In our group this is refered to as the Drop the Rock problem (after the
entertaining section of the C&S rules by that name.) The only solution
we have come up with is that in a potentially hostile enviornment
(terrorists have access to starships, engines can accellerate at 1+G
for months) a system must maintain draconian control over all objects within
a large radius of its inhabited planets.
That means active scanning; passive scanning for energy emitted by
small objects impacting the near-lightspeed asteroid; etc. The
radius must be large enough that the society can apply a large delta
V to the object in question, for example by partical beam, or kinetic
weapon of their own (remember the velocities roughly add in a colision.)
All together though its an arguement against the existance of life on planets
or other objects that can not dodge at least a few miles give a short notice.
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