Jupiter mission reaches most dangerous stage

Galileo space probe: Nasa scientists wait nervously for news of $1.6bn flight to explore the solar system's largest planet

Scientists around the world will wait anxiously tonight for confirmation that the $1.6bn Galileo space mission to Jupiter has successfully completed the most perilous stage of its six-year, 2.3 billion-mile (3.7 billion-kilometre) journey.

The main space probe must first monitor the final, fiery moments of a "daughter" craft which will plunge deep into the Jovian atmosphere. Then, in a quick planetary ballet, it must manoeuvre itself into orbit around our solar system's largest planet.

As the instrument-laden spaceship nears Jupiter's system, it is expected to absorb a dose of radiation 35 to 40 times greater than would kill a human."We have everything crossed we can cross - fingers and toes and all like that. It's certainly a nervous time," said William O'Neil, Galileo project manager at the National Aeronautics and Space Administration's Jet Propulsion Laboratory in Pasadena.

But the scientists at JPL, which manages the Galileo mission, and at Nasa's Ames Research Center in Mountain View, which built the probe, will have to be satisfied with a delayed confirmation that the probe is sending signals and that the orbiter is on course. Radio signals take 52 minutes to travel from Jupiter to Earth.

No pictures or atmospheric data will be available until after information captured on Galileo's tape recorder and its computer is relayed to Earth through Deep Space Network stations in California, Spain and Australia. A first look will not be available until late this month.

Tonight, the cone-shaped atmospheric probe which was released from the orbiter in July should slam into Jupiter's dense, gaseous atmosphere at 106,000mph. If it enters as planned, it will slow down and parachute through ammonia clouds to a realm never before explored. It will tell the mother ship what it sees inside Jupiter's layers of lightning-pierced clouds and may even detect water rain.

But if the scientists do not get the 746lb (338kg) probe into Jupiter's atmosphere at exactly the right angle, it will either bounce into oblivion or burn up too quickly. The plan is that the atmospheric probe will eventually melt and vaporise, but it should transmit up to 75 minutes of data about chemical composition, radiation, temperature, wind, and atmospheric pressure before contact is cut off. The signals transmitted to the 2.5-ton main orbiter for eventual retransmission to ground control will give scientists their first view of the inner reaches of Jupiter, which is 316 times larger than Earth.

After monitoring its daughter craft's fate, the main orbiter should fire its engines for 49 minutes to enter a two-year, looping orbit around Jupiter and eight of its 16 moons.

Those moons include Io, which is highly volcanic, and Ganymede, the largest moon in the solar system.

Planning for the mission started nearly 20 years ago, in 1978. More than $300m had been spent by 1981 on a project that was supposed to be launched in 1984 and to reach Jupiter by July 1987. But the whole mission was delayed and then derailed following the Challenger space shuttle accident of January 1986.