I forget what it's called, "slingshot effect" or something, where the earth is fully orbited several times, picking up more and more speed before being launched off on its trajectory. Normally, the trajectory is *away* from earth, sending a ship to Mars or whatever. Imagine that in reverse, though. Imagine some gigantic chunk of satellite having rockets strapped onto it, refitted to be the precise mass/shape in order to do this. A lot of asteroid impacts happen this way. They are flying through space, they get caught in a nearby planet's gravity, and they begin gradually closing in on the planet as they orbit. Mass burns off gradually as it orbits, but speed continues to pick up until it actually hits atmosphere- remember, in space there's no "friction" with air, hence absurd speeds are possible even with truly massive objects. By the time the asteroid exits the upper atmosphere and comes screaming at the earth, the vast majority of its mass has burned off... but it doesn't need to be much bigger than a car if it's traveling fast enough to hit a planet and cause a fucking nuclear ice age. See: dinosaurs. As with all space shit, it's not easy or simple, and there are so many thousands of variables to consider in each calculation being made. This is why it's frankly astonishing that we were able to supposedly land men on the moon using old as fuck computers, but we are unable to do so again *with supreme ease* using all of the new shit we have at our disposal. This is why some people think space is fake and gay.