SPACE shuttle Atlantis is ready to head for orbit July 31 to inaugurate two new phases of space research.
First, the crew is to deploy the European Retrievable Carrier (Eureca) satellite. This is a new type of reusable laboratory for microgravity research. There are none of the jiggles and jolts of astronaut activity that disturb sensitive experiments in the shuttle's Spacelab laboratory.
But there also is no one on board to adjust equipment. But scientists still can control their instruments and experiments via radio link until a shuttle retrieves the orbiting laboratory next spring.
Once Eureca is on its way, astronauts plan to deploy an Italian-made satellite 20 kilometers (12 miles) above Atlantis on the end of a pencil-thin American-supplied tether. No one knows exactly how this will work in spite of two decades of study. The satellite may move in unexpected ways. Charles Shaw, lead flight director for the mission, has said he expects this Tethered Satellite System (TSS) test will keep astronauts and flight controllers "thinking on our feet." Narrow flight window
The launch window for this pioneering mission opens at 9:56 a.m. EDT July 31. Atlantis is to land at the Kennedy Space Center in Florida six days, 22 hours, and 9 minutes after launch. The crew will spend much of its time trying out the Italian/American TSS.
Tethering spacecraft together is an old concept. The present system has evolved from a 1972 proposal by Mario Grossi of the Smithsonian Astrophysical Observatory at Harvard University.
The late Giuseppe Colombo of the University of Padua, Italy, then made extensive studies of tethered satellite possibilities. Eventually, the Italian Space Agency and the US National Aeronautics and Space Administration formed a joint project to test the concept.
Mission scientist Nobie Stone at NASA's Marshall Space Flight Center in Huntsville, Ala., notes that one of the most intriguing possibilities is that of generating electric power from Earth's magnetic field. That's what this mission will test.
The electrically conducting tether will cut across Earth's magnetic field like the windings in a power station generator. It should convert a small part of the shuttle's orbital energy into electrical energy. Dr. Stone says that, if this proves practical, NASA might be able to "use it as a means of power generation for something like the space station."
He said that studies indicate that such a system could generate something like 75 kilowatts of power. However, he added that it is uncertain how much current the tether would conduct. This is one of many factors the mission will explore.
Tethered satellites also can explore regions not reached by the shuttle. The Italian satellite has instruments to study magnetic fields and charged particles. Such a satellite can also probe atmospheric regions below the shuttle that are too high for instrumented balloons but too low for free-flying satellites. One plan studied for a future mission would deploy a satellite down from the shuttle to about 130 kilometers (80 miles) above ground.
The tether system makes subtle use of orbital dynamics. A satellite travels at a speed that produces an upward centrifugal force that just balances gravity's downward pull at the satellite's height. Actually, it is the center of gravity of the satellite system that moves with this correct orbital speed.
As the 518-kilogram (1,140-pound) TSS satellite and the shuttle separate, the satellite moves above that center of gravity while the shuttle drops a little below it. However, since shuttle and satellite are tied together as a single system, they continue to move with the speed of their common center of gravity.
Now the TSS satellite is moving too fast for its orbital height. The upward centrifugal force is stronger than gravity and the satellite tends to rise. The shuttle, which is not moving quite fast enough to balance gravity, tends to fall. This keeps tension on the tether. As long as more tether is reeled out, satellite and shuttle will continue to separate. Largest satellite yet
While Italian payload specialist Franco Malerba and fellow astronauts test the TSS, the European Space Agency (ESA) will take control of Eureca. ESA mission specialist, Swiss astronaut Claude Nicollier, is to deploy Eureca at an altitude of 425 kilometers (264 miles).
ESA controllers will then use Eureca's thrusters to move it to its working orbit 525 kilometers (326 miles) high. Next spring, they will bring the satellite back down to 315 kilometers (196 miles) height where a shuttle will retrieve it.
The 4.5-ton, $400 million Eureca is the largest ESA-built satellite yet. The agency designed it to carry forward microgravity research begun on shorter flights in the shuttle-carried Spacelab. After refurbishment, Eureca may fly again in 1995.