But the velocity of the impactor must be at least the escape velocity of the impactee, even though orbital velocities "out there" are very much slower than nearer the sun.

So for example, if the earth were where Pluto is, an approaching meteor might come along at an orbital velocity comparable to Pluto's, which is slow (about 0.16 that of earth).

The difference in orbital velocities between the two might be just a few meters per sec. But by the time that slow object fell down the gravity well, it would have sped up to 11 km/sec.

Now granted, Pluto is not as massive as earth, so it's escape velocity is only 1.2 km/sec. But that is still faster than a speeding bullet. Seen from a distance, the colliding worlds would kiss in slow motion, but to an unlucky observer at the point of impact, the incoming would approach at the muzzle velocity of an artillery shell.

http://nssdc.gsfc.nasa.gov/planetary/factsheet/plutofact.html