Under certain circumstances, but since the mass can be (relatively) small due to the extremely high velocity, it can be very hard to spot. Essentially, imagine

Under certain circumstances, but since the mass can be (relatively) small due to the extremely high velocity, it can be very hard to spot. Essentially, imagine trying to pick something the size of a telephone pole, angled end-on at you, from 20 miles away out of the entire night sky. By the time it gets close enough to spot easily, it's too late to do much about it. Edit: jjaredsimpson's comment makes a very good point about velocities much closer to 1c. My comment mostly relates to the more immediately practical kinetic bombardment[0], which works on the same idea. [0] https://en.wikipedia.org/wiki/Kinetic_bombardment https://en.wikipedia.org/wiki/Kinetic_bombardment