Space Invaders: Speed
Let me start by mentioning what happens when a similar sized (1 km) asteroid crashes into the Earth at a typical solar system speed of about 20 km/s. It's velocity and mass are such that any air between it and the surface gets plastered on to the front of the asteroid and smashed into the Earth along with the only very slightly slowed asteroid. The speed of the asteroid is so much greater than the speed of sound (.33 km/s) that the air has no time to get out of the way.
Another data point: It's pretty interesting what a BB sized piece of metal does when it impacts a few inches of armor plate at a few miles/s. It punches through it like paper. If a projectile strikes a material strain waves in that material transfer momentum laterally, distributing the impact. If the projectile is moving much faster than the speed of the strain waves, this does not have time to happen, and material is simply sheared away and pushed ahead until enough mass accumulates to sufficiently slow the projectile. Using this logic, my seat of the pants notion was that the alien spaceship might just drill clean thru the planet.
U of A's Impact Effects Calculator doesn't agree, though it does say that the crater might be a few hundred miles deep. Of course the velocity of my alien spacecraft is much larger than those their models were designed to deal with.