The title pretty much explains the whole topic. But aside from the general flying space rock, what if there were objectives to it? Suppose there was a comet that passed. You could choose to launch yourself to the comet, and mine some rare mineral. There could also be random asteroid events (while you're on the planet or random all the time, so hypothetically you could be on a planet and then leave, come back, and a chunk is missing from where the comet hit.) The comets wouldn't serve the same purpose as the meteors in Terraria, but it could be a random event that brings aliens to a planet or something.
I would suggest closing and maybe deleting this thread, because it's showing up on the suggestions page and getting a ton of votes, even though Vaidred says it's already going to be in the game. This thread's just taking up space and perhaps giving some people the impression that it might not be in the game.
That'd be fun. And I would love to attach engines to a comet and make it my new spaceship, before I attach engines to my homeworld and fly that around. Also, yeah, comets could have all sorts of rare ores, and icy ones might be exceptionally easy to mine with a laser.
http://tvtropes.org/pmwiki/pmwiki.php/Main/PlanetSpaceship There's more than a few people who have done this, so i'm pretty sure that we're fine.
They could be one landing on the planet so large it could flash the screen white and the planet will turn to ocean!