NASA's now abandoned program to send human explorers back to the Moon, and
then on to Mars and other destinations in the solar system. Work on the
program began in 2005 and ended five years later. Key elements of Constellation
were the Ares launch vehicles, the Orion
crew vehicle, and the Altair lunar lander. Of these, Orion has survived
as the basis for the planned Multi-Purpose Crew Vehicle (MPCV).