The European Space Agency slammed a jet-pack accelerated, 20-kg "penetrator" into a 10-metric tone ice cube.
They had their reasons.
The impact decelerated the penetrator to the tune of 24,000 gs. Some folks pass out if exposed to 10 gs, and the human body loses structural integrity around 30 gs (300 m/s^2).
In addition to the obvious grooviness, I see introductory physics problems for homework and tests. You?
Hat tip: Phil Plait's Bad Astronomy