Comparison of Deimos and Phobos as Destinations for Human Exploration and Identification of Preferred Landing Sites

A human mission to one of the two moons of Mars has been suggested as an easier precursor before a mission to land on Mars itself. Astronauts would explore the moon in person and teleoperate rovers on the surface of Mars with minimal lag time, returning samples to Earth. Lockheed Martin evaluated such a mission as part of its Stepping Stones sequence of missions in the spirit of the “Flexible Path” approach advocated by the Augustine Committee. In this paper, we compare Deimos and Phobos as potential destinations, including trajectory design, communications access to Earth and the Martian surface, solar illumination, expected radiation environment, planetary protection issues, and physical access to and from the Martian surface. While prior mission concepts have tended to focus on Phobos, we conclude that Deimos is the better destination for an early teleoperation mission largely because it is farther from Mars than Phobos. This reduces the required mission ΔV by 400 m/s, provides longer communications access and line of sight to 15 deg higher latitudes on the Martian surface, and reduces the frequency and cumulative duration of eclipses by Mars so that a solar powered mission is easier on Deimos than on Phobos. Using a shape model of Deimos, we performed global lighting and communications access analysis and determined that there are two specific regions on Deimos which are the most favorable landing sites. Small areas along the North and South arctic circles on the Mars-facing side of Deimos experience a continuous view of Mars, continuous sunlight for up to ten months during polar summer, and continuous line of sight to Earth during most of the sunlit season. These sites are centered near 60° N 0° W, and 51° S 7° E. A timeline for a mission to these two sites is provided for the 2033-2035 opportunity. This is the easiest opportunity during the next few decades because optimum Earth-to-Mars orbital geometry will likely coincide with the phase of the solar activity cycle that provides the most protection from galactic cosmic rays, reducing the effective radiation dose. During this mission, the crew would land at the southern hemisphere site first, during the middle of the southern summer season. After a four month stay, the crew would depart the surface of Deimos to orbit for 50 days during the equinox and eclipse season, when lighting is unfavorable at any location on the Deimos surface. At the beginning of northern summer, the crew would land at the northern site and stays for ten months before returning to Earth. In this way, the crew can explore both hemispheres of Deimos without requiring advanced power systems.