Challenger poised for 2nd try at retrieving Solar Max satellite
Houston — Heartened by the success of ground controllers in slowing the wobble of the Solar Max, shuttle astronauts are preparing to try again today to retrieve that crippled satellite.
Controllers at the Goddard Space Flight Center in Greenbelt, Md., which manages the Solar Maximum Mission satellite, had turned on its magnetic stabilizers Sunday when the first retrieval attempt failed. These units grab on to Earth's magnetic field. They exert weak torques that orient and stabilize the satellite. Astronaut George D. Nelson had made the satellite spin and wobble a bit faster when he unsuccessfully tried to dock with it. Two attempts to grab Solar Max with the shuttle's mechanical arm also failed. So controllers want to reduce the satellite's wobble and spin.
At the time, the satellite's solar cells were not in sunlight. Controllers were concerned that the backup batteries would be discharged before the satellite could be stabilized. However, by early yesterday morning, the panels had rotated into sunshine. The batteries were recharging. And the undesirable wobble had been stopped.
Goddard controllers then tried to put Solar Max into a slow, controlled spin to orient its grappling fixture toward the shuttle when the astronauts try to retrieve it today.
At this writing yesterday, the astronauts were having a relaxed day aboard the Challenger. They were carrying out chores originally scheduled for this day, which had been intended as a break in the Solar Max repair activities.
Mission officials here have repeatedly explained that the overall schedule for the flight is flexible. ''We've always claimed that the value of having a manned spacecraft was the flexibility. We're using that?the flexibility,'' says lead flight director Jay Greene. ?
He and his colleagues would very much liked to have captured the Solar Max satellite on the first attempt. But they will be more than pleased if it is recovered today.?
They also stress the fact that the major repair to the satellite will be replacement of its attitude control unit. Failure of this unit some 10 to 11 months after Solar Max was launched Feb. 14, 1980, has severely curtailed the satellite's performance because it could not maintain precision pointing.
The astronauts also have planned to install a baffle to improve the performance of an X-ray detector and to replace the electronics box for another instrument called a Coronagraph/Polarimeter.cq Mr. Greene reiterated Sunday that replacing the attitude control unit is the main objective. If that is done, the repair will be considered successful even if the astronauts cannot complete the other, secondary, tasks.
This mission already has some success to its credit. For one thing, the astronauts have deployed the largest (30 feet by 14 feet) and heaviest (21,400 pounds) object yet handled by the shuttle. This is the long-duration exposure facility (LDEF) satellite with some 57 experiments on board. They also have skillfully demonstrated shuttle rendezvous techniques.
Greene called it ''a picture-perfect rendezvous,'' including Dr. Nelson's manned maneuvering unit (MMU) flight between the shuttle and the satellite. It delivered the astronaut at the target right on time. Greene called this a significant demonstration of a capability needed for future missions.
Trouble began when ''Pinky'' Nelson tried to dock with Solar Max. His trunion-pin attachment device failed to latch on to a trunion pin on Solar Max. His efforts made the satellite spin and wobble. He tried to stop this by holding on to one of the solar panels. But that did little good. Mission commander Robert Crippen then maneuvered to try to grab Solar Max with the shuttle arm. This too failed.
By that time, Challenger was running low on propellant for its forward thrusters. These are required for rendezvous work, for maneuvering to capture a satellite, and for possible rescue of an astronaut flying freely with an MMU. (They are not required to bring the shuttle safely back to its base.) Thus mission controllers and the astronauts decided to back off and assess their options.
There is not enough forward propellant to allow Nelson another trip to the satellite. However, it does seem feasible to make one more attempt to grab Solar Max with the shuttle arm.
As of Greene's briefing Sunday, Challenger had 25 percent of its forward propellant left. Greene said it would take about 15 percent to rendezvous with the satellite again.
Another 5 percent would be budgeted for the retrieval attempt, leaving a margin of 5 percent of the original propellant supply for the rest of the mission.
Thus the critical factor in the second retrieval effort - besides reducing satellite motion - is the short supply of forward thruster propellant.
Budgeting that critical commodity will be as severe a test of the astronauts' skill as will the satellite retrieval itself.
To correct the Solar Max satellite problem, mission commander Robert Crippen and pilot Dick Scobee must fly the shuttle to within less than 50 feet of the satellite.
Then mission specialist Terry Hart, operating the shuttle's robot arm from the interior flight deck, must reach up and grab onto a grappling pin sticking out from the side of the satellite.
He will then pull Solar Max inside the shuttle's open cargo bay and place it on a special platform.
Nelson and fellow repairman James D. van Hoften would then go outside Challenger and repair Solar Max, replacing the two major electronic components that failed.
If they succeed, it will be the first repair of an orbiting satellite, opening the way for many more service calls in space.
Mission officials said they were considering extending the six-day mission for a day if they get Solar Max aboard.