By Joel Achenbach
Washington Post Staff Writer
Thursday, February 12, 2009
The Pentagon and NASA are scrambling to assess the risk to spacecraft and the international space station from hundreds of pieces of debris created in the collision Tuesday of two satellites 491 miles above Siberia. NASA's initial estimate is that the space station faces a "very small" but "elevated" risk of being struck.
The situation is unprecedented. Scraps of spacecraft and other orbital junk have crashed together previously, but this was the first incident involving two intact satellites. One was an Iridium satellite launched in 1997 and used for the company's satellite telephone network; the other, a Russian Cosmos satellite launched in 1993, had been non-operational for a decade, NASA and Pentagon officials said.
The space station is in no danger of entering the main debris clouds because it is orbiting just 220 miles above Earth, or 271 miles below the orbit of the collision. The debris will continue to spread, however, and potentially could force the station to make an avoidance maneuver at some point, said NASA spokesman John Yembrick.
"Eventually debris will move and will spread all over. And debris can strike other debris. Eventually debris decays and will lower its orbit. The space station does have the capability of doing a debris-avoidance maneuver if necessary," he said.
Yembrick said the station has maneuvered eight times to avoid debris -- meaning such moves are very rare for a space station that has completed more than 60,000 orbits. Three astronauts are now aboard the still-incomplete orbiting laboratory, two American and one Russian.
A memo from NASA headquarters about the incident said that officials "have determined that the risk to the space station is elevated, and they estimate the risk to be very small and within acceptable limits." The memo called the situation "fluid."
"We're still trying to understand exactly what the true magnitude of the clouds is," said Kelly Humphries, a NASA spokesman at the Johnson Space Center in Houston. He said the astronauts on the station were being briefed about the incident.
About 20 NASA satellites are in orbits that would take them close to the debris cloud, according to the space agency. But there are many hundreds of other satellites -- nearly 1,000 currently in operation, among some 6,600 satellites that have been launched since Sputnik in 1957, according to a 2007 estimate by Jonathan McDowell, an astronomer at the Harvard-Smithsonian Center for Astrophysics.
The military can track space debris as small as a baseball. The U.S. Strategic Command monitors 18,000 distinct pieces of debris, according to Reggie Winchester, spokeswoman for the command. That number will jump by at least 600, the preliminary estimate for the number of pieces from Tuesday's collision.
Even a very small object packs tremendous kinetic energy at orbital velocities, which are on the order of 17,500 mph. Humphries said the space station has "bumpers" designed to shatter an object into tiny pieces before it can penetrate the pressurized interior.
Said Humphries: "It gets down to probabilities. Space being very big, these pieces of debris being very small, the odds are very high that they're not going to collide."
Staff writer R. Jeffrey Smith contributed to this report.