Keen Mac gamers have been watching the development of Kerbal Space Program (KSP) with interest for a while now. 

KSP is a spacecraft building and launch simulation program that combines realistic physics with cartoon characters (called Kerbals). The player builds spacecraft from parts and launches them into space, you can also man missions to other planets and build space stations.

See: Mac OS X games

Although the visual style of the Kerbals are quite cute, the game is anything but a cartoon. It possess realistic Newtonian physics and accurate gravity reactions, and your ships are as likely to fall apart as launch. It's very much a space simulator program.

Kerbal Space Program

KSP is being developed by Squad (Kerbal Dev Team blog here), and has moved from Beta to Alpha but a release date is still a way away. On 14 February Squad announced an updated Alpha for their customers, as well as a new KSP Demo for people to try out.

The game doesn't have any objectives as yet, but is more of an open-sandbox for keen astrophysicists to play with. KSP lists the following features:

The Vehicle Assembly Building and the Space Plane Hangar allows players to build spacecraft out of any imaginable combination of parts.
Fully-fledged, Physics-based Flight Simulation ensures everything will fly (and crash) as it should.
Take your Kerbal crew out of the ship and do Extra Vehicular Activities
Fly out to Moons and other Planets
Procedural Terrain delivers detailed terrain at a vast scale. The Kerbal Planet is 600km in radius!
Mod-enabled, allows players to create new content and modify the game.
Ship systems. Keep an eye out for engine temperatures and fuel levels, and try not to explode.
Build ships with multiple stages, and jettison parts away as they burn out.
Full control over the staging sequence allows for complex ships and advanced functionality.

Click here to download the demo.

The Kerbal Team has also produced this, charmingly homemade, trailer for the game.