25Sep2012

Dreaded Deimos May Disappear!

An enhanced-color image of Deimos taken by the Mars Reconnaissance Orbiter. Image: NASA/JPL/University of Arizona

Fear and dread…! The Martian moon Deimos was discovered by Asaph Hall, Sr. at the U.S. Naval Observatory in Washington, D.C on August 12, 1877.

Deimos’ orbital patterns still aren’t really known to astronomers. However, thanks to new photographic observations taken by ESA’s Mars Express Orbiter, we understand much more about this “dreaded” topic. Could it one day disappear?! Since their discovery, both Deimos and Phobos have been carefully observed by telescopes here on Earth and from spacecraft such as the Mars Exploration Rovers and the Mars Reconnaissance Orbiter.

Phobos’ orbit has been calculated to an accuracy of less than one kilometer, but Deimos remains defiant. Thanks to an advanced technique, researchers from Germany and Russia have now been able to further calculate the small moon’s orbit by comparing images with those taken by Mars Express. What we know is Deimos has an orbit that’s nearly circular and this orbit is at a mean distance of 23,458 km from Mars’ center… but what makes this study unique?

SRC images of Deimos obtained from different orbits. Credit: ESA/ DLR/FU Berlin (G. Neukum)

The answer is the Mars Express orbit. Unlike other spacecraft, it follows an elliptical, near-polar orbit which allows it to periodically obtain exceptional views of Deimos. For slightly over six years, the Mars Express made 50 approaches to Deimos, getting within 14,000 km (and closer) of this small moon. Unfortunately, Deimos, like our own Moon, is tidally locked to Mars, so the orbiter’s observations were mostly of the same surface areas.

According to the news release, “136 images were acquired at different places along Deimos’ orbit by the Super Resolution Channel (SRC) of the High Resolution Stereo Camera (HRSC). The SRC is a 1K × 1K CCD-framing camera which is designed to focus on features of interest within the HRSC image strips, when imaging Mars. In comparison with the HRSC, it magnifies features in the image by a factor of about four. In the case of Deimos, the framing images are ideal for astrometric (positional) measurements of the small Martian satellite.”

With navigational data provided by the European Space Operations Center, the researchers knew exactly what direction the cameras were pointing – giving them exceptional astrometric measurements. “The attitude of the spacecraft (and the pointing of the body-mounted camera) is measured by using two star trackers and three laser gyroscopes. The SRC pointing was verified and corrected for by measuring differences between the observed and predicted positions of background stars in the images. Owing to the SRC’s narrow field of view, usually one or two faint stars per image could be observed. The precise positions of these stars are known from catalogues based on data returned by ESA’s Hipparcos satellite.”

Orbits of Mars Express and the Martian moons. This simulation depicts the relative orbits of Phobos, Deimos and Mars Express around the time of the flyby of Phobos on 7 March 2010. Credit: ESA/DLR/FU Berlin (G. Neukum) and S. Walter/Celestia/NAIF/SPICE

From here, the researchers then tell how they used a new astrometric technique – one which the center point of Deimos was derived by fitting the predicted limb edge to the known limb. By imaging Deimos moving across the field in a sequence of seven or eight shots over a 1.5 to 3.5 minute period, they we able to take in background stars and capture the elusive little moon.

“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, lead author of the paper. “We obtained a set of spacecraft-centred 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 centre 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.”

Why bother? Oddly enough, it is to predict the eventual fate of the Martian moons – dread and fear. Phobos is deeply involved in Mars’ gravity and will probably either impact the planet or shatter. However, more distant Deimos appears to be spiralling away. By learning more about their orbits, we can further our understanding of the gravity field data – allowing us to model their interiors and put clear margins 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 favoured in recent years,” said Olivier Witasse, ESA’s Mars Express project scientist. “Simultaneous modelling 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.”

Original Story Source: ESA Mars Orbiter News Release. Submitted by Tammy Plotner for “Dave Reneke’s World of Space and Astronomy”.

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