Astronomers and space scientists worldwide are grappling with an unwelcome legacy of 32 years of the Space Age - orbiting trash.
Ever since human activity moved into Earth orbit, so has a corresponding shell of space pollution. Material orbiting the Earth includes dead satellites - some nuclear powered - spent rocket boosters, nuts and bolts, paint particles, an American glove and a Soviet screwdriver, to name but a few items.Space pollution poses a number of problems. Orbital debris creates a collision hazard for manned and unmanned spacecraft. Defunct satellites falling from orbit, especially those with nuclear power sources, imperil everyone on the ground. And ground-based astronomers already have had observations marred by light reflected from satellites and other orbiting chunks of material passing in front of telescopes. Further, gamma rays released by nuclear-powered satellites disrupt measurements made by orbiting high-energy astrophysics observatories.
Given the vastness of space, do such pollution issues merit serious attention? The answer from a number of quarters is a resounding "yes." "We worry about trash and billboards down here," Dr. David L. Crawford, of the International Astronomical Union's Committee on Protection of Observatory Sites, says, "but it's worse in space." Dr. Sidney van den Bergh of Canada's Dominion Astrophysical Observatory concurs and asks "Twenty-five years ago, who would have thought we could pollute the oceans?"
The problem is already serious. The U.S. Space Command's Space Surveillance Center at Cheyenne Mountain, Colo., tracks about 7,300 man-made orbiting objects 4 inches or more in diameter. About 1,600 of these are satellites, of which only 300 are active. Nearly everything else is debris from rocket launches.
Many smaller objects orbit the Earth as well. At a 1988 conference on the deterioration of observatory environments, one researcher reported estimates of more than 36,000 objects one-third of an inch and larger in orbit.
Debris in space begets more debris as pieces collide and further fragment. The problem of collision has reached the point that NASA and the European Space Agency have formed working groups with the goal of reducing orbital debris.
Possible solutions include strategies for bringing spent rocket boosters back to Earth. These boosters have a tendency to explode months after use, apparently when corroding tanks allow leftover fuel to mix. In 1986, for example, an ESA Ariane booster exploded, placing debris in orbits from 270 to 840 miles above the Earth. Seven U.S. Delta boosters have blown up. A new National Aeronautics and Space Administration procedure calls for expending all leftover rocket fuel in space.
Space projects at particular risk of damage from the orbital debris problem include the NASA/ESA Hubble Space Telescope, NASA's Freedom space station now under development and all space shuttle flights.
Space shuttle flights are especially vulnerable. In June 1983, a postage-stamp-size paint fleck hit the windshield of the Challenger, leaving a quarter-inch-deep pit. According to a recent study, a fragment no larger than a fingernail, traveling at 7 miles per second, carries the equivalent kinetic energy of a bowling ball moving at 60 mph. NASA scientists calculate a 1-in-30 chance for impact damage on each flight, based on the length and number of planned shuttle missions. If the present increase of orbital junk continues, however, they predict the chance of impact will be one in 10 by the turn of the century and one in four by the year 2010.
As a safeguard during shuttle missions, the Space Surveillance Center employs a computer program to predict possible encounters between the shuttle and any one of the thousands of objects littering space. Predicted encounters are forwarded to NASA for evaluation, and the need for evasive action is assessed. While the SSC can calculate the time of such encounters with great accuracy, the uncertainties of orbital calculations make it impossible to determine whether the rendezvous will be a hit or a near miss.
The $1 billion Hubble Space Telescope, scheduled to be carried aloft by a 1990 shuttle launch, lacks thrusters to move it out of the way of approaching debris. Telescope construction began in 1976 and was completed before space pollution reached its present level.
The HST may be affected not only by physical damage, but also by light reflecting off debris and passing through the telescope's field of view. This can disrupt observations and confuse its guidance system, which relies on target stars for orientation.
The largest collision target of all may be NASA's manned space station, Freedom, scheduled for assembly in low Earth orbit in the next decade. The $28 billion station will measure more than 500 feet in length.
Since Freedom will house as many as eight astronauts, special precautions are being taken to protect it from impacts in space. Engineers and technicians at the Marshall Space Flight Center in Huntsville, Ala., have fired hundreds of small projectiles at simulated station walls to model collisions. The resulting design, reminiscent of one proposed many years ago by Dr. Fred L. Whipple, director emeritus of the Smithsonian Astrophysical Observatory in Cambridge, Mass., calls for double walls and "bumper shields" to surround the station and protect the astronaut modules.
In response to radioactive orbital pollution problems, a joint committee of the Federation of American Scientists and the committee of Soviet Scientists Against the Nuclear Threat has called for a ban on orbiting nuclear power sources, citing scientific, environmental and arms-control arguments.
There is not, however, agreement on this issue. Proponents of the Strategic Defense Initiative ("Star Wars") do not support a ban. According to former SDI Director Lt. Gen. James Abrahamson, failure to develop nuclear power in space could cripple efforts to deploy anti-missile sensors and weapons in orbit.
At least one device being developed - perhaps the ultimate in trash-collection devices - might help reduce orbital debris. The "Autonomous Space Processor for Orbital Debris removal," explains Dr. Kumar Ramohalli, a University of Arizona professor of aerospace and mechanical engineering, is a small, shuttle-launched spacecraft that could seek debris or allow pieces of space trash to come to it. An attached metal-cutting device would chop space junk into pieces small enough to fit in a removable bin.