The hazards of orbiting space junk
As humanity makes more use of space near Earth, a vast amount of debris accumulates there also. Space planners wonder when this will become a hazard to orbital navigation.
Donald J. Kessler at the Johnson Space Center near Houston estimates that, out of 10,000 to 15,000 objects orbiting our planet, only about 235 are working payloads. The rest are junk. They include spent rockets, old satellites, assorted splinters and fragments, and even lost cameras and gloves.
Whizzing around in various orbits out to distances of about 5,000 kilometers, this junk spreads out fairly uniformly around the planet. Even a small piece of it could endanger a functioning satellite or manned space vehicle such as the Soviet Salyut space station or the US reusable shuttle.
At the moment, the likelihood of such a collision is small. One of the latest estimates has been given by L. Schnal and I. Pospisilova of the Astronomical Institute of the Czechoslovak Academy of Sciences. They put the likely collision rate at only about one in 60 years.
As more countries put more satellites into space, however, the orbiting junkyard is likely to grow, and the probabilities of a damaging collision are likely to increase. As David W. Hughes of the University of Sheffield, England, points out in Nature, this is not happening at the moment. Commenting on the Schnal and Pospisilova estimate, he notes that the number of trackable bodies in orbit was growing by about 400 a year between 1975 and 1980. Since then, the number has remained roughly constant, at a little over 4,500. There are probably more than twice as many small bits as well - objects less than 10 centimeters across which radar cannot track.
But this apparent respite in the growth of space debris is probably temporary. Hughes explains that the recent maximum in sunspot and other solar activity caused Earth's outer atmosphere to expand. This increased the atmospheric friction on junk in lower orbits, causing the bits and pieces to reenter and burn up more rapidly. Also, there have been fewer explosive Soviet ''killer'' satellite tests and fewer used-rocket explosions. US Delta launch rockets now are designed to burn up their leftover fuel and to vent residual gases from their combustion chambers to relieve pressure.
Kessler has said that about 50 explosions known to have occurred in orbit have generated around 60 percent of the trackable debris. They probably produced much of the smaller, untrackable, junk as well. While this source of debris now may be curbed, however, a chance collision between larger objects in orbit could also produce many fragments. This is considered to be increasingly likely as more satellites are launched.
Thus, during this decade, something may have to be done to try to clean up space near Earth. Perhaps some kind of automated space broom and wastebasket will be put up by the shuttle and later retrieved. And, as often happens when human beings intrude on what was once wilderness, they may be asked to ''please take your trash with you.''
