by Jim Partan
How is the trajectory for a Mars mission chosen?
Mars is a long way from Earth, between 56 million and 400 million kilometers, depending on the phase of the orbits of Earth and Mars. About every 26 months, the cycle repeats, and Earth and Mars are in approximately the same relative positions. So, what are the best trajectories to go to Mars?
For robotic probes, generally the greatest concern is to reduce the cost of the mission. Robotic spacecraft therefore generally follow a minimum-energy trajectory, called a Hohmann trajectory. These trajectories involve a launch when Earth and Mars are on opposite sides of the sun (called “conjunction”), and take about 8 or 9 months to reach Mars, with a minimum of maneuvering in flight (see diagram). With fewer maneuvers, the spacecraft requires less fuel and therefore has less mass; the design team can then decide either to add more instruments, or to use a smaller, less expensive rocket.
For human missions to Mars, there are several additional factors to be considered, to reduce risks to the crew. The time in transit between Earth and Mars should be relatively short, to reduce the crew’s solar radiation exposure (on both Earth and Mars, the atmosphere provides significant shielding from radiation). Also, although there are ways of providing artificial gravity, shorter transit times will reduce the crew’s time in a weightless or low-gravity environment. Finally, the trajectory should provide a “free return” to Earth: in the event of a serious failure, such as happened in the Apollo 13 mission, the trajectory should naturally return to Earth.
With these considerations, the most practical trajectories are also conjunction-class, but use about twice as much energy as a Hohmann trajectory. The transit time to Mars is reduced to six months, with a two-year free-return to Earth in case the Mars landing is aborted due to serious problems. With this trajectory to Mars, the crew would spend 18 months working on the surface of Mars, and return on a similar six-month trajectory to Earth.