The night sky may look like a pristine black void, but it's littered with everything from paint flakes to burnt-out rocket stages--some 3000 metric tons of space junk. Far from just another sad comment on the slovenliness of humankind, this orbiting trash has the potential to take out satellites, and constellations of telecommunications satellites like Motorola's 66 Iridium satellites are especially at risk, according to a report in tomorrow's issue of Nature. If an Iridium satellite collides with space debris and starts to break up, the researchers say, the debris cloud may trigger a chain reaction that would wipe out other Iridium satellites that orbit at the same altitude.
At present, only 10% of the 100,000 objects wider than 1 centimeter are tracked by ground-based radar. Despite this uncertainty, it's clear that satellites orbiting between 700 and 1500 kilometers from Earth--where most of the space junk floats--face the greatest odds of destruction.
Now three space dynamics experts--Alessandro Rossi and Giovanni Valsecchi of the National Research Council of Italy and Paolo Farinella of the University of Trieste--have for the first time numerically simulated the long-term risks to constellations of telecommunications satellites, like Iridium and GlobalStar, where tens of satellites orbit Earth at the same altitude. They found a 10% chance of a 1-kilogram projectile shattering a satellite in the next decade. And worse, the fragments could within a few years of the collision spread wide enough to threaten other satellites at the same altitude. "If a chain reaction occurs, that altitude region would no longer be suitable for any space activities," says Rossi.
"It's an important issue," says Walter Flury of the European Space Operations Centre in Darmstadt, Germany. Even though the chances of catastrophic impacts are slowly rising as more satellites are put in orbit and as debris accumulates from explosions and collisions of fragments, "space flight can be safe for a long time to come, if adequate measures are being taken." Already, he notes, rocket stages and satellites are built with the capability of being guided into the atmosphere to burn up completely after their mission ends.