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Anticipating Collisions between Spacecraft and Space Junk

With its sophisticated predictive model, NASA can peer hundreds of years into the future at Earth-orbiting objects that could crash into each other.
December 10, 2010

In September, a piece of debris broke off from a 19-year-old nonoperational NASA satellite 330 miles up in the sky. The United States Space Surveillance Network (SSN), which is responsible for monitoring the more than 22,000 satellites and other objects in orbit, detected the event, plotted out the fragment’s orbital path, and determined that it was headed for the International Space Station (ISS). If it hit the $100 billion laboratory, the junk could cause catastrophic damage. Upon receiving the warning, NASA decided to maneuver the spacecraft out of the path of the debris, a task that it now performs about twice a year. The threat of such a collision has more than doubled in just the past two years, says Nicholas L. Johnson, NASA’s chief scientist for orbital debris.

Cluttering space: This computer-generated image shows the vast amount of man-made junk in orbit.

More than half a million man-made objects the size of a marble or larger are now circling Earth—and 15,000 of those are bigger than a fist. This orbital debris, or “space junk,” includes inactive satellites, spent rocket bodies, materials from solid rocket motors, collision fragments, and mission waste. Most operational spacecraft use protective shielding to mitigate the impact of objects less than one centimeter in diameter. But since the larger ones are racing around Earth at speeds of five miles per second, any one of them could destroy any satellite it collided with. The situation imperils the $160 billion satellite services industry, which plays a critical role for international phone calls, television broadcasts, climate and weather data, and military surveillance.

Growth rate in the number of Earth-orbiting debris objects larger than 10 centimeters.

Credit: NASA

To understand how such threats will evolve and to foresee the paths of space junk so that collisions can be avoided, NASA developed one of the world’s most sophisticated predictive models. Called Legend (for “low-Earth to geosynchronous environment debris”), the three-dimensional model simulates the routes of all trackable space objects and even factors in new debris from future crashes. To take uncertainty and randomness into account, hundreds of scenarios are generated using the Monte Carlo method, a set of algorithms that can calculate risk factors in a complex environment. With Legend, NASA scientists use the average of multiple simulations to estimate the number, size, and type of objects that will collide—and approximately how often. Unlike models used by the U.S. Strategic Command Joint Space Operations Center, which detects and tracks large objects and screens active satellites daily for possible collisions within 72 hours, Legend includes smaller fragments and looks far into the future.

In place since 2004, the NASA model is constantly fed with data gathered from the results of ground tests and spacecraft that have broken up in orbit; from telescopes and radars viewing the sky; and from analysis of crater-marked spacecraft surfaces that have returned to Earth. That means new simulations must be run continually. Legend enables scientists to calculate the consequences of a particular breakup or collision and helps them alert managers at the space station that a piece of debris could be in its path. The model also advises soon-to-launch satellites of areas to avoid and will guide scientists as they attempt to develop and launch debris removal technology for the first time.

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.

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