Here's why this is the stupidest plan ever. To land on a giant space rock going 100 miles per second or whatever, you have to come up along side it at 100 miles per second then land on it. That means the spacecraft had the ability to reach 100 miles per second anyway and didn't need the giant rock. There is also no wind resistance or drag in space. So if you can get the spacecraft to that speed anyway, why do you need the rock?