[meteorite-list] Where is Deimos? (Mars Express)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 27 Sep 2012 12:36:55 -0700 (PDT)
Message-ID: <201209271936.q8RJau1t001844_at_zagami.jpl.nasa.gov>


Where is Deimos?
European Space Agency - Mars Express
24 Sep 2012

Despite more than a century of observations, the orbit of the Martian
moon Deimos is still not known to a high degree of accuracy, but a new
study using images taken by ESA's Mars Express orbiter has provided the
best orbital model to date.

135 years have passed since Asaph Hall discovered Phobos and Deimos, two
small companions of the planet Mars. 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 which compares
images taken by Mars Express.

Deimos follows an almost circular, near-equatorial orbit at a mean
distance of 23 458 km from the centre 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 9600 km.
However, since the moon is tidally locked to the planet, the spacecraft
largely observes the same Mars-facing areas on its surface.

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

Any astrometric measurement requires good knowledge of the observer's
location and viewing direction. In the case of the observations from
Mars Express, the position of the spacecraft and the direction in which
the camera was pointing were derived from navigational data provided by
the European Space Operations Centre (ESOC) in Darmstadt, Germany.

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.

In a paper accepted for publication in "Astronomy & Astrophysics", the
researchers describe how they used a new astrometric technique, in which
the centre-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," 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."

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 spiralling 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.

"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."


Andreas Pasewaldt
German Aerospace Center (DLR)
Institute of Planetary Research
Berlin, Germany
Email: Andreas.Pasewaldt at dlr.de
Phone: +49-0-67055-693 (office); +49-176-513-599-01 (mobile)

Olivier Witasse
Mars Express Project Scientist
Research and Scientific Support Department
Directorate of Science & Robotic Exploration
ESA-ESTEC, The Netherlands
Email: Olivier.Witasse at esa.int
Phone: +31-71-5658015
Received on Thu 27 Sep 2012 03:36:55 PM PDT

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