Where is Deimos?

 

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.

Mars Express image of the moons Phobos (foreground) and Deimos (background). Image Credit: ESA/DLR/FU Berlin (G. Neukum)

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 satellite.

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

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.

Simulation of the Phobos flyby of 7 March 2010, showing the relative orbits of Phobos and Deimos and the Mars Express spacecraft. Image Credit: S. Walter/Celestia/NAIF/SPICE; ESA/DLR/FU Berlin (G. Neukum)

“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.”

Source: European Space Agency (ESA)

Curiosity Targets Unusual Rock on Its Journey

 

NASA’s Mars rover Curiosity has driven up to a football-size rock that will be the first for the rover’s arm to examine.

The drive by NASA's Mars rover Curiosity during the mission's 43rd Martian day, or sol, (Sept. 19, 2012) ended with this rock about 8 feet (2.5 meters) in front of the rover.

The drive by NASA’s Mars rover Curiosity during the mission’s 43rd Martian day, or sol, (Sept. 19, 2012) ended with this rock about 8 feet (2.5 meters) in front of the rover. Image Credit: NASA/JPL-Caltech

Curiosity is about 8 feet (2.5 meters) from the rock. It lies about halfway from the rover’s landing site, Bradbury Landing, to a location called Glenelg. In coming days, the team plans to touch the rock with a spectrometer to determine its elemental composition and use an arm-mounted camera to take close-up photographs.

Both the arm-mounted Alpha Particle X-Ray Spectrometer and the mast-mounted, laser-zapping Chemistry and Camera Instrument will be used for identifying elements in the rock. This will allow cross-checking of the two instruments.

The rock has been named “Jake Matijevic.” Jacob Matijevic (mah-TEE-uh-vik) was the surface operations systems chief engineer for Mars Science Laboratory and the project’s Curiosity rover. He passed away Aug. 20, at age 64. Matijevic also was a leading engineer for all of the previous NASA Mars rovers: Sojourner, Spirit and Opportunity.

Curiosity now has driven six days in a row. Daily distances range from 72 feet to 121 feet (22 meters to 37 meters).

This mosaic from the Mast Camera on NASA’s Curiosity rover shows a close-up view looking toward the “Glenelg” area, where three different terrain types come together. Image Credit: NASA/JPL-Caltech/MSSS

“This robot was built to rove, and the team is really getting a good rhythm of driving day after day when that’s the priority,” said Mars Science Laboratory Project Manager Richard Cook of NASA’s Jet Propulsion Laboratory in Pasadena, Calif.

The team plans to choose a rock in the Glenelg area for the rover’s first use of its capability to analyze powder drilled from interiors of rocks. Three types of terrain intersect in the Glenelg area — one lighter-toned and another more cratered than the terrain Curiosity currently is crossing. The light-toned area is of special interest because it retains daytime heat long into the night, suggesting an unusual composition.

“As we’re getting closer to the light-toned area, we see thin, dark bands of unknown origin,” said Mars Science Laboratory Project Scientist John Grotzinger of the California Institute of Technology, Pasadena. “The smaller-scale diversity is becoming more evident as we get closer, providing more potential targets for investigation.”

Researchers are using Curiosity’s Mast Camera (Mastcam) to find potential targets on the ground. Recent new images from the rover’s camera reveal dark streaks on rocks in the Glenelg area that have increased researchers’ interest in the area. In addition to taking ground images, the camera also has been busy looking upward.

Mars has two small, asteroid-sized moons named Phobos and Deimos. From the point of view of the rover, located near the equator of Mars, these moons occasionally pass in front of, or “transit,” the disk of the Sun. Image Credit: NASA/JPL-Caltech/MSSS

On two recent days, Curiosity pointed the Mastcam at the Sun and recorded images of Mars’ two moons, Phobos and Deimos, passing in front of the Sun from the rover’s point of view. Results of these transit observations are part of a long-term study of changes in the moons’ orbits. NASA’s twin Mars Exploration Rovers, Spirit and Opportunity, which arrived at Mars in 2004, also have observed solar transits by Mars’ moons. Opportunity is doing so again this week.

“Phobos is in an orbit very slowly getting closer to Mars, and Deimos is in an orbit very slowly getting farther from Mars,” said Curiosity’s science team co-investigator Mark Lemmon of Texas A&M University, College Station. “These observations help us reduce uncertainty in calculations of the changes.”

In Curiosity’s observations of Phobos this week, the time when the edge of the Moon began overlapping the disc of the sun was predictable to within a few seconds. Uncertainty in timing is because Mars’ interior structure isn’t fully understood.

Phobos causes small changes to the shape of Mars in the same way Earth’s Moon raises tides. The changes to Mars’ shape depend on the Martian interior which, in turn, cause Phobos’ orbit to decay. Timing the orbital change more precisely provides information about Mars’ interior structure.

During Curiosity’s two-year prime mission, researchers will use the rover’s 10 science instruments to assess whether the selected field site inside Gale Crater ever has offered environmental conditions favorable for microbial life.

Source: National Aeronautics and Space Administration (NASA)