For the first time in five years, the international space station changed course on Wednesday to avoid a piece of space junk — in this case, satellite debris that the Russians have insisted wasn't there.
The five-minute maneuver made use of the engines aboard the European Space Agency's Automated Transfer Vehicle, or ATV, which is docked at the Russian end of the station. As a result of the thruster firing, the space station's 18,000-mph progress around Earth was slowed by about 2 mph, lowering the average height of its orbit by about a mile.
The ATV was already being prepared to separate early next month after a highly successful resupply and reboost mission over the past six months. Controllers had planned to put the craft through a variety of tests during three weeks of solo flight before safely plunging it into the atmosphere over the south Pacific.
Because of Wednesday's maneuver, the ATV used up some of the propellant previously reserved for its post-separation test program.
In a status report, NASA said the course change was required because the space debris was predicted to come within about a mile (1.627 kilometers) of the station — bringing the risk of a collision above the threshold for a "debris avoidance maneuver."
Normally, such maneuvers involve raising the station's altitude, to compensate for the orbit's inexorable decay from air drag. Such decay lowers the orbit by 100 to 300 feet per day, and requires periodic engine firings by docked spacecraft or rockets installed on the station itself.
But because the station is now operating near the upper end of its allowable altitude range, any further increase could have exceeded the lifting performance of planned docking missions over the next few months. Hence NASA had to make the unavoidable and wasteful choice to go in the opposite direction.
NASA spokesman Kelly Humphries said the station's most recent previous maneuver to dodge space junk came on May 30, 2003, and the last time the orbit-lowering strategy had to be used was for the remote-controlled linkup of two station modules in 2000, before the first crew came on board.
Russian news reports said Wednesday's maneuver was required to dodge "pieces of space debris" of unspecified national origin. ESA's news release stated merely that the debris came "from an old satellite." NASA's main station news page identified the threatening object as "a spent Russian rocket," but the more detailed daily report called it "part of the Kosmos-2421 satellite" (part of the payload, not part of the booster).
Launched in June 2006, the Cosmos-2421 was a naval surveillance satellite, designed for electronic eavesdropping to keep track of Western military vessels. According to U.S. tracking data, the satellite disintegrated on March 14 into hundreds of pieces — with further fragmentation on April 28 and June 9. More than 500 objects resulted, creating one of the largest debris clouds in space history.
In recent weeks, the station has been cycling through the thickest region of the debris cloud. "It's been giving us fits," said one analyst, who asked that his name not be used because he was not authorized to speak publicly. Although the peak concentration has passed, further avoidance maneuvers may yet prove to be necessary, another source told msnbc.com on condition of anonymity.
This type of Russian satellite has been observed to do this before, satellite watcher Jonathan McDowell told NPR in July. Because the initial breakup usually occurs within range of Russian space tracking stations, experts suspect that the Russians issue some sort of self-destruct command after the satellite's orbital mission ends.
Russian space officials have repeatedly denied that any explosion occurred.
In May, Alexei Zolotukhin, chief of the Russian Space Forces' information service, said the satellite had ended its mission but had not broken up, despite "unverified media reports" to the contrary. And in July, the Interfax news agency quoted Russia's deputy space agency chief, Vitaly Davydov, as saying he saw "no evidence for media reports that claimed, citing NASA, that a Russian military satellite had exploded in orbit and that its fragments threatened the international space station."
Davydov admitted that "there have been vehicles of this type in the past that exploded," but not this time. He blamed the rumors on Western spies: "For some reason, questions of this kind didn't arise in the years when this was happening. ... These days there are such questions all of a sudden. Our interpretation is very simple: there is certain interest in vehicles of this class that are used in the interests of our Defense Ministry."
But as Wednesday's rocket firing showed, Western interest in the satellite's debris cloud was entirely prudent.