LDEF maps orbiting junk.

When NASA launched the Long Duration Exposure Facility (LDEF) in April 1984, it intended to bring the orbiting, unmanned laboratory back to Earth nine months later. But delays after the 1986 Challenger accident kept LDEF waiting until January 1990, when a space shuttle rescued it just days before the craft would have met a fiery demise in Earth's atmosphere (SN: 11/11/89, p.314).

Researchers analyzing the wealth of LDEF data now report that one of its experiments produced the most comprehensive, high-resolution map ever of debris left in Earth-orbit by other spacecraft. LDEF's Interplanetary Dust Experiment, which operated for 11-1/2 months until its recording tape ran out in March 1985, detected some 15,000 collisions between LDEF and particles ranging in size from fine dust to grains of sand, usually clustered in groups. Some of the debris originated in undetermined extraterrestrial sources, such as meteors, but about 90 percent came from orbiting spacecraft, says John P. Oliver of the University of Florida in Gainesville. He and his colleagues recently completed a report on the LDEF mission.

Oliver notes that LDEF's fixed orientation -- one of its sides always faced Earth, while another always faced the craft's direction of motion in orbit -- enabled the dust detectors to record the location and direction of each impact. Every 54 days, the experiment mapped the distribution of dust clouds 250 miles above Earth in a swath extending from the terrestrial latitudes of 28.5 [degrees]N to 28.5 [degrees]S.

For two weeks in the spring of 1984, the detectors recorded sudden encounters with human-made debris--about five "hits" over a 5-minute interval during each of the craft's orbits, says Oliver. For example, among 1,100 impacts recorded by one detector during May and June 1984, about 900 came from an orbital debris cloud now dubbed the "May swarm," which has an incline relative to the equator of 30 [degrees] to 40 [degrees].

On June 4, 1984, LDEF encountered material in a more steeply inclined orbit of 65 [degrees] -- a trajectory typical of U.S. and Soviet military satellites. In a single 5-minute interval, a detector on LDEF's leading edge recorded more than 130 impacts.

The experiment revealed that most orbital debris concentrates in small clumps. That finding, along with further analysis of LDEF data, may enable researchers to predict the nature and exact location of space debris and even measure the amount left by a particular spacecraft, Oliver says. If so, engineers could equip space-bound structures with strategically positioned shields to protect them from the most intense collisions, which can severely degrade instruments over time.

Just as planners of terrestrial projects file environmental impact statements, "we may have to ask people [who send up spacecraft] to file an orbital impact statement," says Oliver. He suggests international agreements may one day be needed to avoid placing debris in a path that endangers long-term space missions.