The probabilistic threshold of concern is so low and the orbits of truly threatening objects are so imprecisely known that no maneuvers at all may ever have been necessary. The probability that there would have been no collision at any time is orders of magnitude larger than the probability that there would have been a collision. One can never prove why something did not happen. It is very, very unlikely that anything we did made any difference. An abundance of caution is wise when lives are at stake, but recognize that it is an abundance of caution, not a necessity. Over time the ISS has been moved to higher altitude, diminishing the risk. Our orbit determination techniques and collision avoidance processes have been refined as well. That there were no maneuvers means many things, but it has little to do with the actual debris environment. Thresholds of concern change, energy management might be more important to sustain the ISS orbit rather than address a smaller risk. There are political. engineering, and other considerations. Colliding with something is far down the list of things that might jeopardize the ISS.
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Space Station Required No Evasive Maneuvers in 2013 Despite Growing Debris Threat
PARIS — The international space station required no collision-avoidance maneuvers in 2013, after a record four such moves in 2012, despite a growing orbital-debris population intersecting its orbit, according to NASA data compiled from the U.S. Space Surveillance Network (SSN) of ground- and space-based sensors.
NASA said the relatively quiet year from a debris-threat perspective reflects “the chaotic nature of the [debris] population,” which has forced the station to fire its engines to avoid a debris threat on 16 occasions in the 15 years it has been in orbit.
In addition to these 16 collision-avoidance maneuvers, one attempted maneuver failed and three others were never undertaken because the debris-proximity warnings came too late. In these three latter instances, the station crew was forced to retreat to the docked Soyuz spacecraft to be ready for an emergency undocking.
Collision avoidance means spending costly fuel to move a facility that is as big as a football or soccer field and weighs some 420,500 kilograms. Maneuver orders are given if there is a greater than one-in-10,000 chance of a debris strike.
SSN data show that the amount of debris that passes through the station’s orbit of 415-420 kilometers in altitude has increased by 60 percent since the first station module was launched in November 1998.
But that figure, now more than 800 objects with a mass ranging from less than a kilogram to more than 1,000 kilograms, includes only those objects that have been identified and cataloged by the SSN. The network has recorded some 5,000 other objects intersecting the station orbit that are awaiting entry into the master catalog.
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