The Collision of Iridium 33 and Cosmos 2251
First published Fall 2015, Crosslink® magazine
A significant concern of all spacefaring nations is that a space-based capability will be hampered or eliminated because of a collision with orbiting space debris. On February 10, 2009, Iridium 33 was struck by Cosmos 2251, marking the first time an active satellite was destroyed by an accidental impact with another satellite.
Iridium 33 was part of a constellation of nominally 66 satellites in 6 orbit planes that provides mobile phone service. An additional 32 satellites were on orbit as spares at the time of the collision. These spacecraft operate in a polar orbit inclined at 86.4 degrees, at an altitude of about 780 kilometers. The satellites are considered medium-sized, measuring approximately 4 meters by 1.8 meters, with a mass of about 700 kilograms each.
Cosmos 2251 was a Russian military communications satellite and was part of a series of spacecraft with similar missions. Cosmos 2251 had been taken out of service in 1995 and was not being actively controlled. It was somewhat larger than the Iridium satellite, having a cylindrical body about 2 meters high and 2 meters in diameter, with a tower that extended its length to 15 meters. It had a mass of 900 kilograms. Cosmos 2251 was in an elliptical orbit of about 750 kilometers by 805 kilometers, inclined at 74 degrees.
The vehicles collided over Siberia at nearly right angles to one another, at a relative speed of 11.65 kilometers per second. The debris from each object spread into a ring centered around the parent orbits, and then over time spread into shells.
The Aerospace Debris Analysis Response Team (DART) was immediately tasked by the U.S. Air Force’s Joint Space Operations Center to assess the event and the potential for risk to other satellite missions. The DART analysis predicted approximately 200,000 1-centimeter debris objects resulting from the collision, with some 3273 being large enough (10 centimeters or greater) to be tracked and therefore added to the resident space object catalog. Such objects are considered large enough to cause the destruction of another object, including active satellites—adding to the likelihood of a space-asset destructive chain reaction, the so-called Kessler syndrome.
Analyses conducted by The Aerospace Corporation have shown that as debris size decreases by an order of magnitude, the number of pieces increases by an order of magnitude. For every 10-centimeter trackable object, there are at least ten untrackable 1-centimeter objects, and one hundred 1-millimeter objects. A collision with a 1-millimeter object is likely to damage a satellite component, while a collision with a 1-centimeter object could very well prove fatal to a mission.
Aerospace’s DART analysis found that much of the debris resulting from the Iridium-Cosmos collision immediately reentered Earth’s atmosphere and burned up. However, it is estimated that 48 percent of the debris remains in orbit, posing hazards to other space-based assets through either direct collision, or impact with other debris, increasing the number of debris objects and thereby increasing the hazard. Although over time much of the debris from Iridium-Cosmos will reenter and cease to be a threat, Aerospace estimates that even a decade after the event, some 18 percent of the debris will still be in orbit.
The Iridium-Cosmos collision is the most problematic space debris event after the Chinese Fengyun-1C antisatellite test in 2007. The collision of Iridium-Cosmos, along with other similar events, has significantly raised awareness of space debris and its potential consequences.
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