Galileo boldly goes to the limit of science

TOM WILKIE

Science Editor

For six years, the Galileo space mission to Jupiter has been alone in the void of space. Now, at the end of its 2.3 billion-mile odyssey, it has company - in the form of the solar system's largest planet, its faint rings, and its 16 known moons.

The space probe's extraordinary travels involved interplanetary snooker, with Galileo looping round Venus once, then heading back to Earth twice to gain enough gravitational energy to carry it across the 360 million miles between the Earth's orbit and that of Jupiter. It has twice come close to asteroids, the tiny planetoids which tend to lie between the orbits of Mars and Jupiter.

But although it is the first man-made probe to examine Jupiter in detail, Galileo is not the first to fly past the planet. The Voyager and Pioneer missions have been to the outer planets before. And, bizarrely, a mission to explore the Sun included a detour to Jupiter.

While scientists await the data from Galileo's six-year-long wanderings - much of it concerned with the Jovian "weather" - attention has switched, within the past month, from the largest planet to the weather on the largest object in the solar system, the Sun itself. The craft sent to probe the Sun have had, like Galileo, to travel bizarre and wonderful paths to reach their destinations.

On 2 December, the Solar and Heliospheric Observatory (Soho) took off aboard an Atlas rocket from the Kennedy Space Centre. After a four month journey covering 1.5 million kilometres, this European Space Agency satellite will take up a unique orbit - not around a planet or the Sun directly - but around the "Langrangian point" the spot where the gravitational pull of the Earth and that of the Sun balance out each other. From this vantage point, it will look at the restless solar surface and listen for internal "sun-quakes".

Soho will be followed in May 1996 by Cluster, a quartet of little satellites which, in orbit around the Earth, will also be examining the effects of the Sun as the solar wind - the continuous stream of ionised gas emitted by the Sun - hits the Earth's magnetic field.

Perhaps the strangest path of all is that taken by the ESA probe, known as Ulysses, which was launched from the shuttle Discovery on 6 October 1990. In order to look at the Sun, it was sent away from the Sun, towards Jupiter. Ulysses sped out to a rendezvous with the planet two years later and used the planet's gravity to shoot up out of the flat disc in which all planets orbit the Sun.

But the Voyager and Pioneer spacecraft have had the longest journeys and are now the most distant man-made objects in the universe. Launched on 2 March 1972, Pioneer 10 was the first of the Jupiter missions. Pioneer 11 followed a year later. The two Voyagers were sent up within a couple of days of each other in 1977. By the late 1970s, the outermost planets of the solar system were lined up in a curve, so that spacecraft could visit them one after another. Voyager 1 passed Jupiter in March 1979 and Saturn in 1980. Voyager 2 visited Jupiter in July 1979, Saturn in 1981, Uranus in 1986, and Neptune in 1989.

They are all now leaving the bounds of the solar system and passing from interplanetary to interstellar space. They may yet yield one final piece of scientific information. Some scientists believe there might be a 10th planet beyond the orbit of Pluto. It may reveal its gravitational pull by deflecting the trajectories of these distant craft from what is expected. But after that, nothing.