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Dr Craig Leff (MSSL)
ExoMars 2016 and 2020 are primarily technology missions rather than full science missions. The 2016 mission comprised the Trace Gas Orbiter (TGO) and the Schiaparelli lander, which was meant to test Entry, Descent and Landing (EDL) techniques for the 2020 mission. A navigational anomaly led to the lander crashing, but TGO successfully entered Martian orbit. It has a science mission to look for trace gases such as methane that could be signs of life, study atmospheric chemistry, look for subsurface water by neutron imaging and obtain high-resolution images of possible landing sites. It will also act as a communications link to Earth for future missions.
The ExoMars 2020 mission will deliver a Russian base station and a European rover to the Martian surface. The rover will search for signs of life and subsurface water, and also test technology for a future sample return mission. It will use a drill to take samples from a depth of 2 metres, below the estimated 1.5 metre layer that has been sterilised by UV light, oxidation and radiation. It will also carry a ground-penetrating radar system. It could discover microbes or organic materials. Special planetary protection protocols will be followed in constructing the probe to minimise the chance of contaminating Mars with terrestrial bacteria.
The landing sites must be old terrain from the Noachian era (hence potentially habitable), have signs of clay or other water-related minerals, be low enough for parachutes to work and be at low latitude so there is enough sunlight to power the rover. With current EDL techniques, only 3 regions meet all these criteria: Oxia Planum, Mawrth Vallis and Aram Dorsum. All of these have been visited before.
There will be 9 instruments including the UK-led PanCam, and 3 instruments in a “warm lab” on the rover to analyse soil samples. PanCam is a multi-camera system with 2 wide-angle cameras plus a telephoto camera with 12 glass filters transmitting different wavelength bands, all mounted on top of a 2-metre high mast. The rover will carry a variety of spatial reference points, and a set of colour calibration targets made from stained glass, which is more stable than paint. The stereo view from the wide-angle cameras will provide navigational data and context for the other images and instruments. The device has been tested in a simulated Martian landscape on Svalbard.
Rovers are not driven in “real time” from Earth: the 10 to 20 minute delay in transmissions to Mars makes this impractical. Operating a rover requires a set of commands to be sent every day, usually while the rover is inactive at night. When it reactivates itself it executes the commands with some local decision-making, for example to stop if it detects a hazard. Rovers use different driving modes. In “blind drives” the rover moves for a short set distance or for a specified time. “Auto-navigation” uses the navigation cameras to move further, but more slowly to allow for unexpected obstacles. “Visual odometry” counts wheel revolutions and compares side-looking images to determine the distance moved. The rover may instead use its instruments to carry out science at its current location. At the end of the sol (Martian day), the rover transmits its science and engineering data to the Deep Space network. Mission controllers review the data and develop the set of commands for the next sol’s activities.
Operating a rover requires detailed planning and careful control of resources both on Mars and on Earth. Rover safety is the highest priority, so controllers have to be reactive if things do not go as planned. Mission planning assumes rovers will have a limited lifetime; however, Opportunity is nearing 5000 sols of operation, Spirit achieved 2210 and Curiosity has so far had over 1800.
Images and data from Curiosity can be viewed at http://www.midnightplanets.com/
Notes and summary by Chris Hooker.
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