UPDATE: 11 October 2010. In a major policy speech, Barack Obama formally ended the Constellation Program. He instructed NASA to begin work on a rocket for deep space exploration. He also directed that commercially based means be used to launch ISS crew members. More information on commercial contractors to NASA can be found on the SpaceX and Orbital pages.
The following is excerpted from a NASA publication, "The Vision for Space Exploration", dated February 2004:
"The Space Shuttle will be critical to completing assembly of the Space Station. With Space Station assembly complete at the end of this decade, NASA will retire the Space Shuttle and put crew and cargo on different launches, a safer approach to crew transport.
NASA will initiate Project Constellation to develop a new Crew Exploration Vehicle for future crew transport. This vehicle will be developed in stages, with the first automated test flight in 2008, more advanced test flights soon thereafter, and a fully operational capability no later than 2014. The design of the Crew Exploration Vehicle will be driven by the needs of the future human exploration missions described in this document. The Crew Exploration Vehicle might also supplement international partner crew transport systems to the Space Station."
With the Space Shuttle slated for retirement in 2010, NASA was moving forward with a replacement transportation system. The goals were to go beyond the ability to send crews into low earth orbit with the aim for a return to the moon no later than 2020, and eventually to Mars and beyond.
There were two major components of the Constellation Program. One was a launch vehicle, the Ares, capable of launching crewed spacecraft or heavy cargo. The second major component was Orion, the crew exploration vehicle. Other components would have consisted of an Earth Departure Stage and Altair, a lunar surface access module.
An artist's drawing of a human exploration base on Callisto, Jupiter's second largest moon. (NASA)