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Rare opportunities with many benefits

As positional arrangements of the Sun, Mars Express and Phobos where the latter is observed at a phase angle of zero degrees are very rare (arrangements with phase angles of less than one degree occur approximately three times a year), the HRSC imaging planners take every opportunity to acquire such images, regardless of the distance from Phobos. They also make the most of phase angles near to (but not quite) zero degrees. In the image with the lowest phase angle here (the brightest picture in the middle of the animation), it is 0.92 degrees. The next opportunities will not arise until April and September 2020. On the latter date, it will be possible to perform an observation at exactly 0.0 degrees, at a distance of 2900 kilometres and with a resolution of 120 metres per pixel.

In addition to the characterisation of the surface and exploration of their origins, the images of the Martian moons also serve to accurately determine their ephemerides – which give their trajectories around Mars. For this purpose, the scientists primarily use images in which one of the moons is shown together with another object, such as a star or planet. Determining the exact location of these bodies is important, not least for the purpose of missions with the moons as their destination. At present, a Japanese mission (Martian Moons eXploration; MMX) is at the planning stage. Not only will it observe the moons, but will also transport a sample of one of them back to Earth. To this end, a rover jointly developed by DLR and the French space agency CNES is set to land on one of the moons and perform a detailed characterisation of its surface, in preparation for the subsequent acquisition of samples. The mission will be carried out by the Japanese Aerospace Exploration Agency (JAXA) and is currently scheduled for launch in 2024. Knowledge about the moons acquired with HRSC and SRC is indispensable for preparing this mission