See Jupiter and die

Nasa's Jupiter probe will melt, be crushed, or both, as it sinks through the clouds. Still, says Peter Bond, what a dramatic way to go
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More than 400 million miles from Earth, America's Galileo spacecraft will today sever the cable linking it to an atmospheric probe. Over the next two days, Galileo will turn to point the probe in the right direction, spin up to 10.5 revolutions a minute, and then release its small companion. If all goes well, three explosive bolts will fire and springs will gently push the probe on its way.

The probe will be setting off on a journey to destruction. Its mission: to send back the first on-the-spot measurements of Jupiter's cloud tops.

These multi-coloured clouds have been a source of fascination for generations of astronomers. Yet, despite hundreds of years of telescopic observations and close-up pictures taken by the Voyager spacecraft, their structure and composition are subjects of theory more than knowledge. The scientists behind Nasa's Jupiter probe hope this mission will fill some of the gaps.

The probe and its mother craft - which will go into orbit around Jupiter in December - have travelled more than 3.68 billion km since their launch in October 1989. They are now approaching the final leg of their interplanetary odyssey at around 28,000kmph.

For the next five months, as the probe coasts towards its destination, a timer will tick away the seconds. Then, six hours before the probe enters Jupiter's atmosphere, it will activate the probe's systems. As Jupiter looms ever larger and the probe encounters the planet's belts of intense radiation, data will be sent back on lightning flashes, bursts of radio waves and charged particles.

The probe is only four feet wide and less than three feet high, and weighs about a third of a ton. But it has been designed to withstand tremendous aerodynamic forces - 350 times higher than the Earth's gravity - and temperatures of up to 15,000C caused by friction. Its outer shell is nearly spherical and consists of two heat shields. Without these, the probe would burn up and disintegrate long before it could send back any useful information.

The probe will arrive at Jupiter on 7 December. By the time it strikes the upper atmosphere a few degrees north of the equator, it will be travelling at around 184,000kmph and glowing white-hot. The angle of approach is critical, says Galileo project manager Bill O'Neil. "If it is too shallow, it will skip out into space; if it is too steep, the probe will burn up."

If the probe survives the intense radiation, sizzling heat and crushing deceleration, scientists expect to gather a host of new data. For 75 minutes, instruments will measure the amount of helium and other substances in Jupiter's cloud layers, monitor the increase in atmospheric pressure, determine the size, number and nature of cloud particles, and record the light and heat from the planet and the Sun at various heights. The information will be stored on Galileo's tape recorder and central computer before being relayed to Earth in mid-December.

Nasa scientists expect the probe will first encounter a haze of white ammonia crystals whipped up by 320kmph winds. As the temperature rises from minus 100C to zero, the ammonia should be replaced by clouds of water ice and then liquid water. The probe may be struck by lightning, drenched by heavy rain and buffeted by hurricane-force winds.

About half an hour after the probe enters Jupiter's atmosphere, it will cross the terminator into night. It should by then be clear of the 80km- thick cloud deck and passing through a warm environment rich in hydrogen and helium, with air pressure seven times higher than on Earth. From then onwards, conditions will become increasingly hostile. No one knows how long the probe will last before it is crushed and vapourised deep in the giant planet's atmosphere.

Meanwhile, 210,000km above, the main craft will be braking into orbit to become Jupiter's first man-made satellite. Despite being half blind and deaf because its main antenna failed to open fully, it will spend the next two years swinging around Jupiter, surveying its swirling cloud decks, planet-sized inner satellites and dusty rings. It should be a fitting main act to follow the probe's magnificent curtain-raiser.