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Phobos Shadows The Surface of Mars

File photo of Phobos
by Staff Writers
Darmstadt, Germany (SPX) Feb 21, 2006
New images taken by the High Resolution Stereo Camera aboard ESA's Mars Express spacecraft show the fast-moving shadow of the moon Phobos as it moved across the Martian surface.

The HRSC obtained the unique images during orbit 2345 last Nov. 10. The observations resulted from close co-operation between the camera team at the Institute of Planetary Research at DLR and the ESA teams, in particular the mission engineers at ESA's European Space Operations Center.

The images confirm the model of the moon's orbit around Mars, as determined earlier in 2004, also on the basis of HRSC images. They also show with accurate planning even moving objects can be captured exactly at their predicted position.

The basis for such observations is the accurate knowledge of the spacecraft position in its orbit, as well as of the targeted location on Mars to within a few hundred meters. Phobos is the larger of the two Martian moons, at 27 kilometers by 22 kilometers (17.3 by 14 miles) in size, and it travels around Mars in an almost circular orbit at an altitude of about 6,000 kilometers (3,840 miles).

Phobos takes slightly more than 7.5 hours to complete a full revolution around the planet. When it is between the Sun and Mars, Phobos casts a small and diffuse shadow onto the Martian surface. To an observer on Mars, it would appear as a very quick eclipse of the Sun. The shadow of Phobos has an elliptical shape on the Martian surface, because the shadow's cone hits the surface at an oblique angle. The shadow appears to be distorted even more because of the imaging technique of the HRSC.

Phobos's shadow moves across the surface at a speed of roughly 7,200 kilometers (4,600 miles) per hour, from west to east. Mars Express travels at a higher speed of about 12,600 kilometers (8,064 miles) per hour on its almost polar orbit from south to north.

Because the HRSC scans the surface synchronized with the flight velocity of Mars Express, it takes some time to cover the shadow in its full dimension, so as the moon moves on, the shape of its shadow is smeared in the HRSC image. The shadow is darker at its center than its edges, because Phobos's small size covers only some 20 percent of the solar disk.

The HRSC team recalculated the orbit of Phobos on the basis of images taken in 2004. With the help of the improved orbit determination - the moon has advanced about 12 kilometers (about 7.7 miles) with respect to its previously predicted position along its orbit - it was possible to calculate those precise times when the shadow observations could be made. The scientists verified the accuracy of the improved orbit determination via the shadow's position in the new images.

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Mars Express Studies Possible Aurorae Above Mars
Paris, France (SPX) Feb 20, 2006
ESA's Mars Express spacecraft has seen more evidence that aurorae occur over the night side of Mars, especially over areas of the surface where variations in the magnetic properties of the crust have been detected. Observations from the ASPERA instrument on board ESA's Mars Express spacecraft show structures (inverted-V features) of accelerated electrons and ions above the night side of Mars that are almost identical to those that occur above aurorae on Earth.

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