According to this(http://www.1728.org/energy.htm
) a 12,500 pound object(the size of a twin otter air craft) traveling at 250,000 Kilometers a second would hit with a force equal to 42,348 Megatons.
What about a Space Shuttle? I don't think it's unfair if we were to talk about the possibility of a real life space ship causing massive destruction if it fell to earth.
I Think the Starfleet shuttle are a lot small and lighter, however if we were to take a landing weight of the space shuttle of 104,328 kg and have it hit at 250,000 kilometer per hour the force would be 792,200 megatons