Humanity seems more prone than ever to heart-pulverizing squabbles and disgusting, knock-down-drag-out wars these days, but at least we can all agree on one thing: we really, really want Starbound to come out already. The wait for the Terraria-alike to end all Terraria-alikes has become unbearable, so you should be happy to hear that you won't have to bear it for much longer. Starbound's first beta is officially on the horizon, with developer Chucklefish gleefully chucking around words like "soon" and "really not very far off now." More importantly, they've outlined how exactly the beta's various phases are going to play out. If you're hoping to join, this is probably required reading.
You can read Chucklefish's full post for nitty gritty details, but here's the quick breakdown: there will be three beta stages, each at successive levels of completion. Stage one, which should kick off very soon, will be overtly buggy and unfinished. Your feedback, however, will absolutely affect the game's development process, so this part is pretty key.
Stage two will also be pretty traditionally "test"-oriented (read: not a glorified demo), but you'll essentially be playing the full game. The main quest line won't be hoisting the whole world on its shoulders yet, but you'll be able to dig around in the game's sandbox to your heart's content.
Stage three, then, will be the culmination of stages one and two, a nearly finished game. If you want a mostly seamless, bug-free experience from your incomplete games (which is a really weird expectation, but whatever), then this is probably where you should jump in. Or you could just wait a little longer for launch. The differences will apparently be negligible at this point.
So that's more or less that. Now all that's left is to wait for Chucklefish to push the big red button that births a billion tiny worlds into existence. The aim is for that to happen "as soon as possible," so keep your eyes peeled. It will, um, probably be pretty hard to miss.