MAR 98-081

Free Return Trajectories for Mars Missions
Christopher Hirata
California Institute of Technology. chirata@alice.caltech.edu

Astronauts on a future mission to Mars will want to be launched on a free return trajectory, that is, one on which the gravity of the Sun and planets alone will return them to Earth in the event of a serious malfunction. Although the total time to travel from Earth to Mars and back is at least two years in free-return trajectories that rely only on solar gravity, this can be shortened somewhat if a Martian gravity assist is used on the return to Earth. For the fall 2011 launch opportunity to Mars, a mission departing Earth on a 132-day fast transfer orbit to Mars can return to Earth after only 670 days, as opposed to 728 days for the non-gravity-assist free-return trajectory. While this difference of two months in a two-year abort trajectory may seem insignificant, in fact, a return to Earth two months earlier could save the lives of the crew.