NASA is carefully tracking some 500 pieces of debris from a Russian intelligence satellite that may pose a hazard for the international space station. The satellite exploded in March; another piece of it broke apart in June.
In recent days, a couple of pieces looked like they might come close enough to the international space station to prompt an evasive maneuver, says Gene Stansbery, of NASA's Johnson Space Center, who helps track debris. Further tracking, however, indicated the debris would pass at a safe distance.
This model of Russian satellite has a history of occasional and peculiar breakups.
"They undergo these mysterious spasms," says Jonathan McDowell, an astronomer at the Harvard-Smithsonian Center for Astrophysics. "And what's unusual about this latest one is that there's so much debris — much more than in any previous explosion of a satellite of this type."
The satellites, known as EORSATs — Electronic Ocean Reconnaissance Satellites — have been in use since the 1970s. Some analysts believe the satellites are used to track U.S. Navy ships from their radio transmissions.
Most analysts believe the breakups are intentional, McDowell says. The initial explosion seems to happen when the satellite is within range of a Russian tracking station, "which leads one to imagine that it's them sending a radio signal going 'blow up now,'" McDowell says.
McDowell says the subsequent breakups may be due to residual fuel igniting.
Stansbery says NASA has contacted Russian officials in the past about the occasional messy breakups of these satellites, but hasn't received a response. Russia is a major partner in the international space station.
The 500 pieces being tracked are at least a couple of inches in size, Stansbery says. He says the debris cloud probably also includes smaller bits that the network can't track.
"In those cases, you don't have any warning if one would come close" to the international space station, Stansbery says, though the risk from those pieces is low.
Stansbery says he's heard the Russians are retiring this model of satellite. The Russian Embassy did not return a call seeking comment.