Orwell Astronomical Society (Ipswich)

The Solar System

Introduction

The Solar System consists of the Sun and the various bodies in orbit around it: the familiar planets and their moons, the asteroids, comets and dark icy bodies far beyond the known planets. One of the major successes of modern astronomy has been to provide observational evidence, through new instruments such as the Hubble Space Telescope (HST) and interplanetary probes, to support theories about the Solar System. As a result, our knowledge of the Solar System has advanced more in the last 20 years of the 20^th Century than in the previous history of astronomy. This article summarises key facts about the Solar System with a particular emphasis on what is visible to the amateur astronomer.

Formation of the Solar System

Immanuel Kant in 1755 was the first to propose the idea of a rotating solar nebula (cloud of gas and dust) that contracted under its own gravitational self-attraction to form the bodies that we know in the Solar System. Modern theories agree in broad terms with Kant's views and, of course, provide a lot more detail and are backed by observational evidence.

The modern view is that the Solar System began with a huge interstellar cloud of extremely tenuous material (approximately 10,000 molecules per cubic centimetre) with an average temperature of some 10-50 K. Gravity was largely responsible for formation of the Solar System from the interstellar cloud; formation began approximately 4.5 billion years ago. The cloud was not uniformly dense but contained local concentrations of material: these became centres of gravitational accumulation. When sufficiently much material accumulated in a relatively small volume, temperature and pressure grew and the object became a proto-star and later a star. The entire interstellar cloud was likely to have had a rotational component to its motion and, as material condensed around centres of accumulation, the law of conservation of angular momentum dictated that the local speed of rotation increased; this resulted in the proto-stars rotating and becoming surrounded by flattened, rotating disks of gaseous material. Once a proto-star accreted sufficient mass, material in its neighbourhood became non-uniform and started to coalesce around discrete lumps. As these grew, they transferred material into the disk surrounding the proto-star.

During 1994-95, the HST captured images of the Orion Nebula (M42) showing the formation of proto-stars surrounded by proto-planetary disks (so-called proplyds) - see figure 1. It may be that, in time, some of these will develop into solar systems similar to our own!

In our local celestial neighbourhood, one of the centres of accumulation together with the discrete lumps in its neighbourhood became the Solar System. Computer simulations of the early evolution of the Solar System show that the discrete lumps are likely to have coalesced into objects roughly the size of the terrestrial planets in a period of some 10 million years (a relatively brief period in astronomical terms) and thereafter continued to sweep up other large lumps of matter for a period of 100 million years or so. It appears that a particularly large lump of matter accreted that was eventually to become the planet Jupiter; the gravitational attraction of this lump appears to have prevented accretion of a large planet at the orbital radius of the asteroid belt and may also have stunted the growth of Mars by sweeping up a lot of material that would otherwise have gone towards its formation.

The solar nebula was hottest near its centre, due to radiation from the proto-Sun and mechanical activity in the nebula itself, and the temperature decreased towards the outer regions. This resulted in three qualitatively different zones around the proto-Sun. Towards the centre of the solar nebula, in the inner zone, temperatures and the flux of the solar wind were too high for ice to exist and objects forming there consisted largely of rocky materials, ultimately forming the terrestrial planets. In the next zone outwards ice could exist and very large, cold planets (the gas giants) formed. In the outermost zone, ice was stable but the material of the solar nebula was less dense than in inner zones and, as a result, the objects that formed were small, icy planitesimals such as the Kuiper Belt objects (including Pluto) and comets.

Towards the end of the period of 10 million years during which the planets coalesced, it's likely that the proto-planets were subject to heavy bombardment by large bodies which resulted in alteration of their axes of rotation and spin rates. Also during this period, gravitational perturbations by the forming gas giant planets resulted in many icy bodies being flung out of orbits relatively close to the Sun into much more extended, highly elliptical orbits, leading to the formation of the Oort cloud, a vast reservoir of icy bodies in orbit around the Sun at distances up to about 100,000 AU (approx 1.5 light years).

The Sun

The Sun is our nearest star, lying at a mean distance of 149,600,000 km from planet Earth. (Approximately 1/260,000 the distance to the next nearest star, Proxima Centauri, in the southern constellation of Centaurus.) The Sun is classified as an unexceptional yellow dwarf star, one of the most prevalent kind in the galaxy. It lies at the centre of the Solar System and its enormous gravitational attraction is the force that keeps the other bodies (planets, comets, asteroids, etc) in their orbits. Its gravity arises from its mass, which accounts for approximately 98% of the mass of the entire Solar System.

The Sun has existed for approximately 4.5 billion years of a 12 billion year lifespan. It has a surface temperature of about 5,800 K, an apparent visual magnitude of -26.7 and an absolute magnitude (magnitude at the standard distance of 10 parsecs) of +4.8. Its spectral class is G2. Its mass is 333,000 times that of the Earth, or some 2x10³⁰ kg. It is composed of 70% hydrogen and 28% helium with oxygen and carbon accounting for the bulk of the remainder. The Sun's diameter is 1,393,000 km and it could accommodate in its volume approximately one million Earths! Its density is 1.41 times that of water. It is travelling, bringing with it the entire Solar System, towards the constellation Hercules at a speed of 19 km/s. It lies in one of the spiral arms of the Milky Way galaxy, about 30,000 light years from the centre, and shares in the general rotation of the galaxy at a speed of some 272 km/s, taking approximately 220 million years to complete a revolution.

In terms of a Hertzsprung-Russell plot of stellar absolute magnitude against spectral type, the Sun is a main sequence star, converting hydrogen into helium. In the core of the Sun, the temperature rises to 15,600,000 K and the density of gas to more than 13 times that of solid lead. These conditions are hot enough to sustain nuclear fusion, and it is this which powers the Sun. Every second, the Sun converts approximately 700 million tonnes of hydrogen into 695 million tonnes of helium, with an energy yield, primarily in the form of heat, light and radio emissions, equivalent to 5 million tonnes mass. No need to worry though, there's enough hydrogen fuel left for another five billion years!

Dark patches on the photosphere called sunspots occur where magnetic flux lines pass through it. The flux lines act to suppress convection and conduction, rendering the photosphere where they cut it locally cooler than average (typically 4500 K versus 5800 K), and thus dark/black in appearance by comparison. The accepted safe way of observing sunspots is to project the Sun's image through a telescope onto a piece of white card. By using this method to follow the progress of sunspots across the solar disc, it is possible to observe the differential rotation of the Sun. The equatorial rotation period is 25 days and 9 hours; however, the polar zones rotate more slowly. Sunspot activity is cyclic and sunspot maximum occurs approximately every 11 years.

In another five billion years, after spending close to 10 billion years on the main sequence, the Sun's hydrogen fuel will begin to run out. As this happens, it will burn its fuel faster and become brighter, hotter and almost 50% larger than it is today. When the Sun's hydrogen fuel falls to a critical level the pressure will be insufficient to hold up the outer layers and the core will collapse. Compression of the core will in turn cause a further rise in temperature, which will cause the outer layers to expand 100 fold as the Sun passes on to the next stage of its existence - that of a red giant star. Sadly, when the Sun becomes a red giant, it will engulf the inner planets of the Solar System, ending all life there! It will burn for another 250 million years as a red giant, during which time its core will fill with helium, contract and rise in temperature. Once the core reaches the critical temperature of 93,000,000 K, helium atoms will fuse and at that moment the core will explode in a helium flash and the Sun will settle down to a brief period as a helium-burning star. Eventually, the Sun will exhaust its helium fuel and the core will collapse again due to the weight of the outer layers. The Sun will not become a carbon-burning star as it is not massive enough to initiate carbon fusion. Instead, it will shed its outer layers far out into space to become a planetary nebula, leaving behind its core as a white dwarf comprising matter so dense that one teaspoon would weigh several tonnes. In time, even the white dwarf will cool to become a black dwarf floating endlessly in space.

Since the mid 1990s, the Sun has been under observation from a variety of satellites, one of the most productive of which has been SOHO (Solar and Heliospheric Observatory). SOHO has detected solar tornadoes forming in the polar regions, some as big as the Earth and with windspeeds up to 14 km/s (average) and gusts up to 10 times faster. It also spotted the phenomenon of "sunquakes", caused by solar flares producing seismic waves in the Sun's interior. These resemble the ripples produced when a stone is dropped into a pool of water, but instead of the ripples travelling outwards at a constant rate, the ripples on the Sun's surface accelerate to a maximum velocity of 400,000 km/h. SOHO has been observing the Sun continuously for longer than any other spacecraft, using an assortment of technologies. Three of its key instruments are the Extreme ultraviolet Imaging Telescope (EIT), the UltraViolet Coronograph Spectrometer (UVCS) and the Large Angle and Spectrometric Coronograph Experiment (LASCO). A sample of results from these instruments is below.

• Figure 2 is a compound picture formed from images captured by the EIT and UVCS. The black circle around the Sun is an artefact of the imaging technique. Outside the circle, the image shows the outer solar atmosphere in ultraviolet light. The light is emitted by electrically charged oxygen flowing away from the Sun to form the solar wind. Inside the circle, the image shows the disk of the Sun and the inner corona in light emitted by electrically charged iron at temperatures near two million degrees Celsius. The image shows dark areas called coronal holes at the poles and across the disk where the highest speed solar wind originates. The structure of the corona is controlled by the Sun's magnetic field, which also is responsible for formation of the bright active regions and the ray-like structures originating in the coronal holes. The image shows structures in the corona extending from its base to millions of kilometres above the solar surface.
• Figure 3, taken by EIT on 31 October 2003, reveals the complex structures that can form on the surface of the Sun.
• Figure 4, captured by LASCO on 05 october 2012, shows a Coronal Mass Ejection (CME) heading towards Earth at approximately 600 km/s. A CME is a massive burst of ejected plasma which, at the Earth, can cause geomagnetic storms and aurorae and disrupt electronic equipment.

Mercury

The closest planet to the Sun is the tiny world Mercury. It orbits the Sun at a mean distance of 57,910,000 km. It has a rotation period of 59 Earth days, about two-thirds of its year of 88 Earth days. It is the smallest of the inner planets, only 4,850 km in diameter. However, it has second highest density of any planet in the Solar System (second only to the Earth), due to a large iron core. Mercury's "atmosphere" is so extremely tenuous that it does not constitute an atmosphere in the usually accepted sense of the word. The planet has an escape velocity of only 4.2 km/s, so any gas at the surface would escape into space in less than a day if the solar wind did not continually replenish it. Replenishment occurs via particles in the solar wind causing evaporation of atoms in the surface of Mercury; once atoms are released, ultraviolet light from the Sun rapidly ionises them and they are then streamed along the lines of magnetic force created by the planet's internal magnetic field and the solar wind. Replenishment and loss into space result together in an atmosphere referred to as the equilibrium atmosphere, comprised mainly of helium with smaller amounts of hydrogen, argon and neon.

On Mercury, the daytime surface temperature in can reach 700 K while at night temperatures can plunge to only 100 K. This gives the greatest range of temperature between night and day of any planet in the Solar System. The large temperature range is attributable to the tenuous nature of the atmosphere which cannot retain any heat, plus the very slow rotation period which enables the surface to cool completely during the night-time period.

The surface of Mercury is lunar in appearance and is composed of the same silicate-type rocks as the Moon. However, there are fewer craters on Mercury and there is evidence of an early, major resurfacing event which covered the planet's surface and obliterated a large proportion of the craters then in existence. Mercury and the Moon were both subject to the late heavy bombardment some 4 billion years ago, which resulted in the formation of many of the visible impact features.

The main surface feature on Mercury is Caloris Basin, a huge impact basin with a diameter of 1340 km, thought to have been caused by a collision with an object approximately 150 km in diameter some 3.85 billion years ago. On the opposite side of the planet, the antipode to Caloris Basin, is an area of jumbled terrain where seismic pressure waves from the Caloris impact were brought to a focus resulting in severe faulting and fracturing of the crust. The Caloris Basin is so named because Caloris is Latin for heat; being near the sub-solar point at perihelion (closest to the Sun) temperatures in the Basin can reach 755 K.

Other surface features on Mercury include valleys, scarps, ridges and mountains. At the south pole of the planet is the crater named Chao Meng-Fu after the Chinese painter and calligrapher Zhao Mengfu (1254–1322), while another crater lying across the 20^th meridian is Hun Kal, meaning  20  in Mayan. (The Mayan people used a base 20 number system.) There is also a crater named Kuiper after the famous planetary scientist who worked on the Mariner 10 project but sadly died before he could see his efforts bear fruit.

It is almost certain that Man will never walk on the surface of Mercury because the very large temperature variations would be very difficult to deal with. To date (late-2012) our detailed knowledge of the topography of the planet comes from two space missions, Mariner 10 in 1974-75 and Messenger in 2011 onwards. Mariner 10 launched on 03 November 1973 with an intent to measure Mercury's environment, atmosphere, surface and body characteristics. Its first encounter with the planet took place on 29 March 1974 and it then entered solar orbit, looping around the Sun once while Mercury completed two orbits, enabling it to flyby the planet a second time on 21 September 1974. A third and final encounter took place on 16 March 1975. Figures 5-7 are based on observations by Mariner 10. Messenger (MErcury Surface, Space Environment, GEochemsitry and Ranging) launched on 03 August 2004 and, after several flybys of Earth, Venus and Mercury, entered orbit around Mercury on 18 March 2011. On 29 March 2011, Messenger returned the first ever image of Mercury from a spacecraft in orbit about the planet, reproduced as figure 8 below. In the figure, the dominant rayed crater in the upper portion of the image is Debussy. The smaller crater Matabei with its unusual dark rays is visible to the west (left) of Debussy. The bottom portion of the image is near Mercury's south pole and includes a region of Mercury's surface not previously seen by spacecraft.

Mercury is a difficult planet to observe from Earth because it never strays further than 27.7° from the Sun and therefore never appears much above the horizon when the Sun is set. (It is too close to the Sun to be viewed by the Hubble Space Telescope.) It is best seen low in the west after sunset or low in the east before sunrise. Observing from a high point can increase the altitude of an object above the apparent horizon and the Orwell Park Observatory therefore makes an ideal observing location because of its relatively unobstructed western and eastern horizons and its altitude.

Venus

Although referred to as the Evening Star or Morning Star depending on which side of the Sun she is on in relation to the Earth, Venus is well and truly a planet. In fact, Venus is the second planet outward from the Sun, lying at a mean distance of 108,200,000 km. At closest approach, Venus comes within 38,000,000 km of the Earth, closer than any other planet. The day on Venus lasts 243 Earth days (Venus' rotation is retrograde), considerably longer than the year of 225 Earth days. In size, Venus is Earth's twin, with an equatorial diameter of 12,104 km compared with 12,757 for the Earth.

Venus is shrouded by a very dense atmosphere, producing on its surface 90 times the atmospheric pressure on Earth at sea level. The atmosphere is so thick that only 2% of incident sunlight reaches the surface. It comprises 96% carbon dioxide with nitrogen, oxygen, argon and a little water vapour accounting for the remainder. The main cloud layer in the atmosphere consists of sulphuric acid. Due to the high carbon dioxide content of the atmosphere, Venus is victim to a runaway greenhouse effect allowing infra-red radiation to reach the surface of the planet but not to leave it, as a result of which the surface temperature is 750 K, nearly 500 K more than it would be if there were no atmosphere. (The temperature at the top of the atmosphere is 240 K.) During the earliest history of Venus, there was probably water on its surface. As the Sun heated up, some 3-4 billion years ago, Venus became so hot that the oceans evaporated, placing more water vapour into the atmosphere, which in turn trapped more heat, which in turn raised the temperature further, and so on. Once in the atmosphere, water vapour would have been broken down by sunlight into its constituent molecules hydrogen and oxygen and the former would largely have escaped into space. Venus today has a very low atmospheric abundance of water and hydrogen. The greenhouse effect plus the very low axial inclination (only 3°) mean that there are no seasons on Venus.

It is impossible to see the surface of Venus visually from the Earth due to the very thick cloud cover surrounding the planet. However, various space probes and terrestrial systems have carried out radar mapping. The most successful space probes were the USSR's Venera soft landers: Venera 9 touched down and returned images of the surface in October 1975; it was followed by Veneras 10, 13 and 14 which provided more images. Figure 9 below was taken by Venera 14 on 05 March 1982. The probe functioned for 60 minutes on the surface of Venus before the hostile conditions overcame it. The base of the probe can be seen in the image. From 14-16 February 1990, NASA's Galileo spaceprobe, en route to Jupiter by means of gravitational slingshots via Venus and the Earth, took a sequence of images while receeding from Venus, at distances from 2.3 to 3.3 million km. Four of the images are reproduced in figure 10; they have been enhanced to show details in the clouds surrounding the planet. The most detailed radar mapping to date was carried out by the Magellan orbiter which mapped the entire surface of the planet from September 1992 to October 1994. It revealed that approximately 85% of the surface consists of flat plains or lowland terrain, while the remaining 15% consists of highland plateau and mountain belts. There are two main areas of highland terrain (or continents): Ishtar Terra in the northern hemisphere is about the same size as Australia and Aphrodite Terra in the southern hemisphere is about the same size as Africa. Radar has also revealed numerous volcanoes; the largest is the massive shield volcano Maxwell Montes in Ishtar Terra. Figure 11 shows the 37 km diameter volcano Grimke.

To the naked eye, Venus is unmistakable, appearing as the brightest object in the sky apart from the Sun and Moon. (It can attain magnitude -4.4.) It is an inferior planet and therefore can show a significant phase, which a small telescope will reveal.

The Earth

Being denizens of planet Earth we tend to scrutinise it more as our home than for what it can tell us about the Solar System! However, the Earth, and in fact the Earth-Moon system, are more amenable to direct study than any other body in the Solar System and can therefore tell us a great deal about the latter.

Earth orbits the Sun at a mean distance of 149,600,000 km taking 365.242 days to complete each revolution. The orbit is nearly circular (eccentricity, e=0.017). The equatorial radius of the Earth is 12,757 km and it rotates on its axis in a period of 23 hours, 56 minutes, 4.0991 seconds. The inclination of Earth's axis of rotation to the plane of its orbit is 23.45°.

Some 4.5 billion years ago, Earth began condensing out of the solar nebula. The early history of Earth was characterised by repeated collisions with bodies of relatively large size. The collisions resulted in a release of kinetic energy which kept the Earth molten and allowed differentiation to take place, whereby the heavier elements sank to the Earth's core and the lighter elements floated to its surface. This differentiation resulted in the interior of the Earth having a layered structure, with four main layers:

• An inner core composed of solid iron and nickel has a radius of 1230 km. The inner core "floats" in a liquid outer core which decouples it from the outer layers; in fact the inner core rotates with respect to the Earth's crust.
• The outer core has a depth of approximately 2350 km and is composed of iron and some lighter compounds.
• Outside the outer core lies the mantle, which contains the bulk of the Earth and which has a depth of 2150 km.
• Above the mantle lies the Earth's rocky crust, some 20 km thick on average.

The coldest part of the Earth is its surface. The rocks on or near the surface of the Earth form the lithosphere, which consists of eight large, rigid strong plates and approximately 24 smaller ones. The plates are floating on a lower layer of hotter, molten (at least in part), plastic material in the mantle. The plates are in relative motion (plate tectonism) driven by convection currents of molten material in the mantle. Plate tectonism is also responsible for continual renewing of the Earth's surface as slabs of lithosphere cool and sink and are replaced by upwellings of hot material from inside the Earth.

The Earth, of course, possesses a dense atmosphere, which is essential to sustain life on the planet. Its main constituents are: nitrogen (77%), oxygen (21%), water (1%), argon (0.94%), other gases (0.06%). Earth's atmosphere and the processes associated with it - wind, precipitation, etc - have degraded the record of early cratering on the planet.

Some of the seminal images of the Earth are reproduced below. Figure 12, taken in December 1968 by astronauts aboard Apollo 8 was one of the first images of the whole Earth. Figure 13 is the first good image of the Earth and Moon taken together from the vicinity of the Moon. Lunar Orbiter 1 obtained the image on 23 August 1966, at 16:36 UT, while approaching the crater Pasteur (centre image)¹. The image shows a crescent Earth, some 383,800 km distant, with sunset terminator running through Odessa, Istanbul and slightly west of Capetown. Figure 14 is a more modern image of the Earth and Moon together in space, taken by the NEAR spacecraft (Near Earth Asteroid Rendezvous, renamed NEAR-Shoemaker after launch) on 23 December 1998 at a distance of 400,000 km from the two bodies.

The Moon

The Moon is our nearest neighbour in space, and it is the only other body in the Solar System upon which Man has set foot. The Moon is so close to the Earth (in astronomical terms) that observers can discern details on its surface with the naked eye; the only other body for which this is possible is the Sun when it occasionally exhibits a particularly large sunspot group.

The Moon orbits the Earth at a mean distance of 384,400 km. Because of its proximity, the Moon shines at a magnitude of up to -12.7 (full Moon) and is second in brightness in the sky only to the Sun. It has a siderial orbital period of 27.322 days. The siderial period is also the time taken by the Moon to rotate on its axis: because of this the Moon is "locked" in orbit and always displays the same face towards the Earth. In fact, the Moon's orbit is noticeably elliptical (eccentricity, e=0.05) and, because of this, the Moon appears to rock slightly from left to right. It also exhibits a small vertical "nodding" motion. The result of the two motions (termed libration in longitude and latitude respectively) is that a total of 59% of the lunar surface is visible from Earth at one time or another. Figure 15 shows a part of the side of the lunar surface never visible from Earth.

The Moon's physical diameter is 3476 km and it exhibits an apparent diameter of approximately 0.5° as seen from Earth. The large apparent size makes the Moon a favourite target for amateur astronomers as, even with the most modest of equipment, a wealth of detail can be discerned on the surface. The most noticeable features of the Moon are its phase and the numerous craters and dark areas. Figure 16 shows the Moon at full phase, with seas and bright craters visible. The phase and the larger dark areas are easily visible to the naked eye; craters and the smaller dark areas are easily observed with binoculars or small telescopes. The features occur as follows:

• The phase is a result of the angle between the Moon, Earth and Sun. As the Moon progresses through an orbit of the Earth, it progressively exhibits the phases new Moon, first quarter, full Moon, third quarter and new Moon again. A cycle of phases is called a lunar month, and has an average period of 29.531 days. This is longer than the siderial period of the Moon's orbit (see below) due to the motion of the Earth as the Moon completes its orbit.
• The craters are ringed structures caused by meteorite impacts. They range in size from small pits to large multi-ringed impact basins hundreds of kilometres in diameter. The whole lunar surface is covered with countless craters.
• The dark areas are termed maria (Latin for seas) and cover 16% of the lunar surface; they are concentrated on the Earth-facing hemisphere. The maria are caused by volcanic outpouring of material from inside the Moon; they are dark because of their high iron content. In a telescope, the maria are seen to be relatively smooth, which implies that they were formed after the end of the heavy bombardment which formed the craters on the remainder of the surface.

Surface objects best placed for observation are on or close to the lunar terminator (the line between illuminated and dark hemispheres) since this provides stark shadows that make detail stand out clearly. This means that the best-placed objects on the surface change from day to day as the Moon progresses through its orbit: in general terms, the best time to observe the Moon is when it's within a few days of first quarter or last quarter. (Of course, the first quarter Moon culminates in early evening whereas the last quarter Moon culminates in early morning: the former is therefore much more conveniently placed for casual observing!)

Over the years, there have been four main theories of the formation of the Moon:

• Co-accretion: the Moon and the Earth formed together out of the primordial solar nebula and have always existed as a pair. This theory now appears problematic because although the Earth and the Moon share a similar bulk composition, there are important differences, for example the Moon lacks volatile elements and elements such as cobalt and nickel which the Earth possesses in abundance.
• Capture: the Moon was originally a body wandering through space which strayed sufficiently close to the Earth that it was captured in orbit by the latter's gravitational field. Computer studies of orbital dynamics suggest that this scenario is all but impossible.
• Fission: the Earth originally had no satellite, but somehow began to spin so fast that a large portion of its mass was flung out of the main body to coalesce and form the Moon. This theory is problematic because the Earth-Moon system simply does not possess enough angular momentum to explain a high spin rate.
• Giant impact: a giant object hit the Earth off-centre some 4.5 billion years ago and blasted a mixture of terrestrial and impactor material into orbit where it coalesced to form the Moon. Theoretical calculations show that the Moon would have coalesced in a matter of only a few years after the impact. This theory was first propounded in 1975 independently by two research groups. It now has a consensus amongst planetary scientists.

The Apollo missions 1969-72 returned a total of 382 kg of lunar rock to Earth. (Figure 17 shows the crew of Apollo 17 on the lunar surface.) Since then, there has been one Luna mission, which returned a mere 170 g of material, and two lunar orbiters (Clementine and Lunar Prospector). Perhaps, eventually, Man will return to the Moon to undertake further exploration and reconnaissance, possibly leading to the ultimate goal of a permanently manned lunar base.

Mars

Mars has been known since ancient times, and because of its red colour has been worshiped as the God of War. It is the fourth planet from the Sun and the outermost of the terrestrial planets. It orbits the Sun at a mean distance of 227,900,000 km, taking 687 Earth days to complete an orbit. It has the longest interval between oppositions of any planet, at 780 days. Mars' orbit is eccentric (2^nd most eccentric after Mercury) and this means that its closest approach to Earth can vary from 56 million km at a perihelic opposition to 101 million km at an aphelic one. Extremely favourable oppositions of Mars occur every 15-17 years, the last (at the time of writing) being on 27 August 2003. Mars' day is just over half an hour longer than Earth's at 24 hours and 38 minutes. Mars' equatorial diameter is only half of that of Earth at 6,970 km.

Mars always displays a pronounced orange-red colour, caused by the high iron oxide content of its surface rocks. In the amateur telescope, it is often disappointing because of its small size (25 arcseconds at most favourable opposition). The only surface features that can be seen clearly from Earth with modest amateur equipment are Syrtis Major, a huge triangular shaped plateau, and the polar ice caps that shrink and expand with the changing seasons. However, professional observatories, the Hubble Space Telescope, orbiting space probes and surface landers have revealed a diverse range of surface features including deserts, mountains, valleys, canyons, volcanoes and features similar to river channels on Earth, providing evidence that there was once water flowing on the surface. (There is no surface water today.) Occasionally, surface features on Mars can be obscured by extensive dust storms, which in extreme cases can obscure the entire disk of the planet.

Among the most impressive features on detailed maps of Mars are its large volcanoes. The tallest volcano on Mars is Olympus Mons, with a height of 25 km (over twice as high as Mount Everest) and width of nearly 550 km. The volcano with the widest base, at over 1,500 km, is Alba Patera. The great size of the shield volcanoes on Mars is due to a thick and stable crust and the absence of tectonic plate activity, which has enabled the volcanoes to grow uninterrupted over geological timescales.

Mars has a very thin atmosphere with a density of approximately 6 millibars, only approximately 1/150 that of the Earth. It is composed of carbon dioxide (95%), molecular nitrogen (2.7%), argon (1.6%), molecular oxygen (0.13%), carbon monoxide (0.07%) plus traces of water vapour, neon, krypton, xenon and even ozone in minute quantities. Mars' atmosphere is too thin to retain heat effectively and the planet lies much further from the Sun than the Earth does. As a result, the surface temperature is much lower than on the Earth: it reaches approximately 0° C in summer but the average daily temperature is circa -50° C and polar winter temperatures can be as low as -120° C. The ground on Mars is permanently frozen to a depth of approximately 1 km. Mars' orbital inclination is 25.19° which is similar to that of the Earth; however, being much further from the Sun and with a more elliptical orbit, the seasons on Mars are longer and more exaggerated.

Mars has two moons. On 10 August 1877, Asaph Hall used the then largest telescope in the world, the 66 cm refractor at the US Naval Observatory, to discover the first of the Martian satellites. Six days later, he discovered the second satellite. He named them Phobos (Fear) and Deimos (Terror) after the two attendants of the God of War. Both moons are thought to be asteroids which suffered orbital perturbations by Jupiter and were subsequently captured by Mars. Phobos is the inner satellite. It is shaped rather like a potato; it measures 20x38x28 km and orbits Mars in a retrograde direction every 7 hours and 40 minutes at a mean distance of 5873 km. It is heavily cratered; the most prominent crater is called Stickney after the maiden name of Hall's wife. It measures 9 km in diameter and appears to have been caused by an impact which nearly shattered the moon and left a series of 500 m wide fractures in its crust. There are six other significant, named craters on Phobos: D'Arrest, Hall, Roche, Sharpless, Todd and Wendall. The orbit of the moon is gradually decaying due to tidal interaction with Mars and it is doomed to collide with the planet in about 40 million years. Deimos is smaller than Phobos at 10x12x16 km and orbits Mars prograde at a mean distance of 20,000 km. It is coated with a lot more surface dust than Phobos and so lacks the clearly defined surface features of the latter. It has two large craters named Voltaire and Swift after the first authors to suggest that Mars would have two satellites (long before their discovery!)

Seen from the Martian surface, Phobos would have approximately half the apparent size of our Moon as seen from Earth and would shine at magnitude -3.9. It would barely cast a shadow. It would travel backwards through the sky running through more than half its cycle of phases in a day. The interval between successive risings of Phobos would be just over 11 hours. Deimos would appear approximately twice the size of Venus as seen from the Earth and would shine at magnitude -0.1. It would remain above the horizon for 2.5 Martian days at a time. A telescope would be necessary to see its phase.

Because of continued interest in the possibility of life on Mars, the planet has been the subject of many space missions. The first was Mariner 4, which executed a flyby on 14 July 1965. Pictures from the spacecraft and from the later Mariners 6 and 7 suggested that the surface of Mars resembled that of the Moon. The first Mars orbiter was Mariner 9; it returned a wealth of images of the surface in 1972. In 1976, Vikings 1 and 2 soft-landed on the planet, returned spectacular images and performed simple chemical tests to look for signs of life (alas, the tests were inconclusive). Several failed missions followed, until Mars Pathfinder landed on 04 July 1997, returned impressive images and deployed a small rover, Sojourner, to analyse rocks near the landing site. (Sojourner remained active until late September 1997.) Later in the same year, Mars Global Surveyor entered orbit around the planet. It completed its primary mission, to map the entire planet in detail from low orbit, in January 2001. Later in that year, the Mars Odyssey orbiter entered orbit around the planet, searching for evidence of water and volcanic activity. In March 2006, the March Reconnaissance Orbiter (MRO) entered orbit around the planet and began detailed monitoring of weather and surface conditions, as well as providing a high bitrate data relay for other missions. In January 2004, two more rovers, Spirit and Opportunity, were delivered to the surface of Mars. Both had planned missions of extent 90 Martian days which they greatly exceeded, Spirit functioning until March 2010 and Opportunity, at the time of writing (late 2012) still operating. On 06 August 2012, the Mars Science Laboratory mission landed and delivered the Curiosity rover; it is by far the largest and most advanced rover sent to Mars to date. Further mapping missions and soft-landers are planned in coming years.

Images of Mars and its moons:

• Figure 18 shows Mars imaged by the Hubble Space Telescope on 26 June 2001 at a distance of 68 million km. This was the best picture of Mars taken from the Earth at the time and, in full resolution, revealed details as small as 10 km across. Note the extent of the north polar cap (top of image). Two dust storms are visible: one near the north polar cap and another in the southern hemisphere (lower right).
• Figure 19, taken on 20 October 1997 by Mars Global Surveyor, shows the volcano Olympus Mons and its central caldera, 66x83 km in size.
• Figure 20 is an image of Phobos, showing the prominent crater Stickney on the right. It was taken by the high-resolution imager on the MRO on 23 March 2008.
• Figure 21 shows two images of Deimos, acquired 5 hours and 35 minutes apart, on 21 February 2009 by the high-resolution imager on the MRO. The sun was to the upper left in the left image and to the right in the right image. The viewing geometry is similar in the two images, but surface features appear very different due to the change in illumination.

The Asteroids

The asteroids are a multitude of small bodies orbiting the Sun at distances ranging from inside the orbit of the Earth to outside that of Saturn. The vast majority (the main belt asteroids) travel around the Sun between the orbits of Mars and Jupiter. By late 2012, in excess of half a million asteroids had been identified, observed sufficiently well that their orbits could be computed, and assigned a number.

The largest asteroid is (1) Ceres with a diameter of 930 km. It was the first asteroid to be discovered (in 1801) and is thus assigned the number 1, by convention for an asteroid, enclosed in parenthesis. It accounts for more than a quarter of the mass of all main belt asteroids. The next two largest asteroids are (2) Pallas and (4) Vesta, each slightly over 500 km in diameter. Only a dozen asteroids are over 250 km in diameter. The bodies are increasingly numerous at smaller sizes. Asteroids come in a variety of shapes, from roughly spherical to highly irregular. Typical asteroids rotate in a period of about nine hours, although there is wide spread about this value. Radar mapping and observations of occultations of stars by asteroids have revealed that some asteroids are, in fact, double.

The asteroids formed along with the major planets of the Solar System out of the primordial solar nebula. However, the gravity of the giant planet Jupiter disrupted the orbits of planitesimals that were forming in its vicinity and prevented them from coalescing into a single solid body. (Jupiter's influence may also have restricted the growth of the planet Mars by perturbing bodies that it would otherwise have swept up and accreted.) Although the asteroids are mostly very small, there are sufficiently many of them that most have suffered collisions during their lifetime. Some asteroids are thought to be little more than orbiting heaps of rubble held together by mutual gravitation attraction.

There are two main types of asteroid: those with compositional properties dictated by those of the solar nebula from which they formed, and those with properties modified by the processes of planetary formation (collisions inducing heating, followed by differentiation). Asteroids of both types are found in very similar orbits, and it is currently an active research topic to find an explanation for this.

The brighter asteroids can be readily observed with small telescopes or binoculars. The BAA Handbook contains ephemeris information, indicating where to search for them. It is possible to detect the movement of an asteroid from night to night by plotting its position on a detailed star map.

Several spacecraft, including Galileo and NEAR, have obtained images of asteroids from relatively close range. Galileo was launched on 18 October 1989 towards Jupiter. En route, it executed flybys of asteroids (951) Gaspra and (243) Ida. On 29 October 1991, it passed within 1600 km of Gaspra (the first close approach of a spacecraft to an asteroid). Figure 22 is a colour-enhanced image captured at a distance of 5300 km, approximately 10 minutes before closest approach. A striking feature of Gaspra's surface is the abundance of small craters. There are also several prominent groove-like linear features, believed to be related to fractures. Two years later, on 28 August 1993, Galileo passed within 2400 km of Ida. Figure 23 was captured on approach at a distance of 10,500 km; it shows Ida to be accompanied by a small moon, named Dactyl. Ida has a highly elongated shape (56x15 km) while Dactyl is spherical, but very much smaller (only 1.5 km diameter). NEAR was launched on 17 February 1996 and, on 27 June 1997, passed within 1200 km of (253) Mathilde. Figure 24 is one of over 500 images captured during the flyby. It was taken at a distance of 2400 km and shows part of the asteroid of approximate dimensions 59x47 km. The large, deeply-shadowed crater in the centre of the image is approximately 10 km deep.

NEAR went on towards the near-Earth asteroid (433) Eros. On 14 February 2000, it entered orbit around Eros and began studying the asteroid with instruments including a multi-spectral camera and a spectrometer. The initial orbital distance was 200 km but, over the following months, Mission Control reduced the height of the orbit enabling NEAR to take images with greater resolution. During January and February 2001, NEAR made a succession of very close passes over the surface of Eros, at heights typically of only a handful of kilometres. On 12 February 2001, it landed on the surface, returning extremely detailed images during its descent. The spacecraft continued to function after landing, returning spectroscopic data about the surface for approximately a week. Figure 25, taken during NEAR's descent, shows regolith on the surface. The portion of surface shown is approximately 12 m across.

Jupiter

The giant planet Jupiter is the largest of the Sun's family - indeed it is more massive than all the other planets in the Solar System put together. Its equatorial diameter is 142,900 km, over 11 times that of the Earth, and its volume is over 1330 times that of the Earth. Its great mass means that its escape velocity, at 60 km/s, is the highest of any planet. The high escape velocity means that Jupiter has been able to retain even the lightest of gasses in its atmosphere. Jupiter orbits the Sun at a mean distance of 778,300,000 km taking 11.8 Earth years to do so. At its closest approach to the Earth, it lies some 585,800,000 km distant.

Jupiter is composed primarily of gas and fluids and has no solid surface like that of Earth. If we could descend from the Jovian cloud-tops down through the atmosphere, the pressure would keep mounting until the atmosphere turned slushy and eventually became liquid. There is thought to be a small rocky core at the centre of the planet. Jupiter's atmosphere consists mainly of hydrogen and helium with trace amounts of ammonia, methane, acetylene, ethane and phosphine. Some water has also been detected. The mean temperature at the cloud tops is 130 K. This is higher than expected on the basis of Jupiter's distance from the Sun, meaning that the planet generates considerable heat internally.

Being a gaseous world, Jupiter does not spin as a solid body; instead, the speed of rotation depends on the latitude. The rotation period at the equator is 9 hours and 51 minutes and at the poles is 9 hours and 56 minutes. Jupiter's rotation period is the lowest of all the planets in the Solar System and such rapid rotation causes an equatorial bulge (oblateness), with the equatorial diameter being 8700 km greater than the polar diameter.

Jupiter comes to opposition every 13 months and has a maximum brightness of magnitude -2.6, second among the planets only to Venus. When observing Jupiter with a telescope, it is the cloud tops that are visible. These have a characteristic banded appearance. The bands are permanent (although changing in detailed shape and structure) and correspond to zonal jets in the atmosphere. The most striking feature in Jupiter's atmosphere is the well-known Great Red Spot (GRS), a huge storm which has been raging for centuries and, at over 48,300 km in diameter could swallow the Earth.

Several probes have been sent to Jupiter: the Pioneers in 1973 and 1974, the Voyagers in 1979 and the Galileo orbiter in 1995. The Pioneers discovered a belt of lethal radiation around Jupiter, the Voyagers discovered Jupiter's thin dark ring system and returned a tremendous amount of information about Jupiter's moons and Galileo returned details of Jupiter's atmosphere and information on its four largest moons.

Jupiter has 67 confirmed satellites (i.e. with well-established orbits). The four largest are called the Galileans after Galileo Galilei (1564-1642) who discovered them on 07 January 1610. The Galileans are fascinating worlds which were extensively studied by the Voyager probes and the Galileo orbiter.

• Io The closest of the Galileans to Jupiter is Io. It orbits inside Jupiter's deadly radiation belt, at a mean distance from the planet of 421,600 km, taking 1.8 days to complete an orbit. Its diameter is 3,642 km. The first close-up pictures of Io resembled a giant pizza with orange and yellow blotches and dark red and black markings - not even remotely similar to any other satellite! Io's strange appearance is due to its volcanism - it is, in fact, the only planetary body in the Solar System apart from the Earth known to be volcanically active. The volcanoes can shoot sulphurous gasses and silicate magma up to a height of 250 km before the material falls back to the surface. The volcanism is caused by tidal heating, which in turn is caused by an orbital resonance between Io and Europa. Europa has an orbital period twice that of Io. Whenever Io is near to Europa in its orbit, the gravity of the latter causes a slight eccentricity in Io's orbit, which in turn generates tidal forces in the moon through the interaction with Jupiter's powerful gravity. The tidal forces heat Io's core to the point where it is liquid and under considerable pressure. When the pressure finds a weak point in the crust, the liquefied sulphurous and silicate lava vaporises and erupts forth with huge force. The volcanic material is ejected at temperatures in the range 700-1800 K against a background temperature of 110-120 K. Because of the volcanism on Io, its features are named after hellish or volcanic themes. For example the largest volcano, Pele, is named after the Hawaiian goddess of fire.
• Europa The next satellite out from Jupiter is Europa. It has a diameter of 3130 km and orbits the planet at a distance of 670,900 km taking 3.6 days to complete one orbit. The moon is covered with an ice crust, which surrounds a silicate mantle and a rocky core. The ice crust may extend to a depth of 150 km and may be frozen solid, or may be liquid at lower depths - scientists are not sure. Europa is involved in the orbital resonance with Io described above. Being further from Jupiter, the tidal forces on Europa are less, but still result in heating of the moon's core and a constant fracturing of the crust, enabling continual resurfacing of the outer crust from icy material underneath. As a result, there are few impact craters remaining on the surface which, instead, resembles a cracked boiled egg, criss-crossed by fault lines.
• Ganymede The next of the Galileans outwards from Jupiter is mighty Ganymede, largest moon in the Solar System. It has a diameter of 5268 km, an orbital radius of 1,070,000 km and takes 7.2 days to complete one orbit. It is much less dense than Io or Europa: in section it consists of an ice crust, an icy upper mantle, a silicate lower mantle and an iron core. Ganymede has an internally-generated magnetic field which indicates that the iron core may be at least partly molten. The surface is characterised by bright and dark terrain, corresponding respectively to a relatively greater abundance of icy material and rocky material. The dark terrain is relatively old, heavily cratered and crossed by furrows some 5-10 km wide, thought to be the result of huge impacts in the early history of the moon. It is also criss-crossed by sets of parallel ridges and valleys caused by tectonic movement and cryovolcanism. The bright terrain is younger, and more sparsely cratered.
• Callisto The outermost of the Galileans is Callisto. It is a heavily cratered world, 4806 km in diameter, which orbits Jupiter every 16.7 days at a distance of 1,883,000 km. Like Ganymede, it is much less dense than the two innermost Galileans. In section, it consists of an ice-rich outer layer surrounding an inactive, icy-rock interior. Callisto does not generate a magnetic field. The most prominent feature on Callisto is the multi-ringed, impact feature Valhalla, some 600 km in diameter.

The other satellites of Jupiter are tiny compared to the Galileans and are well outside the range of typical amateur equipment. Four of them orbit closer to the planet than Io. The remainder orbit beyond Callisto and may be captured asteroids.

The following table summarises data on the satellites of Jupiter, in order of increasing orbital radius.

Table 1. Moons of Jupiter, listed in order of orbital radius.

The following images show Jupiter and the Galilean satellites. (All images of the latter taken by the Galileo spacecraft.)

• Figure 26, captured on 07 December 2000 by the Cassini spacecraft en route to Saturn, shows Jupiter with its cloud bands and the GRS. (The black dot is the shadow of Europa.)
• Figure 27, taken by Voyager 1 on 25 February 1979, at a distance of 9.2 million kilometres, shows the GRS surrounded by complex cloud structures and motions. It reveals features in the cloud deck as small as 160 km across.
• Figure 28, taken on 03 July 1999, is a true-colour image of Io. Detailed comparison with earlier images showed many changes due to vulcanism.
• Figure 29, taken on 07 September 1996, shows the trailing surface of Europa in true colour. The bright feature containing a central dark spot in the lower third of the image is a young impact crater some 50 kilometers in diameter.
• Figure 30 was captured on 26 June 1996 during Galileo's first approach to Ganymede. The dark areas are the older, more heavily cratered regions and the light areas are younger regions. The brownish-grey colour is due to mixtures of rocky materials and ice. Bright spots are geologically recent impact craters and their ejecta. The smallest details that can be discerned in the picture are about 13.4 kilometers across.
• Figure 31, taken in May 2001, shows Callisto's surface to be heavily cratered with variation in colour and brightness. The brighter areas are thought to be mainly ice and the darker areas to be highly eroded, ice-poor material.

Saturn

Far out in the distant reaches of the Solar System, beyond the orbit of Jupiter, lies the gas giant planet Saturn. It was the outermost known planet until William Herschel discovered Uranus in 1781. Saturn takes 29.4 years to orbit the Sun at a mean distance of 1,429,000,000 km. Like Jupiter, it spins rapidly and is gaseous so that the rotation period of the cloud tops depends on the latitude. The rotation period at the equator is 10 hours and 15 minutes while at the poles it is 10 hours and 38 minutes. Because of the rapid rotation, Saturn has an oblate profile with an equatorial diameter of 120,660 km and a polar diameter of 108,000 km.

The composition of Saturn's atmosphere is broadly similar to that of Jupiter (see above) but with a higher abundance of hydrogen and a lower abundance of helium; this is thought to be due to helium migrating to the core of the planet at low temperature during planetary formation. Like Jupiter, Saturn radiates more heat than it receives from the Sun; this is thought to be associated with convective mechanisms involving the hot, molten hydrogen core of the planet. Saturn has an atmospheric temperature of 95 K.

Saturn has the lowest density of any planet in the Solar System. In fact, it is less dense than water, so given a sufficiently large ocean it would be possible to float the planet!

The most prominent feature of Saturn is undoubtedly its magnificent system of rings. The Voyager space probes revealed that all of the gaseous giant planets have ring systems; however Saturn's is the only one that can be seen from Earth with a small telescope. The rings consist of countless particles in orbit around the planet in the plane of its equator, beginning at just 7000 km above the cloud tops and with a diameter of some 275,000 km. Despite their vast extent, the rings are only some 100 m thick. The particles in the rings are icy, and range in size from centimetres to a few metres in diameter. The ice in the rings gives them a high albedo.

There are three main theories capable of explaining the formation of the rings:

• Destruction of a satellite due to gravitational tides. The Roche limit is an important parameter in the context of this theory: it defines the distance within which a satellite, held together by its own gravity, will disintegrate due to tidal forces generated by the gravity of the parent planet. Inside the Roche limit, gravity prevents debris and dust from forming a satellite, causing it to remain in place as a ring. During the formation of the Solar System, while Saturn was growing by accreting mass, it is possible that a satellite existed for a time outside the Roche limit but, as the planet grew, so too did its Roche limit, which eventually "overtook" the satellite, resulting in the disintegration of the latter. Another possibility is that a satellite was drawn towards Saturn, perhaps through orbital interaction with other satellites; when it got too close, it was torn asunder by the planet's gravity to form the ring system that we see today.
• Shattering of a satellite through collision. It is conceivable that a satellite was shattered by collision with another body during the early formation of the Solar System and that the resulting debris formed the rings. Once debris forms a ring system around the planet, it can be spread to the full extent of Saturn's rings by gravitational perturbations induced by passing interplanetary bodies (for example, comets).
• Incomplete accretion. According to this theory, the ring system formed by local condensation of the proto-planetary cloud out of which the Solar System formed. The local mass concentration that became Saturn itself attracted its own flattened disk of gas, dust and other material which eventually became the rings.

There are three main rings discovered from Earth, labelled A, B and C in order of proximity towards Saturn. The A and B rings are relatively bright and have essentially been known since the time of the earliest telescopes. The C ring, also known as the Crepe Ring, is semi-transparent and was discovered by the American astronomer William Bond in 1850. More recently, the Pioneer and Voyager space probes discovered the D, E, F and G rings which are all extremely tenuous and faint. Between the A and B rings there is a gap in which few ring particles orbit. This gap is known as the Cassini Division after its discoverer, Giovanni Domenico Cassini (1625-1712) who first spotted it in 1675. The division is about 4,000 km wide, and can be seen in small telescopes. There is a much smaller gap within the A ring called Encke's Division after its discoverer, Johann Franz Encke (1791-1865). A moderate to large telescope is necessary to see Encke's Division. The gaps in the rings are controlled and maintained by the gravitational effects of Saturn's family of satellites.

The Voyager probes returned pictures and data which changed forever our view of Saturn's rings. Previously, the accepted wisdom had been that the rings consisted of a small number of wide rings with divisions cut through them. Instead, the Voyager images revealed tens of thousands of separate ringlets, each different in character. At the same time it became apparent how big a part the moons of Saturn play in maintaining the structure of the rings. Some of Saturn's very small satellites are situated within the ring system itself and closely control the structure of nearby ring arcs; these moons are known as shepherd moons. The famous "spokes" in the rings seen in the Voyager images are thought to be caused by electromagnetic forces within Saturn interacting with dust particles within the rings.

The rings rotate in the plane of the planet's equator, which is tilted at an angle of 26.7° to the ecliptic. Because of this, as the Earth and Saturn move in their orbits, the rings are presented edgewise-on to the Earth at alternate intervals of 13.75 years and 15.75 years (the difference is due to the eccentricity of Saturn's orbit). The rings are so highly reflective that they are responsible for a difference in magnitude of the planet of 1.1 between fully open and edge-on aspects. When edge-on, the glare is reduced and opportunities to observe the faint moons are improved. Between 1655 and 1980, a total of 13 of Saturn's moons were discovered while the rings were edge-on. When the rings were edge-on in May 1995, the Hubble Space telescope discovered four new small moons.

Figures 32 and 33 illustrate Saturn and some of the structure of the ring system. They were taken by the Cassini spacecraft, the former on 23 July 2008 at a distance of approximately 1.1 million km and the latter on 01 June 2005 at a distance of approximately 2.5 million km. (Small moons are visible in both images, when displayed at full scale.)

Saturn has 62 moons with confirmed orbits, 53 of which have names, and only 13 of which have diameters larger than 50 km. The largest is Titan, orbiting the planet at a mean distance of 1,221,800 km. Titan is larger than Mercury and, in fact, is the second largest satellite in the Solar System after Ganymede. At just over 5150 km in diameter, it is the only moon in the Solar System with an apprecable atmosphere, consisting mainly of nitrogen and methane. Its atmosphere is thought to be similar in composition to that of the early Earth, so there is considerable speculation as to whether Titan harbours primitive life forms, or could harbour life if it were in orbit around a planet closer to the Sun and hence warmer.

Saturn's other satellites are all less than 1500 km in diameter, but span a huge range of characteristics. The following table summarises details for the nine largest satellites. Images are by the Cassini orbiter, except where credited otherwise.
 

Metis

1789

12.9

185

199

Mimas is an icy sphere covered with craters, mostly under 30 km in diameter. It has one large crater, named Herschel (after the discoverer of the moon), of 100 km in diameter.

Image taken on 13 February 2010 at a distance of 50,000 km.

Enceladus

1789

11.7

238

249

Parts of Enceladus' surface are covered with craters while others are relatively smooth. Enceladus is in an orbital resonance with Dione which results in it undergoing tidal heating and associated tectonic activity. It is thought that ice volcanoes have covered the surface with a layer of frost particles in geologically recent times.

Image taken on 21 November 2009 at a distance of 2000 km. It shows the south polar region of Enceladus, with fractures that release icy particles and water due to heating.

Tethys

1684

10.2

294

529

One side of Tethys is heavily cratered, and includes a 400 km wide impact basin called Odysseus and a huge fault valley named Ithaca Chasma. Other regions of Tethys are less heavily cratered, and are thought to be associated with cryo-volcanism which has obliterated surface features.

Image taken on 24 September 2005 at a distance of 1500 km. It shows the south polar regions of Tethys, with Ithaca Chasma running up the bottom half of the image, slightly to the left of centre.

Dione

1684

10.4

377

560

Dione is roughly the same size as Tethys, but considerably more dense, indicating a higher proportion of rocky material. It has a cratered surface, but most craters on Dione are under 30 km in diameter. There are bright streaks on the surface which are thought to be water ice.

Image taken on 08 February 2008 at a distance of 211,000 km.

Rhea

1672

9.7

527

764

Rhea's surface is saturated with craters, including a very high proportion of relatively small ones (<20 km in diameter). It does not participate in an orbital resonance with another satellite and thus has no mechanical source of heating. Therefore, over geological time, it has cooled and its crust has contracted, preventing any resurfacing.

Mosaic of images taken on 26 November 2005 at distances ranging from 79,200 to 58,700 km.

Titan

1655

8.3

1,222

2575

Christiaan Huygens discovered Titan in March 1655. In the 1940s, Gerard Kuiper used spectroscopy to discover that Titan has a methane atmosphere. It is the only satellite in the Solar System known to have a significant atmosphere. Voyager 1 approached Titan in Autumn 1980 and revealed that an opaque haze surrounds the entire satellite, preventing any surface details from being seen. However, radar can penetrate the haze, and radar observations in 1990 revealed the existence of bright and dark features on the surface. On 14 January 2005, the Cassini spacecraft released the European Space Agency probe Huygens to land on the surface of Titan; during the descent it revealed vast methane lakes and widespread stretches of wind-sculpted hydrocarbon sand dunes.

The image is a "fish-eye" projection taken with the descent imager/spectral radiometer onboard the Huygens probe, when it was about 5 km above Titan's surface on 14 January 2005.

Hyperion

1848

14.2

1,481

185x113x 140

Hyperion orbits just outside the orbit of Titan. It tumbles chaotically as it orbits Saturn: this, together with its highly irregular shape indicate that it has probably suffered major collisions during its formation; it may be the single remaining significant fragment of a much larger moon. Hyperion is locked in an orbital resonance with Titan. Much of the debris blasted off Hyperion by collisions plunges onto Titan, however some impacts Rhea and may be responsible for the large number of very small craters on the moon.

Image taken on 26 September 2005 at a distance of approximately 33,000 km. It shows a low density body blasted by impacts over the eons.

Iapetus

1671

10.2-11.9

3,561

720

The surface of Iapetus is divided into bright and dark areas. The bright terrain is heavily cratered and occupies the trailing hemisphere. The dark material occupies the leading hemisphere. This dichotomy points to bombardment of the leading surface by dark material. It is thought that meteorite bombardments of Phoebe (the next moon out) dislodge debris which spirals in towards Saturn, some of which is swept up by Iapetus.

First high-resolution image by Cassini of the bright trailing hemisphere of Iapetus. In false colour. Taken on 10 September 2007 at a distance of approximately 73,000 km on the outbound leg of its encounter with the moon. The most prominent topographic feature, in the bottom half of the image, is a 450 km wide impact basin, one of at least nine such large basins on Iapetus.

Phoebe

1898

16.4

12,952

115x105

Phoebe is similar in colour to main-belt asteroids. It has a retrograde orbit around Saturn, inclined at 30° to the equator of the planet. It is though to be an object captured by Saturn's gravitational field.

Mosaic of images taken in 2004 at distances ranging from 16,000 to 12,400 km. The unusual variation in brightness over the surface is due to the existence on some crater slopes and floors of bright material, thought to contain ice.

Table 2. Largest moons of Saturn, listed in order of orbital radius.

To the naked eye, Saturn appears slightly yellowish in colour. At maximum brightness the planet shines at magnitude -0.3. The aspect of the rings significantly influences the apparent brightness of the planet. In an amateur telescope, it is usually possible to discern some atmospheric belts on Saturn; however, these are less pronounced than on Jupiter and there are fewer of them. With a typical amateur telescope of 150 mm aperture or greater it is possible to observe some of the more prominent moons.

Uranus

Uranus was the first planet to be discovered telescopically. It was discovered by William Herschel on 13 March 1781 using a 16 cm reflector of 2.13 m focal length with a magnification of x227. Herschel had a fascinating background. He was born on 15 November 1738 in Hanover. His father was an oboist in the band of the Hanovarian Foot Guards and his formal education was at the garrison school. At the age of 15, he joined the guards band as oboist and violinist. He stayed with the band for only four years before leaving and settling in England, where he started a successful musical career, working in London, Sunderland and Leeds as a composer, teacher and military band instructor before eventually taking up the much-prized post of organist at the Octagon Chapel in Bath in 1766. While in Bath, Herschel's interest in astronomy began to dominate. Being dissatisfied with commercially available telescopes he set about constructing his own, with considerable success, his telescopes being much prized for their excellence. Between ten and eleven o'clock at night on 13 March 1781, he noticed in the constellation Gemini a faint "star" which appeared to grow larger when he switched to an eyepiece with higher magnification: this meant that it was definitely not a star (which would appear as a point of light at any magnification) so he noted it in his journal as a curious either nebulous star or perhaps a comet. On 17 March he observed the object again and noticed that it had moved. At this point he thought that it was a comet, but later observations by other observers and calculation of the body's orbit proved that it was indeed a new planet. Once an approximate orbit had been established, it was possible to determine where the object had been in earlier times, and a search of astronomical journals revealed that John Flamsteed, first Astronomer Royal, had observed it (mistaking it for a star) on 23 December 1690.

Uranus orbits the Sun at an average distance of 2,875,400,000 km. It is the third largest planet in the Solar System after Jupiter and Saturn, with an equatorial diameter of 51,118 km. It spins on its axis in a period of 18.0 hours. Its axis of rotation is tilted at an angle of 98° to its orbital plane so, during each orbit, it presents each pole face-on to the Sun for a time. It takes just over 84 years to orbit the Sun. As a gas giant planet, Uranus has a low density (1.3 g/cm³). It consists of a core of heavy elements (mainly silicon and iron) surrounded by a mantle of water, methane and ammonia which in turn is surrounded by a layer of hydrogen and helium. The methane absorbs red wavelengths of light and gives the planet its characteristic blue-green colour. The temperature at the cloud tops is 57 K.

Six years after Herschel discovered the planet, on 11 January 1787, he found the first two moons of Uranus, subsequently named Oberon and Titania. He also glimpsed a third satellite, Umbriel, in 1802. On 24 October 1851 the English amateur astronomer, William Lassell, using a 61 cm reflector, recovered Umbriel and discovered Ariel. Almost a century later, on 16 February 1948, Gerard Kuiper, using the 208 cm reflector at the McDonald Observatory in Texas, discovered Miranda, the innermost and faintest of the relatively large, spherical satellites. In 1986, Voyager 2 on its fly-by of the planet discovered a further 10 small satellites inside the orbit of Miranda. Additional discoveries by modern terrestrial telescopes and the Hubble Space Telescope have subsequently brought the total number of satellites to 27.

On 10 March 1977, Uranus occulted the star SA0 158687. Thirty minutes before the planet obscured the star, the latter was seen to "wink" five times and, after the planet moved on to reveal the star again, there followed a second series of symmetrical "winks". The occultation, together with subsequent occultations of other stars, revealed the existence of a total of nine separate rings, most with a diameter of only 10 km or so. Voyager 2 revealed that a host of faint ringlets and sheets of dust accompany the main rings.

Uranus' high axial inclination together with images of its inner moon, Miranda, point to a violent early history of the Solar System, when collisions by large bodies were common. The unusual axial inclination is thought to be due to a single large impact which re-orientated the entire Uranian system. Miranda shows evidence of being shattered by a large impact, and the fragments then reassembling under their mutual gravitational attraction.

Uranus is of magnitude circa 5.5 (depending on its distance from the Earth) and thus is visible to the naked eye from a dark-sky site under good atmospheric conditions. At its closest to Earth, Uranus presents a disc of 4 arcseconds diameter. Its disc can be discerned with binoculars or a small telescope. A moderate telescope will show the blue-green colour, but no detail is visible on the cloud tops except in the very largest telescopes. On 28 July 1997, the HST took its first images of Uranus with its Near Infrared Camera and Multi-Object Spectrometer (NICMOS); these are reproduced in figure 34. The image on the right of the figure was taken 90 minutes after the one on the left, showing the rotation of the system during that period. The figure shows the rings of Uranus which, although extremely faint in visible light, are prominent in the near infrared. It also shows eight of the small satellites of Uranus discovered by Voyager 2 (all orbit relatively close to the planet).

Figure 35 shows Miranda. It was taken by Voyager 2 on 24 January 1986 from a distance of about 31,000 km, shortly before the spacecraft's closest approach to the moon. It shows a wide variety of fractures, grooves and craters, as well as features of different albedo, some exposed via grooves and troughs in the surface reaching to depths of a few kilometers. The great variety of directions of fractures and troughs, and the different densities of impact craters on them, signify a long, complex geologic evolution.

Neptune

Much of of our knowledge of Neptune and its system of moons has been obtained from the Voyager 2 probe, which executed a flyby in August 1989. Neptune is the furthest of the gas giant planets from the Sun. It orbits the Sun at a mean distance of 4,504,500,000 km, taking 163.7 years to do so. Its rotation period is 18.8 hours. Neptune is slightly more dense than Uranus at 1.6 g/cm³; it has a very slightly smaller equatorial diameter at 49,532 km. Its atmosphere too is very similar to that of Uranus but contains in addition a haze of aerosol particles. Its atmosphere, like that of Jupiter, is very dynamic with huge bands and depressions. The largest feature in the atmosphere of Neptune is the Great Dark Spot, in size slightly in excess of planet Earth. Another cloud system imaged by Voyager 2 is named The Scooter: it is a fast-moving depression which circles the planet in just a few hours.

Neptune was the second planet to be discovered telescopically. Unlike the chance discovery of Uranus, the discovery of Neptune was a triumph of mathematical prediction based on explaining the deviation of the orbital motion of Uranus from its predicted path. The discovery was not without human interest and the chief protagonists were Jean Couch Adams (1819-92), born the son of a tenant farmer from Launceston, Cornwall and Urbain Jean-Joseph Le Verrier (1811-77), a gifted French mathematician born in Normandy.

At an early age, Adams showed great talent for mathematics. He later went to Cambridge University and it was during his studies there, while browsing in a bookshop in the town one day in June 1841, that he came across an article by George Airy (written in 1832 for the British Association, before he became Astronomer Royal) describing the anomalous motion of Uranus. The planet refused to adhere to predictions of its orbital position and Airy believed that the anomaly was due to errors in measurements of its position, or to inaccuracies in the Newtonian theory of gravity; he rejected the possibility of there being another planet perturbing its motion. The anomalous motion of Uranus looked to Adams like an interesting mathematical puzzle and he wondered if it could be explained by the influence of a planet as yet unknown. By 1845, he had refined a solution for the orbit of the supposed new planet and predicted where it would be visible in the sky. He delivered his results by hand to the Astronomer Royal (the post by this time occupied by George Airy) at Greenwich. Unfortunately, Airy had retained his scepticism about the possibility of an unknown planet being the cause of the peculiar motion of Uranus and did not respond quickly to Adams. It was not until November 1845 that Airy replied and, even then, only to ask if Adams' calculations explained discrepancies in the distance of Uranus as well as its motion. Feeling snubbed, Adams did not reply.

Meanwhile, in France, Le Verrier was also working on the problem and, in June 1846 (about eight months after Adams' predictions), published a paper attributing the peculiar motion of Uranus to the effect of an unknown, remote planet and discounting all other potential explanations. He had calculated the co-ordinates of a ten degree arc of the zodiac where he believed that the new planet would be found. Although French astronomers applauded the work, they did not take up the search for the unknown planet. In late June 1846, Airy, who had read of Le Verrier's work, wrote to him asking the same question that he had of Adams, but without mentioning the latter's work. Le Verrier replied and, in July, Airy asked the astronomer James Challis, Director of Cambridge Observatory, to begin searching for the new planet. Challis began the task, seemingly without real enthusiasm.

Back in France, Le Verrier was still experiencing difficulty interesting French astronomers in taking up the search! Eventually, he found a letter from an aspiring assistant astronomer, Johann Gottfried Galle (1812-1910), working at Berlin Observatory. Seeing in Galle a prospect of undertaking the search, Le Verrier wrote asking him to do so. The letter reached Galle on 23 September and, on that night, together with student assistant, Heinrich d'Arrest (1822-75), he used the 23 cm Fraunhofer Refractor at Berlin Observatory to search the sky where Le Verrier had indicated. Galle called out the position of the stars and d'Arrest noted them against a star map; after some time the student shouted excitedly that star is not on the map. Neptune was discovered! On the following night, Galle and d'Arrest checked that the object had moved by the predicted amount and that a planetary disc could be discerned. Le Verrier's prediction had been accurate.

In Cambridge, on 29 September, Challis found a star that looked very disk-like, rather than the usual stellar point. However he did not follow through the observation that night and decided instead to go to bed. The following day he heard news of Neptune being discovered at the Berlin observatory six days previously. He then went back through his notebooks and discovered that he had observed the planet on two separate occasions in early August but had not identified it for what it was!

Although the discoverers based their search on Le Verrier's predictions, nowadays both Adams and Le Verrier are given equal credit for the prediction of Neptune.

The English amateur astronomer, William Lassell, using a 61 cm reflector, discovered Neptune's largest satellite, subsequently named Triton, on 10 October 1846, less than a month after the discovery of the planet itself. (Lassell was later to discover, with the same telescope, the satellites Umbriel and Ariel of Uranus.) Spectroscopic observations from Earth show Triton to have a surface of frozen methane, carbon dioxide, water and nitrogen. Triton orbits in a retrograde direction at a mean distance of 354,000 km from Neptune. At 2720 km, it has a larger diameter than Earth's Moon, but its visual magnitude is only 13.5 which, combined with its proximity to Neptune, makes it a very difficult object for all but the largest amateur telescopes. Analysis of the trajectory of Voyager 2 near Triton showed the satellite to have a density of 2 g/cm³. The interior of the moon is a mix of ice and rocky material; pictures of the surface from Voyager 2 showed few impact craters but revealed evidence of widespread cryovolcanism (the upwelling of icy/slushy material from the interior) which resulted in extensive resurfacing in geologically recent times. The cryovolcanism has given the satellite a young, highly reflective surface with an albedo of 72% at visible wavelengths. The orbital geometry of the Neptune/Triton system causes seasonal extremes on Triton occurring in a cycle approximately 688 years long. Extreme summer seasons occur with the Sun directly overhead at 50° north or south latitude on the satellite; they result in significant changes in the temperature distribution on Triton. During an extreme summer, local cryovolcanism is stimulated and nitrogen, carbon monoxide and methane migrate from the summer pole towards colder areas of the planet: this re-instates the reflective surface on colder parts of the satellite.

The famous planetary observer, Gerard Kuiper, discovered a second moon of Neptune, named Nereid, in 1949. Nereid moves in a distant, highly eccentric orbit. (In fact, it has the most eccentric orbit of any satellite in the Solar System - its distance from Neptune varies from 1,400 million km to 9,700 million km.) It is thought that the orbital configuration of Triton and Nereid indicates a violent history in Neptune's family of satellites: the planet may have captured Triton from a previous heliocentric orbit; such an event would have disrupted any pre-existing family of satellites and could account for the highly eccentric orbit of Nereid. The orbit of Triton is decaying and the moon will collide with the planet in about 100 million years' time.

Voyager 2² discovered six small satellites of Neptune orbiting inside the orbit of Triton: five of them are very small and are probably captured asteroids; the sixth, Proteus, is a larger satellite (about the size of Mimas, one of the moons of Uranus), approximately 400 km in diameter. Proteus has an irregular shape and is much darker than Nereid, reflecting only some 6% of incident sunlight (against 12% for Nereid). In 2002 and 2003, surveys using large ground-based telescopes found five additional outer moons, bringing the total to thirteen.

During the early 1980s, Earth-based observations of occultations by Neptune suggested that the planet is surrounded by a system of rings and ring arcs. Voyager 2 confirmed the existence of the ring system and also revealed a great deal of dusty material orbiting with the rings themselves. There are five continuous rings and one discontinuous ring. All are extremely faint and not visible directly from Earth. The brightest two rings are located approximately 53,000 and 63,000 km from the centre of the planet.

Neptune is an eighth magnitude object and hence is visible in a small telescope or binoculars. It has an apparent diameter of some 2.3 arcseconds and, like Uranus, in a moderate to large aperture telescope presents a blue-green disc. Earth-based telescopes are not capable of discerning any detail in the cloud tops of Neptune; however the Hubble Space Telescope can show some detail.

Figure 36 was taken by Voyager 2 on 20 August 1989 at a distance of 7.2 million km from Neptune, five days before closest approach. The picture shows the Great Dark Spot and its companion bright smudge at the centre; on the west limb the fast moving bright feature called the Scooter and the little dark spot are visible. Figure 37 is a mosaic of images of Triton captured during Voyager 2's flyby of the Neptune system in late August 1989. Colour is synthesised. The pinkish deposits constitute a vast south polar cap believed to contain methane ice, which would have reacted under sunlight to form pink or red compounds. The dark streaks overlying the pink ice are believed to be an icy and perhaps carbonaceous dust deposited from huge geyser-like plumes (some of which were found to be active during the flyby). The bluish-green band extends all the way around Triton near the equator; it may consist of relatively fresh nitrogen frost deposits. The greenish areas includes what is called the cantaloupe terrain, whose origin is unknown, and a set of "cryovolcanic" landscapes apparently produced by icy-cold liquids (now frozen) erupted from Triton's interior.

Pluto

In 1893, Percival Lowell, a wealthy Bostonian with an avid amateur interest in astronomy, decided that the science was to be his life's work. He commissioned an observatory atop Mars Hill in Flagstaff, Arizona and, in 1894, began observing there. He concentrated initially on the planet Mars, taking over from the renowned observer of the planet, Giovanni Schiaparelli (1835-1910), who had recently retired due to failing eyesight. By 1900, Lowell's interest had turned from Mars to the search for a possible Planet X orbiting in the distant outer limits of the Solar System beyond the orbit of Neptune. For the next 16 years, he searched from Flagstaff for Planet X without success. He died suddenly of a stroke on 12 November 1916 and it was only after some 13 years of legal wrangling over his legacy that Flagstaff Observatory was able to resume the search in earnest. The observatory hired Clyde Tombaugh (1906-97) to undertake an exhaustive photographic search. He exposed large photographic plates (26x43 cm) at night and, during the day, examined them with a device called a blink comparator to compare photographs of the same area of the sky taken hours or days apart thereby revealling any moving objects. Each plate contained an average of 160,000 stars! By this approach he found several asteroids and a comet. Eventually, on 18 February 1930, on examining photographic plates taken during January and February of the year, he found, close to the naked eye star Delta Geminorum, a magnitude 15 point of light which moved in the blink comparator. He had discovered Pluto! (The name was taken from the God of the Underworld.) Following further checks, the discovery was announced to the world on 13 March 1929.

Although Pluto was found as a result of a search for a body to explain apparent perturbations in the motion of Uranus (after allowing for the effects of Neptune), it eventually became clear that it was far too small to fulfil such a role. Subsequent work based on an analysis of the trajectory of Voyager 2 around Neptune showed that, in fact, Neptune was very slightly less massive than had previously been thought and this reconciled the positions of the planets in their orbits to the known bodies in the Solar System: the discovery of Pluto based upon supposed perturbations of Uranus turned out to be a happy accident!

Pluto takes 248 years to orbit the Sun. Its orbit is more eccentric than that of any of the planets: perihelion is at a distance of 4,425,100,000 km and aphelion at 7,375,100,000 km. For 20 years during each orbit, Pluto is closer to the Sun than Neptune; however, there is no fear of a collision because Pluto's orbit is inclined at 17° to the ecliptic and it has a 3:2 orbital resonance with Neptune so that the two bodies never come close to each other. Pluto lay inside the orbit of Neptune from 08 February 1979 until 11 February 1999. On 05 April 2231, it will next come inside the orbit of Neptune. Pluto's mass is only 0.0022 that of Earth. Its surface is thought to be rocky and covered with a surface layer of methane ice. The surface temperature is about 50 K. Pluto's atmosphere is composed primarily of methane; it is very thin (in fact, approximately as tenuous as the air at a height of 80 km above the Earth) because of the low escape velocity of only 1.2 km/s. The atmosphere does thicken during Pluto's summer months as some of the methane surface ice evaporates into gas.

On 22 June 1978, James Christy, working at the US Naval Observatory, noticed that several images of Pluto appeared to be elongated in one direction. He quickly concluded that Pluto must have a satellite, which was subsequently named Charon (after the boatman in Greek mythology who ferried souls across the River Styx to the Underworld). Charon rotates around Pluto in 6.7 days, a period that matches exactly the rotation period of the parent body. Thus, in the Pluto-Charon system, both primary and secondary bodies have attained synchronous rotation through tidal interaction and keep the same faces turned towards one another. On 15 May 2005, astronomers working with the HST discovered two further moons, subsequently named Nix and Hydra. On 20 July 2011, a fourth moon was discovered, and on 07 July 2012, a fifth.

The axis of rotation of Pluto and the pole of the orbit of Charon lie approximately in the plane of the ecliptic (a similar situation to Uranus). This orbital geometry meant that Charon and Pluto passed in front of one another as seen from the Earth during the late 1980s. By studying how the light curve of the combined system varied as Charon passed in front of Pluto and vice versa, astronomers were able to obtain precise estimates of the sizes of the bodies and to map major features on their surfaces. The accepted values nowadays are as follows:

• diameter of Pluto: 2302 km,
• diameter of Charon: 1186 km,
• distance between bodies: 19,636 km,
• combined density of the system: 2.0 g/cm³.

Pluto and Triton share similar physical characteristics. This led astronomers to suggest that they may have had a common origin; however it is difficult for a theory to explain also the existence of Charon and the 3:2 orbital resonance between Neptune and Pluto. It may be that Pluto/Charon and Triton formed originally as separate Kuiper belt objects and then were perturbed by the gravitational influence of Neptune. The perturbation of Triton (if that is what happened) resulted in its capture by Neptune, which in turn would have disrupted any system of large moons that was already in orbit around the planet.

When Pluto was discovered, it was considered to be a planet orbiting alone in its region of space. However, over the last few decades, powerful ground and space-based observatories have completely changed the understanding of the outer Solar System. Instead of being the only object in its region of space, Pluto and its moons are now known to be just one example of a vast number of similar icy objects, occupying a zone called the Kuiper Belt extending from the orbit of Neptune to a distance of 55 astronomical units. Astronomers estimate that there are at least 70,000 Kuiper Belt Objects (KBOs), with the same composition as Pluto, measuring 100 km or more in diameter. In 2005, Mike Brown and his team at Caltech discovered a KBO, subsequently named Eris, slightly larger and more massive than Pluto and orbiting at a greater distance from the Sun. This brought into sharp relief the question of the status of Pluto and, at the XXVI^th General Assembly of the International Astronomical Union in 2006, a resolution was passed which had the effect of reclassifying Pluto and other large KBOs as dwarf planets.

Pluto is of magnitude 14.5 and accordingly requires a telescope of aperture at least 250 mm to be glimpsed. Few amateur astronomers have seen it!

Figure 38, photographed by the HST in 2005, shows Pluto, Charon and the two newly-discovered moons Nix and Hydra. Figure 39 is the first surface map of Pluto. It was assembled from four separate images of Pluto's disk taken by the HST in late June and early July 1994. The map covers nearly the entire surface of Pluto and shows a dark equatorial belt and bright polar caps. The brightness variations may be due to topographic features such as basins and fresh impact craters but are more likely produced by the complex distribution of frosts that migrate across Pluto's surface with its orbital and seasonal cycles.

Comets

Mankind has observed comets for thousands of years. For example Halley's comet, the most famous and most studied comet ever, has been observed since at least 240 BC as evidenced from the Chinese records of its apparition in that year. Figure 40 shows images of the nucleus of Halley's comet captured by the European Space Agency's Giotto probe on approach to the comet on 13 March 1986.

Comet observers have been particularly fortunate at the end of the 20^th Century to have enjoyed, within the space of a few years, the impressive comets Hyakutake and Hale-Bopp, and the aftermath of the strike by comet Shoemaker-Levy 9 on Jupiter.

A comet typically consists of a nucleus of some 1-10 km diameter and two vast tails. The nucleus is generally an irregular "dirty snowball", comprising dust, rocky material and snow or ice. When the comet approaches the Sun, solar heating causes ice in the nucleus to sublime and release trapped dust grains. This creates an atmosphere of gas and dust called the coma surrounding the nucleus. The dust particles are generally no larger than one micron in diameter. Pressure of sunlight on the dust particles causes them to stream away from the coma forming a dust tail which shines by reflecting sunlight and typically appears yellow-white. The dust tail can extend for up to 10 million km and is generally gently curved because of the comet's motion around the Sun. The solar wind accelerates ions in the coma along magnetic lines of force: this forms a plasma tail which generally looks bluish because of the presence of carbon monoxide which fluoresces in sunlight with a blue glow. The plasma tail is generally straight and can attain a length of up to 100 million km. Plasma tails sometimes become disconnected from the nucleus of the comet.

Kenneth Edgeworth in 1949 and Gerard Kuiper in 1951 suggested that a vast belt of comets in nearly circular orbits could exist in the far reaches of the Solar System beyond Pluto. At the time, the idea did not gain credence. However, in the 1980s, astronomers used powerful computers running orbital models to study the potential role that such a reservoir of bodies could play in sourcing short-period comets. They found that it could explain the orbital characteristics of many short-period comets; thus the existence of the belt, which became known as the Kuiper Belt, was postulated and searches were undertaken for objects belonging to it. In August 1992, astronomers found the first Kuiper Belt object (KBO), a magnitude 23 body of 320 km diameter. It takes almost 300 years to orbit the Sun at a mean distance of 44 AU. Further KBOs were found in subsequent years. The most distant Kuiper Belt object discovered to date has a highly eccentric orbit with a period of almost 800 years and a maximum distance from the Sun of 133 AU. Many KBOs are in orbital resonances with Neptune (in the same way that Pluto is). Extrapolating from the sample of KBOs discovered to date indicates the existence of some 200 million such objects in total.

While the Kuiper Belt is the source of short period comets, another reservoir of material is the source of long period comets. This is the Oort cloud, a collection of some 190 billion comets in orbit around the Sun at a distance of 20,000-100,000 AU. At such great distances from the Sun, perturbations from passing stars can alter orbits and send comets into the inner Solar System where they are visible to observers on Earth. Although close passages of stars are rare, over the lifetime of the Solar System sufficiently many have occurred to stir up the Oort cloud and launch numbers of comets into new orbits passing through the inner Solar System.

Footnotes

Adapted from articles by Roy Gooding and Joe Walsh

Original: Newsletters June, July 1976; June, July, October 1988; January, April, May, August, October 1989; January 1990; July, November 1991; February 1992; April, July, August 1993; May, October 1995; April 1997; April, May, November 1998; April 1999
Updated: 23 March 2023