Tim Furniss/LONDON
IF IT DOES NOT WORK THIS time, NASA will probably ditch the project. The Italian Tethered Satellite System (TSS), it is hoped, will be deployed successfully from the Space Shuttle Columbia/STS75 on 24 February, with the aim of reaching a distance of 20km at the end of its tether above the Orbiter.
Four of the seven-man crew of the STS75 were aboard the STS46/Atlantis mission which made the first, aborted, attempt to deploy the TSS four years ago. Commander Andy Allen (STS46 pilot), mission specialists (MS) Jeff Hoffman, Franklin-Chang Diaz and European Space Agency/NASA MS Claude Nicollier will be determined to succeed this time.
The 22 February lift-off of the Columbia/STS75 will be the 50th Shuttle launch since the Challenger accident in January 1986. After three days of the STS57 flight, the USA will pass 1,000 days of manned space flight experience. The flight will be the first to be managed from a new mission-control centre at the Johnson Space Center, Houston.
When the TSS was given its first space try-out in July-August 1992, the $140 million, Alenia-built, 520kg TSS 1 reached a maximum distance of only 256m. The tether twice snagged on a 6mm-diameter bolt in the base of the 12m-high deployment-mast mechanism built by the former Martin Marietta.
The STS46 crew eventually freed the tether by retracting and extending the deployment mast, but NASA and Italian Space Agency officials erred on the side of caution and cancelled the primary mission, which was to have seen the TSS 1 generating electrical power by the motion of the conducting tether through the Earth's magnetic field, 20km from the Orbiter.
The goal of the STS75 re-flight dubbed the TSS 1R, is to prove the theory of Italian professor Giuseppe Colombo, that each of two space craft, connected by a tether and constrained at the same angular velocity relative to the Earth, but 20km apart, experiences different centrifugal force and gravity, thereby creating tension in the tether.
Tethered systems could not only be used to generate electricity, but also to generate thrust, to move a satellite, or to raise a satellite's orbit by imparting motion from the tether to a spacecraft. A craft could also be trawled through the lower atmosphere, below the Shuttle, like a fishing net from a trawler, for use in studying the ionosphere for longer periods than is possible during brief forays of sounding rockets.
The TSS could also create artificial gravity aboard a spacecraft - achieved briefly by the Gemini 11 and 12 manned spacecraft in 1966 - or to use the tether-generated-current system to study the natural currents in the polar regions of the Earth's magnetosphere, which are responsible for auroras.
DELICATE DEPLOYMENT
The primary objective for the STS75 is to demonstrate the ability to control the tethered satellite in the near-zero gravity of orbit. The STS46 astronauts experienced predicted difficulties during the short TSS 1 mission, confirming that some of the pre-mission worries that oscillations, such as "skip-rope" and pendulum effects could end up wrapping the tether around the orbiter (see box). If there are any serious problems, the tether can be guillotined, or the whole system, including the deployment mast, can be cast off. The TSS 1R is equipped with 70kg of science instruments.
The delicate deployment of the 1.6m-diameter TSS 1R will begin with the rotary-action erection of the 12m-long tubular-truss structure on which the actual Tethered Satellite is mounted. With the Columbia initially at 180¡ pitch-up and moving backwards, the satellite will begin to creep out of the truss with the aid of small nitrogen-gas thrusters, to keep the slowly unreeling tether taut, as the Columbia gradually pitches down to 13¡. On hand with the TSS veterans on the flight deck will be rookie crew members, pilot Doc Horowitz and two Italians, the European Space Agency/NASA MS Maurizio Cheli and payload specialist Umberto Guidoni.
The reel is equipped with a level-wind mechanism for uniform winding, a brake assembly and a 600RPM drive motor. Eight thrusters on the satellite will allow it to rotate around the tether as well as fore and aft. As it unreels - at about 0.1m/s initially, and eventually, at 2m/s - the electrical current running through the tether will induce motion in the tether which may force it into a skip-rope oscillation, deflecting the tether by 100m at its peak point. This can be damped out with the Columbia's yawing manoeuvres.
If the TSS 1R is deployed a great distance, new territory will be encountered during the reel-in process, when the skip-rope may be converted into a pendulum motion. Particular tether lengths have resonant frequencies at which there is a worse risk. The most extreme, but unlikely, consequence would be the tether wrapping itself around the Orbiter. The Tethered Satellite's gyros and accelerometers should be able to damp out severe oscillations by commanding its on-board thrusters.
Source: Flight International
