All autumn, the night sky has been dominated by that greatest of the planets, Jupiter. It is so bright that as the evening begins shading off into night, Jupiter glows in solitary splendour for a while until the darkening sky reveals the bright stars. With Venus currently the Morning Star, Jupiter reigns unopposed as the brightest object in the night sky. Even Sirius, brightest of the stars, now setting ever closer to sunset as the season draws on, is dimmed by Jupiter, for all that the latter, being a planet, shines by reflected light only.
By the end of winter, Jupiter itself will be lying in the western sky at sunset and setting earlier each night. However, its great companion Saturn, now rising about four hours before Jupiter sets, will ensure that we are not alone with the stars. As the year runs on we are seeing the rings ever more obliquely as the ring-plane swings slowly towards us. Not until 1995 will we lose sight of them altogether, when we will pass through the ring-plane, and it will be edge on, invisibly thin.
Now out of sight, and almost out of mind, is the space probe Galileo speeding on its way to Jupiter at about 150,000 km/h after three years of looping around the inner solar system using the Sun, Earth, Venus and the Moon to redirect its trajectory and accelerate it. This long prologue to the main journey was necessary because, following the Challenger disaster, the liquid-fuelled Centaur upper stage, which was the proposed booster to send the vehicle on its way to Jupiter, was deemed too dangerous a cargo for the shuttle, and the mission was completely redesigned. Instead of relying on brute force to hurl Galileo at Jupiter, an elegant set of falls through the gravitational gradients of the inner solar system was used as a substitute for thrust. While greatly lengthening the time for the journey, these extra visits enabled astronomers to make a set of observations of Venus, Earth, the Moon and the asteroid Gaspra.
In February 1990, four months after being launched, the space craft swung close around Venus and photographed the striking 100 per cent cloud cover. In December, it made a close pass of the Earth and took a series of oblique images of Antarctica, which were reconstituted by computer processing to yield a remarkable picture of the great southern weather engine (below). Here we can see the great Antarctic depressions which produce the familiar succession of cyclonic storm fronts that scythe from south-west to north-east across New Zealand.
The result of this flyby of Earth was to sling Galileo out beyond our orbit towards the asteroid belt and its rendezvous with asteroid 951, Gaspra. The asteroids, the majority of which lie in a zone between the orbits Mars and Jupiter, have been the subject of much speculation since the first of them, Ceres, was discovered by Piazzi in 1801. What are they made of? Are they the congealed fragments of material that never coalesced to form a planet or are they the fragments of a planet which broke up?
At one point in the mission, it seemed that Galileo would be able to shed little light on these questions. After the craft's close pass of Earth, disaster struck, for the high-gain antenna failed to deploy on command. Looking like a gossamer umbrella, this directional antenna was to focus the satellite's transmissions towards Earth so that the signal would be strong enough to transmit the high-density data. Without it, the only other aerial was the omnidirectional low gain antenna which has but a fraction of the data capacity because of the weakness of the signal by the time it reaches the Earth. Facing disaster, the project scientists have reconfigured the transmission programmes and receiver schedules so that the low power antenna will be able to relay about 70 per cent of the intended data.
Until this brilliant patch-up was done, it was feared that fewer than 20 per cent of the observations would be recovered, and that virtually the whole of the visual data would be lost. But, by waiting for a year, by which time Galileo had fallen back to the Earth for its final acceleration and redirection towards Jupiter, the stored data and images of Gaspra were able to be transmitted at distances so short that even the low gain antenna could transmit at a reasonable speed, and we recovered not only coloured images of the asteroid but also views of the north pole of the Moon. In spite of the fact that dozens of space craft have visited the Moon, because of the energy requirements they have all orbited in the lunar equatorial plane. This has meant that they have had no better view of the polar regions than we have from Earth, and all this while we have lacked near vertical views of the area.
Now, with this job done, Galileo is on its way to its intended objective. However, there remains one further target of opportunity, the asteroid Ida, located deep within the asteroid belt, observations of which will be a valuable supplement to those already made of Gaspra Scheduled for August 8, 1993, this will be Galileo's last major activity until December 1995, when Jupiter is approached and the probe is released to drop on its parachute through the turbulent outer atmosphere. For perhaps one hour, this instrument package will swing beneath its parachute, transmitting data back to the orbiting Galileo. Then the instruments will fail under the assault of rising temperatures and pressures, and the main vehicle will be left to burn its engines and lift into the first of its many orbits around the planet.
Using the mass of one of the moons, these orbits will be altered from time to time so that not only is Jupiter itself kept under review, but also the three outer Galilean moons: Europa, Ganymede and Callisto. Io, the innermost, famous for its volcanoes of sulphur, lies within the Io torus, a doughnut-shaped space around Jupiter which contains energetic electrons and ions and which is the source of electromagnetic waves. This is no place for electronic equipment to linger, so Galileo will orbit beyond this hostile zone while it continues to feed us with information.
Unlike the Voyagers, which were mere fly-by-night visitors, for all that they yielded us some of the most dramatic astronomical pictures ever published, Galileo will orbit the planet until its systems fail. Barring accidents during this time, we will accumulate sufficient data to resolve many of our present questions about this planet and its system of 16 moons. However, going on past experience, we will also be presented with evidence which demands a whole new set of theories to explain it. Just as, in 1610, Galileo's discovery of the four largest moons of Jupiter demonstrated that, contra the wisdom of the ancients, the world was not the only centre of rotation in the universe, so the satellite bearing his name will doubtless throw a spanner in the works of one or more of our cherished theories.
For example, much additional evidence is needed before we are satisfied that we understand the nature of the three outer Galilean moons. Before the Voyager programme there was a consensus that these moons were all rocky bodies bearing, like our own, the evidence of impact cratering from early in their history.
The reality turned out to be quite unexpected: Europa, covered with a virtually smooth sheet of ice, Ganymede, whose dirty ice surface bears not only traces of early impacts but also a complicated system of furrows and faults suggestive of plate tectonics, and, at the outer edge, Callisto, covered completely by shallow impact craters but, unlike our moon, showing no signs of the great depressions or maria.
Jupiter itself shows us nothing but cloud tops—a vast system of cyclonic storms and circumplanetary jet streams. The Great Red Spot is itself no more than an anticyclone having a diameter three times that of the Earth which has been blowing for at least 300 years. Its colour, as that of the various zones which band the planet, is due to unknown contaminants of the gases roiling up from the interior.
Believed to be composed largely of hydrogen and helium, with other elements in much the same proportion as found in the Sun, the atmosphere of Jupiter nevertheless shows significant differences from that of the Sun. The very much lower temperature allows both ammonia and carbon compounds to exist, and the Sun's unfiltered ultraviolet radiation dissociates simple compounds such as methane, leaving free radicles which can react with other molecules or radicles to form more complex compounds.
Just what a prolonged inspection will reveal is unknown, but it will more than likely be surprising. Each year the number and complexity of molecules found in interstellar space is increased as the sophistication of our detectors and methods of analysis improves.
Only 40 years ago, the interstellar medium was thought to be limited almost entirely to the elements blown there by the terminal catastrophes of nova and supernova explosions. Quite unexpected were the host of carbon compounds, some exotic and some, such as formaldehyde, a terrestrial commonplace. Also, a new field has been opened up with the discovery of Fullerenes, colloquially known as "Bucky balls," highly stable spherical configurations of carbon atoms the extraterrestrial existence of which is yet to be proved, though not unlikely.
Galileo may lack the shallow appeal of heroes in space: the star warrior riding skywards on a pillar of flame to conquer the unknown—that most publicised demonstration of the right stuff. But, barring any further accidents, then that few hundred kilos of light alloys, complex computers and refined sensors will tell us tales as fascinating as any Nordic saga. Out there, in the realms of eternal ice, lie pages from the first days of the solar system and the accumulated wonders of five billion years of existence.