Quoting Bioware announcement:
To date, over 1.6 million gamers have explored the engaging sci-fi universe of Mass Effect. Beginning on March 10, fans everywhere will be able to expand the “Mass Effect” experience with the release of the first ever downloadable content for the award-winning saga.

The Bring Down the Sky downloadable content pack will be released to Xbox LIVE Marketplace on Monday, March 10 at 2 a.m. PST, and will be available for 400 MS points. This is the first in a series of planned downloadable content that further expands the Mass Effect universe and continues the adventures of Commander Shepard and the Normandy crew.

Bring Down the Sky includes a new uncharted world that introduces the notorious and feared alien race of the Batarians. A Batarian extremist group has hijacked a mobile asteroid station in the Asgard system, setting it on a collision course with the nearby colony world of Terra Nova. Only Commander Shepard can save the millions of innocent civilians before the asteroid completes its deadly descent.

Bring Down the Sky contains approximately 90 minutes of heart-pounding action and a new Achievement worth 50 Gamerscore points.