WHEN astronaut Bonnie Dunbar plucked the long-stranded LDEF satellite from orbit Jan. 12, she also retrieved the hopes of 200 principal investigators in nine countries. Now that the experiments carried by the Long Duration Exposure Facility finally are back in their hands, those hopes of new insight into the space environment are being realized. Before the shuttle Columbia made the historic rescue, LDEF chief scientist William Kinard of the National Aeronautics and Space Administration's Langley Research Center predicted that none of the experiments would be ``a dead loss.'' He expected the value of most of them ``to be substantially increased by the prolonged exposure'' in space. That turns out to be the case.
The shuttle Challenger left LDEF on orbit April 6, 1984. A second mission was to retrieve the 22,500-pound satellite 11 months later. But schedule shifts and Challenger's loss stranded LDEF for five years and nine months. It made 32,594 revolutions of Earth, accumulating an unplanned bonus of space experience.
Prof. Donald Kinser of Vanderbilt University in Nashville is typical of scientists who are reaping that bonus. He had two trays with 120 glass samples on LDEF. With the extended exposure, he says he now can make a much more accurate prediction of the effects on glass of radiation in earth orbit.
It will take 18 months to make precision measurements of radiation damage. But Professor Kinser reports that already he can see the samples are not darkened as he had expected. ``So,'' he says, ``it would be correct to say that there appears to be less radiation damage than we anticipated when we first launched the samples.'' He also will make stress tests to see how LDEF sample breakage compares with that of ``control'' samples left on Earth.
The most famous LDEF experiment - 12.5 million tomato seeds from Park Seed Company - also appear to have survived well. They are being distributed to schools for student experiments. Planting the space seeds along with ``control'' seeds left on Earth, these students will be able to compare the resulting plants and fruit later this summer.
Space station designers are concerned about environmental damage especially to systems mounted outside the station and to the station fabric itself.
Micrometeorites made many holes and craters on LDEF's exterior. Atomic oxygen and other hazards of the orbital environment attacked materials. Yet the damage was mostly superficial. Beneath protective thermal blankets, the experiments survived well. Even a thin plastic covering provided an effective micrometeor buffer.
Meanwhile, chemistry Prof. John Gregory of the University of Alabama at Huntsville is studying 120 samples of different materials to see what atomic oxygen did to them. He explains that the highly reactive atoms ``tend to eat up surfaces of materials like Kapton, carbon, and other plastics that have been used in the past for space coatings.''
Carbon, for example, combines with the oxygen to form carbon monoxide and dioxide and then evaporates. Since carbon fibers are planned for use on Space Station Freedom, Dr. Gregory says, ``we now know that it's going to have to be properly protected from erosion.'' The quick examination at the Kennedy Space Center in Florida suggests that silicon coatings may provide such protection.
The Kennedy team has completed its study of the LDEF frame. This is a bus-size 8,500-pound structure shaped as a 12-sided prism 30 feet long by 14 feet in diameter. NASA will store the frame in a canister with a controlled environment while it decides whether to fly it again. At stake is a major decision as to whether and how to try to provide an ongoing LDEF service for experimenters on a commercial basis.