Rockets can be detected and jammed or intercepted easily enough. If the planet is anything like Earth, the rocket will have to fight gravity and drag to reach orbit. These aren't cheap Palestinian rockets; their job isn't easy, and ships will fight back with jamming, evasive action, chaff, and point-defense. There will be plenty of time to react to them. On the other hand, all the ship needs to do is let go of a piece of metal. Boom. Now, if the planet has little gravity or atmosphere, things will be different. The rockets will have an easier time of it, and orbital projectiles won't fall as quickly. A fleet will have to keep its distance. But such a planet would be incapable of supporting life without carefully designed structures. A few hits to those and everyone in the colony is dead. An attacking fleet could stand off and launch swarms of projectiles at the colony. Surely a few will hit.