In previous versions of Angry Birds, gravity behaves as it does in our daily life: You launch birds up, and they come down. Angry Birds Space does away with that mundane concept, opting for gravity wells instead. "Up" and "down" become entirely relative: In each level the action happens on one or more asteroids that each have their own gravity fields and affect your birds' trajectories in all sorts of interesting ways.
On some levels, your birds and the evil pigs share the same planet, making the game similar to the old Angry Birds. On other levels, you launch your birds from a floating asteroid toward one or more other asteroids. The asteroids' gravity fields make the trajectory curve, and you must take that into account when aiming. For example, let's say that you aim a bird under an asteroid, and launch. The bird enters the asteroid's gravity well and goes into partial orbit, then escapes the gravity well and shoots off in an entirely different direction. Many of the new levels would not be possible with the old physics model, and figuring them out can be challenging even if you're an Angry Birds veteran.
For aiming assistance, Rovio has introduced a new trajectory tool: When you pull the slingshot back to aim, you can see a dotted line curving to show exactly what the initial trajectory looks like. If a bird's trajectory puts it in the path of a gravity well, then the dotted line curves accordingly so that you can determine where your bird will go.
The game ships with three level packs (Pig Bang, Cold Cuts, and Danger Zone), and each pack contains 90 levels of varying difficulty. If you have a Samsung device, you get to play Danger Zone right away; this worked for me, even though I am not running an official Samsung ROM on my Galaxy S II. You'll also find levels titled "Eggsteroids," in which you can view and play your golden eggs. The two golden eggs I uncovered were riffs on Space Invaders and Mario, and were both extremely enjoyable to play. The game ships with a golden egg level already unlocked, promoting the Samsung Galaxy Note (its icon looks like a Galaxy Note).
Along with new gameplay mechanics, Rovio has introduced several new birds. A new version of the yellow bird, like the original, goes into "jet mode" when you tap the screen. The difference, however, is that now instead of just shooting straight, the bird will fly toward whatever point you tapped--a bit like a feathered guided missile. Another new bird, the light blue one, freezes rocks, making them much easier to smash using other birds. The amount of rock it freezes depends on when you tap the screen, so your timing must be just right for maximum effect.