Space debris mitigation measures and risk assessment B. Protection strategies of space systems 2. Collision avoidance Current space surveillance systems do not reliably track objects in LEO with a radar cross-section of less than 10 cm in equivalent diameter. Collision avoidance strategies are most effective when the uncertainty in the close approach distance is kept small, preferably less than 1 km.
Collision avoidance is always probabilistic.
NASA uses an acceptable risk criterion of 1 in 100,000 as a chance of collision. For example, if an object is predicted to pass through a box measuring 5 km x 25 km x 5 km oriented along the flight path of the United States Space Shuttle, the SSN sensor network intensifies its tracking of the potential risk object. If the improved fly-by prediction indicates a conjunction within a box measuring 2 km x 5 km x 2 km, an avoidance manoeuvre may be performed. During the period 1986-1997, the United States Space Shuttle executed four such evasive manoeuvres. The Russian SSS and the Russian Space Agency perform similar collision avoidance assessments for the Mir space station.