> Using the Wikipedia example of a 1km, 5600g superconducting rail gun that launches at about 10km/s, we just need about a 10km gun to achieve 30km/s

That's a lot longer gun than I expected! Didn't the navy get to 6 km/s with a 5m barrel? That's a lot more than 5.6kg, of course.

> But you could launch a whole swarm of microprobes which could be a very effective distributed observation platform with a gigantic baseline.

For the beginning I'm going to assume each shot will end up vaporizing a significant fraction of the gun. Getting something to 30km/s once is certainly going to be much easier than doing it hundreds/thousands of times.