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.
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.
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.