Mars Express helps create best orbital model of Deimos

A new study using images taken by ESA’s Mars Express orbiter has provided the best orbital model of the Martian moon Deimos to date.

Washington: A new study using images taken by ESA’s Mars Express orbiter has provided the best orbital model of the Martian moon Deimos to date.

Asaph Hall discovered Phobos and Deimos, two small companions of the planet Mars, 135 years ago.

Since that time, the satellites have been imaged innumerable times from the Earth and from spacecraft, including recent measurements by the panoramic cameras on the Mars Exploration Rovers and instruments on the Mars Reconnaissance Orbiter.

Although the orbit of the inner moon, Phobos, has been calculated to an accuracy of less than 1 km, the path of more remote Deimos is less well known. In order to improve the orbital models for Deimos, researchers from Germany and Russia have developed a new technique that compares images taken by Mars Express.

Deimos follows an almost circular, near-equatorial orbit at a mean distance of 23,458 km from the center of Mars. Unlike other Mars orbiters, Mars Express follows an elliptical, near-polar orbit, which occasionally enables it to obtain excellent views of Deimos.

Between July 2005 and July 2011, the spacecraft made 50 approaches to Deimos, passing within 14,000 km of the satellite. The closest approach was in March 2011, when the orbiter closed to a range of about 9,600 km. However, since the moon is tidally locked to the planet, the spacecraft largely observes the same Mars-facing areas on its surface.

The Super Resolution Channel (SRC) of the High Resolution Stereo Camera (HRSC) acquired 136 images at different places along Deimos’ orbit.

In the new study, the researchers describe how they used a new astrometric technique, in which the center-of-figure of non-spherical Deimos was determined by fitting the predicted limb (visible edge) of the satellite to the observed limb.

Over a period of 1.5 to 3.5 minutes, a sequence of seven or eight images was acquired as Deimos moved across the field of view. In all cases, the first and the last image were taken with long time exposures (about 500 ms) to capture faint background stars (magnitudes ranging from 3.4 to 8.8). From the five or six short-time exposures, two to four images usually included Deimos.

“From 50 sets of observations, we fortuitously had nine in which stars were sufficiently bright to be seen in all images. We obtained a set of spacecraft-centered Deimos coordinates with accuracies between 0.6 and 3.6 km,” said Andreas Pasewaldt, a PhD student at the Institute of Planetary Research in Berlin, lead author of the paper.

“Using a shape model, together with nominal data on Deimos’ position and rotational state, we predicted the limb that would be observed from the spacecraft. This limb was projected onto the SRC image, and then fitted to the observed limb during a series of manual and automated steps. This eventually gave us the precise position of the center of figure for Deimos.

“Comparisons with current orbit models indicate that Deimos is ahead of, or falling behind, its predicted position by as much as +3.4 km or -4.7 km, depending on the chosen model. The data obtained by our ‘limb fit method’ should considerably improve the models of its orbit,” he noted.

There is considerable interest in the orbital tracking of the Martian moons. Phobos, moving deep within the gravity field of Mars, is strongly affected by tidal interaction with the planet. This will eventually cause the moon to crash into Mars or break apart, creating a ring of debris. In contrast, Deimos is far enough from Mars to take more than one Martian day to complete each orbit, so it is spiraling slowly outwards.

Improved knowledge of their orbits will also shed new light on the history of the satellite system. Such knowledge is particularly important in the interpretation of gravity field data, acquired during very close flybys. This enables the researchers to model the interiors of the moons and put constraints on their origin.

“Simultaneous modeling of both orbits may provide strong constraints on the origin and evolution of Phobos and Deimos,” said Olivier Witasse, ESA’s Mars Express project scientist.

“Better orbital models are also important for future satellite missions, such as automated sample returns currently being studied at ESA, when high navigational accuracy is needed,” he added.

A paper describing the study has been accepted for publication in Astronomy and Astrophysics.