Two United States astronauts are poised to become the world's first rocket-powered human satellites. Together with rendezvous maneuvers between their space shuttle, Challenger, and a 6.5-foot-in-diameter balloon, their excursions will begin the development of an important new astronautical skill. This is the ability to deliver astronauts to a given location in space and then enable them to move about and work as individuals unattached to their mother ship.
At this writing, the countdown was under way to launch the mission, dubbed 41 -B, at 8 a.m. Eastern daylight time today. On that schedule, mission commander Vance Brand and pilot Robert Gibson would perform the rendezvous operations Sunday and Monday. Then on Tuesday and again on Thursday, mission specialists Bruce McCandless and Robert L. Stewart would try out the maneuvering backpacks, aided by mission specialist Ronald McNair from inside the shuttle.
The immediate object of these exercises is to rehearse for mission 41-C next April, when astronauts will retrieve and repair the Solar Maximum Mission satellite.
The astronauts will have to adjust to the subtleties of maneuvering in an environment where, in moving from point to point, you generally can't go straight to the target. Challenger, the balloon, and each astronaut with an MMU (manned maneuvering unit) is an independent satellite. They move according to the restraints of orbital mechanics as well as in response to rocket thrust.
The major factor governing their motion is what physicists call the energy of their orbits. This energy is divided between the energy of a satellite's speed and the energy associated with its height above Earth, its gravitational energy.
The greater the height, the larger the gravitational energy. And since every orbit has a specific amount of energy, this means there is less energy represented by the satellite's speed. That is why satellites move more slowly as they raise the height of their orbits. Conversely, as orbital height is lowered, a satellite moves faster. It's like a comet speeding up as it approaches the sun.
Thus, in maneuvering toward another orbiting object, the astronauts have to remember that the lower satellite has the faster track.
Brand and Gibson will maneuver Challenger so it drifts about 140 miles from the balloon. The shuttle will be above the balloon's height as it does this. Then, when they move back toward their target, the shuttle pilots will put Challenger on a course that carries it below the balloon's orbit. Eventually, they will approach the balloon from below.
Likewise, when astronauts McCandless and Stewart move out from Challenger in excursions of 150 and 300 feet, they will move above or below a straight plane as they ''fire'' their MMU rockets. These use compressed nitrogen gas rather than a burnable fuel.
The astronauts may think in terms of moving toward or away from a target. But they, in fact, will be changing their orbital energy slightly. This can make them drift upward or downward. Astronauts still on the shuttle will track them by radar and call for adjustments if this drift becomes excessive.
Such guidance will be vital next April to help astronaut George D. Nelson avoid overshooting when he approaches the Solar Max satellite. This time, McCandless and Stewart will practice docking with an instrumented pallet fixed to the shuttle's 50-foot arm. They will be learning how to fine-tune their movements while operating under the same laws of energy conservation and transformation which govern the motion of planets and comets in our solar system.