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A guide to the Solar System
Compiled by Sydney Observatory June 2010
A guide to the solar system
Contents The solar system ........................................................ 3 The sun........................................................................ 4 Mercury ....................................................................... 6 Venus........................................................................... 7 The Earth..................................................................... 8 The Moon.................................................................. 10 Mars ........................................................................... 16 Asteroids................................................................... 18 Jupiter........................................................................ 20 Saturn ........................................................................ 23 Uranus ....................................................................... 25 Neptune .................................................................... 26 Pluto .......................................................................... 27 Comets ...................................................................... 28 Dwarf planets and the Kuiper Belt........................ 29 Solar system scale model ....................................... 30 Useful webpages...................................................... 31 Contact details ......................................................... 32
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A guide to the solar system
The solar system Located far from the centre of the galaxy called the Milky Way is a relatively insignificant star which we call the Sun. Surrounding it is a collection of planets, moons and other objects which together form our solar system. The Sun lies at the centre of the solar system and dominates all the other objects in it. It generates all of the light, most of the heat and contains over 98% of all the matter in the solar system. Apart from the Sun, the eight planets are the largest objects in the solar system. The planets Mercury, Venus, Earth and Mars are small and rocky and are called the terrestrial planets because they resemble the Earth. Jupiter, Saturn, Uranus and Neptune are extremely large and gaseous. They are called the Jovian planets because they resemble Jupiter. Pluto is a small, icy world which resembles the large moon of Neptune, Triton, and belongs to the class of ‘dwarf planets’. In addition, we know of at least 170 moons circling the planets, thousands of asteroids orbiting the Sun, mainly in a belt between Mars and Jupiter, and hundreds of icy bodies orbiting the Sun in the Kuiper Belt, beyond Neptune’s orbit. Also, countless rocky meteoroids randomly move around the Sun and a halo of icy comets exists beyond the Kuiper Belt, known as the Oört cloud.
formed the planets and other objects that we find in the solar system today. Is our solar system unique? Probably not. As of May 2010 astronomers had found 454 candidate planets around other stars. Many were detected using the 3.9 m Anglo Australian Telescope (AAT) near Coonabarabran, NSW, using a ‘Doppler wobble’ technique. Each planet pulls on its parent star and causes it to wobble back and forth in space. Careful and sometimes lengthy observations with sensitive telescopes like the AAT, can detect this wobble which can be used to calculate the planets mass. Earth-like planets are currently impossible to detect at this stage but the race to find them is intensifying with larger telescopes being built and improved observation techniques. Earth is the third planet from the Sun. It is unique as the only one on which we know there is life, and where we have observed running water. Each of the other planets and objects has its own unique features and exploring these is the great adventure of our age.
Most astronomers believe that our solar system formed from a huge cloud of dust and gas about 4600 million years ago. Over millions of years gravity caused this cloud to collapse and eventually condense to form a massive ball of gas with a disc of smaller clumps of dust and gas circling around it. As this central ball of gas collapsed even further, the temperature of the gases increased rapidly. Eventually, temperatures in the centre reached about 15 million degrees, which is high enough to cause nuclear fusion reactions to start. Vast amounts of energy were released and the Sun began to shine. Innumerable collisions between the clumps of gas and dust in the circling disc
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A guide to the solar system
The Sun Diameter: 1 392 530 km Average distance from Earth: 149 597 900 km Period of rotation: 25 days at the equator; 35 days at the poles Planets: 8 Dwarf planets: 4
At the centre of the solar system lies the closest star to the Earth — the Sun. Although only an average star, it appears spectacular when viewed from our close vantage point on the Earth. With a diameter of 1 392 530 km, the Sun is a huge ball of gas which contains 99% of all the mass in the solar system. It has 333 000 times the mass of the Earth and is composed of 92% hydrogen, 7.8% helium and less than 1% heavier elements. It is the source of all the light, heat and energy that is necessary to create and sustain life on the Earth. Like the planets, the Sun rotates on its axis, but due to its gaseous nature different parts rotate at different rates. This is known as differential rotation. The equator rotates fastest, in a period of 25 days while the poles rotate the slowest at 35 days. Astronomers believe the Sun was formed out of a giant cloud of gas and dust approximately 4600 million years ago. As the cloud collapsed the material was compressed, raising the temperature at its centre to about 15 million degrees, which is high enough for nuclear fusion reactions to commence and at which point the star is said to be born. In this fusion process four hydrogen atoms combine, or fuse, to form one helium atom in a process known as the proton-proton reaction. In the Sun, each second 635 million tonnes of hydrogen are converted into 630 million tonnes of helium. The remaining 5 million tonnes of matter is converted into energy. There is enough material in the Sun to continue this process for another 5000 million years or so.
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Photo courtesy NASA
Eventually however, the hydrogen in the core available for fusion will be exhausted. The core will contract until the temperature rises high enough for the remaining helium to be converted into heavier elements. Meanwhile, the outer layers will cool and expand and the Sun will become a red giant star so large that it will engulf the Earth. After a few million years, all nuclear activity will cease and the Sun will blow off its outer layers, leaving its white hot core exposed to space. The Sun will end its life as a white dwarf star. The Sun’s interior Within the Sun there is a definite structure. At the centre and extending out to about a quarter of the Sun’s radius, is the core. Temperatures within the core are approximately 15 million degrees and it is here that all the energy of the Sun is created. Outside the core and extending out to within 140 000 km of the Sun’s surface is a region known as the radiative zone. In this region the intense energy from the core is radiated outwards through a complex process of individual atoms absorbing and then re-emitting the energy at longer wavelengths which have less energy. The ‘bouncing around’ of the energy in this extremely dense zone means that it may take as long as 100 000 years for the energy created in the centre to weave its way to the surface. Surrounding the radiative zone is the convective zone. The difference in temperature from the bottom to the top of this zone means that energy is transported to the surface in giant convection cells. Convection involves the overturning of hot gases, that is, the hot gases from the bottom rise
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to the top layer where they cool and sink back to the bottom again to repeat the cycle.
latitudes. Nobody currently knows why this happens, although there are many theories.
At the edge of the Sun lie two relatively thin layers. The first is the photosphere, possibly only about 200 km thick. The photosphere is the ‘surface’ that we see and is the source of all the visible light. It has a temperature of about 6000 °C. The density decreases abruptly at its outer limit, giving it the appearance of a sharp edge when seen from the Earth. When viewed up close, it has a turbulent, granulated surface, reflecting vigorous convective motion.
Also associated with this activity are other features such as prominences and flares. A prominence is a mass of ionised (charged) gas carried from the surface to the corona. Since ionised gases conduct electrical charges very easily, the prominences form great loops along the magnetic field lines in much the same way as iron filings follow a magnet’s lines of force. Prominences can best be seen along the edge of the Sun where they are very prominent, hence their name.
Above the photosphere is the chromosphere (colour sphere). The thickness of this reddish-pink layer varies, reaching up to 30 000 km in places. It is most easily seen during solar eclipses when the Moon covers the Sun. The temperature ranges from 4200 °C at the lower edge of the sphere to about 1 000 000 °C at its higher edge. The outermost part of the Sun is the corona (crown). It is extremely tenuous (not very dense) and extends several million kilometres into space. In addition, the corona has a temperature of over 2 000 000 °C. Since it is 100 000 times dimmer than the photosphere, it can only really be studied during solar eclipses. Interesting features Due to its intense light, observations of the Sun are extremely dangerous, but when the proper precautions are taken, various features on the Sun’s surface can be seen. The most easily visible are sunspots. These are places where strong magnetic fields have calmed the turbulent gases of the photosphere and prevented hot gases from rising to the surface. They appear dark because they are cooler (only 3500 °C) and thus radiate only 20% as much energy as equal areas of the rest of the photosphere. Sunspots always appear in pairs, since they form the north and south poles of magnetic loops, just like an ordinary bar magnet. The size of these spots can vary from between 1000 km to 100 000 km across. The position and number of sunspots vary over an 11-year cycle. At the beginning there are very few sunspots and they form at mid-latitudes. As the cycle progresses the number of sunspots increases and they progressively form at lower
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Solar flares are usually considered the most spectacular feature of the Sun’s activity. They are thought to result from violent explosions in the solar atmosphere that are triggered by the sudden release of energy from tangled and twisted magnetic fields, much like a rubber band that has been stretched and twisted, then released. Flares can release as much energy as a 1000 million one-megaton hydrogen bombs in just a few seconds, and eject as much as 9000 million tonnes of matter. Energetic particles reaching the Earth from solar flares interact with the Earth’s magnetic field and cause the northern and southern lights. They also interrupt radio communications and cause confusion in the navigation of migratory birds. Larger than solar flares and indeed sometimes larger than the Sun itself are coronal mass ejections or CMEs. Only detected from the mid 1970s onwards CMEs are billion-tonne clouds or bubbles of electrified, magnetic gas that solar eruptions hurl into space at speeds ranging from a few hundred to 2000 km/s. Earth-directed CMEs can trigger magnetic storms when they strike our planet’s magnetic field, distorting its shape and accelerating electrically charged particles trapped within. At solar maximum 2–3 per day can occur and if they strike the Earth they can cause spectacular auroral displays and even effect sensitive electronic equipment.
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Mercury Diameter: 4878 km Average distance from Sun: 58 million km Orbital period: 88 days Rotation period: 59 days Number of known satellites: 0 Photo courtesy NASA
Mercury is the closest planet to the Sun and is an airless world closely resembling the Moon in appearance. It was known well before recorded history and because of its relatively rapid motion in the sky was named after the Roman messenger of the gods.
by the Sun. By comparison the Earth’s atmosphere is 1 000 000 000 000 times more dense. Since this atmosphere is negligible there is no erosion to wear away the craters formed by meteorite impacts. Consequently, any craters that do form will remain preserved for a very long time.
Of the planets visible to the naked eye, Mercury is the most difficult to observe because it always appears close to the Sun and is usually lost in its glare. When it is possible to see Mercury, it can only be seen close to either the western horizon shortly after sunset, or the eastern horizon shortly before sunrise. Since Mercury orbits closer to the Sun than the Earth, it is sometimes possible to see its unlit side. As a result, Mercury can be seen to display phases in the same way as the Moon and Venus.
One surprise of the Mariner 10 mission was the discovery of a weak magnetic field around Mercury. The Earth has a magnetic field believed to be caused by its rapidly spinning molten iron core acting like a dynamo and generating electricity. Scientists, however, thought that Mercury rotated too slowly to have a magnetic field. The only explanation they have is that Mercury must have a very large solid iron core, perhaps as large as 75% of its total diameter and which has retained this ‘fossil field’.
Being the closest planet to the Sun, the surface temperature on Mercury can reach a scorching 427 °C during the day. Owing to the lack of any substantial atmosphere, heat cannot be retained during the night and consequently temperatures can plunge to –184 °C. In 1974, the Mariner 10 spacecraft became the first and so far only spacecraft to fly past Mercury. This status will not change till 2009 with the arrival of the Messenger (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) spacecraft which was launched in August 2004. Mariner 10 beamed back our first close-up pictures of the planet, revealing a heavily cratered surface and huge cliffs crisscrossing the planet, apparently formed when Mercury’s interior cooled and shrank, cracking the planet’s crust. The spacecraft also found that Mercury has a very thin atmosphere composed entirely of captured particles given off
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Mercury has the second most elliptical (oval) orbit of all the planets. At its most distant point from the Sun (aphelion), Mercury is 50% further away than when it is at its closest point (perihelion). This elliptical orbit also means that Mercury’s speed in its orbit is always changing, depending on its distance from the Sun. When it is close to the Sun it travels much faster than when it is further away. On Mercury, a ‘day’ is longer than a ‘year’. It takes 88 Earth days for Mercury to travel once around the Sun. That is, Mercury’s ‘year’ is 88 Earth days long. The time for Mercury to rotate once on its axis is 59 Earth days. This, however, is not the length of its day. The period from one sunrise to the next is 176 Earth days. So, one Mercury day would actually be two Mercury years long!
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Venus Diameter: 12 104 km Average distance from Sun: 108 million km Orbital period: 225 days Rotation period: 243 days Number of known satellites: 0 Photo courtesy NASA
Venus appears as the brightest object in the night sky after the Moon. This is because it passes closer to the Earth than any other planet and has a cloud layer that reflects most of the sunlight that falls on it. This brilliant appearance is what led the Romans to name it after their goddess of love and beauty. It is often called the morning or evening star because it can be seen either just before sunrise in the east or shortly after sunset in the west. Venus has a dense carbon dioxide atmosphere with a surface pressure 90 times that of the Earth’s. This is equivalent to the pressure experienced 1000 m under the Earth’s ocean surface. In addition, Venus is perpetually shrouded in a mantle of thick clouds composed mainly of sulphuric acid. This hellish concoction of gases contributes to the presence of what is termed a runaway greenhouse effect which has, over time, raised the surface temperature of Venus to around 480 °C. This is more than twice the maximum temperature of a normal household oven. At these temperatures, lead would flow as a liquid on the surface of Venus. Venus is only a little smaller than the Earth but, until recently, little was known of its surface features. The dense clouds of Venus completely obscure the surface, making normal telescopic observations impossible. This situation changed with the arrival of Soviet and American spacecraft. In 1978, the US Pioneer Venus Orbiter mapped the surface by radar. More recently, in 1990 the Magellan spacecraft was able to image almost the entire surface with radar to an unprecedented high resolution. This enabled scientists to create three-dimensional images which revealed a surface much flatter than the Earth’s. Most of the
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surface turned out to be rolling plains with some low lying basins and only three major highland regions. Beginning in 1975, a number of Soviet Venera spacecraft have soft landed onto the surface of Venus and beamed back the first closeup pictures. These showed scenes of complete desolation, with angular boulders and flat slabs of rock stretching to the horizon. Colour pictures showed the rock-strewn surface bathed in the light of an orange sky. From the evidence of these space probes, scientists now believe that Venus still has active volcanoes, making it the third body in the solar system known to have active volcanism. The other two are the Earth and Io, one of the moons of Jupiter. Lightning has also been detected which is believed to be related to this volcanic activity and is almost continuous over the regions suspected to contain volcanoes. Venus has some interesting characteristics. Firstly, the planet rotates about its axis in 243 Earth days in the opposite direction to its orbital motion around the Sun. Consequently, from the surface of Venus, the time from one sunrise to the next is 116 Earth days which is about half as long as its year of 225 Earth days. Venus displays phases the same way as the Moon and Mercury. Any planet or moon that lies between the Earth and the Sun will show phases. When Galileo Galilei observed the phases of Venus in 1610, it convinced him that the Sun was indeed at the centre of the solar system.
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The Earth Diameter: 12 756 km Average distance from Sun: 149 597 900 km Orbital period: 365.256 days (1 year) Rotation period: 24 hr (1 day) Number of known satellites: 1 (natural), 7000+ (artificial) Photo courtesy NASA
Earth is the third planet from the Sun and is the largest of the small, rocky, terrestrial-type planets. Of all the worlds in our solar system, it is unique in that the composition of its atmosphere and the presence of great oceans of water allow life to exist and flourish on its surface. The interior of the Earth has three very distinct parts — the core, the mantle and the crust. The core of the Earth is composed of an inner core and an outer core. The inner core is made of solid iron and nickel and accounts for just 1.7% of the Earth’s mass. It is extremely hot and has a pressure 3.7 million times greater than the air pressure on the Earth’s surface. This solid inner core is surrounded by the liquid outer core which accounts for 31% of the Earth’s mass. It has a temperature of 4100 °C. The outer core is a liquid because the pressure here is considerably less than at the centre. Giant circular currents (convection) within the liquid outer core generate electrical currents that produce the Earth’s strong magnetic field. The mantle surrounds the outer core and makes up 67% of the Earth’s mass. It extends from 70 km to 2890 km below the Earth’s surface. The mantle is mostly solid, but the upper layer appears to be partially molten. It is composed of rocks similar to basalt which is a common volcanic material. The crust rests on the mantle and has just 0.4% of the Earth’s mass. It is quite thin and has the same relative thickness as the skin of an apple has to its diameter. The continents are embedded in the crust and have an average thickness of 70 km. The crust is not a single continuous shell, but a mosaic of tightly fitting plates. Geologists believe there are eight large plates and a few dozen
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smaller ones. The plates are in constant motion and are being pushed against each other. Along the boundaries of these plates, earthquakes frequently occur as stresses build up in the rock. At some of these plate boundaries, the crustal material from one plate gets pushed down beneath the other plate. As the rock melts, it rises to the surface and forms volcanoes. At other plate boundaries, the crustal material gets folded and uplifted to form great mountain chains. In the oceans, material from the mantle wells up along giant ridges and pushes the plates along. At these sites new crustal rock is deposited to make up for the material pushed under at the plate boundaries. This great movement of the crust is due entirely to convection currents (circular currents of rising hot magma) in the mantle. The study of this movement is known as plate tectonics. The atmosphere of the Earth is composed of 78% nitrogen, 20% oxygen, 1% water vapour and less than 0.5% carbon dioxide. In the past the composition of the atmosphere was very different. Oxygen was just a small component of the atmosphere, with carbon dioxide comprising a large proportion. Over thousands of millions of years, chemical reactions with calcium, hydrogen and oxygen (which form limestone in the oceans) resulted in the carbon dioxide being removed from the atmosphere. As plant life increased, oxygen accumulated in the atmosphere through photosynthesis (the process by which plants convert sunlight into chemical energy). Plants absorb carbon dioxide and water and use sunlight to convert these to sugar and oxygen. More than 80% of all photosynthesis occurs in the oceans, making them the principle habitat of life.
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At the top layers of the atmosphere, the oxygen combines together to form ozone. This ozone layer reflects most of the ultraviolet (UV) light from the Sun, and protects life from its harmful effects. If the Earth had no atmosphere and was a bare, rocky world like the Moon, it would have an average temperature of –18 °C. However, the average temperature of our planet in the layer of air just above the surface is 15 °C. The blanket of air keeps our planet some 33 °C warmer than it would otherwise be. This is due to the water vapour and carbon dioxide in the air, absorbing the reflected heat of the Sun from the Earth, and radiating it back towards the surface. This means the heat does not escape the Earth and the surface temperature rises. This process is known as the greenhouse effect. Without the greenhouse effect temperatures would be too cold and life would not exist on the Earth. The environmental debate in recent years has been whether man is adding one or two degrees to the greenhouse effect and not whether we are creating it. As a consequence of the greenhouse effect, the surface temperature of the Earth is raised sufficiently high for water to exist as a liquid (above 0 °C), but not too high for it to boil away (below 100 °C). The oceans cover 70% of the Earth’s surface. They contain many minerals dissolved from rocks and soil and carried there by rivers. Sodium chloride, or common table salt, composes 3.5% of ocean water. Fresh water is
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essential for most living things not found in oceans. Its source is rain, which comes from pure water vapour evaporated from the oceans. Only 2% of the total water of the Earth is fresh water and of this, about 80% is frozen at the poles. It is the presence of these great bodies of water that has allowed life to evolve and in its own way, change the planet slowly to accommodate it further. The seasons The Earth follows a near circular path around the Sun. The tiny difference of 3% between its greatest and smallest distance from the Sun, does not account for the temperature difference between the seasons. The seasons arise because of the tilt of the Earth’s axis, which is 23.5° off the perpendicular to the plane of its orbit. When one hemisphere is tilted towards the Sun, the Sun’s rays strike it at a steep angle. As a result, any given unit of surface area absorbs a lot of heat and warms up. This produces the season we know as summer. The other hemisphere tilted away from the Sun, receives the Sun’s rays at a lower angle. For an equal area as above, the surface absorbs less energy and does not heat up as much and results in winter. Six months later, when the Earth is on the opposite side of the Sun, the situation is reversed, and each hemisphere experiences the opposite season. Midway between these two extremes, the Sun appears over the equator and each hemisphere receives equal amounts of heat leading to the seasons of autumn and spring.
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The Moon Diameter: 3476 km Average distance from Earth: 384 402 km Orbital period: 29 days 12 hr 44 min (synodic) or 27 days 19 hr 18 min (sidereal) Rotation period: 27 days 19 hr 18 min (sidereal) Photo courtesy NASA
Believed by the Romans to be their moon-goddess, Luna, the Moon is the only natural satellite of the Earth and is by far the nearest celestial body. The Moon is the fifth largest moon in the solar system. Relative to its parent planet it is quite large, being almost one-third the size of the Earth. For this reason astronomers sometimes regard the Earth-Moon system as a double planet. The Moon travels around the Earth in an elliptical (oval shaped) orbit. Its distance from the Earth varies from 356 000 km to 407 000 km. Furthermore, having only one-eightieth of the Earth’s mass, means that the surface gravity of the Moon is just one-sixth of the Earth’s. This means that a 60 kg person would weigh* just 10 kg on the Moon. * Strictly, the mass of 60 kg would not change but the force or weight of the person would vary.
Origin and structure Over the centuries many theories have been put forward as to how the Moon was formed. One of the earliest was the fission theory which suggested that the Earth and the Moon were originally one body, and that the Moon was thrown off as a result of rapid rotation. Another popular theory suggested that the Moon formed elsewhere in the solar system, independent of the Earth, and was subsequently captured during a close approach to the Earth by its gravity. Yet another theory suggested that the two formed together at the same time, from the same material and in the same region. However, the current most popular model uses new information gleaned from manned and unmanned space missions, and is called the giant impact hypothesis. This theory suggests that soon after
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the Earth had formed about 4600 million years ago, a Mars-sized planetoid (a small planet in the process of forming) collided with the Earth and vaporised each of the two planets’ recently formed crusts and mantles. The iron core of the planetoid eventually settled into the Earth and became part of the Earth’s iron core. The resulting debris of the impact was blasted into space and eventually formed a disc of material orbiting the infant Earth. This material slowly coalesced to form the Moon. The Moon has no atmosphere because its gravity is too weak to hold on to any gases it may originally have had. As a result, temperatures vary considerably from –184 °C at night to 101 °C at noon. Since the poles receive constant light and heat from the Sun, they have a constant temperature of –96 °C. The lack of an atmosphere means that little erosion occurs on the Moon and consequently surface features can remain preserved for hundreds of millions of years. The features most widely associated with the Moon are the craters. These are formed when meteorites (chunks of rock and metal left over when the solar system was formed) collide with the Moon. The size of a crater is determined by the size and impact speed of the meteorite. Craters are always circular, because they are gouged out by the shock waves from the impact, moving through the Moon at constant speed in all directions, in much the same way as circular ripples move on the surface of a pond. In the past there were many more impacts by large meteorites. Today however, there are very few because most of all the large meteoroids have been swept up by the
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planets and there aren’t many left. The largest crater on the Moon is Bailly (on the far side) which measures 295 km in diameter. On the Earth-facing side of the Moon, the largest crater is Clavius which has a diameter of 232 km. The Moon’s surface is divided into two distinct types of regions. The highlands are the oldest parts of the Moon dating back to about 4000 million years. They are very heavily cratered (which indicates their great age) and cover about 84% of the Moon’s surface. The maria, or seas, are low lying, relatively flat areas which formed more recently, about 3000 million years ago. These areas are called seas because when Galileo and his contemporaries first viewed the Moon with telescopes they said they resembled seas. Today we know they are giant lava plains, formed when small asteroids collided with the Moon causing the crust to crack open. Lava from the interior of the Moon came flooding out, covering any features that were originally there. They remained relatively free of large craters, because by the time they were produced, the meteorites that formed large craters were all used up. They appear slightly darker than the highland regions because the rocks in the maria contain more elements like iron and magnesium, which are darker. The far side of the Moon is almost devoid of these maria regions and the reason for this is not yet clear.
and nature of its core has not yet been fully established. Lunar exploration The first spacecraft to visit the Moon was the Soviet Luna 1, which swept past the Moon in January 1959. Later that same year the Luna 3 space probe beamed back the first ever pictures of the far side of the Moon. In 1964, the Americans followed with the Ranger probes which photographed the Moon up close as they approached for crashlandings. In 1966, the Soviets soft landed their Luna 9 probe and the Americans their Surveyor craft on the Moon, and beamed back detailed photographs from the surface. Meanwhile, in order to search out safe landing sites for future manned missions, the American Lunar Orbiter probes orbited the Moon and photographed almost the entire surface with unprecedented clarity. More recently, in 1994, the Clementine mission was able to survey the entire surface including the poles. A new stage of unmanned exploration was begun in 1970 when the Soviet probe Luna 16 returned soil samples from the Moon. Later that year, the Luna 17 probe landed the Lunokhod 1 vehicle on the Moon. It was able to drive across the surface, analysing the soil and returning high-quality television pictures as it went along.
As seen with the naked eye, the lunar maria are the most easily seen features of the Moon. Their distribution over the disc of the Moon means that with good imagination we can see the face of a man looking down on us. This is commonly known as the man in the moon.
By far the most important stage of lunar exploration was the Apollo manned lunar landings. Begun as a Cold War space race, it proved to be a boon for astronomers and scientists because it enabled them to sample the lunar environment first-hand and on a scale many dared not even dream of before.
Seismometers (instruments to measure earthquakes) left on the Moon by Apollo astronauts indicated that the Moon is almost solid in its interior. Whenever a moonquake occurred or a meteorite struck, the Moon would vibrate for days on end much like a gong. This indicated that the interior must be solid since a molten (liquid) interior would dampen vibrations very quickly. It is believed that the crust of the Moon varies from just 10 km under the maria, to about 160 km under the highland regions on the far side. The Moon’s solid or partly molten mantle probably makes up much of its interior. The size
The first lunar flight was Apollo 8 in December 1968. It orbited the Moon ten times and returned with high-resolution pictures of the surface. The first landing was the Apollo 11 mission in July 1969, when Neil Armstrong and Edwin ‘Buzz’ Aldrin became the first men to walk on the Moon. They landed in the Sea of Tranquillity and deployed several scientific instruments which measured moonquakes, solar activity and the Earth to Moon distance. In addition they returned with extremely valuable soil and rock samples. The next landing was Apollo 12 in the Ocean of Storms, very close to the Surveyor 3 spacecraft which had
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landed there three years earlier. The astronauts stayed longer on the surface and deployed more sophisticated instruments. Apollo 14 followed next by landing near the Fra Maura crater. Apollo 15 landed in the Apennine Mountains of the Moon, and used the Lunar Rover for the first time to travel further afield in search of soil and rock samples. The astronauts on this mission also deployed new instruments to measure heat flow from the interior of the Moon. Apollo 16 landed in the highlands of the Moon near the crater Descartes. In December 1972, Apollo 17 became (for now) the last manned mission to the Moon and landed in the Taurus-Littrow mountains. It is not commonly known that the Moon has a smell. When the astronauts re-entered the lunar module after their moon walks, they reported a strange smell in the cabin which was very similar to black powder (or gunpowder). They quickly realised that what they could smell was the odour from the very fine moon dust that had adhered (stuck) to their spacesuits. The Apollo missions represented a turning point in the history of humanity. It was the first time that a person had stood on another world and it changed forever the way we see our own fragile planet. Recent missions have included Clementine 1994, Lunar Prospector 1999 and the latest mission SMART-1 2003. Over 71 days in orbit, Clementine systematically mapped the 38 million sq km of the Moon. In addition, the spacecraft took 620 000 high-resolution, about 320 000 mid-infrared thermal images and mapped the topography of the Moon with laser ranging equipment. After successfully completing the lunar mapping phase of the mission, Clementine suffered an onboard malfunction that resulted in the activation of its altitude thrusters. This exhausted all the hydrazine fuel for altitude control and left the spacecraft spinning at 80 revolutions per minute. This prevented Clementine from performing the planned close fly-by on the near-Earth asteroid (1620)Geographos. Beginning on 15 January 1998, Lunar Prospector spent one year mapping the entire surface of the Moon from a distance of about 100 km. The data collected during this phase of the mission greatly improved on the quality of data collected
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previously. Among the early returns from the instruments were those from the neutron spectrometer indicating significant amounts of water ice at the lunar poles. Originally, the mission was to have ended with the spacecraft crashing into the Moon when its fuel ran out. As the mission neared its end, however, the suggestion was made to use the crash as part of an experiment to confirm the existence of water on the Moon. In 1999 the spacecraft was successfully crashed into a crater near the lunar south pole, thought to be a likely location for ice deposits, but no water was detected in the resulting impact plume. In September 2003 the European Space Agency launched its first lunar mission. The main purpose of the SMART-1 mission was to flight-test the new solar-electric propulsion technology — a kind of solar-powered thruster that is ten times more efficient than the usual chemical systems employed when travelling in space. This system could be used to provide the propulsion system for future ESA missions into deep space, such as BepiColombo due to be launched in 2014. NASA’s Lunar Reconnaissance Orbiter (LRO) and Lunar Crater Observation and Sensing Satellite (LCROSS) were launched aboard an Atlas 5 401 launch vehicle on 18 June 2009 and separated from one another less than an hour later for the journey to the Moon. LRO was designed to map the surface of the Moon from a height of 30-70km for at least one year and possibly up to 5. It will characterise future landing sites in terms of terrain roughness, usable resources, and radiation environment with the ultimate goal of assisting the return of people to the Moon. LCROSS was designed to search for water ice on the Moon's surface by crashing the Centaur upper stage rocket into the dark floor of one of the Moon’s polar craters (Cabeus) on 9 October 2009. The impact ejected material from the crater’s floor to create a plume that specialised instruments were able to successfully analyse for the presence of water (ice and vapor), hydrocarbons and hydrated materials.
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Interactions The phases of the Moon Having no light of its own, the Moon shines because of the light it reflects from the Sun. At any one moment the hemisphere turned away from the Sun will be dark. The apparent shape of the Moon in the sky (its phase) therefore depends on what position it has reached in its orbit around the Earth. When the Moon lies between the Sun and the Earth, we cannot see the sunlit side of the Moon and it is said to be new (position 1 on diagram). As the Moon continues in its orbit, we begin to see a little of the day side of the Moon and it appears as a crescent (position 2). Eventually the Moon has moved through a quarter of its orbit and it appears as a half moon and is said to be at first quarter (position 3). When the Moon continues it appears between half moon and full moon, and is said to be gibbous. Eventually the Moon is at position 6 (opposite the Sun) and is then full. The process of the Moon slowly increasing its phase is called waxing. Once past full moon, the phase of the Moon then begins to diminish, as indicated in the diagram above. This process is called waning. Eventually, the Moon returns to the new moon position, and the cycle is repeated. The period from one new moon to the next is referred to as the synodic or lunar month and is on average 29 days 12 hr 44 min long. However,
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relative to the background stars, the period of one complete orbital revolution is 27 days 19 hr 18 min and is referred to as the sidereal month. This happens to also be the Moon’s period of rotation about its axis. Combine this with the Moon spinning on its axis in the same direction as it moves in its orbit, and only one side of the Moon ever faces the Earth. We never see the far side. This is known as captured rotation. During the crescent phase of the Moon, the unlit part of the Moon can be seen to be bathed in a soft light. This phenomenon is due to the reflection of sunlight onto the Moon from the Earth. It is known as earthshine (similar to moonlight on the Earth) and was explained by Leonardo da Vinci. As seen from the Earth, the Moon appears to travel from west to east along its orbit. This means that every day it has moved further east and hence rises about 50 minutes later each day. The blue moon Rare events are said to occur once in a blue moon. A blue moon normally occurs seven times in each 19-year Metonic cycle. Until the 1930s, the blue moon was described as the third of four full moons in a season. In recent times the term has been used to describe the occurrence of a second full moon in a calendar month.
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A guide to the solar system
Eclipses Often, when the moon is new it passes directly between the Sun and the Earth resulting in the long conical shadow of the Moon falling onto the Earth’s surface. Standing at that point, observers would see the disc of the Moon completely covering the Sun. This is known as a total solar eclipse. By a quirk of nature, the Moon is just the right distance from the Earth to make it appear the same size as the Sun, that is, even though the Moon is 400 times smaller than the Sun, the Sun is in turn 400 times further away, and as a result they both appear the same size as viewed from the Earth’s surface. Observers only slightly removed from the Moon’s shadow would only see part of the Sun covered by the Moon and this is known as a partial solar eclipse. Solar eclipses are very dangerous to observe with the unaided eye, and can result in blindness. However, with proper precautions much can be learnt about the Sun’s atmosphere that can only be observed during solar eclipses.
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When the Moon is full, it sometimes passes behind the Earth and into its long conical shadow. This can be seen from the entire moonfacing hemisphere of the Earth and is known as a total lunar eclipse. Often however, the alignment is not exact and only a part of the Moon’s disc is covered by the Earth’s shadow and is known as a partial lunar eclipse. During lunar eclipses, it is fascinating to watch the Earth’s circular shadow slowly move across the Moon’s face. This simple observation led the ancient Greek philosopher Pythagoras to conclude that the Earth must be a sphere since its shadow is always circular. Since the Moon’s orbit is inclined 5° to the Earth’s orbit, eclipses don’t occur every time the Moon is new or full. Consequently, eclipses can only occur when the Moon is near the points where it crosses the Earth’s orbit and is at the same time, either new or full. The next total solar eclipse visible from Sydney is in July 2028 whereas the next total lunar eclipse is December 2011.
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The tides It has been known since ancient times that the Moon influences the tides, but it was left to Sir Isaac Newton in 1686 to explain how. The Moon exerts a gravitational force on the Earth. The Moon-facing side of the Earth is attracted more strongly to the Moon than the centre of the Earth, so the liquid oceans on that side bulge toward the Moon. The side furthest from the Moon is gravitationally attracted more weakly, so it gets ‘left behind’ and consequently the oceans on that side bulge away from the Moon. In this process the movement of the ocean waters deplete the areas midway between the bulges on either side of the Earth. As the Earth rotates on its axis, it progressively moves through the tidal bulges
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and depleted areas, thus resulting in a cycle of two high tides and two low tides every day. The Sun’s gravity also produces tides on the Earth, in exactly the same way as the Moon. Although the Sun’s gravity is 180 times stronger than the Moon’s, the Moon is very much closer to the Earth. As a result, the Moon’s gravitational force across the Earth is 2.2 times greater than the Sun’s. The pair of bulges produced by the Moon is thus greater than the pair raised by the Sun. As the Moon orbits the Earth, these two pairs of bulges get in and out of step. When the Moon is full or new, the bulges are in step and combine to produce large tides called spring tides. When the Moon is at first or last quarter, the bulges are out of step and they produce lower than normal tides called neap tides.
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A guide to the solar system
Mars Diameter: 6787 km Average distance from Sun: 228 million km Orbital period: 687 days Rotation period: 24 hr 37 min Number of known satellites: 2 Photo courtesy STScI
Often called the ‘red planet’ because of its bloodred appearance in the night sky, Mars is named after the god of war in Roman mythology. It is the second closest planet to the Earth but because of its small size (about half as big as the Earth), it is only the third brightest planet in the sky after Venus and Jupiter. Mars is best observed when it is closest to Earth, that is, at opposition (opposite the Sun and on the same side of its orbit as the Earth). Oppositions of Mars occur roughly every two years and two months. Due to its eccentric orbit, it comes closer to the Earth during some oppositions than at others. The most recent favourable opposition was in August 2003. Of all the planets, Mars is the most Earth-like and has many similarities. A day on Mars is almost the same length as on Earth (24 hr 37 min). The tilt of its axis is also very similar at 25.2°, compared to Earth’s 23.5°. The almost identical axial tilt means Mars experiences seasons just like the Earth, but since it takes twice as long to orbit the Sun, each season will be roughly twice as long as on Earth. The atmosphere of Mars is very thin and has a surface pressure less than 1% of the Earth’s. It is composed mainly of carbon dioxide (95%) with only minute traces of oxygen and water. The thin atmosphere and its distance from the Sun means Mars is very cold. The temperature at noon on a summer’s day seldom rises above 20 °C at the equator, while during winter at the south pole it drops to below –125 °C, the freezing point of carbon dioxide. Viewed from the Earth, Mars is the only planet for which the surface is visible through telescopes. In
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the past, astronomers observed lines crisscrossing the disc and mistakenly identified these as being canals built by Martians to divert water from the poles to the barren wastes of the equator. Today we know these canals were simply optical illusions. The most striking features seen from Earth are the polar icecaps which can be seen to grow and shrink with the ebbing of the seasons. Another feature often seen during the summer months on Mars, are dust storms which can grow to encompass the entire planet and hide the surface from view for weeks on end. Detailed views of the surface had to await the arrival of space probes, with the first being Mariner 4 in 1965. It beamed back the first closeup pictures of the surface and surprised astronomers with unexpected views of craters, making Mars look like our Moon. In 1969, the Mariners 6 and 7 flew past Mars and returned even more detailed photographs. The first spacecraft to orbit Mars was Mariner 9 in 1971 which was able to photograph the planet for a complete Martian year and observe its changing aspects. It discovered giant extinct volcanoes, with one, Olympus Mons reaching 24 km in height or approximately three times the height of Mt Everest. In addition, a 4000 km long valley, called Valles Marineris was discovered. This valley dwarfs the Grand Canyon on Earth. Erosion due to wind and sandblasting by dust storms were also observed. The most puzzling features however, were the sinuous stream-like structures which suggested water once flowed on the surface. Since the atmosphere of Mars is too thin to support liquid, it means that the atmosphere in the past must have been more
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dense than at present. Great changes must have occurred on Mars, the precise nature of which is yet to be resolved. In 1976 the Viking 1 and 2 spacecraft, after safely reaching Mars orbit released probes that soft landed on the surface and returned pictures of the rock-strewn Martian surface. In addition, instruments on board the landers analysed the soil and discovered the reason for the red colour of Mars. They found that the rocks and soil are covered by iron oxide, commonly called rust. In 1984, a meteorite was found in the Allan Hills icefield of Antarctica with properties that strongly suggested it was Martian in origin. The meteorite possibly found its way to Earth after being ejected from Mars by a very large meteorite impact. Most importantly, in 1996, this meteorite was found to possess tiny features that some scientists have interpreted as being microscopic fossils resembling bacteria on Earth. These are very exciting findings, although much more research needs to be done before conclusive evidence is obtained for life ever having existed on Mars. On 4 July 1997 the Mars Pathfinder landed and dispatched a small robot rover called Sojourner, which over a three-month period, undertook 15 chemical analysis experiments and returned 550 surface pictures back to Earth along with the 16 000 from Pathfinder. Sadly the next two missions failed with the Mars Climate Orbiter braking too late and exploding in the Martian atmosphere in 1998 and in 1999 the Mars Polar Lander crashed into the surface due to a programming error. Fortunately the Mars Global Surveyor which arrived at Mars in 1997 sent excellent data back to Earth including images that strongly support the idea of water flowing on Mars as recently as 10 000 years ago. In late 2003 and early 2004 four different probes headed for Mars, Nozomi from Japan was lost on route, Beagle 2 from the European Space Authority reached Martian atmosphere but was then lost, probably on landing. The Mars Express orbiter which dropped Beagle 2 continues to send back useful information, including the first evidence
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of water which it found frozen at the South Pole of Mars. The two NASA exploration rovers Spirit and Opportunity landed on Mars in January 2004 and have sent back many images and information about the surface features of the planet. In early March of that year NASA announced that the second rover Opportunity had discovered the presence of sulphate compounds in some rocks indicating that they had been altered by exposure to liquid water after their formation. After nearly 2000 days, or Sols, they are still exploring the surface of Mars however Spirit became bogged on Sol 1899. The latest spacecraft to examine Mars from orbit is the Mars Reconnaissance Orbiter which arrived at Mars on 10 March 2006. The orbiter has the most powerful telescopic camera ever taken to another planet, plus five other scientific instruments. Some of its recent photographs have revealed bright new deposits seen in two gullies that suggest water carried sediment through them sometime during the past seven years. The most recent spacecraft to land on Mars was Phoenix which landed in the north polar region on 28 May 2008 and successfully found frozen water ice just below the surface before falling silent in November probably as the result of the extreme cold of a Martian winter. NASA engineers will make another attempt to re-establish communication with Phoenix in October 2009. Mars has two small irregularly shaped moons — Phobos (fear) and Deimos (panic). Both these moons are quite small with Phobos being just 27 km long and Deimos 15 km long. These moons are most likely captured asteroids which have wandered too close to Mars and become trapped in orbit. Phobos is the nearer of the two moons to Mars and orbits just 6000 km above the surface in just 7 hr 39 min. As a result, it would appear from the surface of Mars to rise in the west and set in the east twice each day. Also, it always has the same face turned to the planet, like the Earth’s Moon. Deimos orbits 20 000 km above the planet in 30 hr 18 min, almost the same as a Martian day, and so from the surface of Mars, it appears to rise in the east and set in the west two and a half days later.
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Asteroids Asteroids are believed to be leftover material from the formation of the solar system. Around 4000 have known orbits but there may be as many as 40 000 strewn between the planets. The majority of asteroids lie between Mars and Jupiter in a region known as the asteroid belt. Photo courtesy NASA
As the solar system was forming 4500 million years ago, giant chunks of rock and metal (planetesimals) were colliding and sticking together. By this process, the planets slowly grew and developed into their current sizes and positions. Not all of these planetesimals however, were used up to form the planets. It is believed that a planet should have formed between the orbits of Mars and Jupiter. However, the powerful gravity of Jupiter ‘stirred up’ the planetesimals in this region, and prevented them from forming a planet. These planetesimals became the asteroids. Asteroids differ in colour from one another, which may indicate differing compositions. About threequarters of the asteroids are extremely dark and reflect only 3.5% of the sunlight falling on them. They are probably composed of carbon-rich rocks, and are classed as C-type asteroids. Roughly onesixth of the asteroids have moderate brightness and reflect 16% of the sunlight falling on them. They are reddish in colour and resemble stony meteorites in composition. They are classed as S-type asteroids. A third type of asteroid has moderate brightness and is rich in metallic nickeliron. These are known as M-type asteroids. Most asteroids are relatively small with only a few having diameters greater than 250 km. The larger asteroids are often referred to as minor planets. The largest asteroid is (1)Ceres is 1003 km in diameter and was reclassified as a ‘dwarf planet’ in August 2006. It is a C-type asteroid and contains one-third of all the matter in the asteroid belt. The brightest asteroid however, is (4)Vesta. It is 538 km in diameter and is an S-type asteroid. Of the asteroids, only Vesta is bright enough to be visible to the naked eye.
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Not all the asteroids lie within the asteroid belt. Some are found further away than Jupiter, and others lie closer to the Sun than Mars. The first asteroid known to come within the orbit of Mars was (433)Eros. It is about 10 km in size and can come within 23 million km of the Earth. In 1977, an asteroid named Chiron was discovered between the orbits of Saturn and Uranus. It has a diameter of about 300 km, and appears to resemble Pluto more than the asteroids. It’s true nature is still a mystery. The asteroid (1566)Icarus actually approaches the Sun within the orbit of Mercury. It is only about 1 km in size and can get as close as 28 million km to the Sun. This extreme closeness means that its surface temperature can reach 500 °C. Asteroids that cross the orbit of the Earth are known as Apollo asteroids. Most members of this group are very small and can be observed only when close to the Earth. They are named after the asteroid Apollo, which was discovered in 1932 and is just 1 km in size. The asteroid Icarus also belongs to this group. Another group of asteroids are the Trojans. These lie along the orbit of Jupiter in two areas, one 60° ahead and the other 60° behind the planet. They lie at the points on the orbit where Jupiter’s gravity is cancelled by the Sun’s gravity. These Trojan asteroids are all very dark and appear to have the same origin. The first asteroid to be discovered was (1)Ceres. On 1 January 1801 (the first day of the 19th century), the Italian astronomer Giuseppe Piazzi working in Sicily, came across a small world in an orbit between Mars and Jupiter. He named it Ceres after the patron goddess of Sicily. The German astronomer Johann Bode, organised a group of amateur and professional astronomers, whom he
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called the ‘Celestial Police’, to track down more of these minor planets. Eventually they found a second object, (2)Pallas, followed by Vesta and then (3)Juno. Sir William Herschel (the discoverer of Uranus) looked at them with his largest telescopes, but could only see starlike images. He proposed to call them asteroids, which means ‘starlike’. Other astronomers preferred the term minor planets. Today both terms are used to describe these objects. The first spacecraft to visit an asteroid was the Galileo Jupiter probe. In 1991, en route to Jupiter, it passed by the small asteroid (951)Gaspra, and returned the first ever close-up images of an asteroid. This asteroid proved to be irregularly shaped about 19 km long and with many small craters. Following this encounter, in 1993 the Galileo spacecraft flew past the asteroid (243)Ida. The images returned, showed that it was about 56 km in size, with a very heavily cratered surface. Most surprisingly of all, Ida was found to have a small moon, now called Dactyl (see right side of the photograph p 16). This small moon is 100 km from the centre of Ida and takes about 24 hours to orbit it once. Both Ida and Dactyl appear to be S-type asteroids. In February 1996, the Near Earth Asteroid Rendezvous-Shoemaker (NEAR-Shoemaker) space probe was launched toward the asteroid belt. It was the first long-term, close-up study of an asteroid. Using innovative sensors and detection equipment NEAR-Shoemaker collected information on Eros’ mass, structure, geology, composition and gravity. The NEAR mission sought to answer fundamental questions about the nature and origin of the many asteroids and comets close to Earth’s orbit. These ‘near Earth’ objects may contain clues about the formation of Earth, other planets, even the whole universe. Eros’ pristine surface offers a look at conditions in space when Earth formed more than 4.5 billion years ago. It landed on Eros on 12 February 2001 and functioned until 28 February 2001. Meteoroids Meteoroids are interplanetary particles. The vast majority are just a few millimetres in size. A meteoroid entering the Earth’s atmosphere heats the air around it so much (through friction) that it glows. From the ground it appears as a streak of light called a meteor or shooting star (sometimes
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even a falling star). The fragments are usually so small that they burn up at a height of about 100 km. Often however, some fragments are large enough to survive and strike the ground. We call these meteorites. In the course of its journey around the Sun, the Earth collides with hundreds of thousands of these meteoroids every day. We see them mainly at night, when it is dark. More are visible after midnight than before, because we are then facing the direction of Earth’s motion around the Sun. Consequently, more meteors are seen in this direction. This is very similar to rain striking the front windscreen of a car more often than the back windscreen as it is moving along. Meteoroids appear to be related to asteroids. One theory is that they are particles that have broken off asteroids as a result of previous impacts. Their composition appears to be very similar. Stony meteorites are similar to S-type asteroids. Stony-iron meteorites resemble C-type asteroids and iron meteorites are comparable to M-type asteroids. Most meteorites are as old as the solar system — 4500 million years old. Some however, are considerably younger and resemble moon rocks and Mars soil samples in composition. A recent theory suggests that these young meteorites may be surface fragments of the Moon and Mars. These fragments may have been ejected from the surfaces of these planets by large meteorite impacts, eventually finding their way to Earth. When Earth’s orbit intersects a stream of meteoroid particles, a meteor shower is observed. Perspective causes the parallel paths of the meteors to appear to radiate from a point in the sky (in the same way sunrays appear to fan out). This point of divergence is called the radiant. Meteor showers are related to comets. The dust blown off them as they approach the Sun becomes strewn out along their orbits. If the Earth crosses such an orbit, a meteor shower occurs. The ancient Greek philosopher Aristotle, thought meteors were weather phenomena. He named them meteors from the Greek word for the sky (the word meteorology means the study of the weather).
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A guide to the solar system
Jupiter Diameter: 142 984 km Average distance from Sun: 778 300 000 km Orbital period: 11.86 yrs Rotation period: 9 hr 55 min Number of known satellites: At least 63 Photo courtesy STScI
Named after the Roman king of the gods and father of Mars, Jupiter is the largest planet and contains about 71% of all the material of the solar system, excluding the Sun. By any standard, Jupiter is a big world. It has 318 times the mass of the Earth, about 1300 times the volume and 11 times the diameter of the Earth. The extremely rapid rotation of 9 hr 55 min flattens the poles of Jupiter and makes the equator bulge, forming a shape known as an oblate spheroid. This means that the equatorial diameter is 8600 km greater than the polar diameter. This flattening of the poles is clearly seen even with small telescopes from the Earth. Jupiter is composed mainly of hydrogen with about 10% helium. The core of Jupiter is estimated to be 10 to 20 times the mass of the Earth. Since Jupiter emits about twice as much heat as it receives from the Sun, the core temperature is probably about 20 000 °C, around three times greater than the Earth’s core temperature. This heat is thought to be generated from the slow compression of the planet which contracts about 2 cm a year. The core is thought to be composed of melted rock and may have a pressure 100 million times greater than the pressure on the surface of the Earth. In a layer above the core and extending to 20 000 km below the cloud tops is the mantle. The intense pressure causes the hydrogen present here to take on the properties of a metallic liquid, which conducts electricity well. The currents circulating in this layer are believed to generate Jupiter’s powerful magnetic field which is 20 000 times stronger than the Earth’s.
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Surrounding the mantle is a layer of ordinary liquid hydrogen. Above this, hydrogen behaves as a gas, circulating in large currents generated by the planet’s rapid rotation. The atmosphere is topped by three layers of clouds, which give Jupiter its distinctive appearance. At a temperature of just –153 °C, the upper layer is the coldest and is composed of ammonia ice crystals. The middle layer is a mix of ammonia and hydrogen sulphide. In the lower layer the temperature and pressure are high enough that clouds of water ice may exist. The clouds in Jupiter’s atmosphere move in east-west belts, parallel to the equator. Wind speeds vary in these zones, which are influenced by the planet’s rapid rotation and heat welling up from the interior. Wind speeds of up to 540 km/h have been recorded. These belts appear as cloud bands in small telescopes. In addition to the cloud bands, quite a number of oval features are present. These are thought to be stable circular currents (eddies), or cyclones which are trapped by the opposing winds of the cloud bands. The largest of these eddies is the Great Red Spot, which is three times the size of the Earth and has been observed for over 300 years. It rotates once in six days, in an anticlockwise direction. In 1979, a series of three very thin rings were discovered by the Voyager 1 and 2 spacecraft. They are composed of ice coated dust particles which are on average 1/1000 mm in size. The rings are only 30 km thick and extend from about 100 000 km above the cloud tops out to about 200 000 km.
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The Galilean moons Jupiter has, as far as we know at the moment, more moons than any other planet with a total of 63. A record ten of these were found at the same time in January 2001 by astronomers from the University of Hawaii using a relatively small 2.2 m telescope atop Mauna Kea. Most are relatively small with some probably being captured asteroids or comets. Four of the moons however, are very large and were discovered by Galileo in 1610. They can easily be seen with binoculars and are known as the Galilean moons. Of the total, 38 of the moons have names, while the rest are designated by numbers, the last four were announced in May 2003. Io (pronounced ‘eye-oh’) is the innermost of the Galilean moons. It orbits Jupiter at a distance of 421 600 km and has a diameter of 3630 km, making it only a little bigger than our own moon. It is believed that Io is a rocky body with a large metallic core. Io is the most volcanically active body in the solar system, with nine active volcanoes known to exist. It has 200 volcanic craters over 20 km across. This is ten times as many as the Earth has in that size range. The largest volcano is called ‘Pele’ and is 1400 km across. Volcanic material is ejected from vents at speeds of around 3600 km/h and the resulting umbrella-shaped plumes suggest that the volcanoes function much like geysers on the Earth. The orange colour of Io may come from sulphur contained in the rocks coating much of the surface. The reason Io is so active is because of its nearness to Jupiter. Jupiter’s gravitational attraction subjects Io to huge forces. These forces create a bulge that moves across Io’s surface, causing its crust to flex and bend, back and forth much like a tennis ball being continually squeezed. Astronomers refer to these as tidal forces. This movement generates enough heat to melt the interior and produce the widespread volcanism. Europa orbits Jupiter at a distance of 670 900 km. At just 3138 km in diameter it is the smallest of the Galilean moons. Europa’s surface of water ice is extremely flat and smooth. A network of dark grooves and ridges cover the surface and appear
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to be cracks in the ice crust that are filled in with water. The crust of Europa is probably only about 100 km thick and the absence of impact craters indicate that the surface must have been flooded by water, removing any evidence of craters. It is thought that tidal forces heat the interior and periodically melt the surface just as they do on Io, but to a much lesser extent. Ganymede is the largest moon of the solar system. With a diameter of 5262 km it is larger even than the planet Mercury, so if it wasn’t orbiting Jupiter it would be a respectably sized planet in its own right. It orbits Jupiter at a distance of 1 070 000 km and is composed of half ice and half rock. Ganymede has two types of surface which differ in brightness. Heavily cratered dark regions are the oldest parts of the surface and their origins are not clearly understood. Lighter regions have fewer craters, which indicate they are younger than the darker terrain. They contain parallel curving grooves that have no counterparts on the earth-like planets. The crust of Ganymede is thought to be about 75 km thick and beneath this lies a mantle of ice with a rocky core. Callisto is the near twin of Ganymede. It has a diameter of 4800 km and orbits Jupiter at a distance of 1 884 600 km. Callisto is the most heavily cratered body in the solar system. Valhalla is the largest of its impact craters. It is a bright ringed structure 3000 km in diameter and was probably formed when an asteroid-sized object struck the moon eons ago. Until the advent of the space age, little was learnt about Jupiter and its retinue of moons. The first space probe to reach Jupiter was Pioneer 10 in 1973. Its sister ship Pioneer 11, arrived soon after, in 1974. These spacecraft beamed back our first ever close-up pictures of Jupiter and its moons. They also relayed data on the temperature and pressure within Jupiter’s atmosphere. These early pictures were of relatively low quality and few in number. Pioneer 11 used the gravity of Jupiter to fling it on towards an encounter with the planet Saturn in 1979. Voyagers 1 and 2 flew by the planet in 1979. These two spacecraft returned about 30 000 images and a wealth of other data. The achievements of these
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two highly successful probes were numerous. Several new small moons were discovered, the Jovian ring system was clearly identified, the structure and composition of the Jovian atmosphere was studied, and the nature of the Galilean moons was investigated for the first time in great detail. Io was found to possess several active volcanoes, with some actually erupting at the time of the probe fly-bys. The information gleaned from these two encounters is still leading astronomers to learn much about Jupiter and its moons. Once past Jupiter, the gravity of the giant planet flung the two Voyagers onto encounters with the planet Saturn in 1980 and 1981. Voyager 2 was later even able to visit the planets Uranus and Neptune in what became known as the Grand Tour of the solar system. In December 1995, the Galileo space vehicle became the first spacecraft to orbit Jupiter. Its primary mission was to conduct a series of complicated orbits about Jupiter in order to successively swing it past the large Galilean moons and study them at extremely close range. Also, it is to investigate the atmosphere and weather of the planet over this same period. However, the most exciting phase of the mission occurred in December 1995, when an atmospheric probe released by the Galileo spacecraft several months earlier, plunged into the Jovian atmosphere. It relayed back to Earth information about the atmospheric composition, temperature and pressure for 59 minutes before it was crushed by the planet’s atmosphere and ceased transmitting. In 1994, en route to Jupiter, the Galileo spacecraft was able to study the effects of the collision of Comet Shoemaker-Levy 9 with Jupiter. Unfortunately, the impacts were not able to be seen directly from the Earth, owing to them being on the far side of the planet and hence, out of view from Earth. Galileo however, was in such a position for it to have a direct line of sight with the impact areas. The resulting impacts confirmed theories that the formation of the solar system by cometary and asteroid bombardment. On 28 December 2000 Galileo completed its 29th orbit of Jupiter as it flew by the biggest moon in the solar system, Ganymede. Shortly afterwards the Cassini spacecraft on its journey to Saturn also
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observed Jupiter in the weeks surrounding the ‘Millennium Flyby’ on 30 December thereby achieving for the first time ever, simultaneous observations of an outer planet by two spacecraft. Galileo’s successful mission ended on 22 September 2003 when it was deliberately put on a collision course with Jupiter. This ensured the spacecraft would never be able to contaminate the large moons thought to be some of the most likely environments off the Earth to sustain life. In addition to spacecraft visiting the planet, the Hubble Space Telescope has allowed astronomers to view Jupiter and its moons with almost the same level of clarity. This remarkable telescope will continue to aid astronomers in their investigations of Jupiter and the other planets of the outer solar system for many more years to come. In May 2010, one of Jupiter's two main cloud belts completely disappeared. Known as the South Equatorial Belt (SEB), it is a band twice as wide as Earth and more than twenty times as long. The loss of such an enormous "stripe" can be seen with ease halfway across the solar system even through a small telescope. It is not the first time this has happened, most recently in 1973-75, 1989-90, 1993 and 2007. One possible cause is that that some ammonia cirrus cloud has formed on top of the SEB, hiding it from view. How long it will be gone for is unknown and the return can be just as dramatic.
View of a chain of craters named Enki Catena on Jupiter’s moon, Ganymede. This chain of 13 craters probably formed by a comet which was pulled into pieces by Jupiter’s gravity as it passed too close to the planet. Soon after this break-up, the 13 fragments crashed onto Ganymede in rapid succession. Anatomy of a Torn Comet. Cat No. PIA01610. Galileo image courtesy of NASA and Brown University
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Saturn Diameter: 120 540 km Average distance from Sun: 1 429 400 000 km Orbital period: 29.42 years Rotation period: 10 hr 40 min Number of known satellites: 61 According to Roman mythology Saturn was the god of agriculture and the father of Jupiter. Saturn is the second biggest planet, but the first in terms of beauty. It is 9.4 times the diameter of the Earth and 752 times its volume. Its magnificent ring system makes it the ‘jewel’ of the solar system. Saturn is the root of the English word ‘Saturday’. Saturn is a Jovian-type planet which means it has many characteristics similar to that of Jupiter. Like Jupiter, Saturn is composed mainly of hydrogen but with just 3% helium, less than half of Jupiter’s amount. As a result, Saturn is not as dense as Jupiter and in fact has a density less than water. This means that alone among the planets, Saturn would float if it were put into a large enough pool of water. Saturn’s axis is tilted 26.7° and rotates about its axis in just 10 hr 40 min. This rapid rotation of Saturn about its axis means that it bulges at its equator or appears flattened at the poles. Its equatorial diameter is 11 800 km greater than its polar diameter. This suggests that Saturn has a rocky core about 10 to 15 times the mass of the Earth. The core is surrounded by a mantle of metallic liquid hydrogen, which in turn is surrounded by ordinary liquid hydrogen. As the temperature and pressure decrease, the liquid hydrogen becomes gaseous. The temperature at the top of this gaseous layer (the cloud tops) is about –185 °C. In the core, the temperature probably reaches 12 000 °C with a pressure about 12 million times the pressure on the Earth’s surface. The top gaseous layer is thought to consist of three layers, but because it is much colder these cloud layers are less complex than those found on Jupiter. These clouds appear yellow in colour
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Photo courtesy Hubble heritage Team
and move in zones parallel to the equator. Though Jupiter has many more of these zones, the wind speeds on Saturn are much higher and are capable of reaching 1800 km/h. In addition, Saturn has fewer oval shaped circular currents than Jupiter. Saturn emits about 80% more heat than it receives from the Sun. The heat is thought to be from the sinking of what little helium is present, towards the core. As this happens the helium loses energy in the form of heat. This process is known as ‘helium rain’. Galileo was the first person to observe the rings of Saturn. His primitive telescope however, didn’t enable him to see the rings clearly and he incorrectly identified them as puzzling ‘ears’ extending on either side of the planet. In 1659 the Danish astronomer Chistiaan Huygens observed Saturn with his improved telescope and correctly identified the extensions as a ring. In 1675, the Italian astronomer Jean-Dominique Cassini discovered a gap in the rings, which today is known as Cassini’s Division. Today astronomers have identified seven main rings. Each of the rings is denoted by a letter from A to G, given in the order in which they were discovered. Closest to the planet is the faint D-ring. It is 6700 km from the cloud tops. Next is the tenuous C-ring, followed by the B-ring which is the densest and brightest of the rings. Cassini’s Division is 4700 km wide and separates the B-ring from the A-ring. Following this is the very peculiar and narrow F-ring which is believed to be kept in place by two small satellites, Prometheus and Pandora, which are known as shepherd satellites. Beyond is the very faint G-ring and the even
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A guide to the solar system
more tenuous E-ring, both of which were discovered by the Voyager spacecraft, and are around 420 000 km above the cloud tops. In relation to their total span of about 400 000 km, the rings are extremely thin at just 10 to 100 m thick. They are composed of thousands of millions of ice particles which vary in size from 1 cm to 1 km. If all the particles were collected to form a single moon, it would be about 100 km in diameter. In addition, the Voyager spacecraft discovered that the main rings consist of many tiny rings or ringlets — about 1000 have been identified. Also, above the main B-ring the voyagers discovered dark radial spokes, which are believed to be charged dust particles suspended above the ring and carried around by the planet’s magnetic field. Saturn is believed to have in total at least 50 moons with possibly a few more that are still uncertain. Many of these moons are very small and less than 200 km in size. Only 30 of the moons have names. Of the bigger moons, Titan is the largest at 5150 km in diameter. It is the second biggest moon in the solar system after Ganymede of Jupiter. It orbits Saturn at a distance of 1 222 000 km. Like Ganymede and Callisto of Jupiter, it is thought to be composed of half water ice and half rock, but unlike them it has an atmosphere. This atmosphere is 80% nitrogen (similar to the Earth) and is four times more dense than the Earth’s. It contains orange clouds of carbon monoxide (a common example of which is car exhaust) that completely obscure the surface. Astronomers believe Titan’s atmosphere today is similar to the Earth’s atmosphere as it was billions of years ago.
The first spacecraft to reach Saturn was Pioneer 11 in September 1979. It passed within 21 000 km of Saturn and beamed back the first close-up pictures of the planet and the rings. It discovered several small moons as well as the F-ring. The more sophisticated Voyager 1 and 2 probes passed Saturn in November 1980 and August 1981 respectively. Voyager 1 passed within 124 123 km of Saturn before heading out of the solar system. Voyager 2 passed within 101 335 km of Saturn and used the gravity of the big planet to hurl it toward an encounter with the planet Uranus in 1986. The Voyagers provided an incredible wealth of information, which have increased our understanding of the Saturnian system. The European Space Agency (ESA) and NASA recently cooperated on a mission to Saturn called Cassini-Huygens. Launched in 1997 it reached Saturn in July 2004. On 14 January 2005 the Huygens probe successfully entered Titan’s upper atmosphere and descended under parachute to the surface. The descent phase lasted around 2 hr 27 min with a further 1 hr 10 min on the surface. Throughout the mission data was collected from all instruments providing a detailed picture of Titan’s atmosphere and surface. The Cassini orbiter has already exceeded its original mission length of four years and is planned to continue collecting data until after the August 2009 Saturnian equinox through to September 2010. It will make repeated close flybys of Titan and Enceladus, both to gather data about the moons and for gravity-assist orbit changes that will enable it to obtain views of Saturn’s higher latitudes.
Excluding Titan, the six largest moons in order of increasing distance from Saturn are: Mimas (390 km diameter), Enceladus (500 km diameter), Tethys (1060 km diameter), Dione (1120 km diameter), Rhea (1530 km diameter) and Iapetus (1460 km diameter). These moons are mainly all composed of water ice, with some methane, ammonia and nitrogen ices. They are all heavily cratered, with Mimas having one crater (named Herschel) nearly one-third of its diameter. It was formed from a meteorite impact that almost shattered the moon.
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A guide to the solar system
Uranus Diameter: 51 120 km Average distance from Sun: 2 875 000 000 km Orbital period: 83.75 years Rotation period: 17 hr 14 min Number of known satellites: 27 Photo courtesy STScI
Uranus is named after the Roman god of the heavens who was also the father of Saturn. In March 1781, the English amateur astronomer Sir William Herschel, accidentally discovered Uranus, thus making it the first planet discovered since ancient times. Uranus is four times the Earth’s diameter and 14.6 times its mass. Its volume is 64 times that of the Earth and it orbits twice as far from the Sun as Saturn. Uranus rotates once every 17 hr 14 min and its axis is tilted 98°. This means that Uranus is tipped on its side. Consequently, at different times during its 84-year orbital period, one pole of Uranus is pointing directly toward the Sun, followed by the other. No-one knows why this is so, but one theory is that an ancient collision with an earth-sized object knocked Uranus on its side. Uranus is a gas giant, but unlike Jupiter and Saturn its composition is not dominated by hydrogen and helium. Hydrogen accounts for only about 15% of the planet’s mass. Methane, ammonia and water contribute to a greater percentage of the mass. Uranus is thought to have a dense rocky core. The mantle is probably liquid hydrogen with a small amount of liquid metallic hydrogen deep in the interior. It is believed that an ocean of water and methane lies at the base of the atmosphere. The atmosphere is composed mainly of hydrogen and helium with some methane. Ammonia and water clouds form at the top layer. These methane clouds give Uranus its distinctive blue-green colour. Uranus’ atmosphere lacks zones and cloud bands as on Jupiter and Saturn, but wind speeds have been measured as high as 600 km/h. Uranus has a uniform temperature of –214 °C.
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Rings were discovered in 1977 and confirmed by Voyager 2 in 1986. They total 11 in number and are very narrow and dark. Most of the rings are no more than 10 km wide and range from 10 to 100 m in thickness. The dark ring particles range from just 10 cm to 10 m in size. The innermost ring is 7700 km from Uranus and the outermost ring is 26 000 km above the cloud tops. Uranus has a total of 27 moons. Prior to the Voyager 2 encounter in 1986, Uranus was thought to have just five moons. Most of the moons discovered by Voyager 2 are very small and are less than 160 km in size. The largest moons are Titania (1610 km), Oberon (1550 km), Umbrial (1190 km), Ariel (1160 km) and Miranda (485 km). These big moons are thought to consist of mainly rock, with a smaller percentage of water ice than on Saturn’s similar sized moons. All the moons of Uranus are named after characters in Shakespeare plays and the writings of Alexander Pope. Inspired by the discovery of Uranus, Martin Klaproth, a German chemist, changed the name of his newly discovered element from Klaprothium to Uranium.
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A guide to the solar system
Neptune Diameter: 49 530 km Average distance from Sun: 4 504 400 000 km Orbital period: 163.73 years Rotation period: 16 hr 7 min Number of known satellites: 13 Photo courtesy NASA
Neptune is named after the Roman god of the sea. It is too faint to be seen with the naked eye and was only discovered in 1846 by the German astronomer Johann Gottfried Galle. This was after its position was independently predicted mathematically by the young Englishman John Couch Adams and the French astronomer Urbain Leverrier. The first man to see Neptune however, was Galileo in 1610, but he failed to identify it as a planet because of his primitive telescope. In terms of size, composition and mass, Neptune is almost the identical twin of Uranus. Its diameter is 3.9 times greater than the Earth’s, and has 17 times the mass of the Earth. It is the smallest of the gas giant (Jovian) planets. Its axis is tilted 29.6° and rotates once in 16 hr 7 min. Like Uranus, hydrogen accounts for just 15% of the planet’s mass, with water, methane and ammonia in great abundance. Neptune’s rocky core is probably slightly larger than Uranus’. A mantle of liquid hydrogen surrounds the core and lies beneath what may be an ocean of water mixed with methane and ammonia. Neptune’s atmosphere is a mixture of hydrogen, helium and methane. It is the presence of methane which gives Neptune its blue colour. Clouds of ammonia and water ice also exist in the upper layers. Neptune radiates more heat than it receives from the Sun. This internal heat source makes the atmosphere of Neptune more active than Uranus’. It has cloud bands and large oval structures. In addition, Neptune has the strongest winds of all the planets, with speeds often reaching 2000 km/h. In 1989, Voyager 2 discovered a great storm on Neptune called the Great Dark Spot. This appears to be similar to Jupiter’s Great
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Red Spot. Recent observations with the Hubble Space Telescope however, indicate that this dark spot has since vanished. Like the other Jovian planets, Neptune has rings. In 1989, Voyager 2 discovered seven very dark and narrow rings. The innermost ring is 42 000 km from the cloud tops. The outermost ring is 63 000 km above Neptune and was found to have three unusually dense areas. The rings consist of very dark, non-reflective particles which range from microscopic dust to about 10 m in size. Before the Voyager 2 encounter, in 1989, Neptune was thought to have just two moons. However, Voyager discovered six more moons which were all less than 400 km in diameter. The outermost moon is Nereid, discovered in 1949. Triton is the largest moon and was also discovered in 1846. With a diameter of 2700 km it is only a little smaller than our own Moon. It is believed to be composed mainly of rock with about 25% water ice. Voyager 2 was able to photograph the surface in great detail and revealed one of the most complex surfaces of any moon. Its surface temperature is the coldest in the solar system at just –245 °C. A thin atmosphere of nitrogen and methane surrounds Triton and volcanic activity in the form of geysers was also observed. In addition, the south polar cap was found to contain frozen methane and nitrogen. All this suggests that Triton is probably very similar to the planet Pluto, since it has a similar size and composition.
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A guide to the solar system
(134340)Pluto Diameter: 2274 km (approx) Average distance from Sun: 5 915 800 000 km Orbital period: 248.03 years Rotation period: 6 days 9 hr 18 min Number of known satellites: 3 Photo courtesy STScI
Pluto, or as it now known (134340)Pluto, is named after the Roman god of the underworld. Formerly known as the ninth planet it now belongs to a new category of objects known as ‘dwarf planets’. Others include (136199)Eris (see p 28) and (1)Ceres (see p 16). Pluto is very small, even smaller than our Moon, and from its great distance, the Sun only appears as a bright star. It was discovered in 1930 by Clyde Tombaugh, after the position for a possible ninth planet was calculated by Percival Lowell and a photographic search was begun at his observatory. One reason why this new object was named Pluto is because the first two letters are the initials of Percival Lowell (PL). Since then Pluto has proved to be too small and too distant to be the planet calculated by Lowell. Its discovery was just a matter of good luck and hard work on the part of Clyde Tombaugh. Pluto’s orbit is far more eccentric (oval) compared to the eight planets in our solar system. It approaches as close as 4425 million km from the Sun and as far as 7375 million km. Having such an elliptical orbit means that for 20 years of its nearly 248-year orbital period, Pluto moves closer to the Sun than Neptune. A collision with Neptune is not possible because Pluto’s orbit is inclined 17.2° to that of the eight planets. Also, for every one orbit that Pluto makes, Neptune races ahead making 1.5 orbits. As a result, at the times their orbits appear to cross, Pluto is always hundreds of millions of kilometres above Neptune’s orbit and on the opposite side of the Sun. From February 1979 to February 1999 Pluto’s orbit brought it closer to the Sun than the orbit of Neptune.
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In 1978, a ‘moon’ was discovered and it was named Charon, after the mythological boatman who ferried souls to the underworld. Charon is 1270 km in diameter, which makes it over half the size of Pluto. The Pluto-Charon system was considered by many astronomers to be a double planet system. Charon is 19 100 km from Pluto and is in a synchronous orbit. That is, both Pluto and Charon rotate about their respective axes and also orbit about each other in the same time of 6 days 9 hr 18 min. Consequently, the same sides of both Pluto and Charon always face each other. If you were standing on Pluto, Charon would appear stationary in the sky and you would only ever see one side of it. Conversely, the same is true for the view of Pluto from Charon. Studies have indicated that Pluto has a thin atmosphere of nitrogen and methane which extends all the way to Charon, suggesting that the two share the same atmosphere. Methane frost has also been found on the surface of Pluto, indicating that when Pluto is furthest from the Sun, its atmosphere may freeze and condense on the surface. Charon’s surface appears to be bluer than Pluto’s, so it’s possible that it has a different composition. Both also appear to have bright polar caps. Recent pictures from the Hubble Space Telescope show dark and bright markings on Pluto’s surface (see photo above). Pluto’s axis is tilted 122.5°, so like Uranus it is tipped on its side. One theory suggests that when Charon was captured by Pluto’s gravity, it knocked Pluto over and made its orbit eccentric (oval).
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A guide to the solar system
Comets Comets are the erratic wanderers of the solar system. In the past they were seen as omens of doom and disaster, but today they are objects of wonder. The ancient Greeks described comets as ‘hairy stars’. The Greek word for hair is kome, hence the word comet (and comb etc). Comets today are described as ‘dirty snowballs’. They consist of a nucleus, or solid body, that is largely composed of various ices and dust, and may possibly contain a rocky core. Comets are typically 1–10 km in size, with the brighter comets being slightly larger. When far from the Sun the ices remain solid. However, as the comet approaches the Sun, it begins to warm up. This causes the ices to sublimate, that is, change directly from a solid to a gas. This venting of gases creates a cloud that surrounds the nucleus of the comet. This cloud is called the coma and can be anything from thousands to hundreds of thousands of kilometres in diameter often it blocks the nucleus from view. As the comet continues to approach the Sun, the charged particles emanating from the Sun (called the solar wind), blow the gases of the coma away from the comet and form the tail. Since the tail always points away from the Sun, a comet that is moving away travels tail-first. The tail has only enough material to fit in a suitcase but it can stretch for hundreds of millions of kilometres. Comets are thought to originate in a region called the Oört Cloud named after the Danish astronomer Jan Oört who proposed it in 1950. This cloud is believed to have a radius of about 100 000 times the Earth-Sun distance, and may extend halfway to the nearest star. It is estimated that the Oört Cloud contains more than a trillion comets. The innermost region of the Oört Cloud is called the Kuiper Belt, named after the American astronomer Gerard Kuiper. Gravitational forces can change the orbit of a comet in the Oört Cloud, into a new orbit closer to the Sun. Comets are of two types — short-period and long-period. Short-period comets have orbital periods of less than 200 years. They originate from the Kuiper Belt. Halley’s comet is the most famous
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Photo courtesy Martin George
and brightest of these and has a period of just 76 years. Long-period comets often have orbital periods of 100 000 years or more, and they originate in the outer-regions of the Oört Cloud. Some of these have long, arching (parabolic) orbits and may never return after their once-only visit to the Sun. In January 2004 the Stardust probe passed though comet Wild 2 to within 240 kilometres of the comet, catching samples of comet particles and scoring detailed pictures of Wild 2’s pockmarked surface. It returned these particles to Earth on 15 January 2006. Halley’s comet In 1705 the English astronomer Edmond Halley used Sir Isaac Newton’s newly developed theory of gravity to calculate the orbits of bright comets that had been observed in 1531, 1607 and 1682. He concluded that all three were in fact the same comet with a period of 76 years. He predicted that the comet would return in 1758 and sure enough, the comet was sighted on Christmas day 1758. Halley died 17 years earlier and did not live to see the return of the comet which now bears his name. There have been a total of 30 recorded sightings in history, with the earliest being in 240 BC. The next return will be in 2061. In 1986, a flotilla of spacecraft were sent to study the comet on its return. The Soviet Vega 1 and 2, and the Japanese Susei and Sakigake probes studied it from thousands of kilometres away. The European Giotto probe however, passed within 605 km of the nucleus and returned the first ever images of the nucleus of a comet. It revealed a potato shaped nucleus about 16 km long and 8 km across at its widest. The surface was darker than coal and had hills and craters.
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A guide to the solar system
Dwarf planets and the Kuiper Belt
The first Kuiper Belt Objects (KBOs) were discovered in 1992 and since then more and more objects were found, some of them nearly the same size as Pluto. To date over 800 KBOs have been found but there may be over 70 000 KBOs with diameters over 100 km.
In 2008 the IAU went further to classify all dwarf planets beyond the orbit of Neptune to be known as plutoids.
KBOs are typically smaller than Pluto yet similar in that they are icy worlds whose orbits are eccentric (very oval) and tilted to the orbital plane of the eight planets.
(1) Ceres — the largest of the asteroids in the Asteroid Belt between Mars and Jupiter, it is 950 km in diameter.
In 2003 a KBO was discovered and designated 2003UB313. Recently named (136199)Eris, it is more than twice Pluto’s average distance from the Sun and, with a radius of 2400 km, it is slightly larger. It was initially labelled as the tenth planet in our solar system because of its size. However, this label raised the prospect that similar objects found in the future could increase the number of planets into the hundreds. As a result a need for a precise definition of the term planet was needed. In 2006, the International Astronomical Union (IAU) defined what constitutes a planet in our solar system. For an object to be classified as a planet it must pass all four of the following tests. Pluto failed number three and became part of a new class of ‘dwarf planets’. Planets must: 1. orbit around the Sun, therefore limiting this definition to our solar system 2. have sufficient mass for self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape 3. have cleared the neighbourhood around its orbit; and 4. not be a satellite.
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There are now five dwarf planets in our solar system:
(134340) Pluto — previously categorised as a planet (see p 25). (136108) Haumea previously categorised as a KBO with the designation KBO 2003EL61 is approximately 2000 km long and 1500 km wide, (136199) Eris — previously categorised as a KBO with the designation 2003UB313 is approximately 1300 km in diameter. (136472) MakeMake previously designated as the plutoid 2008FY9 is approximately 1900 km long and 750 km wide There is an increasing number of solar system bodies that potentially qualify as ‘dwarf planets’. The largest of these are (with approximate diameters): Sedna (1500 km), Orcus (1350 km), Charon (1207 km), Quaoar (1150 km), 2002TC302 (≤1200 km). There is little doubt that in the years to come the numbers of ‘dwarf planets’ will increase dramatically with improved telescopes and observational techniques.
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A guide to the solar system
Solar system scale model It is difficult to realise just how big the solar system is. To help you, try building this model around your school or local suburbs. The size and distance of the planets from the Sun are given below in actual distance and for a model of scale 1.4 billion:1
Body
Body diameter (km)
Model diameter (mm)
Orbit radius (km)
Scaled orbit radius (meters)
1 391 900
1000
Mercury
4 866
3.4
57 950 000
42
Venus
12 106
8.6
108 110 000
78
Earth
12 742
9.1
149 570 000
107
Mars
6 760
4.8
227 840 000
164
Jupiter
139 516
100.2
778 140 000
559
Saturn
116 438
83.6
1 427 000 000
1025
Uranus
46 940
33.7
2 870 300 000
2 062
Neptune
45 432
32.6
4 499 900 000
3 233
Sun
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A guide to the solar system
Useful webpages Below are some webpages that may be useful for further information on astronomy and the world: Sydney Observatory http://www.sydneyobservatory.com Powerhouse Museum http://www.powerhousemuseum.com Australian Astronomy http://www.astronomy.org.au NASA website http://www.nasa.gov/ Jet Propulsion Laboratories http://www.jpl.nasa.gov/index.html European Space Agency http://www.esa.int/esaCP/index.html Astronomy Picture of the Day http://antwrp.gsfc.nasa.gov/apod/astropix.html Bureau of meteorology homepage http://www.bom.gov.au Geoscience Australia http://www.ga.gov.au/ Planetary Satellite information http://www.ifa.hawaii.edu/%7Esheppard/satellites/ Astronomy Exploration Timeline http://sseforum.jpl.nasa.gov/educators/index.cfm?Display=SSE_Timeline The Nine Planets http://seds.lpl.arizona.edu/nineplanets/nineplanets/nineplanets.html
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A guide to the solar system
Contact details Sydney Observatory Watson Road, Observatory Hill, The Rocks Tel: (02) 9921 3485 / 9241 3767 Email: [emailprotected] www.sydneyobservatory.com.au Opening hours Open 10.00 am – 5.00 pm daily (except Good Friday and Christmas Day) Free entry for general admission Admissions • 3-D Space Theatre Monday – Friday: 2.30 and 3.30 pm Weekends and school holidays: 11.00 am, 12.00 noon, 2.30 and 3.30 pm Cost: $7 adults, $5 child/concession, $20 family (No bookings required) • Night viewings Nightly Cost: $15 adult, $10 child, $12 concession, $45 family (Bookings are essential)
The information contained in this guide is correct at the time of printing. © 2010 Trustees of the Museum of Applied Arts and Sciences This publication is copyright. Apart from fair dealing for the purposes of research, study, criticism or review, or as otherwise permitted under the Copyright Act, no part may be reproduced by any process without written permission. The Powerhouse Museum, part of the Museum of Applied Arts and Sciences also incorporating Sydney Observatory, the Powerhouse Discovery Centre and the NSW Migration Heritage Centre, is a NSW government cultural institution.
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