Microsoft is counting on the alien-shootout game, the final chapter in a trilogy, to boost the company’s money-losing entertainment unit to profit this fiscal year.

The game which debuted this week is also key to Microsoft’s quest to widen its lead over rival Sony in the battle for industry dominance of the current generation of game machines.

One of the strength’s of Microsoft’s Xbox 360 console is its ability for online play, and Microsoft in a statement said that more than a million members of the Xbox Live online service played Halo 3 over the Web within the first 20 hours. September 25 was the most active day ever for the online platform.