A slight 'over performance' during a manoeuvre last week had put the spacecraft on a trajectory that included an occultation by Phobos. This meant that Mars Express would pass behind Phobos as seen from Earth. As this would jeopardise the tracking measurements, it was decided to perform another manoeuvre to position the flyby at a slightly higher altitude than originally planned.
After the closest flyby, the work is not over. Mars Express will sweep past Phobos a further seven times before the campaign is complete. In addition to the tracking experiment, known as MaRS for Mars Radio Science, the MARSIS radar has already been probing the subsurface of Phobos with radar beams. "We have performed a preliminary processing of the data and the Phobos signature is evident in almost all the data set," says Andrea Cicchetti, Italian Institute of Physics of Interplanetary Space, Rome, and one of the MARSIS team.
The camera, HRSC, will be used on the 7 March flyby, when Mars Express passes over the daylight side of Phobos at an altitude of 107 km, and will continue to be used during all the subsequent flybys, obtaining high-resolution images of the moon's surface. The other instruments will also get their chance to work.
ASPERA is already studying the way charged particles from the Sun interact with the surface of Phobos. SPICAM, PFS, OMEGA are characterising the surface of the moon, with PFS also aiming to measure the temperature of Phobos' day and night sides. HRSC will pay particular attention to the proposed landing site for the Russian Phobos-Grunt mission, which is expected to launch in 2011/12.
"All the experiments on Mars Express have something to say about Phobos," says Olivier Witasse, Mars Express Project Scientist, ESA. This is a bonus for science, considering that none of them were originally designed to study Phobos the moon, only Mars the planet. The science results from these flybys are expected in subsequent weeks or months, when the various teams have had time to analyse the data.
This animation shows how the orbit of Mars Express has been influenced by the gravitational influence of Phobos during the spacecraft's flybys of the moon in summer 2008. Since the orbital deviation strictly depends on the mass and shape of the moon, scientists could use this very deviation to determine the mass of Phobos with unprecedented accuracy (1.072 1016 kg, or about one billionth the mass of the Earth).
(Photo Credit: MaRS team/Observatoire Royal de Belgique)
On July 23, 2008, the High Resolution Stereo Camera on board the ESA's Mars Express took the highest-resolution full-disc image yet of the surface of the moon Phobos.
The image data was acquired from a distance of 97 km with a spatial resolution of about 3.7 m/pixel in orbit 5851. These images have surpassed all previous images from other missions in continuous coverage of the illuminated surface at the highest spatial resolution of 3.7 m/pixel.
This image is photometrically enhanced to bring out the features in the less illuminated part.
(Photo Credit: ESA/ DLR/ FU Berlin (G. Neukum))
Source: European Space Agency