"The first piece of downloadable content (known as DLC) was announced on February 6, 2008. The Bring Down The Sky pack was released on March 10, 2008 for the Xbox 360 as a paid download and on July 29, 2008 for Windows as a free download. It should also be noted that as of late January 2009, the DLC was made compatible with the Steam version, titled revision a. As with retail box copies, players will need to register their Steam serial number on BioWare's website in order to receive the serial number required for installing the DLC.
Bring Down the Sky includes a new uncharted world that introduces the notorious and feared batarians, a humanoid species with four eyes. 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. This mission is concurrent with the main storyline. BioWare has stated that the adventure should take ninety minutes to complete. The expansion also features an additional achievement awarded upon completion of the mission titled as "Colonial Savior" with a value of 50 points. Additionally, in the PC version of the game, a message may be found in a small music outpost on the asteroid when the computer inside is read twice. The message appears to be a humorous jab by the developers at those who stirred up the controversies surrounding the game's romance scenes upon its initial release."
Source: Wikipedia, "Mass_Effect," available under the CC-BY-SA License.