Ok everyone - let's say there's a gigantic anti-gravity machine attached to the asteroid so that it actually does hit the earth at the speed of a truck backing up. What happens to the earth?

My first thought is, shit would get real fucked up before it even hit the ground. A rock that's thousands of miles in diameter would cause serious gravity effects as it slowly traveled down to earth, like local tides would be completely fucked.

In terms of impact i think it would pretty much just F=ma equation. According to a few rough estimate numbers i put into ai, it says the object would have 2.4x10^20 joules of kinetic energy. To put that in perspective, the nuclear bomb in Hiroshima released only 63x10^12 joules