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What is the threshold of detection for small orbital debris? How small can something get before it can no longer be reliably tracked (and therefore avoided)?

Given that collision energy is dependent on the delta-V of the parties to a collision, and that some extreme cases may be unlikely or absurd e.g. geosynchronous vs "retrograde geosynchronous(?)", how much collision energy could an untrackable debris item deliver to an unsuspecting space vehicle? In other words, how bad could the "hidden hazard" actually get?

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The main method for detecting small objects is radar. An object's visibility to radar is expressed as its radar cross-section (RCS). This is a measure of how much radar energy is reflected by the object to the radar antenna.
There is no direct relationship between size and RCS. A few examples: a highly reflective object (e.g. a corner reflector) has a large RCS even if it's small, a stealth aircraft (like the B-2) has a small RCS despite its large size.
There is one limit: a radar can't detect objects much smaller than its wavelength. The AN/FPS-80 Cobra Dane radar works in the L band with a wavelength of 15-30 cm.

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    $\begingroup$ So one could imagine a bolt - say 10cm long x 1 or 2 cm diameter whizzing about, and potentially striking a valued (perhaps human-occupied) orbital craft with a closing delta-V of several kms/sec, and such an object being completely untrackable and therefore unavoidable. Kind of a scary prospect. $\endgroup$ – Anthony X May 16 '15 at 19:49

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