'Ulysses' plays billiards in space to see behind the Sun

A spacecraft named Ulysses will this week fulfil the Greek hero's task as described in Dante's Inferno: 'To venture the uncharted distances . . . of the uninhabited world behind the Sun.'

On Tuesday, Ulysses will pass over the South Pole of the Sun, a place where no man- made instrument has been before. The extraordinary enterprise, likened by one scientist to 'a celestial game of billiard balls', marks a milestone on a billion-kilometre voyage that has taken almost four years of travelling and 35 years of planning.

The mission is to explore the solar winds coming from the poles of our own star. Scientists hope the data collected will help them to understand - and perhaps predict - how the Sun causes the magnetic storms on Earth that can result in major electrical disturbance and even power black-outs.

Vast amounts of radiation and sub-atomic particles are spewed into space from the Sun's boiling surface. At temperatures of a million degrees, atoms from the Sun's corona are stripped of their electrons and their intense energies enable the charged particles to burst away from the star to form the solar wind travelling at about 400km/sec.

Although scientists are able to monitor and measure the solar wind coming from the Sun's equator, the poles have until now been off-limits because of the enormous difficulties of getting a spaceprobe into a solar orbit.

Because the Earth travels round the equator of the Sun at 30km/sec in an elliptical orbit, it is necessary to quit this orbit first before entering an orbit around the solar poles. Unfortunately, existing rockets on their own are too feeble to break out of this trajectory.

Ulysses managed it by being shot into space from a space shuttle at the fastest velocity ever achieved by a spacecraft. It then travelled away from the Sun towards Jupiter where it used the planet's gravity as a 'slingshot' to swing it into a trajectory that would take it over the solar poles.

'It's like a game of celestial billiard balls,' said Richard Marsden, the Ulysses project scientist at the European Space Agency's science centre in the Netherlands. 'We used the gravity of Jupiter to swing Ulysses into another trajectory at 90 degrees to the initial one.'

Ulysses's instruments will be used to compare the solar wind from the poles with the wind coming from other regions of the Sun. From its vantage point above the Sun's South Pole it will also monitor the more intense solar radiation given off from solar flares, which Dr Marsden likened to storms on the Sun's surface.

Radiation from solar flares can interfere with the Earth's own magnetic field, squeezing it like a lemon and generating the terrestial magnetic storms that can interfere with electrical appliances such as televisions and power cables. 'One of the aims is to be able to predict when these magnetic storms are likely to happen on Earth,' Dr Marsden said.

A better understanding of the solar cycles - such as sunspots - is also expected to help scientists studying the Earth's climate. When these cycles of sunspot activity stopped in the latter half of the 17th century, for instance, the Earth suffered a mini-Ice Age.

Perhaps the greatest achievement of Ulysses is being launched at all. Scientists first proposed such a mission in 1959, two years after the launch of the first Sputnik satellite. It suffered three major delays, from 1980 to 1986, when the shuttle Challenger exploded soon after lift-off. A launch date of 1986 was postponed and Ulysses finally got off the ground in February 1990.