After sifting clues for five years, a team of scientific sleuths has found that puzzling clouds of junk orbiting the Earth are made up of radioactive debris leaking from a large group of orbiting Soviet nuclear reactors.
It is the first major case of nuclear pollution in space and one of the messiest environmental legacies of the Cold War.
The atomic debris, estimated at 70,000 detectable particles and perhaps millions of smaller ones, poses no danger to people, experts say. But it threatens to damage working satellites and will force engineers to add more shielding to help protect new spacecraft.
The cloud is seemingly destined to grow, though by how much is unclear.
"We're worried about it," said Dr. Donald J. Kessler, the senior scientist for orbital debris studies at NASA. "It looks like it could be pretty bad."
Orbital junkyard
Experts say the situation reflects the dangers posed by dead satellites, shattered rocket stages and millions of other bits of man-made debris that speed around the Earth in an orbital junkyard.
The mess has grown so great over the decades that it now threatens to erupt into a chain reaction in which a speeding piece of scrap would hit a large object, shattering it into hundreds of pieces that would repeat and amplify the process in a cascade of destruction.
The Soviet reactors are threatening to wreak havoc in the most crowded orbit in the heavens, roughly 600 miles up. The band is jammed with satellites for navigation, surveillance, weather tracking and observation of the Earth's natural resources.
The leaks are of a radioactive and highly corrosive coolant, a mixture of sodium and potassium in liquid metal form. The danger is due to the speed at which the drops of coolant are traveling rather than their radioactivity.
Though the droplets are circular and small -- the observed ones are 0.6 centimeters to 2.0 centimeters, or up to about the size of a quarter -- they still pack an enormous punch that can ruin or damage satellites, especially such fragile parts as sensors and solar energy panels.
"Liquid droplets moving at 10 kilometers a second relative to a spacecraft might as well be solid," Dr. Kessler said. "They do just as much damage." He added that the droplets were too small to completely shatter a whole spacecraft and thus would be unlikely to pose a risk of a chain reaction of colliding debris.
For more than two decades, starting in 1967, nuclear reactors were used by the Soviet Union to power low-flying radar satellites that spied on the movements of Western warships. The nuclear reactors made more electricity than the solar cells of regular satellites, allowing the use of energy-hungry radar equipment.
In all, 33 nuclear-powered spy satellites were launched into orbits about 150 miles above the Earth, where they worked for a few months at best before their reactors were switched off.
As a safety precaution, Moscow boosted the old reactors into parking orbits roughly 600 miles up, keeping them from re-entering the atmosphere for hundreds of years and allowing their most deadly nuclear fission products to decay. Sending the reactors higher than that, into less-crowded orbits, would have required more fuel and was probably the main factor that made the parking orbits relatively low.
In 1978 a complete failure brought one satellite crashing down on northern Canada, scattering radioactive debris across the tundra. The last of the spy class quit working in 1988.
First hint of trouble
The first hint of trouble came in the late 1980s as NASA began detailed studies of orbital debris in preparation for building a large space station, which was seen as potentially threatened with damage or destruction by space junk.
NASA's investigative powers increased markedly in 1990 when it began getting regular deliveries of data from a 100-foot dish radar in Tyngsboro, Mass., that is run for the Defense Department by the Lincoln Laboratory of the Massachusetts Institute of Technology.
Known as Haystack, it can search through space to find the proverbial needle -- objects a centimeter or less in length, or smaller than a half-inch. And it does so over distances of more than 600 miles.
Almost immediately, Haystack began to find signs of widespread debris.
The band of debris extended from 850 kilometers to 1,000 kilometers from the Earth's surface, or from about 530 miles to 620 miles high. It was most densely packed around 600 miles up.
Dr. Kessler and his colleagues at the Johnson Space Center studied published designs of the Soviet space reactors and found that they were filled with sodium-potassium coolant -- the only liquid aboard the craft in sufficient quantities to account for an estimated mass of about 66 pounds spread through space.