| NASA developed and has been using collision avoidance procedures for high value targets such as the International Space Station, and NASA’s Earth Observation System. |
Note: A “conjunction,” in this context, is a close approach between two orbiting objects. “Conjunction analysis” is the art of combining measurements of the location and uncertainties in location of the objects and in propagating that information forward in time to the point of closest approach. “Collision avoidance” is the act of maneuvering a satellite to decrease the probability of collision.
Collision avoidance is an important, but limited, tool for protecting spacecraft in orbit. In 2009, the US Space Surveillance Network was tracking ~19,000 objects larger than 5-10 cm in diameter. Of those objects, less than 5% are operational satellites capable of maneuvering. In addition, maneuverable spacecraft can only avoid the other 19,000 tracked objects, a small fraction of the estimated 500,000 hazardous orbital debris larger than 1 cm.
Another issue is the accuracy of position measurements for the satellite and the conjuncting debris. Although progress has been made in using special perturbation orbital data and in predicting the effects of atmospheric drag, significant uncertainties remain in the predicted locations of the orbiting objects and the probability that they will actually collide. Typically, satellites cannot afford to maneuver unnecessarily or for low probability conjunctions.
Regardless of the limitations, many high value satellites choose to perform conjunction analyses and, if necessary, to execute collision avoidance maneuvers. The NASA Johnson Space Center developed and has been using collision avoidance procedures for many years to support Space Shuttle missions and the International Space Station. Later, the NASA Goddard Space Flight Center developed a conjunction assessment and collision avoidance process for robotic spacecraft, e.g., NASA’s Earth Observation System (EOS) in low Earth orbit and the Tracking and Data Relay Satellite System (TDRSS) in geosynchronous orbit.
In 2008, NPR-8715_006A, NASA Procedural Requirements for Limiting Orbital Debris, began requiring that NASA “shall have conjunction assessment analyses performed routinely for all maneuverable Earth-orbiting spacecraft with a perigee height of less than 2000 km in altitude or within 200 km of GEO.”
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The "Afternoon Constellation" or (A-Train) of Earth Observation Spacecraft orbits at ~700km altitude, near the peak in the orbital debris population. |
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