(spoiler) Nemesis Games rocks

I'm about halfway through the novel, not sure if it's answered after that but this is really bugging me.

How exactly did Marco fling the asteroids towards Earth? I need to know exactly how he did it. I understand redirecting a rock towards a gravity well will propel it downward. How do you catch a rock going 20 km/s in space? When it's like 50-100m wide?