BOX 9.1
On-Orbit Collision of Iridium 33 and Cosmos 2251

On February 10, 2009, the satellite communications company Iridium lost contact with one of its spacecraft, Iridium 33. Earlier that day, Iridium had received a prediction of a close approach of 584 m (1,916 ft) between Iridium 33 and another orbiting spacecraft, the non-operational Russian communications satellite Cosmos 2251. Iridium had received close approach reports before, and the one on February 10 was not particularly alarming or deemed a “top predicted close approach” compared to other predicted close-approach events for that week. Nevertheless, at the time the close approach was predicted to occur above northern Siberia, Iridium abruptly stopped receiving telemetry from its spacecraft.

The destruction of Iridium 33 was confirmed when the U.S. Space Surveillance Network (SSN) detected debris clouds in the orbits of both Iridium 33 and Cosmos 2251, marking the first payload-to-payload collision in the history of spaceflight. (See Figure 9.1.1.) The collision of Iridium 33 and Cosmos 2251 added an additional 2,181 trackable pieces of debris to the approximately 19,000 objects larger than 10 cm already in orbit in 2009.1 (See Figure 9.1.2.) Today, more than 22,000 pieces of debris are being tracked,2 plus an estimated population of approximately 500,000 particles between 1 and 10 cm, and more than tens of millions of particles smaller than 1 cm orbiting Earth.3 Some of the debris from that collision has reentered Earth’s atmosphere: as of May 2011, the SSN had cataloged 547 pieces of debris associated with Iridium 33 (down from 594 originally) and 1,488 pieces of debris associated with Cosmos 2251 (down from 1,587 originally).4 Particles smaller than 10 cm are very difficult to track reliably with current capabilities.5

Iridium created its own collision analysis process during the initial development and launch phase of the Iridium constellation of satellites. Although the data available for tracking a satellite’s position are the best the U.S. government can offer, those data were not designed to be used for conjunction analysis, although they are being used for that purpose today. Prior to the Iridium–Cosmos collision, Iridium had never made an on-orbit maneuver with one of its satellites in response to a close approach prediction. Between February 2009 and February 2011, Iridium made 41 collision mitigation maneuvers based on 46 close approach warnings;6 however, all of these actions were integrated into normal constellation maintenance actions and so had little impact on Iridium operations.

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1 NASA Orbital Debris Program Office, “Orbital Debris Frequently Asked Questions,” available at http://orbitaldebris.jsc.nasa.gov/faqs.html#3.

2 Government Accountability Office, Space Acquisitions: Development and Oversight Challenges in Delivering Improved Space Situational Awareness Capabilities, Report to the Subcommittee on Strategic Forces, Committee on Armed Services, House of Representatives, GAO-11-545, Washington, D.C., May 2011.

3 NASA Orbital Debris Program Office, “Orbital Debris Frequently Asked Questions,” available at http://orbitaldebris.jsc.nasa.gov/faqs.html#3.

4 See T.S. Kelso, “Iridium 33/Cosmos 2251 Collision,” updated May 13, 2011, available at http://celestrak.com/events/collision/.

5 J. Lyver, NASA, presentation at the Workshop to Identify Gaps and Possible Directions for NASA’s Micrometeoroid and Orbital Debris Programs, March 9, 2011, National Research Council, Washington, D.C.

6 J. Campbell, Iridium, presentation at the Workshop to Identify Gaps and Possible Directions for NASA’s Micrometeoroid and Orbital Debris Programs, March 9, 2011 National Research Council, Washington, D.C.



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