Orbital Collision Won't Be The Last

Satellite operators are at risk of a loss to orbital debris like the one experienced by Iridium Satellite LLC last week about twice a decade, and the danger grows every time there is another collision involving orbiting objects.

The collision between an operational Iridium communications satellite and an old Russian military store-and-dump satcom bird was the first between intact spacecraft. But it was the fourth known accidental collision between man-made space objects since 1991, and "it was by far the most severe," says Nicholas Johnson, who monitors orbital debris at NASA's Johnson Space Center.

The Iridium 33 satellite - one of 66 in the "Big LEO" low Earth orbit satcom constellation - and the 16-year-old Russian Cosmos 2251 - a Stela-2M communications spacecraft - collided broadside over northern Siberia with a closing velocity of about 7 mi. per second shortly before 5 p.m. GMT (noon EST) Feb. 10 at an altitude of about 790 km. (491 mi.).

Both spacecraft - the 800-kg. (1,764-lb.) Cosmos and the 689-kg. Iridium - shattered into debris clouds that continued on the paths of their respective circular orbits, spreading out along the original directions of flight and extending upward and down into a region of space heavily used by civilian and military weather satellites and other Earth-observation spacecraft.

Among them is the U.S. ? ?NOAA-N Prime spacecraft just launched into an 860-km. orbit (see p.