orbit to Mars is also possible. As with the Moon, this option is mostly considered for crewed missions. Robert Zubrin, in his book The Case for Mars, discusses various trajectories to Mars for his mission design Mars Direct. The Hohmann transfer orbit can be made free-return. It takes 250 days (0.68 years) in the transit to Mars, and in the case of a free-return style abort without the use of propulsion at Mars, 1.5 years to get back to Earth, at a total delta-v requirement of 3.34 km/s. Zubrin advocates a slightly faster transfer, that takes only 180 days to Mars, but 2 years back to Earth in case of an abort. This route comes also at the cost of a higher delta-v of 5.08 km/s. Zubrin writes that faster routes have a significantly higher delta-v cost and free-return duration (e.g. transfer to Mars in 130 days takes 7.93 km/s delta-v and 4 years on the free return), and so he advocates for the 180-day transfer. A free return is also the part of various other mission designs, such as Mars Semi-Direct and Inspiration Mars. There also exists the option of two- or three-year free-returns that do not rely on the gravity of Mars, but are simply transfer orbits with periods of 2 or 1.5 years, respectively. A two-year free return means from Earth to Mars (aborted there) and then back to Earth all in 2 years. The entry corridor (range of permissible path angles) for landing on Mars is limited, and experience has shown that the path angle is hard to fix (e.g. +/- 0.5 deg). This limits entry into the atmosphere to less than 9 km/s. On this assumption, a two-year return is not possible for some years, and for some years a delta-v kick of 0.6 to 2.7 km/s at Mars may be needed to get back to Earth. NASA published the Design Reference Architecture 5.0 for Mars in 2009, advocating a 174-day transfer to Mars, which is close to Zubrin's proposed trajectory. It cites a delta-v requirement of approximately 4 km/s for the trans-Mars injection, but does not mention the duration of a free return to Earth. See also Gravity turn in orbital redirection Patched conic approximation Distant retrograde orbit References External links Gravitational n-body simulation of a lunar free-return trajectory Three-body orbits Earth orbits Apollo program Apollo 13 Human missions to Mars