During the last of a series of eight encounters with the martian moon Phobos, the High-Resolution Stereo Camera (HRSC) on ESA's Mars Express spacecraft acquired a detailed view of the martian satellite. German DLR has compiled an enormous 40mp image of the small moon

The orbiter flew past Phobos at a distance of only 100km on 9 January 2011 and imaged the southern hemisphere of the irregularly-shaped moon. Researchers at the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) planned the image acquisitions, during which the orbiter was moving past Phobos at 2.3 kilometres per second, and processed the resulting data.

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Since Mars express flies in an elliptical orbit around the Red Planet, it regularly moves away from Mars and approaches Phobos, which orbits the planet at an altitude of roughly 6000km, about every five months. With data obtained during this flyby, the researchers were able to image large parts of the southern hemisphere of the moon for the first time at a resolution of 3,8m/pixel. HRSC scanned the martian moon, which is just over 20km across, with five of its nine sensors that are located sequentially in the camera. The sensors were active for a total of about a minute per sensor; Phobos was in the field of view for only 9 seconds.

Klaus Dieter Matz, a researcher at the DLR Institute of Planetary Research (Institut für Planetenforschung), planned the image acquisition. To obtain the sharpest possible images given the high speed of the spacecraft during the flyby, Mars Express had to slew during the rendezvous. For the computation of this complicated manoeuvre, the researchers considered the orbit of the moon and the exact path of the Mars Express orbiter, which ESA determines from a variety of parameters. Corrections during image acquisition were not possible as control signals transmitted from ground stations on Earth would have taken 19 minutes and 47,4 seconds to reach the spacecraft, but he manoeuvre was accomplished without any problems. However, "The maximum speed at which ESA could turn Mars Express during the flyby was 0,15 degrees/second. For ideal images, the spacecraft must be rotated at 0,26 degrees per second during the high-speed flyby," explained Matz. The effects caused by the limited slew rate were corrected later during processing

Source: DLR