Voyages to the Outer Solar System

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Gresham Lecture, Wednesday 24 November 2010
Voyages to the Outer Solar System
Professor Ian Morison
Perhaps one of the greatest achievements of unmanned space flight has been the wealth of information – not to say stunning images – that have resulted from NASA’s programme to send probes to the outer parts of our Solar System. Here we will chart nearly 40 years of exploration, from the Pioneer and Voyager probes in the 70’s and 80’ to the Gallileo spacecraft’s study of Jupiter around the turn of the millennium and, more recently, the Cassini and Huygens probes studying Saturn and Titan. Finally, as this is written, the New Horizons spacecraft is on it way to Pluto, where it will arrive in mid 2015, and then travel further into the Kuiper Belt.
Pioneer 10
Pioneer 10 was launched from Cape Canaveral on March 2nd, 1972 and was the first spacecraft to travel through the asteroid belt to reach Jupiter. It had entered the Asteroid Belt on July 15th that year - a region 280 million km wide and 80 million km thick. The material in the Belt encompasses sizes from dust particles up to the major asteroids travelling at speeds up to 72,000 km/h and scientists had feared Pioneer 10 might not be able to negotiate its way through. It was even thought that the debris within the Asteroid Belt would be so thick that any spacecraft would be destroyed.

Arriving at Jupiter on December 3rd 1973 at an approach speed of 131,000 km/h, Pioneer 10 was the first spacecraft to make direct observations and obtain close-up images of Jupiter. It mapped out the giant gas planet's intense radiation belts, located the planet's magnetic field, and showed that Jupiter was predominantly composed of liquids. Following its encounter with Jupiter, Pioneer 10 continued flying outward to explore the outer regions of the Solar System, where it studied the solar wind - and outflow of energetic particles from the Sun - along with cosmic rays - highly energetic particles that enter the Solar System from stellar explosions within the Milky Way galaxy. In 1983, Pioneer 10 became the first human-made object to pass the orbit of Pluto, then the most distant planet from the Sun (now sadly downgraded to the status of a “Dwarf Planet”) and continued to make valuable scientific contributions in the outer regions of the Solar System until its science mission ended officially on March 31, 1997.

Contact was lost with Pioneer 10 in 2003 and it is now heading in the direction of Aldebaran in the constellation Taurus. It was, by some definitions, the first artificial object to leave the solar system and, as it could theoretically be found by another advanced civilisation, Carl Sagan proposed that it should carry a Plaque that would tell them something about ourselves and our Earth. The symbol at the top left symbolises the radio line transition of the hydrogen atom and so gives a length scale - that of the wavelength of the emitted photon of 21cm. On the extreme right this is used to give the height of the female human being. The “chart” in the centre left shows the position of Sun relative to the directions of 14 pulsars that lie in the plane of the galaxy. Thus, in the same way that one could locate the position of a ship from its bearings to known lighthouses (identified by their flash rate), so the position of our Sun could be found. In fact, as the period of the pulsars, indicated by marks of the bearing lines, change with time, they could even tell when the probe was launched. Neat! The Solar System diagram shows the path of the spacecraft and that we live on the third planet from the Sun. Although the original was slightly censored by NASA, it received some opprobrium from the American Ladies League of Decency!

The Pioneer Plaques
Pioneer 11
Launched a year later, in April 1973, Pioneer 11 reached Jupiter in December 1974. It passed within 42,500 km of Jupiter's cloud tops and, despite receiving intense bombardment from Jupiter's radiation belts (which are 40,000 times more intense than Earth's) happily survived. As Pioneer 10 had achieved all of the main mission objectives at Jupiter, Pioneer 11 used Jupiter to provide a “gravitational slingshot” to increase its speed and send it on a course toward Saturn where, in 1979, it became first spacecraft to fly past Saturn before beginning a long journey out of the Solar System in the direction of the constellation Aquilla. It flew within 21,000 km of Saturn and discovered a new ring and two new moons as well as detecting a thick atmosphere on Titan, Saturn’s largest moon. Its instruments measured the heat radiation from Saturn's interior, and probed Saturn’s magnetosphere, magnetic field and, from the gravitational effects on the spacecraft’s trajectory, its interior structure.
The measurements of Jupiter's radiation environment made by the two Pioneers enabled the many later missions to Jupiter and Saturn to have suitably “hardened” electronic systems so that could withstand the intense radiation belts that exist around these planets.
An aid to SETI (The Search for Extra-Terrestrial Intelligence)
The most sensitive and sophisticated SETI program ever undertaken was Project Phoenix which, from 1998 to 2003, observed over 800 sun-like stars searching for any signals from that might come from ET. It used two of the world’s largest radio telescopes, the 300m Arecibo dish in Puerto Rico and the 76m Lovell Radio Telescope at Jodrell Bank in the UK where the author was the project scientist. The use of two very widely spaced telescopes making simultaneous observations meant that local interference would not cause spurious detections. The rotation of the Earth meant that a signal at a specific transmitted frequency would, due to the Doppler effect, be received at different frequencies at the two telescopes. This enabled us to eliminate any signals received from satellites orbiting both the Earth the Sun. But how could we prove that the system was operating perfectly? Each day, prior to the 12 hour observing period, we detected the very weak signal still being transmitted by the Pioneer 10 space probe, then over 11 billion km from Earth. Its last signal was detected on January 23rd 2003 from a distance of 1 billion miles – almost twice the mean distance of Pluto!
The Pioneer Anomaly
Doppler shift measurements of the received signals from the two spacecraft as they passed out of the solar system indicated that they were slowing down slightly more than expected: each year they travelled about 5,000 km less in distance. The slowing down was extremely small and the increase in gravity to explain it is equivalent to just one ten-billionth of the gravity at Earth's surface. However, although tiny, the effect persisted over several decades so that when last heard from, Pioneer 10, was a quarter of a million miles closer to the Sun than expected. Thought it is suspected that there may be some spacecraft related explanation, such as a gas leak providing a tiny thrust, some scientists have considered the possibility that our understanding of gravity may need to be revised.
Voyagers 1 and 2
Voyager 1 was launched by NASA on September 5, 1977, two weeks after its twin spacecraft, Voyager 2, but, as it was sent on a shorter trajectory, reached Jupiter and Saturn first. Amazingly, it is still in communication with Earth pursuing an extended mission to locate and study the Kuiper belt and the outer boundaries of the Solar System. It has been realised that a rare alignment (once every 175 years) of the outer planets would enable the voyager probes to utilise a technique called “Gravity Assist” to gain speed from the gravitational energy of the planets it passed and be able to undertake what was then called “The Grand Tour”. This was a linked series of gravity assists that would enable a single probe to visit all four of the Solar System's giant planets within a period of just twelve years rather than thirty! [If a spacecraft passes close behind the path of a planet, the gravitational pull tries to make the planet fall into its surface and so the probe gains speed along the direction of the planet’s orbit. However, if it is moving fast enough, it will not impact the planet but continue onwards with a new trajectory and significantly greater speed. This is also sometimes called a planetary or gravitational “slingshot”]
Voyager 1 began imaging Jupiter in January 1979 and its closest approach was on March 5, 1979, just 276,000 km above its cloud tops. Over a 48 hour period, it studied Jupiter’s moons, rings, magnetic fields and radiation belt and made the exciting discovery of volcanic activity on Jupiter’s innermost moon, Io.

Voyager 1 images of the Great Red Spot and Io, showing an active volcano.

The gravitational assist trajectories at Jupiter were successfully carried out by both Voyagers, and the two spacecraft went on to visit Saturn. Voyager 1 reached Saturn in November 1980, when the space probe came within 123,000 km of Saturn's cloud-tops. A year earlier, Pioneer 11 had detected a dense atmosphere on Titan and it was thought worthwhile for Voyager 1 to investigate this further rather than continue the Grand Tour on to Uranus and Neptune. This close fly-by deflected Voyager 1 out of the plane of the Ecliptic (within which lie the planets) and so ended its planetary quest. [Had they not done this, Voyager 1 could have flown passed Pluto!]

Voyager 2
Voyager 2 was launched on a slower, more curved, trajectory and remained in the plane of the ecliptic so that having passed Jupiter and Saturn it could continue on the on to Uranus and Neptune by means of the gravity assists gained during its fly-bys of Saturn and Uranus in 1981 and 1986. It is probably the most productive single

unmanned space voyage carried out so far, having visited all four of the outer planets and their systems of satellites and rings. Voyager 2 carried cameras for imaging along with instruments to make measurements at ultraviolet, infrared, and radio wavelengths. It was also able to measure the density of subatomic particles, including cosmic rays, in outer space.

On July 9, 1979 Voyager 2 came within 560,000 km of the Jupiter’s cloud tops and showed that the Great Red Spot was a complex, anticlockwise rotating storm nestling within Jupiter’s complex banded cloud systems along with many other smaller storms and eddies. Perhaps the most exciting discovery made by the two Voyager spacecraft was that of volcanism on Io. Together, the Voyagers observed the eruption of nine volcanoes and found evidence that other eruptions had occurred between the two Voyager fly-bys.
Voyager 1 had observed a large number of intersecting linear features on the surface of Jupiter’s second innermost moon Europa and scientists had thought that the features might be deep cracks. However, high-resolution photos from Voyager 2 showed that the "might have been painted on with a felt marker." The idea arose that Europa might have a thin crust of water ice, possibly floating on a deep ocean kept liquid by the tidal heating due to its proximity to Jupiter. Voyager 2 also found three new small satellites: Adrastea, Metis and Thebe.
Voyager 2 at Saturn
Just over 2 years later Voyager 2 passed behind Saturn. As the radio signals it was transmitting had to pass through Saturn’s atmosphere as it disappeared and reappeared, scientists were able to gather information on its atmospheric temperature and density profiles. At its cloud tops the temperature was ~ -203 C whilst at the lowest depths measured it increased to -130 C.

What we know about Uranus
Uranus was the first planet to have been discovered in modern times and though, at magnitude ~5.5, it is just visible to the unaided eye without a telescope it would have been impossible to show that it was a star rather than a planet, save for its slow motion across the heavens. Even when telescopes had come into use, their relatively poor optics meant that it was charted as a star many times before it was recognised as a planet by William Herschel in 1781.
Uranus revolves around the Sun once every 84 Earth years at an average distance from the Sun of roughly 19 Astronomical Units. The surface cloud layers are seen to rotate with a period of as little as 14 hours, but this is due to high winds in the upper atmosphere and the nominal rotational period of Uranus is 17 hours, 14 minutes. Whilst for the majority of planets the rotation axis is roughly at right angles to the plane of the Solar System, Uranus has an axial tilt of 98 degrees, so in effect “rolls” around the Sun. Each pole gets around 42 years of continuous sunlight followed by 42 years of darkness. In contrast to the Earth, this makes the poles warmer than the equator.
Uranus is the least massive of the giant planets at 14.5 earth masses and has the second lowest density 1290 kg/m3. It probably has a central rocky core of about 2 Earth masses above which is a mixture various ices, such as water, ammonia, and methane along with an outer gaseous layer made up of about 1 Earth mass of hydrogen and helium. As the ices make up a far great proportion of its mass than gas, Uranus is often termed an ice giant rather than a gas giant.
The Rings of Uranus
On March 10th 1977, observations were to be made of the occultation by Uranus of a star, SAO 158687, using a telescope mounted in the Kuiper Airborne Observatory. Just before the star’s light is lost its light will have passed through the atmosphere of Uranus and comparing the spectra of the stars at this time with that prior to the occultation it is possible to learn about the planet’s atmosphere.
The telescope was observing the star well before the expected time of occultation when the astronomers were somewhat perturbed as the star’s light suddenly, disappeared. The signal did return after a rather tense period but this was then followed by four partial losses of signal. Now reasonably confident that it was not their equipment that was faulty, they continued to observe the star following the occultation when the sequence was seen to repeat in the inverse order. They realised that the light from the star must have been eclipsed by material in five rings about Uranus with the outermost (called the epsilon ring) being the thickest. From the times of the ring’s occultations they could calculate the diameters of the rings and found that the outermost was ~44,000 km from the centre of Uranus.
Voyager 2 at Uranus
On January 24, 1986, when Voyager 2 came within 81,000 km of the planet's cloud tops it directly imaged the ring system and more rings were discovered bringing the number up to 11. Observations showed that the Uranian rings are distinctly different from those at Jupiter and Saturn. The Uranian ring system appears to be relatively young, and it did not form at the same time that Uranus did. It is thought that the particles that make up the rings might be the remnants of a moon that was broken up by either a high-velocity impact or torn up by tidal effects.

Images of Uranus and its rings and its moon Miranda taken by the Voyager 2 spacecraft in 1986.
The radiation belts of Uranus were found to be of similar in intensity to those of Saturn. This radiation is such that “irradiation” would darken any methane that is trapped in the icy surfaces of the inner moons and ring particles and may have contributed to the darkened surfaces of the moons and the ring particles, which are almost uniformly dark grey in colour. A high layer of haze was detected around the sunlit pole of Uranus. This area was also found to radiate large amounts of ultraviolet light, a phenomenon that is known as "dayglow." The average atmospheric temperature is about −213 degrees Celsius.
The Uranian moon Miranda, the innermost of the five large moons, was shown to be one of the strangest bodies in the Solar System. Voyager 2’s detailed images showed huge canyons made from geological faults as deep as 19 km, terraced layers, and a mixture of old and young surfaces. It is thought that Miranda might consist of a reaggregation of material following an earlier event when Miranda was shattered into pieces by a violent impact.
Voyager 2 at Neptune

Neptune showing the clouds, both light and dark in its atmosphere, and its moon, Triton, as imaged by Voyager 2 in 1989.

Neptune can be seen in even a small telescope and had even been observed by Galileo: whilst observing Jupiter on the 28th December 1612 he recorded Neptune as an 8th magnitude star and a month later observed it close to a star on two successive nights. He noted that their separation had changed and could easily have reached the conclusion that this was because one was not a star but a planet!

The honour of Neptune’s discovery is shared by Adams at the University of Cambridge and Le Verrier at the Paris Observatory who had both computed its position from the perturbations its gravitational attraction had caused to the orbit of Uranus. Neptune is the fourth largest planet by diameter, and the third largest by mass, slightly more massive than its near-twin Uranus. Neptune's atmosphere is primarily composed of hydrogen and helium along with ~ 1% of methane which may help contribute to its vivid blue colour. Winds in its atmosphere can reach 2000 km/hr, the highest of any planet. As Voyager 2 passed Neptune in 1989 it observed a Great Black Spot comparable to Jupiter’s Great Red Spot. It measured the cloud top temperature to be -218 C. Neptune’s sidereal rotation period is roughly 16.11 hours long and it has a similar axial tilt to Earth.
Neptune and Uranus are often considered "ice giants", given their smaller size and greater percentages of ice in their composition relative to Jupiter and Saturn.

Neptune’s core is composed of rock and ice, having about one Earth mass. The mantle is made up of ~12 Earth masses largely made up of water, ammonia and methane. The atmosphere contains high clouds that cast shadows onto the blue coloured surface layers.

Like Uranus, Neptune also has a ring system. The rings, which have a reddish hue, may consist of ice particles coated with silicates or carbon-based material. In sequence measuring from the centre of Uranus are the broad, faint Galle Ring at 42,000 km, the Leverrier Ring at 53,000 km and the narrow Adams Ring, at 63,000 km. The largest of Neptune’s 13 moons, and the only one massive enough to be spherical, is Triton which, unlike all other large planetary moons, has a retrograde orbit. This implies that it has been captured from what is called the Kuiper Belt, a region containing many small bodies beyond Neptune’s orbit. It keeps one face towards Neptune and is slowly spiralling inwards where it will be eventually torn apart when it reaches the Roche limit, so giving Neptune a more extensive ring system. Triton consists of a crust of frozen nitrogen over an icy mantle believed to cover a substantial core of rock and metal, its surface is relatively young. Part of its crust is dotted with geysers that are believed to be erupting nitrogen. [As seen in the lower half of Triton’s image above.]
Its Voyage Continues
Its planetary mission over, Voyager 2 is now continuing to travel outwards from the Sun and is now at a distance of over 13.6 billion km - more than twice as far from the Sun as Pluto but not yet beyond the outer limits of the orbit of the dwarf planet Eris.

It is not headed toward any particular star but should it pass near the star Sirius, currently 8.4 light years from the Sun in about 296,000 years. It is hoped to be able to receive its (now very weak) radio signals until at least 2025 – a space mission that will then have lasted over 48 years since its launch!

The Voyager Message
The Voyager spacecraft became the third and fourth human artefacts to escape entirely from the solar system. Following the example of the Pioneer Plaques, NASA placed a more comprehensive message aboard Voyager 1 and 2 to make a kind of time capsule, intended to communicate something of ourselves and our world. It was in the form of a record whose contents of the record were selected by a committee chaired by Carl Sagan of Cornell University. They put together images, sounds and music along with greetings in fifty-five languages.

The Voyager record and cover.
The images include many photographs and diagrams both in black and white and colour. The first images are scientific, showing mathematical and physical quantities, the solar system and its planets, our genetic code, and human anatomy and (very discretely) reproduction. [NASA, following the criticism the line drawings of a naked man and woman on the Pioneer Plaque, only allowed a silhouette of the couple to be included.] Images of out human race depict a broad range of cultures going about their lives. There are also images of landscapes, architecture and of animals, insects and plants. The sounds include those made by surf, wind, and thunder along with animal sounds such as birdsong and from whales. The musical selection featured composers such as Beethoven, Mozart, Bach, and Stravinsky. The gilt record cover showed a diagram of the hydrogen atom - to give a length and frequency reference - along with pulsar map - both of which had been on the Pioneer plaques. In the upper left-hand corner is an easily recognized drawing of the phonograph record and the stylus carried with it. Electroplated onto the record’s gold-plated copper is a pure sample of the isotope uranium-238 which has a half-life of 4.51 billion years. Any civilization that encounters the record will be able to use the ratio of remaining uranium to its daughter elements to determine its age!
The Galilleo Mission to Jupiter
The Galileo spacecraft was launched in 1989 and arrived at Jupiter on December 7, 1995 following gravitational assist flybys of Venus and Earth. Amongst its achievements, Galileo conducted the first asteroid flybys (Gaspra and Ida), discovered the first asteroid moon (Ida’s moon Dactyl). It also launched a probe into Jupiter's atmosphere. En route it was able to image the impacts of the fragments of comet Shoemaker-Levy 9 into the atmosphere of Jupiter - these impacted beyond Jupiter’s visible limb as seen from Earth. Due to its distance from the Sun, solar panels would not have been practical, so power was provided by two radioisotope thermoelectric generators which powered the spacecraft through the radioactive decay of plutonium-238. (Prior to the launch, anti-nuclear groups, concerned over what they perceived as an unacceptable risk to the public's safety from the plutonium should the spacecraft crash, sought a court injunction to prohibit Galileo’s launch.)

The asteroids Gaspra at left and Ida with its moon, Dactyl, on right.
On arrival at Jupiter for its 2 year prime mission, Galileo orbited Jupiter in extended ellipses so allowing it to sample different parts of the planet's extensive magnetosphere. The orbits also allowed it to carry out close flybys of Jupiter's largest Gallilean moons. Having successfully completed its prime mission, the spacecraft made a number of very close flybys of Jupiter's moons Europa and Io.
In July 1995, five months before reaching Jupiter, Galileo released a probe to enter Jupiter’s atmosphere. It collected nearly an hour’s data as it descended through 150 kilometres of atmosphere until the pressure reached 23 times that of the Earth’s and the temperature rose to 153 Celsius. The probe found that the atmosphere through which it had passed was rather more turbulent and hotter than expected.
During the 8 years of its primary and extended missions made many key observations and discoveries. Some of these were:
it made the first observation of ammonia clouds. The atmosphere creates ammonia ice particles from material coming up from lower depths.
it confirmed that the moon Io had extensive volcanic activity that is 100 times greater than that found on Earth. The heat and frequency of eruptions are reminiscent of early Earth.
it provided significant support for the theory that liquid oceans exist under Europa's icy surface.
it showed that Ganymede possesses its own, substantial magnetic field.

it showed that Jupiter's ring system is formed by dust created as interplanetary meteoroids smash into the planet's four small inner moons.

its star scanner discovered that the second magnitude star Delta Velorum was occulted by a companion star for 8 hours becoming the brightest eclipsing binary known with a period of 45 days.
During its mission, Galileo carried out some experiments not directly related to its planetary studies. As it passed Earth on its second gravity assist flyby it tested the feasibility of the communication to and from satellites using pulses from powerful optical lasers. This proved very successful, and the author believes that some military satellites are now using this technique to rapidly download their data to Earth in a way that would be very difficult to eavesdrop. The late Carl Sagan devised a series of experiments to see if Galileo could detect signs of simple or advanced life here on Earth as a way of indicating how we might detect life on other planets. It found, for example, very strong absorption of red light over the continents due to the chlorophyll in photosynthesizing plants along with narrow band radio transmissions that could only come from advanced life.
Finally, when its plutonium powered generator could no longer supply sufficient power, Galileo was intentionally commanded to crash into Jupiter to eliminate any chance of a future impact with Europa that could contaminate the icy moon. After 14 years in space and 8 years surveying Jupiter and its moons, it finally dived into the Jovian atmosphere at a speed of ~48 km per hour on September 21st 2003 – the end of one of NASA’s most successful missions!
What do we now know about Jupiter and its Moons?
With Saturn, Uranus and Neptune, Jupiter is one of the Gas giants of the Solar System and its mass is exceeds that of all the other planets combined by two and a half times. Its interior mass is primarily made up of hydrogen (~71%) helium (24%) with ~5% of heavier elements. Its composition thus closely follows that of the solar nebula from which it was formed. Interestingly, if Jupiter were to acquire more mass, its diameter would actually decrease, so it is about as large as a planet of its composition could be.
Jupiter is thought to consist of a dense core surrounded by a layer of liquid metallic hydrogen lying under an outer layer, about 1000 km thick, composed very largely of molecular hydrogen. Jupiter is perpetually covered with a cloud layer about 50 km thick. The clouds are composed of ammonia crystals arranged into bands of different latitudes made up of light coloured zones between darker belts. The orange and brown colours in the Jovian clouds are caused by compounds containing phosphorus and sulphur exposed to ultraviolet light from the Sun. At differing latitudes, the darker clouds so formed deeper within the atmosphere are masked out by higher clouds of crystallizing ammonia producing the pale zones seen between the belts.
The Great Red Spot
Wind speeds of up to 100 m/s are common in the atmosphere and opposing circulation patterns caused, in part, by Jupiter’s rapid rotation rate cause storms and turbulence in the atmosphere. The belts and zones are seen to vary in colour and form from year to year, but the general pattern remains stable. The best known feature in the atmosphere is undoubtedly the Great Red Spot. It is a persistent anticylonic storm, more than twice the diameter of the Earth, that has been observed since at least 1831. It rotates in an anticlockwise direction with a rotation period of about 6 days and is thought to be stable and so has become a permanent, or at least a very long term feature of the Jovian atmosphere. It is not however fixed in position, and though staying at latitude 22 degree south has moved around the planet several times since it was first observed. Similar, but smaller, features are common; with “white ovals” of cool clouds in the upper atmosphere and warmer brown ovals lower down. These smaller storms can sometimes merge to form larger features, as happened in 2000 when three white ovals, first observed in 1938, combined into one. In the following years its colour has reddened and it has been nicknamed Red Spot Junior.
The Rings of Jupiter.
Jupiter has a very faint planetary ring system composed of three main segments: an inner halo, a brighter main ring, and an outer "gossamer" ring having two distinct components. They appear to be made of dust with the main ring probably made of material ejected from the satellites Adrastea and Metis as a result of meteorite impact. Jupiter’s strong gravitational pull prevents the material falling back onto their surfaces and they gradually move towards Jupiter. It is thought that the two gossamer rings are produced in similar fashion by the moons Thebe and Amalthea.
Jupiter’s Galilean Moons

Jupiter's Moons: Io, Callisto and Ganymede [Europa is in a later image]
Even a very small telescope can detect the four major moons of Jupiter as they weave their way around it. In order of distance from Jupiter, they are called Io, Europa, Ganymeade and Callisto and are comparable in size to our Moon. Discovered by Galileo in 1610, they showed him that Solar System objects did not all have to orbit the Sun, giving further evidence for the Copernican model of the Solar System.
Observations in 1676 made by the Danish astronomer, Christensen Roemar, of the times of their eclipses as they passed behind Jupiter led to the first determination of the speed of light. An eclipse of Io occurs every 42.5 hours - the period of its orbit - and it thus provides a form of cosmic clock. However, Roemar observed that the 40 orbits of Io during the time that the Earth was moving towards Jupiter took a total of 22 minutes less than when the Earth was moving away from Jupiter ~6 months later. The change in apparent period is due to the Doppler effect and this enabled him to calculate the ratio of the velocity of light to the orbital speed of the Earth around the Sun. He derived a value for this ratio of ~9,300. As the orbital speed of the Earth is ~30 km/s this gave a value (actually calculated by Christiaan Huygens from Roemar's observations) for the speed of light of ~279,000 km/s.
The two innermost Moons, Io and Europa are of great interest. Io is the fourth largest moon in the Solar System with a diameter of 3642 km. When high resolution images of Io were received on Earth from the Voyager spacecraft in 1979, astronomers were amazed to find that Io was pockmarked with over 400 volcanoes. It was soon realised that giant tidal forces due to the close proximity of Jupiter would pummel the interior, generating heat and so give Io a molten interior. As a result, in contrast with most of the other moons in the outer Solar System which have an icy surface, Io has a rocky silicate crust overlying a molten iron or iron sulphide core. A large part of Io's surface is formed of planes covered by red and orange sulphur compounds and brilliant white sulphur dioxide frost. Above the planes, are seen over 100 mountains, some higher than Mt Everest - a strange world indeed.
In contrast, Europa, the sixth largest moon in the Solar System with a diameter of just over 3000 km, has an icy crust above an interior of silicate rock overlying a probable iron core. The icy surface is one of the smoothest in the Solar System. Close up images show breaks in the ice as though parts of the surface are breaking apart and then being filled with fresh ice. This implies that the crust is floating above a liquid ocean, warmed by the tidal heating from its proximity with Jupiter. This could thus conceivably be an abode for life and some ambitious proposals have been made for a space craft to land and burrow beneath the ice to investigate whether any life forms are present!

The surface of Europa showing cracks caused by tidal flexure and “icebergs”.
The Cassini mission to Saturn
The Cassini–Huygens spacecraft is a joint NASA/ESA/ASI mission, launched in 1997, which continues to study Saturn and its satellites. It was composed of two main elements: the NASA Cassini orbiter and the ESA-developed Huygens probe which was to descend to the surface of Saturn’s largest satellite, Titan. Cassini-Huygens entered into orbit around Saturn on July 1, 2004 and, on December 25 of that year projected the Huygens probe towards Titan which it reached on January 14, 2005. Huygens made a descent through Titan's atmosphere to the surface making the first landing ever accomplished in the outer solar system.

En route to Saturn, Cassini made a close approach to Jupiter produced the most detailed global colour portrait of Jupiter yet – showing features just 64 km across.

Cassini’s image of Jupiter and artist’s impression of its arrival at Saturn.
Cassini’s observations of the light scattering by particles in Jupiter’s rings showed the particles were irregularly shaped and, most likely, result from ejecta released by micrometeorite impacts on the Jovian moons Metis and Adrastea.
Tests of Einstein's Theory of General Relativity
On its way to Jupiter, Cassini passed behind the Sun, so giving a way of testing Einstein's Theory of General Relativity. Due to the curvature of space cause by the Sun, the radio signals that reached us from Cassini have to travel along a longer path than if the Sun were not present. This causes a delay in their arrival time of about 200 microseconds (called the Shapiro delay) and which agreed with Einstein’s theory to an accuracy of about one part in 50,000 - one of the very best tests of his theory made to date.
Arriving at Saturn
After travelling for seven years, on July 1, 2004, the spacecraft flew through the gap between the F and G rings and, having passed within 33,600 km of Saturn’s cloud tops, went into orbit. Just a day later, it had its first (though distant) flyby of Saturn’s largest moon, Titan. Images showed methane clouds above the south pole and many surface features. Radar studies of Titan made in October 2004 showed a relatively smooth surface having a height range of just 54 metres. Not surprisingly perhaps, Cassini has discovered several more moons orbiting Saturn – given names such as Methone, Pallene, Polydeuces, Daphnis and Aegaeon. On June 11, 2004, Cassini flew by the moon Phoebe and the, very bright, close-up images indicated that a large amount of water ice exists under its immediate surface.
Cassini’s primary mission ended on July 30, 2008, but given the excellent condition of the orbiter, the mission was extended to the end of June 2010 and then in February 2010 was extended again until 2017 - the time of summer solstice in Saturn's Northern Hemisphere.
What we now know about Saturn and its Moons.
Galileo first observed Saturn with his telescope in 1610 and became somewhat perplexed. He described the planet as having “ears” and composed of three bodies which almost touched each other with that at the centre about three times the size of the outer two whose orientation was fixed. Galileo became even more perplexed when, two years later the outer two bodies had gone. “Has Saturn swallowed his children?” he wondered. He became further confused when they reappeared in 1613. In 1655 Christiaan Huygens observed Saturn with a far superior telescope and suggested that Saturn was surrounded by a ring system. He wrote that “Saturn is surrounded by a thin, flat, ring, nowhere touching, inclined to the ecliptic".
As telescopes improved, more details could be seen and, in 1675, Giovanni Domenico Cassini observed that Saturn's ring system was composed of a number of smaller rings separated by gaps the largest of which has become known as “Cassini’s Division”. In the mid 1800’s, James Clerk Maxwell showed that a solid ring could not be stable and would break apart so that they must be made up of myriads of particles individually orbiting Saturn. This would imply that different annuli of the rings would be moving at different speeds around Saturn and this was proved when James Keeler of the Lick observatory made spectroscopic studies of the ring system in 1895.
There is no doubt that, due to its ring system, Saturn is the most beautiful objects in the Solar System that can be observed with a small telescope. The key to understanding Galileo’s confusion lies in Huygens’s description that the ring system was inclined to the ecliptic due to the Saturn’s axial tilt. Assume that Saturn’s North Pole was, at some point in its orbit tilted closest to the Sun. Close to the Sun, we would see much of the northern hemisphere and the rings at their most open. Just under 15 years later, Saturn will be on the opposite side of its orbit and the north pole would be tilted away from the Sun. We would then see the southern hemisphere best and the rings would also be wide open. Half way in between these extremes we see the rings edge-on and, just as Galileo observed, they effectively disappear. So the Earth will lie in the ring plane twice every orbit, about once every15 years.

It is not surprising that the rings effectively disappear as it is thought that they are less than 1 km in thickness! The ring particles range in size from dust particles up to boulders a few metres in size and are largely composed of water ice (~93%) along with amorphous carbon (~ 7%). Three rings can be observed from Earth that extend from 6,630 km to 120,700 km above Saturn's equator. The outer ring, A ring, has a significant gap within it called Enkes Division whilst Cassini’s Division separates the A from the middle B, or Bright Ring. Inside the B ring is the fainter C, or Crepe Ring. Two further rings have been discovered more recently; within the C ring there is a very faint D Ring, whilst outside the A Ring is a very thin F Ring.

The rings are thought to have been formed when a moon either came within the Roche Limit of the planet where tidal forces broke it apart, or was impacted by a large comet or asteroid to give the same result. (As a small body nears a massive one, the gravitational force on the nearer side of the body exceeds that on the far side. There is thus a differential force across the body which tends to pull it apart. The Roche Limit is the distance from a planet at which this force would break up a typical small body.)
The structure we see within the rings is due to the cumulative effect of the gravitational pull of Saturn’s many moons. Where a Moon has a period which is a simple multiple to that of particles at a certain (nearer) distance to the centre of Saturn, a “resonance” occurs which clears particles from that part of the ring system. In this way, the moon Mimas clears particles from the Cassini Division.

Saturn observed by the Cassini spacecraft as Saturn eclipsed the Sun. The far side of Saturn from the Sun is partially lit by light reflected from the rings.
Titan is the largest moon of Saturn and the only the moon in the Solar System known to have a dense atmosphere. It is also only object other than Earth for which there is evidence of surface liquids, in the form of hydrocarbon lakes, in the satellite’s polar regions. It is about 50% larger than our Moon and 80% more massive, and is second only to Jupiter’s moon Ganymede in size and larger (but not as massive) as Mercury. Like our Moon, it is tidally locked and always presents the same face to Saturn. Titan has a relatively smooth crust composed of water ice which overlays a rocky interior. The atmosphere is quite dense, largely made up of nitrogen (~98%) giving a surface pressure of more than one and a half times that of the Earth. Within the atmosphere are clouds of methane and ethane and an orange haze made up of organic molecules that result from the break up of methane in the atmosphere by ultraviolet light from the Sun. The source of this methane is somewhat of a mystery, as the Sun’s ultraviolet light should eliminate methane from the atmosphere in about 50 million years. It is not impossible that it has a biological origin!
Observations, first from the Hubble Space Telescope and then from the Cassini spacecraft (in infra-red light to observe through the haze) show that Titan's surface is marked by broad swaths of bright and dark terrain. The largest feature is Xanadu, about the size of Australia.

The Huygens Probe
On December 25, 2004 the Huygens Probe separated from the Cassini Orbiter that had carried it to Saturn and, having been lowered through its atmosphere by parachute, landed on the surface of Titan on January 14, 2005. Images of the surface taken from a height of ~16 km showed what are considered to be drainage channels in light coloured higher ground leading down to the shoreline of a darker sea or plain. Some of the photos even seemed to suggest islands and a mist shrouded coastline. There was no evidence of any liquids at the time of landing, but strong evidence of its presence in the recent past.

A Cassini view of the surface of Titan and an image of the Huygen's probe on the surface.
As the spacecraft landed, a penetrometer studied its de-acceleration. It was initially thought that the surface had a hard crust overlaying a sticky material. One scientist compared the colour and texture of the surface to that of a Crème Brûlée. Another, with stepping on a cowpat! However, it may have been that the craft landed on, and then displaced, a pebble on the surface giving the effect of a surface crust and the surface may, in fact, consist of “sand” made up of ice grains forming a flat plain covered with pebbles made of water ice.
Lakes on Titan
Data from Voyager 1 and 2 had showed that Titan had a thick atmosphere of approximately the correct temperature and composition to support lakes of liquid hydrocarbons (ethane or methane) on the surface. During a Titan flyby on July 22, 2006, the Cassini spacecraft's radar imaged the northern latitudes and a number of large, smooth regions were seen dotting the surface near the pole. The Cassini–Huygens team concluded that the imaged features are almost certainly hydrocarbon lakes some of which lie in depressions and appeared to have channels leading into them. This was confirmed when, on July 8, 2009, a specular reflection in infrared was seen off the southern shoreline of a lake called Kraken Mare. The Cassini data indicates that Titan hosts within its polar lakes "hundreds of times more natural gas and other liquid hydrocarbons than all the known oil and natural gas reserves on Earth". The desert sand dunes along the equator, while devoid of open liquid, nonetheless hold more organics than all of Earth's coal reserves!

Radar Image of a lake of Ethane or Methane near Titan’s pole and a reflection or “glint” from one of the lakes.
Enceladus, discovered in 1789 by William Herschel, is Saturn’s sixth-largest moon. The Voyager spacecraft had shown that Enceladus’s icy surface reflects almost all of the sunlight that strikes it. Voyager 2 revealed that despite the moon's small size, just 496 km in diameter, it had a wide range of terrains ranging from old, heavily cratered surfaces to regions as young as 100 million years old.
In 2005, as the Cassini spacecraft performed several close flybys of Enceladus, it discovered a water-rich plume venting from the moon's south polar regions. This discovery, along with the fact that there are very few, if any, impact craters in this region, showed that Enceladus is geologically active. Rather as the reason that Jupiter’s moon, Io, shows volcanism, it is thought that its proximity to Saturn results in tidal heating of the satellite's interior. Analysis of the venting gas suggests that it originates from a body of sub-surface liquid water, which along with the interesting chemistry found in the gas plume, has fuelled speculation that Enceladus might even be able to support simple life forms.

Water Vapour Plumes venting from Enceladus and a model that might explain their presence.
The New Horizons mission to Pluto and its Moons - Charon, Nix and Hydra
Pluto was discovered by Clyde Tombaugh from a pair of plates taken in January 1930. Its name was suggested by Venetia Burney, the 11 year old daughter of an Oxford Professor, when she was told of its discovery the next day. Pluto was the Roman God of the underworld who was able to make himself invisible. As the first two letters of its name were the initials of Percival Lowell, at whose observatory it had been discovered, this suggestion was eagerly accepted.
In the 1978, James Christy, working at the Naval Observatory in Washington, discovered a satellite of Pluto, now called Charon, and in 2005, the Hubble Space Telescope, discovered two further moons, now called Nix and Hydra. Observations of Charon enabled Pluto’s mass to be determined - which was far less than originally thought and there is no doubt that had it been discovered recently, it would never have been afforded the status of a planet, but is has since become part of our culture and the author was somewhat saddened when it was demoted in 2006.

Pluto with its three moons, Charon discovered in 1978 and Nix and Hydra discovered in 2005 and an infra red image of Jupiter and Io made by the New Horizons spacecraft as it passed Jupiter on its way to Pluto.
New Horizons was originally planned as a voyage to what was then the only unexplored planet in the Solar System as, when the spacecraft was launched, Pluto was still classified as a planet. It was launched on January 18th 2006 for a 9 year voyage to Pluto and beyond. Having flown over 4.8 billion km, it will flyby Pluto and Charon around July 2015 and then hopefully visit some Kuiper Belt objects in an extended mission. New Horizons passed Jupiter on February 28th, 2007 using the planet's gravitational pull to increase its speed to ~ 83,000 km/hr in what is called a “slingshot” or “gravitational assist” manoeuvre and, in passing, made infrared images of Jupiter and its moons. In addition to the scientific equipment, several cultural artefacts are carried by the spacecraft. These include an American flag, a Florida state quarter and some of Clyde Tombaugh’s ashes – a nice thought!
©Professor Ian Morison, Gresham College 2010

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