SCIENCE The American space probe Galileo is orbiting Jupiter. The information it is sending back to Earth may change scientific perceptions of the solar system's largest planet
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The American space-probe Galileo is now returning spectacular pictures of the giant planet Jupiter and its moons. This is the first chance that scientists have had of seeing Jupiter close up for 17 years. The two Voyager probes made fleeting fly-bys in 1979, but Galileo is there to stay. On 7 December last year it became the first artificial object to go into orbit around Jupiter; at first, a highly elliptical orbit but, over the coming months, one that will become more circular and bring it more frequently into the Jovian system, round the giant planet itself and past each of its main moons.

To say that Jupiter is no ordinary planet would be incorrect. The things we most often imagine when we hear the word "planet" are rocky bodies like our own Earth, or chilly Mars, or even steamy Venus. But these are merely cinders left close to the Sun after its formation. If there were such a thing as a typical planet it would be Jupiter, a great gas-bag made almost entirely of hydrogen and helium.

As the Sun formed out of a vast cloud of gas about 4.8 billion years ago, and began to shine, the building rubble left from its foundation started to blow away. Only the more heat resistant, rocky material was left in the inner solar system - material that eventually built up to form the terrestrial planets Mercury, Venus, Earth and Mars. The bulk of the solar left-overs collected into the gas giants, of which Jupiter is by far the largest. It is more massive than all the other planets put together, and, of the others, Saturn, Neptune and Uranus are all far bigger than little Earth. In fact, if Jupiter were just 50 times more massive still, its core would have become hot enough to begin nuclear reactions that would have made it shine like a second star in the solar system. As it is, Jupiter is still 318 times more massive than the Earth and the temperature at its core is probably about 35,000C. The nuclear and gravitational processes at work in the interior generate twice as much heat as the planet receives from the Sun.

But Jupiter is more important than just a failed star. It plays a crucial gravitational role in the solar system, stirring up the thousands of lumps of rock that make up the asteroid belt, deflecting comets in their orbits and sometimes sending them hurtling towards the Earth. It was in the 1930s that astronomers first spotted asteroids that cross the Earth's orbit and so pose a potential hazard to us. Now hundreds are known, millions suspected and many astronomers are warning that catastrophic impacts, releasing energy equivalent to maybe 1 million megaton nuclear explosions, may be comparatively frequent, occuring every 10,000 or 100,000 years rather than every few hundred million years as was once thought.

Every few months it seems that another big one is discovered, sometimes surprisingly close to Earth - occasionally passing closer than the Moon. The second biggest, an asteroid called Eros, is 22km across, and recent calculations made by French and Italian scientists suggest that it may crash into the Earth in one to two million years time. If that seems safely far in the future, asteroid spotters such as Duncan Steel of the Anglo- Australian observatory point out that Eros is just one of many. Although erosion has obscured the evidence on Earth, we only have to look at the pock-marked surface of the Moon to realise how widespread such bombardment must have been in the past.

Evidence is continuing to mount that periods of mass extinction in the history of our planet may have been caused, at least in part, by asteroid or cometary bombardment. The demise 65 million years ago of most of the dinosaur species that existed at that time is now famously attributed by many scientists to a massive impact in the Gulf of Mexico. Such large impacts would not only be bad news for any dinosaur or anybody directly underneath, they would throw so much water, dust and rock- vapour into the atmosphere that they could dramatically affect the climate. Jupiter may be directly or indirectly responsible for many such impacts. Even from afar it exerts a slight gravitational influence over millions of asteroids and comets, making their precise orbits hard to predict. Sometimes objects pass much closer, and then the giant planet can play one of two roles. Either it will fling the smaller object like a slingshot, perhaps in the direction of the Earth, or it will exert such a pull that the comet or asteroid is captured from its orbit around the Sun and prevented from coming our way.

In July 1994 astronomers witnessed a cosmic spectacle as at least 21 fragments of comet Shoemaker-Levy 9, previously torn apart by Jupiter's gravity, crashed into the planet itself Such collisions at least afford the inner planets, including Earth, some protection.

IN 1610 GALILEO turned the recently invented telescope upon Jupiter and discovered its four largest moons, now collectively known as the Galilean satellites. It is fitting that the first man-made satellite of Jupiter should also be called Galileo. But the spacecraft Galileo seemed for a long time as if it were under a curse. It was ready for launch in 1986 when the explosion of the space shuttle Challenger set back the US space programme by several years. And there were concerns about possible environmental dangers posed by Galileo's nuclear powered energy source in the event that it too should suffer a mishap. Nonetheless the launch finally went ahead on 18 October 1989.

Physical and safety limitations on the size of the rocket meant that it had to take a long, tortuous route to Jupiter, passing the Earth twice and Venus once, gaining speed by using the gravitational field of each to propel it like a slingshot. After passing Venus it was judged sufficiently clear of solar radiation to open the umbrella-like main antenna that was to provide its main means to communicate data back to Earth. But messages from sensors revealed that the antenna had not opened fully. Despite numerous attempts to nudge it loose with its own unfurling mechanism, and brief firings of Galileo's engines, it remained shut.

What engineers now believe happened is that during the journey from the shuttle launch site in Florida back to NASA's Jet Propulsion Laboratory in California after the Challenger accident, and during the return to the launch site a few years later, a protective coating had rubbed off where one of the antenna's umbrella spokes rested on the central shaft. In the cold vacuum of space following launch, the exposed areas of metal had cold-welded together. Just a sharp tap with a hammer might have freed it. As it was, mission scientists had to work out how to make do with the much smaller secondary antenna by increasing the sensitivity of the deep space radio telescope network on Earth.

As if the problem with the antenna was not enough, as Galileo approached Jupiter, the tape recorder that stores data before it is slowly beamed back to Earth appeared to jam. The mission scientists have now managed to overcome this by reprograming the computers controlling the tape-recorder to avoid the particular places where the tape was jamming. Because of the technical difficulties, some planned pictures and time-lapse cine- sequences of Jupiter may not now be possible, although project scientist Terrence Johnson believes 75-80 per cent of the mission can still be accomplished.

On December 7 1995, Galileo finally arrived in the Jovian system and within a few minutes passed Io, the innermost moon. Jupiter's gravitational field then acted to give a slingshot effect in reverse, reducing Galileo's speed sufficiently to place it in an elliptical orbit around the planet. At the same time, a probe released from the main craft five months earlier was now plummeting through Jupiter's cloud tops.

Richard Young of NASA's Ames Research Centre says that this was the hardest atmospheric entry ever attempted. Once it had left Galileo, there was no way to control the probe. Yet the angle at which it entered the Jovian atmosphere - 8.5 degrees from the horizontal - had to be so precise that a mere half a degree error in one direction would have caused it to bounce off into space, and half a degree in the other and it would have been destroyed in the atmosphere in seconds. Travelling at 106,000 miles per hour, the temperature in front of the protective nose-cone reached 15,500C, and the probe experienced a deceleration 230 times the force of gravity on Earth.

As it began to feel the effects of the atmosphere, automatic sensors were meant to release a parachute but, due to another fault, there was a 53-second delay. Instruments intended to make measurements of the tops of the ammonia clouds that form the visible exterior of Jupiter were therefore activated too late. By the time they started to function, the probe was already at four times the atmospheric pressure on Earth. Nevertheless, data continued to stream up to the parent craft for over an hour, revealing many of Jupiter's secrets.

Jupiter does not have a surface in the same way that the Earth does. Deep inside it there is probably a rocky core about the size of the Earth, but, as far as the probe was concerned, it was gas all the way down. There are several layers of cloud visible to telescopes on the Earth, a deck of ammonia crystals at the top, and below that, clouds of ammonium hydrosulphide. Then come clouds believed to be of water vapour, like clouds on Earth, and then there is a thick layer of fluid molecular hydrogen. Below that, the hydrogen behaves more like a liquid metal that conducts electricity and operates like a dynamo to produce Jupiter's powerful magnetic field.

The composition of Jupiter is believed to resemble that of the cloud of material from which the entire solar system formed in the remote past, and so should in many ways be similar to the composition of the upper layers of the Sun. Measurements by the probe show that the ratios of helium to hydrogen do seem to match, although the concentrations of other inert gases are more confusing.

One of the biggest surprises from the probe was that it measured a very low ratio of oxygen to hydrogen, indicating far less water than is present even in the outer layers of the Sun. The measurement may not have been typical though because, as luck would have it, the probe descended into what is known as a "5 micron hot-spot", that is to say an area that gives out infra-red radiation but is comparatively clear of clouds. It could represent a hole in the clouds, marking a downward draught of cold, dry gas. That is what the scientists hope. If they are wrong they may need to reformulate their theories about Jupiter.

Another discovery was that, unlike the Earth, wind speeds increase with depth into the atmosphere. The probe reached depths where gusts of 400mph seem commonplace. There appears to be less thunder and lightning than on the Earth but what there is seems more violent.

Just before the probe entered the atmosphere it passed through two intense radiation belts similar to the Van Allen belts surrounding the Earth. Without protection these could pose a hazard to future missions, but the probe passed through them so quickly that no damage was done. The atmospheric probe was vented so that the pressure of the Jovian atmosphere would not crush it. What finally destroyed it must have been the heat. Even the titanium components would have vapourised, so that the probe has now quite literally become part of the planet.

Last month the main Galileo craft completed its first orbit and came in close to the heart of the Jovian system for a second time. This time its route took it within 800km of the largest moon, Ganymede. Ganymede is in fact the largest moon in the solar system, almost as big as the planet Mercury and made of a mixture of rock and ice. Pictures radioed back from Galileo show that throughout its history, the pockmarked face of Ganymede has been extensively bombarded by comets and meteorites. Much to the scientists' surprise, it has also been torn and wrinkled by forces that must be similar to those which create mountains on Earth. Voyager had previously revealed a network of cracks across the surface, but Galileo showed that the spaces between them, instead of being smooth, were crisscrossed with a whole network of fault lines and furrows in what may be muddy ice. The pictures that were sent back are causing planetary scientists to revise their assumptions about this far-distant world. It is clear, for example, that Ganymede is very old and that it probably has a magnetic field. This means that it might even have a molten iron core like the Earth's.

Some of the latest pictures released are of the closest moon to Jupiter, Io. For two moons orbiting the same planet, Ganymede and Io could not be more different. Though the cracks on Ganymede suggest tectonic activity some time in the last few million years, other regions are so dark and covered in craters they must have been exposed to cosmic bombardment for billions of years.

Io by contrast looks like a mouldy orange. Its smooth orange surface is pock-marked by black, yellow and white blotches. The interior must be churned about and melted by the powerful tidal forces of nearby Jupiter, making Io 100 times more volcanically active than the Earth. A previous close-up view of Io had come in 1979 from the Voyager probes, and the latest pictures from Galileo show how much the moon has changed since then: new craters flooded with black lava; mountain ranges turned white with ash and great patches of fresh yellow sulphur spray. Voyager had seen a geyser of sulphur dioxide fountaining 300km above the surface, and there is evidence that Galileo may have flown through a volcanic plume 900km high. Over the coming months Galileo will be mounting a volcano watch in the hope of catching an eruption on Io taking place.

There may be even more exciting discoveries to be made from the moon Europa. Voyager had revealed it to be smoother than a billiard ball, with a surface that had clearly been melted and re-worked many times. The best of the Voyager pictures show it to be covered in cracks reminiscent of those on sea-ice on the Earth and that has led many people, including Professor Steve Squyres of Cornell University, to propose that there may be oceans of liquid water underneath the ice, perhaps kept liquid by tidal forces. Steve Squyres says, "The surface is extremely young: we don't know whether it formed a million years ago or a week ago Tuesday".

There is another intriguing possibility: that of life on Europa. There is no way that sunlight could penetrate the thick ice in sufficient quantity to sustain life in any form as we know it, but Europa's insides may be churned about sufficiently by tidal forces to make the moon volcanically active. Steve Squyres has descended deep in the Earth's oceans in the submersible Alvin to look at the life that has evolved around volcanic hydrothermal vents on the sea floor. There, whole communities of organisms have developed that depend not on sunlight but on sulphur compounds released by the volcanic springs. The bacteria that make use of the sulphur compounds serve as food for white worms, shrimps and sea anemones, none of which ever sees the light of day, In his two sequels to 2001: A Space Odyssey Arthur C Clarke portrays life developing in the icy oceans of Europa in rather the same way. It may yet turn out that fact is almost as strange as fiction.

! Martin Redfern is executive producer of the BBC World Service Science Unit.