Orbital Impact, Iridium 33 vs Cosmos 2251

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A possible explanation why Iridium fared better:

Satellite collision could have been far worse

23:38 15 April 2009 by Jeff Hecht

The unprecedented collision between an Iridium communications satellite and a derelict Russian Cosmos 2251 communications satellite on 10 February could have been far worse. One satellite probably did not hit the other's massive body directly but only rammed into a protruding solar panel or antenna, NASA has concluded.

At the end of March, NASA had catalogued 823 large pieces of debris from the collision, well below the roughly 1300 fragments expected from a direct collision of the 900-kilogram Cosmos with the 560-kg Iridium. More pieces are still being catalogued.

Observations show about two-thirds of the pieces came from the Cosmos, a compact cylinder with solar panels covering its circumference. Iridium satellites are longer, thinner cylinders with appendages such as solar panels and antennas projecting from them.

"The Iridium is underrepresented" in the debris, says Mark Matney of NASA's orbital debris programme, so it's possible that something projecting from the Iridium smashed into the Cosmos. That collision, at a velocity of more than 11 km/s, broke up the Cosmos more severely than the Iridium.

"Body-to-body collisions are the worst case. As near as we can tell, that's what happened with the Chinese anti-satellite test," Matney told New Scientist. That test shattered a 750-kg Chinese weather satellite in January 2007, producing thousands of pieces of debris.

"Each breakup has a character all its own" because satellites have different structures, says Matney. Studying the events should help NASA better understand collisions, which are inevitable as satellite and debris populations grow.

So far most fragments remain near the 790-km elevation where the collision occurred. There they threaten the 70 remaining Iridium satellites (including spares) and important Earth-observing satellites. Few of the pieces have reached lower orbits occupied by the International Space Station and the space shuttle.
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