After a long cruise and eventful arrival, the ExoMars mission gets down to work next week. The Trace Gas Orbiter (TGO) will make its first observations of Mars during two orbits to test and calibrate instruments in preparation for a detailed study of the atmosphere of the Red Planet beginning in 2018.

The unmanned ESA/Roscosmos mission, which arrived at Mars on October 19, is in a highly elliptical orbit at an altitude of between 230 and 98,000 km (143 to 69,000 mi) with a period of 4.2 Earth days. It's currently carrying out aerobraking maneuvers, where the spacecraft periodically skims the upper reaches of the Martian atmosphere. This slows it down and will make its orbit more circular until it reaches an altitude of 400 km (250 mi) by March 2018. At that point, the TGO will begin the science part of its mission.

In the meantime, it will be either too far from the planet or moving too fast for useful operations. However, the orbiter won't be twiddling its metaphorical thumbs. Starting next week, mission control will bring the spacecraft's instruments online and carry out calibration tests to make sure they're working properly and sending back accurate information.

One of the most important series of tests will involve the neutron detector, which has already been taking observations en route from Earth to Mars. This check is especially important after the launch of the Schiaparelli lander changed the spacecraft's configuration.

The neutron detector is designed to seek out neutrons created by the impact of cosmic rays on Mars, which will allow scientists to learn more about the composition of the Martian surface. It will also aid in seeking ice or water to a depth of two meters.

In addition, the orbiter's other two primary instruments will be used to make a preliminary inventory of the trace gases in the Martian atmosphere, with special emphasis on finding methane – a key indicator of present-day microbial life. This will include looking straight into the atmosphere and at an oblique angle to catch the Sun shining through it in order to gauge the effect of sunlight on the atmosphere's constituents.

How the Trace Gas Orbiter takes stereo pair images(Credit: ESA)

Along with the instruments, the TGO's imaging camera will take its first pictures of Mars and of the stars, to help calibrate the camera's position in relation to the surface. Because it will be moving very fast, timing the images will be tricky, but ESA hopes to take images in sequence to produce stereo pairs. This will produce three dimensional images of the surface and help scientists to measure the height of any features discovered. Even if the images don't come out as planned, the exercise will still allow mission control to test the camera's filters and stereo rotation mechanism. On the second orbit, ESA hopes to carry out observations of the Martian moon Phobos.

Ahead of the main phase of its mission, the TGO will also be kept busy relaying data from NASA's Curiosity and Opportunity rovers.

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