launch date  22 October 1977

The International Sun-Earth Explorers were launched to explore the Sun’s effects on the Earth’s magnetic field.

ISEE 3 (ICE) (International Cometary Explorer)

Launch date  12 August 1978

The first spacecraft to achieve a “halo orbit” – an orbit around a point in space as opposed to a particular celestial object. ISEE 3 was also the first craft to travel through the tail of a comet – the Giacobini-Zinner Comet in 1985; the craft also observed Halley’s Comet in 1986. This resulted in the craft’s name change to the International Cometary Explorer.

IUE (International Ultraviolet Explorer)

Launch date  26 January 1978

Still in operation, the International Ultraviolet Explorer was launched to study ultraviolet emissions from stars and galaxies.

EUVE  (Extreme Ultraviolet Explorer)

Launch date  7 June 1992

This orbiting ultraviolet observatory has successfully located sources of high-energy ultraviolet radiation – some of which were located outside the Milky Way Galaxy.



Reconnaissance for Apollo

In 1961, US President John Kennedy announced that an American would land on the Moon before the decade had ended. However, little was known about the lunar surface, and no American had even orbited the Earth. To find out if spacecraft could land safely on the Moon’s surface and take off again, robot probes were dispatched. In total, 13 Ranger, Surveyor, and Lunar Orbiter probes travelled to the Moon, gathering the vital information needed to land the Apollo craft there.


The Ranger Series

Launched between 1961 and 1965, the Ranger probes were intended to hit the surface of the Moon. The first  three probes did not reach their target. The fourth did crash into the Moon but failed to return any images. After another two fruitless missions, Rangers 7, 8, and 9 were successful, providing accurate, close-up images of the Moon. These enabled scientists developing Apollo to select suitable landing sites for their craft.

The Lunar Orbiter Probes

Between august 1966 and August 1967, five Lunar Orbiter spacecraft orbited the Moon. The probes returned detailed pictures of the surface and measured levels of radiation and space dust. The first three looked for suitable Apollo landing sites along the Moon’s equator. The last two Lunar Orbiter probes travelled in an orbit that took them over the poles of the Moon.

The Surveyor Series

Between June 1966 and January 1968, five Surveyor spacecraft soft landed on the Moon’s surface. During the mission, the probes tested the lunar soil to determine its composition and assess whether the surface was strong enough for the proposed Apollo missions to land. Each of the probes in the series returned information and images detailing the prospective landing sites. In November 1969, Apollo 12 landed 180 m (600 ft) from Surveyor 3; the astronauts on the mission returned parts of the probe to Earth for analysis.

Kennedy’s Speech

‘I believe that this nation should commit itself to achieving the goal, before this decade is out, of landing a man on the Moon and returning him safely to the Earth.’

The First Apollo Missions

The aim of the Apollo program was to land the first person on the Moon. However, when the program was announced, in 1961, only two people had actually been into space. Scientists were faced with a daunting task -Êto construct a rocket powerful enough to reach the Moon and a spacecraft that could travel there and back.

Saturn V

Built for the Apollo missions, the giant Saturn V rocket was one of the most powerful rockets ever built. It was extremely successful, with 13 perfect launches and no failures, even during its test flights. Saturn V carried the Apollo craft into Earth orbit; then the third and final stage fired again to provide an extra boost to place it on the correct trajectory for the Moon.

The Apollo Spacecraft

Three main components made up the Apollo spacecraft. The command module (CM) held supplies and contained the crew’s quarters, which had just enough room for the astronauts to move around in. Attached to the CM was the service module (SM), which housed the craft’s main engine. It supplied electrical power and controlled the crew’s life-support system. On returning to Earth, the SM was jettisoned; the CM, with the astronauts inside, returned to Earth. The third part of the craft was the lunar module.

First Around the Moon

The three Apollo 8 astronauts, Frank Boorman, James Lovell, and William Anders, became the first people to travel around the Moon. Their space-craft was launched on 21 December 1968; 3 days later they were in lunar orbit. To return, Apollo 8’s engine was fired while the craft was behind the  Moon and out of contact with the Earth. Ground control then had to wait for Apollo 8 to reappear to be certain that it was safely on the way home.

Preparing for Touchdown

The Apollo Lunar Module first flew in space in March 1969 on the Apollo 9 mission. In Earth orbit, the Apollo 9 astronauts practised docking, undocking, and firing the Lunar Module’s (LM) engines to test the procedures for a lunar landing mission. Two months later, Apollo 10 flew to the Moon, and the LM swooped down to 15 km (9 miles) above the surface. The only thing the mission did not do was actually land on the Moon.

Apollo 11 and After

On 20 July 1969, Apollo 11 landed two astronauts in the Moon’s Sea of Tranquillity. Neil Armstrong was the first to climb down on to the surface, followed by Edwin “Buzz” Aldrin. They spent about 22 hours on the Moon, 2 1/2 of which were spent outside the lunar module. They then took off to rejoin Michael Collins, who was orbiting in the command and service module.

Emergency – Apollo 13

An oxygen tank exploded in the service module as Apollo 13 drew close to the Moon. This left the astronauts without sufficient oxygen to breathe, and without their power supply. They were now unable to fire the main engine. For the rest of their mission, the astronauts had to live in their lunar-module “lifeboat” and use its engine to speed them back to Earth.

Lunar Rover

A four-wheeled electric car powered by batteries, the lunar rover had a top speed of only 14 km/h (9 mph). It carried cameras to record the lunar missions, antennae for communicating with ground control, and bags to carry rock samples. The buggy travelled to the Moon folded against the outside of the lunar module.

The Apollo Experiments

All the Apollo missions carried experiment packages to discover as much as possible about the Moon. Much of the equipment was left on its surface to continue collecting data after the astronauts had left. The Apollo Lunar Scientific Experiments Packages – ALSEPs – had their own power supplies and radio transmitters to send results back to Earth. Specially designed equipment was taken to the Moon to collect samples of rock and soil. Studies of the Moon rocks that were returned to Earth revealed that the Moon contains elements similar to those found on the Earth.

The Apollo-Soyuz Programme

In July 1975, the USSR and the USA co-operated in space for the first time. After being launched by the Saturn 1B rocket, an orbiting US Apollo spacecraft carrying three astronauts docked with a Soviet Soyuz capsule that had two cosmonauts on board. A unique docking module was used, which also had to function as an airlock as the US craft contained oxygen, but the Soviet craft had a mixture of oxygen and nitrogen at a higher pressure.

The Voskhod Programme

There were only two missions in the Voskhod programme, but both broke records. Voskhod 1, in October 1964, was the first spacecraft to carry more than one person. Its crew of three included a spacecraft designer, a physician, and a pilot. Voskhod 2, a two-person mission, featured the first  space walk. The Voskhod spacecraft was a modified Vostok, the first piloted Soviet spacecraft. To make room for the three cosmonauts, the Vostok’s ejector seat was replaced by three couches. This meant that the cosmonauts landed inside the capsule, instead of parachuting to the ground separately.

First Space Walk

Aleksey Leonov, one of two cosmonauts on Voskhod 2, was the first person to leave

a spacecraft and “walk” in space. On18 March 1965, Leonov prepared himself in the craft’s inflatable airlock. He then entered space to spend 10 minutes there. Leonov had difficulty getting back into the airlock as his suit had expanded; he struggled for 8 minutes to get back inside.

The Gemini Programme

Between March 1965 and November 1966, there were 10 piloted Gemini missions. The aim of the programme was to test as many of the techniques as possible that would be needed for the planned Apollo missions. Among the aims of the project were to rendezvous and dock two craft and achieve successful space walks by astronauts. Unlike the earlier Mercury craft, Gemini capsules were built so that they could be manoeuvred in any direction by the crew. The Gemini craft were built to take two astronauts; each craft was launched by a Titan II rocket.

First Space Docking

On 16 March 1966, an Agena rocket was sent into orbit, followed 90 minutes later

by Gemini 8. The astronauts on the mission, Neil Armstrong and David Scott, guided their Gemini craft to dock successfully with the Agena; however, the two craft then began to spin rapidly and tumble. Fortunately, the astronauts managed to separate and stabilize the capsule, and they returned home safely.


The Luna Programme

The Luna spacecraft were robot explorers sent by the USSR to the Moon between 1959 and 1976. Of the 24 missions, Luna 1 was the first craft to escape the Earth’s gravity, flying past the Moon and into orbit around the Sun; Luna 2 was the first probe to hit the Moon, and Luna 10 orbited it, to become its first artificial satellite.

Collecting Moon Rock

Three Luna probes collected samples from the Moon’s surface in 1970, 1972, and 1976. After landing on the Moon, the probes extended a mechanical arm that drilled into the surface to extract soil and rock. This was placed in a capsule at the top of the spacecraft, which then returned the cargo of Moon rock to scientists on Earth.


Two Luna spacecraft delivered robot vehicles, Lunokhods 1 and 2, to the Moon in 1970 and 1973. These mobile laboratories crawled along, exploring and photographing the surface. They tested soil samples and sent back images and information to Earth. During the lunar night they stopped moving and closed their lids to conserve energy.


Rocket Power

Space rockets work in the same way as fireworks; fuel is mixed with a material called

an oxidizer, which contains oxygen, the gas necessary for combustion. This mixture, the propellant, burns to form hot gases, which expand, escaping through a nozzle and causing the rocket to move upwards. This reaction was first described by the 17th-century English scientist Isaac Newton in his Third Law of Motion.He stated that for every action (the escaping gases) there is an equal and opposite reaction (the rocket’s movement).

Gravity and Thrust

Gravity, more properly referred to as gravitational force, is a force that pulls any two objects together. The force is stronger between more massive objects and gets weaker as they move further apart. The Earth’s gravity pulls everything on or near its surface down towards its centre. A ball thrown into the air falls to the ground, drawn by this pull of gravity. The harder the ball is thrown, the higher it goes before falling. If you could throw it hard enough, it would escape the pull of gravity and travel into space. To achieve this force, you need the thrust of a rocket engine.

Gravity and Thrust

Escape Velocity

A satellite in orbit around the Earth is held there by the Earth’s gravitational force. The speed necessary to go into orbit around the Earth is 28,000 km/h (17,500 mph). Escaping into outer space requires a higher speed, which is called the escape velocity – the Earth’s is 40,320 km/h (25,000 mph). Since the pull of gravity is different for each planet and moon, it follows that the escape velocity is also different. For example, the Moon’s escape velocity is about a fifth that of the Earth’s. A probe that escapes from the Earth’s gravity is held in a solar orbit by the gravitational pull of the Sun unless its speed is great enough for it to escape the Solar System.

Launching a Satellite

Satellites ride into space on the top of powerful rockets. The heat-resistant rocket nose cone protects the satellite from friction when travelling up through the atmosphere. Once above the atmosphere, this protective layer is jettisoned into space, and the final rocket stage pushes the satellite into the correct orbit. Many satellites then use their own rocket engines to put them into the higher orbits where they operate. The US Space Shuttle carries satellites into orbit in its payload bay. The satellites are then released into space from their cradle.

Kelompok 9

How a space probe works

Probes are built to carry cameras and instruments to gather information about distant planets and moons and send it back to Earth. The launch rocket gives the probe its initial speed, but it has its own rocket motor that allows it to change direction. Power is supplied by solar cells for probes exploring the inner planets. But for probes in the outer Solar System, where the sunlight is weaker, electricity is generated using heat from a radioactive material. A radio antenna always points towards the Earth to send back pictures and information, and receive instructions.

What the orbiter does

A probe in orbit round a planet can observe it closely, sending back pictures that map the entire surface in detail. Probes orbiting Venus used radar to penetrate the thick cloud cover and map its surface. Since a probe may remain with a planet for several years it can spot changes over time. The Viking orbiters, for example, operated for up to 4 years and recorded Martian dust storms in detail.

If the mission includes a lander, the orbiter can relay the lander’s messages from the surface to the Earth where they will be analysed and interpreted by ground-station computers.

Space debris

Each time a rocket is launched, more rubbish joins the debris in Earth orbit and the risk of future craft suffering collisions increases. Orbital debris includes discarded satellites and fragments of satellites that have disintegrated. Debris in near-Earth orbit falls back into the atmosphere. Smaller pieces burn up, but larger pieces can fall back to Earth, as parts of the Skylab space station did in 1979. Further from the Earth, debris stays in orbit for many years – it is thought that satellites in geostationary orbit could remain there for up to a million years. About 7,000 orbiting objects are tracked by radar and fewer than 400 of these are working satellites. Many pieces of debris are too small to be tracked but still large enough to be dangerous.




Why go into space?

In the 1950s, both the USA and USSR developed missiles powerful enough to launch a satellite into space. Both nations intended to launch a satellite during the International Geophysical Year of 1957. The USSR was the first to achieve this with Sputnik 1, launched on 4 October. From this point on, space exploration became a race between these two nations, with politicians on each side aware of the military and political advantages of having satellites in orbit and reaching the Moon first. Initially, the Soviets seemed to be ahead in the race; in addition to the first satellite, they achieved the first person in space, Yury Gagarin in 1961, and the first space walk, by Aleksey Leonov in 1965. However, as the 1960s progressed, the USA gained the lead.


Future space travel

So far, the only other world that human beings have visited has been the Moon. The next step would be to travel to a planet, but the enormous distances involved are a major obstacle. Astronauts would have to take supplies of food, fuel, and water for a journey that would take months or even years. None of the other planets provides Earthlike conditions for human existence. However, it may be possible to use materials on a planet to manufacture fuel for the return journey and extract water to drink or oxygen to breathe. Trips to the stars are unlikely because spacecraft travel at a fraction of the speed of light, and it would take thousands of years to visit even the nearest stars.


Everyday Life in Space

The most striking difference between life on the Earth and life in space is weightlessness, which affects every aspect of a space traveller’s life. Some find that being confined inside a small place such as Mir can cause problems during a long mission. To make things more comfortable, the space station is decorated in soothing colours. Mir’s inhabitants do have some privacy, with cabins for rest and relaxation. They are supplied with books, music, and a porthole in order to see the Earth. Supply craft bring letters and gifts from families back home and luxuries such as fresh fruit.


All food that is eaten in space is transported from the Earth, and, to keep the weight of the craft down, most is dried and sealed in individual packets. The food is dehydrated, meaning that water has to be added before the food can be eaten. Meals are then heated up in an oven; canned foods are also eaten. To drink, astronauts must suck liquids through straws from closed bottles or cups; without a lid, the liquid would float out of a cup.


In space, astronauts do not need beds to lie on. Instead, they use sleeping bags, which are fixed to a wall to keep them from floating around and bumping into things. Their arms would float if they were not secured, so they are strapped down during sleep. There are no ups and downs in a weightless environment, so astronauts can sleep at any angle. Spacecraft are noisy so some astronauts use ear plugs to help them get some peace and quiet.


Colonies in space

In the future, people may not always have to live on the Earth. They could live in colonies on the Moon, on another planet, such as Mars, or in huge orbiting space cities. Nowhere else in the Solar System provides Earthlike conditions to support human life, so space colonists would have to build an artificial environment in which human beings could live and work in comfort. The basic needs of any community would be air to breathe, food and water, and power to keep the colony running. Large permanent space colonies could not rely on the Earth to provide such essentials, so any space settlement would have to be completely self-sufficient.



Astronomers conduct most of their detailed studies of space from observatories. The site of an observatory is one of its most important features, as telescopes must be situated far from city lights, which overwhelm faint starlight. They are often constructed near oceans as the air there is steadier, so the stars “twinkle” less, providing clearer images.

Mauna Kea

Located on an extinct volcano in Hawaii, Mauna Kea is a modern observatory used by astronomers from many different countries. It is situated 4,200 m (13,800 ft) above sea level, higher than most clouds, on an island surrounded by the Pacific Ocean.

These conditions make Mauna Kea one of the best space-observation sites in the world, providing extremely clear images of celestial objects. Because the observatory is situated at such a high altitude, its telescopes are able to collect infrared and microwave radiation, which are blocked by the lower layers of the atmosphere.

Fact File

Stars appear to twinkle in the sky because the light they emit travels through the Earth’s atmosphere. This blanket of shifting gases bends the starlight in different directions, causing the “twinkling”. Seen from above the atmosphere, the stars shine steadily.


Telescopes collect light from celestial objects, focusing the light waves to form an image. Most modern astronomers do not in fact look through their telescopes. Instead, they attach equipment to them, such as cameras, computers, and spectrometers, to retrieve as much information from the light as possible. There are two different types of telescope: refracting telescopes, which use lenses to collect light, and reflecting telescopes, which use mirrors.

Refracting Telescopes

In a refracting telescope, a lens called an objective lens collects and focuses light to form an image. This image is viewed through a second lens, the eyepiece, which makes the image appear larger. Big lenses tend to change shape slightly when mounted in a telescope, resulting in image distortion, so astronomers are unable to build very large refracting telescopes.

Reflecting Telescopes

Reflecting telescopes use a curved mirror to focus light from objects in space. The focused light forms an image in front of or behind the main mirror – the larger the main mirror,

the more detailed the image viewed.  All the largest telescopes are reflectors, because it is possible to mount larger mirrors than lenses. There are different kinds of reflecting telescope: the most frequently used are the Cassegrain, Newtonian, and Coudé arrangements.


The only way astronomers can learn about very distant objects in space is by extracting as much information as possible from their light waves and other radiation collected by telescopes. By splitting starlight in the same way as a prism splits daylight, a spectrometer indicates a star’s composition and temperature. A star’s spectrum looks like a rainbow, but it is marked with patterns of dark lines. The positions of these lines reveal which gases surround the star.

Recording Images

Early astronomers used telescopes or their eyes alone to look at space and would then draw the objects they had seen. Now, both amateur and professional astronomers use photographic equipment to acquire  permanent images. Often, pictures are computer-processed, providing astronomers with highly detailed information. Amateur astronomers can also achieve excellent results with the most basic of equipment.

New Telescopes

Ordinary telescope mirrors are very thick and heavy, so new types of mirror are being constructed to make them lighter, more powerful and easier to manoeuvre. The Keck telescope at the Mauna Kea Observatory in Hawaii, USA, is one such modern telescope. It is segmented into 36 small mirrors, which combine to produce a single image. Another new type of telescope has a thin mirror with computer-controlled supports that adjust its angle. A third type of modern telescope resembles a honeycomb and is constructed of strong, light mirrors that are supported by air pockets.

Multiple telescopes

The light from several mirrors or telescopes can be combined to simulate one larger mirror. These mirrors act as one reflecting surface, providing more detailed images than they would separately. The Multiple-Mirror Telescope is made from six 1.8-m (6-ft) diameter mirrors, which combine to provide a 4.5-m (15-ft) reflecting surface. This telescope is situated at the Whipple Observatory in Arizona, USA. The Multiple-Mirror Telescope is currently in the process of being converted into a telescope with a single mirror, which will be 6.5 m (21 ft) in diameter. The Multiple-Mirror Telescope will be taken out of service in the middle of 1996; the new single-mirror telescope will come into operation in 1997.

 The Very Large Telescope

The Very Large Telescope is under construction at the European Southern Observatory in Chile. It will combine light from four large telescopes, each with a mirror 8.2 m (25 ft) in diameter, to make detailed pictures of very faint objects. The first of these telescopes is due to go into operation in late 1997.

Space Observatories

In Space, optical telescopes can produce clearer, more detailed pictures than they can on Earth, where the atmosphere distorts the light from distant objects. Satellites can also be

deployed in space to collect the types of radiation that are blocked by our atmosphere. Space observatories are launched by rockets, or released into orbit from the Space Shuttle.

Hubble Space Telescope

On April 25, 1990, the Hubble Space Telescope was launched into orbit 600 km (373 miles) above the Earth. The telescope has a mirror 2.4 m (8 ft) wide. Initially, Hubble’s images were disappointing, as its mirror was incorrectly shaped, but the telescope has now been fixed.

Hubble Servicing Mission

In 1993, the US space shuttle was sent on a repair and servicing mission to the Hubble Space Telescope. One of the most important of the repairs was the attachment of a set of

small mirrors, which were added to compensate for the faulty main mirror. Hubble now produces images of exceptional clarity; it has revealed many distant celestial objects and produced huge quantities of information, which astronomers will study for years to come.

Hipparcos Satellite

Constructed by the European Space Agency, Hipparcos recorded the position and brightness of 120,000 stars, building an up-to-date star catalogue. Although it failed to reach the correct orbit, its measurements were more accurate than any taken from Earth.

What is Invisible Astronomy?

Stars are visible because they emit light, which our eyes detect. However, there are other types of radiation from space that we cannot see. This invisible radiation contains information about objects such as  black holes. Although some of it is collected by ground-based telescopes, astronomers must send their instruments above the atmosphere

to study most invisible radiation.

Electromagnetic Spectrum

ALL OBJECTS IN SPACE emit electromagnetic waves. The hotter an object, the more energy it radiates. Very hot cosmic objects emit radiation concentrated towards high-energy, shorter wavelengths; cooler objects emit radiation with lower-energy, longer wavelengths. Gamma rays are the most energetic electromagnetic waves; radio waves carry the least energy.

Radio Astronomy

Radio waves from space were discovered in 1931 by Karl Jansky. They are collected by radio telescopes – huge, curved dishes that point towards the sky. The dish collects and focuses radio waves, just as a mirror focuses light in a reflecting telescope. Among the objects that radio dishes can detect are clouds of gas between stars that cannot be detected in visible light.

Radar Astronomy

In addition to collecting radio waves, a radio telescope can also transmit bursts of signals. These bursts are directed towards an object in the Solar System, producing an echo, which is reflected back to the radio dish. The time the echo takes to return to the dish tells astronomers how far away the object is. Radar equipment on the Magellan probe, orbiting Venus, produced maps and images of the planet’s surface, which is constantly covered by a band of thick cloud. Radar also revealed that Venus was spinning in the opposite direction to the other planets in the Solar System.


Signals collected by two or more radio telescopes can be combined to pinpoint the origin of radio signals – a method called interferometry. The further apart the telescopes are, the more accurate their measurements will be. To build up a radio image of the sky, more telescopes are needed.  The Very Long Baseline Array (VLBA) has 10 radio telescopes, and stretches from Hawaii in the Pacific Ocean to North America and the Caribbean.


Very Large Array

Situated in Socorro, New Mexico, USA, the Very Large Array is made up of 27 separate radio dishes arranged in a Y shape. It uses interferometry, combining the signals from each dish  to make radio “pictures” of the sky that are accurate enough to be compared with optical images. Each dish is 25 m (82 ft) across. The array is expandable and can span over 36 km (23 miles).

Microwave Astronomy

Microwaves, unlike radio waves, are unable to penetrate the lower layers of the atmosphere. Telescopes situated on mountain tops, such as Mauna Kea in Hawaii and La Silla in Chile, are able to detect them, as are satellites. Microwaves can tell astronomers which substances are contained in the clouds of dust and gas between the stars.


Kelompok 10


Infrared Astronomy

All objects emit some infrared radiation. Water vapour in the lower part of the atmosphere absorbs infrared, so, to detect it, telescopes must be placed at high altitudes or on satellites. By measuring infrared, astronomers can observe objects surrounded by dense clouds of dust, such as those found in the Orion Nebula, where new stars are being born. They can also observe the rings of gas around stars where planets may form.

Ultraviolet Astronomy

Hot stars emit ultraviolet radiation. This is usually prevented from reaching the ground by the Earth’s atmosphere. Therefore, ultraviolet telescopes are always placed on satellites. Instead of glass, which would absorb this type of radiation, a mineral called quartz is used to make the telescopes’ mirrors. These have special coatings that are able to reflect the ultraviolet.

X-Ray Astronomy

Studies of x-rays in space are carried out by satellites or rockets. This is because the Earth is protected from this type of radiation by its atmosphere. X-rays come from extremely hot gases found in the remnants of supernovae, or from pairs of stars where one is a white dwarf or a black hole. As X-rays would pass through a conventional mirror, telescopes collecting them use an array of concentric, cylindrical mirrors, which reflect the rays at a shallow angle.

Gamma-Ray Astronomy

Collected by satellites orbiting above the Earth, gamma rays consist of very-high-energy radiation. They come from various cosmic sources, including pulsars and the nucleus of the Milky Way Galaxy. Very short, intense gamma-ray emissions, known as gamma-ray bursts, have puzzled astronomers since their discovery in 1967, as they appear at random and their exact origin is unknown.

Astronomy in Practice

Professional astronomers do not spend all their time using telescopes; they can spend many months analysing the images and data they have accumulated at observatories. Sometimes, astronomers do not need to visit a telescope; the Isaac Newton telescope in the Canary Islands, for example, can be remotely controlled from Cambridge, England. Many amateur astronomers also watch and photograph the night sky. Their equipment is usually not sophisticated enough to explore distant galaxies, but they can observe the Solar System.

Getting Started

For an amateur astronomer, a good pair of binoculars is more effective than a small, cheap telescope. With binoculars, you can identify the mountains and craters on the Moon and see many more stars than with the naked eye, particularly if you focus on the Milky Way. Through binoculars the stars still resemble points of light, but the nearest planets resolve into discs. You can observe the phases of Venus in addition to those of the Moon. The planets are more difficult to find than the stars as their positions constantly change; however, astronomy magazines and some newspapers tell you where to look for them each month. Look out for Jupiter’s four large moons, and monitor their movements as they orbit the planet. Binoculars will also provide you with more detail in star clusters, such as the Pleiades, and nebulae such as the Orion Nebula. Watch out for comets, as often these are named after the person who discovered them. You never know your luck – you could be one of those people.

Starting your stargazing

Although you will be able to see many interesting sights from your garden or through a window, you will see even more if you can find an observation site away from the glare of street lights. If you join a local astronomical society, you will find companions with whom you can study the stars, and with whom you can go on stargazing trips. A star map will help you find particular objects. You also need a watch to tell the time and a compass to find the right direction. Your eyes will take about 30 minutes to adapt to the dark, but do not spoil your night vision by using an ordinary torch to read a star map; instead, cover your torch with red transparent paper so that it emits red light. Clear nights provide good conditions for stargazing, but they may also mean that the air is cool, so remember to wear warm clothing.



What is a Space Probe?

Space probes are spacecraft sent without a crew to explore the Solar System. They carry instruments and cameras to collect information to send back to Earth as radio signals. Probes have visited Halley’s Comet, two asteroids, and all the planets except Pluto, and have flown close to the Sun. Usually, they fly past a planet or moon, or they orbit to map the surface, or they make a landing to study the environment in greater detail.

Fact File

Derelict space probes that have either landed or just crashed on to the surface, can be found on Mars, Venus, and the Moon. Four space probes have now left the planetary Solar System and are speeding away towards the stars.

Galileo Probe

Launched in OCTOBER 1989, Galileo twice used the Earth’s gravity to help propel it on its 6-year journey to Jupiter. It parachuted a smaller probe down into Jupiter’s clouds, and this sent measurements back up to the orbiting main probe until crushed by atmospheric pressure after 57 minutes. The main probe studied Jupiter’s constantly changing cloud patterns and its four largest moons.

Giotto Probe

Halley’s comet returns to swing around the Sun every 76 years or so, and on its last visit, in 1986, the ESA probe Giotto was one of five spacecraft launched to meet it. Giotto made the closest approach, flying right through the coma, the comet’s surrounding cloud of dust and gas, only 605 km (376 miles) in front of the

comet nucleus. Giotto provided the first

ever views of a comet nucleus.

Fact File

Although protected by a double dust shield, Giotto was nevertheless hit by dust from Halley’s Comet that knocked it sideways and damaged some of its instruments. After it left Halley’s Comet, Giotto was deactivated in April 1986 and then reactivated in July 1992 for an encounter with the comet Grigg-Skjellerup.

Magellan probe

Launched from THE Space Shuttle Atlantis on 4 May 1989, the Magellan probe mapped the whole of Venus, using radar to penetrate its thick clouds. Orbiting Venus, Magellan gathered information on its surface that was sent back to Earth once during each orbit. It showed a landscape with mountainous volcanoes. On 12 October 1994, Magellan plunged into Venus’s atmosphere and was destroyed. Mapping Venus Magellan completed its mission with a sixth survey in late 1994.

The Mariner Series probe

Seven of the ten Mariner probes sent to explore Mercury, Venus, and Mars were very successful. In 1964, Mariner 4 became the first probe successfully to fly past Mars. Mariner 9, in 1971, was the first probe to orbit another planet, photographing the whole of Mars’s surface..

The Pioneer Series probe

Beginning in 1958, Pioneers 1 to 4 were unsuccessful attempts to explore the Moon. However, Pioneers 5 to 9, launched between 1960 and 1968, all successfully orbited the Sun, studying the solar wind, cosmic rays, the Earth’s magnetic field, and particles in space. Two of these probes continued to send back data for over 20 years. Pioneers 10 and 11 became the first probes to venture beyond the asteroid belt.

Sakigake and Susei probe

Japan’s first-ever space probes, Sakigake and Susei, joined three others in an international mission to meet Halley’s Comet in 1986. They were launched in 1985 and projected into orbits around the Sun that would take them close to the comet. Sakigake flew between the Sun and the comet, measuring the solar wind. Susei flew closer to the comet, studying the effects of the solar wind on its coma.

Vega probe

 Launched in 1984, the two Vega spacecraft had a dual mission. In June 1985 they each dropped a separate landing probe on to Venus to analyse its soil. Each lander released a balloon probe into the planet’s clouds. The probes floated above the surface in order to measure wind strength and direction. The main probes then met Halley’s Comet in March 1986, pinpointing the exact position of the comet nucleus (so that the later ESA probe Giotto could be aimed to pass close to it) and studying the gas in its coma and tail.

The Venera Series

The first three probes of the Russian Venera series to Venus were crushed by the planet’s enormous atmospheric pressure before they could reach the surface. In 1970, Venera 7 became the first to send back measurements from the surface, and in 1981, Veneras 13 and 14 analysed the soil. In 1983, Veneras 15 and 16 used radar to map the planet while in near-polar orbit.

Viking probe

When the two Viking spacecraft arrived at Mars in 1976, they orbited the planet before parachuting their landing probes on to the surface. The landers tested the soil for any signs of life. Scientists concluded that there were none. The landers also monitored the weather on Mars, measuring winds and temperatures, while the orbiting spacecraft photographed the surface.

Voyager probe

Launched in 1977, the two Voyager probes undertook successful missions to explore Jupiter, Saturn, Uranus, and Neptune, out to the very edge of the planetary Solar System. Voyagers 1 and 2 went to Jupiter and Saturn, and Voyager 2 continued alone to Uranus and Neptune. As the probes flew past, they sent back highly detailed pictures of the planets themselves, their rings, and many of their moons, discovering new moons and rings at each encounter.

Leaving the Solar System

All four of the probes to the outer planets, Pioneers 10 and 11 and Voyagers 1 and 2, have now left the planetary Solar System and are heading towards the stars. Both Pioneers carry a plaque showing the position of the Sun and the Earth, and a picture of a woman and a man whose hand is raised as a sign of peace. The Voyagers carry video discs containing pictures and sounds from the Earth.

Probes Update

The first probe to leave the planetary Solar System was Pioneer 10, which crossed the orbit of Neptune on 13 June 1983. It is heading towards Ross 248 in the constellation of Taurus, and is expected to continue  communicating until the year 2000. Pioneer 11 is journeying out of the Solar System in the opposite direction to Pioneer 10. Voyagers 1 and 2 should both be sending back information on the Sun’s energy field until about the year 2010.

Future Probes

There are many plans to explore planets, comets, and asteroids using smaller, more standardized, and less expensive spacecraft than previously used. The United States and the Russian Federation are planning probes to orbit and land on Mars with roving vehicles and balloon-borne instruments, preparing the way for a later, piloted mission. The NEAR (Near-Earth Asteroid Rendezvous) probe will orbit the asteroid Eros. A probe may also visit Pluto before its orbit takes it much further from the Sun. In 2003, the Rosetta probe will orbit round Comet Wirtanen for about 4 years.


What is a Satellite?

A satellite is any object that orbits another object. Any spacecraft that orbits a planet or moon is an artificial satellite. Although this term covers piloted craft, such as the Space Shuttle, most artificial satellites are small and carry only instruments. All have the same basic components, but they have many different uses, ranging from weather and communications satellites to telescopes in space.

Communications Satellites

Almost all communications satellites are in geostationary orbit. Satellite dishes on Earth send telephone and TV signals up to the satellite, which processes and sends them back down to another ground station. Satellites can send signals either to an entire continent or to just a localized area. Direct-broadcast satellites send out television signals either to individual dish receivers or to televisions connected by cable to larger dishes.

Weather Satellites

There are two types of weather satellite. Those in geostationary orbit keep a constant watch over about one third of the Earth. Others, in polar orbits, can cover the whole surface about once every 12 hours. Weather satellites measure ground and air temperatures, record wind speeds and cloud movements, and map rainfall areas. This information allows meteorologists to forecast the weather.

Earth-Resources Satellites

Earth-resources satellites monitor conditions on the Earth. Flying in a near-polar orbit, they can observe the whole Earth regularly. They provide measurements for mapping purposes, geological surveys, mining activities, and oil exploration. They record different crops, places where crops are not growing well, and areas threatened by pests, such as locusts. They detect oil spills, forest fires, the destruction of rain forests, and air and sea pollution.

Astronomical Satellites

Astronomical satellites provide information about space that cannot be obtained from the ground. They study the Earth’s magnetic field, its radiation belts, and the solar wind. From distant stars and galaxies, they detect radiation, including gamma rays, ultraviolet, X-rays, and infrared radiation. This often reveals information about objects such as quasars, invisible gas clouds, black holes, and the remains of exploding stars.

Rockets Around the World

Rockets are used to lift satellites, probes, and astronauts into space. There are two main types: conventional rockets that can be used only once, and reusable, winged shuttles, which take off  like a rocket and land like a glider. The USA uses both, but other countries use only conventional rockets. A number of countries, including China and India, have built rockets for launching satellites.


What is a Space Station?

Space stations orbit the Earth, providing a place for crews to live and work in for weeks or months at a time. The station offers everything the crew needs to keep alive and well while they conduct scientific investigations.

The Salyut Series

The world’s first space station, Salyut 1, was launched in 1971. It was the first in a series of seven space stations put into orbit by the USSR, and cosmonauts travelling from Earth in Soyuz spacecraft occupied all except  Salyut 2. Over time, their visits increased in length from a few weeks to over 6 months. Salyuts 6 and 7 were given an extra docking port for cosmonauts to visit the resident crew, and for Progress craft bringing extra supplies from Earth.


America’s first space station, Skylab, was launched on 14 May 1973. Minutes after the launch, a meteoroid shield and one of the solar panels were torn off by the effects of air pressure. Skylab’s first crew repaired the damage so that the station was fit to live in. Over the next year, three crews stayed for 28, 59, and 84 days each. Skylab fell back to Earth in 1979; most of the station was destroyed as it entered the atmosphere, but some debris fell on Australia. Fortunately no one was hurt.

The Soyuz Programme

Soyuz spacecraft are used to ferry Russian cosmonauts to and from space. The three crew travel in the middle section of the craft, the descent module, which is covered with a heat shield because it must withstand very high temperatures on re-entry into the Earth’s atmosphere. At the front, the orbital module carries food and supplies. The rear instrument module contains the main engine, rocket engines for re-entry, and communications and control equipment.


Mir (meaning “peace”) is the Russian Federation’s latest space station, following the Salyut series. Mir’s main module, containing the station controls and living area, was launched in 1986. A multiple docking adapter with side-facing docking ports has allowed extra modules to be added to the main module, while ports at each end are used for visiting Soyuz craft and Progress supply ferries.

Living space

The living and work compartment of the main Mir module usually has three crew, but visiting cosmonauts can increase this to six.

US Docking

On 29 June 1995, the Space Shuttle Atlantis carried five US astronauts and two Russian cosmonauts up to Mir, where a crew of one American and two Russians had

been living for several months. This was the first docking ever made by

the Shuttle, and a special docking system was installed in Atlantis’s payload bay. After 5 days, the Shuttle returned to Earth with six Americans and two Russians, leaving Mir with

a fresh crew of two.

US Space Shuttle

The US Space Shuttle, first launched in 1981, was the world’s first reusable spacecraft. Of its three components – the Orbiter space plane, rocket boosters, and external fuel tank – only the fuel tank is not recovered after a mission. Special heat-resistant tiles prevent the Orbiter from burning up when it reenters the Earth’s atmosphere. The remote manipulator

arm in the Orbiter’s payload bay can put satellites into space, recapture them from space, and act as a stable platform for astronauts working in the bay.

Challenger Disaster

On 28 January 1986, millions of television viewers all over the world watched in horror as the Space Shuttle Challenger exploded less than 2 minutes after its launch. It was totally destroyed, and all seven crew members were killed. One of the crew, Christa McAuliffe, was a teacher who had intended to conduct lessons from space. An inquiry into the disaster found that the seal between two sections of a booster rocket had failed, causing a gas leak, which then ignited. The Shuttle programme was grounded for nearly 3 years after the accident while its safety was improved.

The Future – Alpha

Construction of an international space station involving the USA, the Russian Federation, Canada, Japan, and Europe is due to begin in 1997. The station, known as Alpha, will be assembled in space over a period of 5 years, starting with the launch of a Russian-built control centre. As well as being used for scientific experiments and materials research, Alpha, it is hoped, could provide an intermediate base for spaceflights to Mars.


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