Space predictions: This chart shows the cumulative number of collisions between objects 10 centimeters and larger. The slope of the curve beginning in 2009 is based the Legend model’s prediction of approximately 0.14 debris-producing collisions per year between catalogued objects. Actual collisions occurred in 1991, 2005, and 2009. (Three scenarios are modeled: BAU=business as usual; PMD=post mission disposal; NFL=no future launches)
In his talk at the first Orbital Debris Removal Conference last year, J.C. Liou, NASA’s lead scientist for Legend, said that the model predicted 178 collisions in the next 200 years, 83 of them catastrophic ones in low Earth orbit. Collisions are expected about once every five years, on average. The most recent crash happened in February 2009, when an operating Iridium telecommunications satellite smashed into a retired Russian Cosmos satellite at more than 15,000 miles per hour. The collision was the first to destroy an active satellite, and it generated more than 2,000 new pieces of trackable debris. The event led to a change in a policy that had been in place since the creation of NASA more than 50 years ago. Under the “big sky theory,” space junk was not seen as a major threat, because space is so vast. “The big sky theory is no longer a viable concept for space operations,” says Chris Moss, the director of the Joint Space Operations Center at California’s Vandenberg Air Force Base. Satellites are now being concentrated into specific orbits, Moss points out. “It’s no longer trivial that any two objects could collide. We are taking the problem seriously and spending our resources on ways to solve it. ”
That’s a daunting task, especially since the smaller pieces of space junk cannot easily be tracked from the ground. “We are now entering a time when the orbital debris environment will increasingly be controlled by random collisions,” says Donald Kessler, a consultant who retired from NASA 14 years ago. In 1978, as a young NASA astrophysicist, Kessler predicted that by 2000, fragments from random collisions in low Earth orbit would become a significant source of small debris that would increase the likelihood of still more collisions. His landmark paper Collisional Frequency of Artificial Satellites: The Creation of a Debris Belt led to the creation of NASA’s Orbital Debris Program Office, where he spent 17 years as lead scientist for orbital debris research. Over that time, the scenario he predicted became known as the Kessler Syndrome.
Kessler recently published a new paper verifying that his 1978 predictions were roughly correct, and he compared them with the space environment of today. His conclusion: we face a serious problem. “If we maintain the current population of orbiting spacecraft without adding anything else, just the result of random collisions would be producing debris faster than atmospheric drag could clean out low Earth orbit,” he says. His recommendation: it’s time to start actively removing the largest debris objects from the sky. “The Legend model shows that even if we did collision avoidance with all of the operational spacecraft successfully,” he says, “it would make little difference in the outcome, because there is so much debris that cannot maneuver.”
The message from the model is finally getting through. In his 2010 National Space Policy, President Obama directed NASA and the Department of Defense to clean up space and provided a charter to conduct research on how to do it. “Remediation is the next step,” says Johnson. Initial funding of space cleanup efforts is expected in 2011.