GMAT

Do you want to go Mars but don't know when to leave or how much to bring? Do you want to study Earth protection by planning a mission to a near-Earth asteroid? Do you want to place a telescope in an orbit that is always looking outwards from the Earth and Sun? The General Mission Analysis Tool (GMAT) is an open-source space mission design tool to help you answer these and many other space flight challenges. GMAT is developed by a team of NASA, private industry, and public and private contributors. It is used for real-world engineering studies, as a tool for education and public engagement, and (after completion of final acceptance testing in Sept. 2013) to fly operational spacecraft.

Web Site: http://gmatcentral.org/

GMAT Web Site