While reading this article about fast moving space dust: http://story.news.yahoo.com/news?tmpl=story&cid=96&e=14&u=/space/20040712/sc_space/cassinistruckbyhailstormofringparticlesatsaturn
Pretty dang small, I’d imagine. NASA’s had a couple of close encounters with space debris, one of the shuttle windows had a pock mark in it after it got hit by a paint chip while in orbit. It’s simply a matter of time before some of the junk up there kills someone. (It’s thought to have knocked out at least one satellite, IIRC.)
Getting hit by one of these would probably hurt:
How much mass would a bit of fast space debris need to hit with the same energy as the bullet?
E = 1/2 mv[sup]2[/sup]
Bullet:
150 grains = 9.72 grams
2820 ft/s = 860.1 m/sec
E[sub]bullet[/sub] = 9.72 X 860.1
E[sub]bullet[/sub] = 7190584
Dust:
20 km/sec = 20000 m/sec
E[sub]dust[/sub] = m[sub]dust[/sub] X 20000[sup]2[/sup]
E[sub]dust[/sub] = m[sub]dust[/sub]X 4 X10[sup]8[/sup]
Equate the energies:
E[sub]bullet[/sub] = E[sub]dust[/sub]
7190584 = m[sub]dust[/sub]X 4 X10[sup]8[/sup]
0.018 = m[sub]dust[/sub]
So an 18 milligram dust particle traveling at 20 kps would have the same energy as a bullet from a Winchester .308
18 mg is a bit big to call dust. That’s about the mass of a 3.2 millimeter diameter sphere of water.
Actually Squink you forgot the 1/2 bit of the E = 1/2mv[sup]2[/sup] equation, but since that applies to the bullet and the dust, you got away with it!
Good thing I didn’t tack standard units onto the answer then. 
People have been struck by meteorites. The thing about really small particles is that they burn up in the atmosphere. And for the slightly bigger ones that arrive at the surface the size of a grain of rice after having mostly vaporized away, they have slowed down quite a bit.