Astronauts in most dangerous test yet

ASTRONAUTS on board the Atlantis space shuttle today attempt the most novel and potentially the most dangerous of all the shuttle's experiments to date.

This evening, the astronauts are due slowly to unwind a satellite on the end of a 12-mile tether, like a kite on the end of a string. This is a unique engineering experiment - nobody knows how the tether will behave, although its behaviour has been predicted on computer models. A Nasa spokesman yesterday described the experiment as 'probably the most fraught with unknowns than any we've done before'.

The aim of the 30-hour experiment is to generate an electric current along the tether, measure its characteristics, then pull the satellite in again for a return to Earth. Electrons sweeping across the tether, made of conductive material, are expected to generate 5,000 volts.

The most dangerous time will come later this week, when the 1,1001b satellite, called Tethered Satellite System, TSS-1, approaches the shuttle as it is winched back in. The satellite could crash into the spacecraft or wind itself around it on the end of the tether.

Nasa scientists want to know how much power they can generate via the tether. On this trip, the current will be jettisoned back into the ionosphere. But in the future it may be possible to use the power to help keep satellites in orbit, or slow them down as they re- enter the Earth's atmosphere.

TSS-1, an Italian-built satellite, is being deployed a day late because of problems with a second satellite that travelled with it into space on this latest shuttle. This is a dollars 426m ( pounds 220m) European satellite, called Eureca (the European Retrievable Carrier), a re- usable satellite with some 50 biology and physics experiments on board, designed to exploit the lack of gravity in space.

Scientists at the European Space Agency said Eureca was stable yesterday morning, and they were still trying to work out why it initially wobbled into too low an orbit. They hoped to shift it into its correct position early this morning, where it will stay until it is retrieved by another shuttle next spring.

Nasa said yesterday that it did not expect to extend the mission, despite the delay, and that it should be able to complete all the top priority experiments planned for TSS-1 in time for the shuttle to land at Kennedy Space Center as planned on Friday. But the space agency will see how the tether is behaving and how much fuel the shuttle has left tomorrow before it decides whether an extension beyond Friday is necessary.