The low-lying giant and the high-flying kite: America's Atlantis space shuttle yesterday launched the first of two innovatory European satellites. Peter Bond reports

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CO-OPERATION between the United States and Europe has reached a new high point this week, with the inclusion among the crew of seven astronauts on board the Atlantis shuttle's 49th flight of Franco Malerba, the first Italian in space, and the Swiss-born Claude Nicollier, representing the European Space Agency.

Neither does the European interest stop there. Tucked inside the shuttle cargo bay at take-off on Friday were two unusual satellites, scheduled for release during the seven-day mission. The initial part of the mission was dedicated to the release of the first of these: the 4.5-tonne Eureca scientific platform, the largest satellite ever built in Western Europe.

On Sunday morning, after a 24-hour delay due to communication problems, Nicollier used the shuttle's robot arm to deploy Eureca (a name derived from European Retrievable Carrier). Under the watchful eye of the European Space Operations Centre at Darmstadt in Germany, Eureca was to use its own propulsion system to raise its orbit by about 60 miles (100km). There it will remain until spring, when another shuttle will be sent up to retrieve it for scientific examination back on Earth.

Unlike most European and US investigations of microgravity, which tend to last only a matter of days, Eureca's 10-month period of isolation should give scientists the opportunity to gauge the real potential of conducting experiments in low Earth orbit, paving the way for more ambitious research on the international Freedom space station planned for the late Nineties.

The 15 research facilities crammed on to Eureca's platform are designed to operate automatically. Eighty per cent of the satellite's 1,000kg (2,200lb) complement of experiments is taken up with microgravity, notably the production of pure, uniform semiconductor and protein crystals. The other main area of research will be life sciences, involving studies into how exposure in space affects biological samples.

Eureca will also carry a wide variety of experiments outside the field of microgravity. These include instruments to detect gamma-ray and X-ray sources, to measure the impact of cosmic dust particles and to study solar radiation. There are also plans to test an experimental thruster system that uses ion propulsion, and to try out a new two-way transmission system for satellite telemetry by bouncing signals off the Olympus relay satellite.

Until now, all satellites have been released from their launch vehicle once they reach their operational altitude, then left free to go their own way. A few, such as Eureca, have been designed for retrieval by the shuttle, but information about the vast majority of satellites is only available via their on-board telemetry.

All this is about to change with the launch of the second satellite, which is due to be released on Tuesday evening. Instead of being jettisoned from the cargo bay like its predecessors, the first Tethered Satellite System (TSS-1) will be raised aloft like a kite at the end of a string, allowing it to sample the space environment, then lowered back by the Atlantis crew into its housing for a return to Earth.

This novel and potentially dangerous attempt to investigate orbital dynamics is a joint venture between Nasa and ASI, the US and Italian space agencies.

The Italian-built satellite is divided into two hemispheres. Included in the upper half are three experiments to measure electrodynamic and magnetic-field changes, an accelerometer and a current meter. Other experiments are mounted on a metre-long instrument boom and two extendable booms. There is also a short mast to carry the communications antenna. The lower section contains the power, data-handling, telemetry and navigational equipment.

Although some rather unsuccessful experiments with short tethers took place during the crewed Gemini missions of the mid-Sixties, most of our knowledge about the behaviour and possible uses of a line linking two orbiting objects has come from theoretical studies and computer simulations. No one really knows how such a novel system will behave.

Some scientists fear that the satellite might crash into Atlantis during the final stages of retrieval or even wind itself around the spacecraft. Daniel Goldin, newly appointed administrator of Nasa, was so concerned that he ordered an extra safety review a few weeks prior to launch.

Manipulating a satellite at the end of a tether 20km (12 miles) long is a unique engineering challenge. The tethered satellite operates in an electrically charged environment and is controlled by gravitational influences, so its 1.6m-diameter (5.1ft) sphere had to be covered in a special electrically conductive paint which also serves as a passive cooling system. A hurried respray was required a few weeks ago when engineers discovered that the original water-based paint had lost its conductive properties.

Deployment of TSS-1 will be a long and tedious process. The first stage involves unfolding a lattice structure 12m (38.4ft) long while the satellite is perched on top. By firing its thrusters, the satellite will gently rise from this small launch tower.

Further use of the thrusters will be required out to a distance of about 1,000m (900 yards). There will be little or no tension in the tether between the shuttle and the satellite until this separation has been attained, because their orbital speeds and the forces acting upon them will be almost identical.

As the crew reel out the line, they will carefully monitor the status of the tether and the satellite. Once the satellite passes 6km, it will be spun at 0.25rpm so that on- board sensors can record the system's response as it travels through the electrical and magnetic fields of the ionosphere.

Six hours into the experiment, the satellite will reach its fully deployed position 20km (12 miles) above the spacecraft. After a 10-hour session of tests on tether dynamics, the satellite will be set spinning again so that the deployable booms can measure the surrounding sea of charged particles.

The system's motion through Earth's magnetic field will generate up to 5,000 volts across the tether. Although only 2.5mm wide, the tether has five different layers. Near its core is an electrically conducting copper wire which will carry current from the satellite to Atlantis. Two electron accelerators will be able to shoot a 0.75-amp current coming down the tether back into the ionosphere, thus completing the circuit.

Taking advantage of their ability to vary this current and control the electrical potential of the satellite, scientists hope to gain a new insight into a whole range of processes, including the creation of the aurora, or Northern Lights.

Some tether movement will be created by the flow of current through the copper strip, but unexpected oscillations could occur at any time, so the crew will have to remain alert throughout the mission. The disturbance regarded by the mission team as most likely to occur is a 'skipping rope' type of movement, though a pendulum-like swing is also possible. The commander and pilot could be in continual demand as they try to damp down these irregular motions by manual manoeuvring of the shuttle. Most dangerous of all is the final period of satellite retrieval, when the line goes slack and a collision is a distinct possibility.

Italian researchers would dearly like to bring the dollars 160m ( pounds 83m) TSS back to Earth for a second mission a few years from now. However, Atlantis is equipped with two independent pyrotechnic systems to cut the line should danger threaten.

Plans already exist to lower a longer tether into the elusive regions of the upper atmosphere which are too high to be studied from aircraft or balloons, and too low to be probed by normal satellites. Looking even further ahead, tethers could stretch as far as 100km (60 miles) above or below an orbiting spacecraft. Long-term applications for such lines include power generation for an orbital space station, elevators for low-cost space transportation, and the creation of artificial gravity by rotating two satellites at either end of a tether.

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