A collision avoidance maneuver will be performed for a conjunction predicted by the United States Space Command if the predicted miss distance is less than 2 km radially, 5 km down track, and 2 km out-of-plane and if the maneuver does not compromise either primary payload or mission objectives. Propellant redlines will not be budgeted for any potential maneuvers.
If NASA flight controllers decide a maneuver is necessary, the orbiter uses its on-board propulsion system to execute the maneuver. On average, the orbiter changes its velocity by about 30 cm/s to avoid a collision, which requires the expenditure of about 11 to 14 kg of propellant (Loftus, 1997). Between shuttle missions STS-26 and STS-82, the orbiter logged approximately 527 days of on-orbit operations. During that time, nine cataloged objects penetrated the 2 km× 5 km×2 km box. In five of these nine cases, avoidance maneuvers were not performed because they would have interfered with primary mission objectives. In the other four cases, evasive maneuvers were performed by the shuttle. On two other occasions, maneuvers were performed when penetrations of the larger 5 km×25 km×5 km alert box, but not the 2 km×5 km×2 km maneuver box, were predicted (NASA, 1997).
The current technique for determining the threat of collision between the orbiter and a tracked object does not directly take into consideration the geometry of the conjunction or uncertainties about the position of either the orbiter or the other object. In the future, NASA plans to switch from this deterministic approach to avoiding collisions to a probability-based approach, which will be used for ISS operations.
The new method is based on the probability of collision (Pc), which is defined as the probability that an object will penetrate a sphere around the spacecraft. The calculation of Pc is based on the uncertainties of the positions of the spacecraft and the other object at conjunction and the geometry of the predicted conjunction.
In this probability-based approach, the U.S. Space Command’s computation of misses between orbits (COMBO) program will be run with current data for all cataloged objects for 72 hours into the future. The SSN will increase the