Deimos Orbital Model Improved
September 26, 2012

Scientists Work To Improve Orbital Model Of Martian Moon Deimos

April Flowers for - Your Universe Online

135 years ago, a scientist named Asaph Hall discovered Phobos and Deimos, two small moons orbiting Mars. Since then, the moons have been photographed innumerable times from Earth and from spacecraft, including recent measurements by the Mars Exploration Rovers panoramic cameras and the Mars Reconnaissance Orbiter.

The orbit of Phobos, the inner moon, has been calculated to an accuracy of less than 1 kilometer. But, despite this century of interest and study, the orbit of Deimos is still not known to a high degree of accuracy. However, a new study by ESA's Mars Express orbiter has provided the best orbital model to date. The study will be published in an upcoming issue of Astronomy & Astrophysics.

Researchers from Germany and Russia have developed a new technique to compare images taken by Mars Express in an attempt to improve the orbital models for Deimos.

Deimos' orbit is nearly circular and near-equatorial, at a mean distance of 23,458 km from the center of Mars. In contrast to other Mars orbiters, Mars Express follows an elliptical, near-polar orbit, giving it exceptional views of Deimos at times.

Passing within 14,000 km of the moon, Mars Express made 50 approaches to Deimos between July 2005 and July 2011. The closest was in March 2011, when Mars Express closed to a range of about 9,600 km. Deimos is tidally locked to Mars, meaning that Mars Express largely observes the same Mars-facing areas on its surface.

Mars Express acquired 136 images at different places along Deimos' orbit using the Super Resolution Channel (SRC) of the High Resolution Stereo Camera (HRSC). The SRC is a 1K x 1K CCD-framing camera designed to focus on elements of interest within the HRSC image strips. Unlike the HRSC, the SRC magnifies the features by a factor of about four. The framing images are ideal for astrometric measurements of Deimos.

Any astrometric, or positional, measurement requires solid knowledge of the observer's location and the viewing direction. In this case, those aspects were derived from navigational data provided by the ESA Operations Center (ESOC) in Darmstadt, Germany.

Mars Express' attitude and the pointing of the body-mounted camera are measured by using two star trackers and three laser gyroscopes. Using the difference between the observed and predicted positions of known stars, the SRC pointing was verified and corrected. SRC has a narrow field of view, so usually only one or two faint stars could be seen per image. The precise position of these stars is known from catalogues based on data from ESA's Hipparcos satellite.

The research team developed a new astrometric technique, in which the center-of-figure of non-spherical Deimos was determined by fitting the predicted visible edge, or limb, of the satellite to the observed limb.

A sequence of seven or eight images was obtained over a period of 1.5 to 3.5 minutes as Deimos moved across the field of view. On each pass, the first and last image of each set was taken with a long time exposure (about 500 ms) to capture faint background stars with magnitudes ranging from 3.4 to 8.8. Two to four of the short time exposure 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," said Andreas Pasewaldt, a PhD student at the Institute of Planetary Research in Berlin. "We obtained a set of spacecraft-centered Deimos coordinates with accuracies between 0.6 and 3.6 km."

"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."

Astronomers are very interested in the orbital tracking of the Martian moons. Phobos moves deep within the gravity field of Mars and is strongly affected by the tidal interactions with the planet. Eventually the moon will collide with Mars or break apart, creating a ring of debris. Deimos, however, takes more than one Martian day to complete each orbit and is slowly spiraling away from Mars.

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

"It is unclear whether they are asteroids that were captured by Mars or whether they coalesced from a ring of material that formed around the planet after a large object collided with Mars, although the latter scenario seems to be favored in recent years," said Olivier Witasse, ESA's Mars Express project scientist. "Simultaneous modeling of both orbits may provide strong constraints on the origin and evolution of Phobos and Deimos."

"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."