The ExoMars Trace Gas Orbiter arrived at Mars in October 2016 to investigate the origin of trace gases in the atmosphere. However, upon arrival, the spacecraft was in a highly elliptical four-day orbit at an altitude ranging from 200 to 98,000 km, which was not suitable for its mission. To alter this orbit, in March 2017 the TGO started a manoeuvre called aerobraking, which involves repeatedly hitting the very top layer of the Martian atmosphere to reduce its velocity.
“Since March 2017, we’ve been conducting a terrifically delicate aerobraking campaign, during which we commanded it to dip into the wispy, upper-most tendrils of the atmosphere once per revolution, slowing the craft and lowering its orbit,” says European Space Agency’s flight director Michel Denis.
The aerobraking campaign was completed on 20 February, when the spacecraft fired its thrusters for about 16 minutes to raise the closest approach to the surface to about 200 km. This effectively ended the campaign, leaving it in an orbit of about 1050 x 200 km. This orbit will be tuned over the next month in a series of 10 thruster manoeuvres to bring it into its final mission orbit at about 400 km altitude in mid-April.
Searching for traces of life on Mars
Once the team finalizes the tests and calibrates the spacecraft’s instruments, the TGO will begin its primary mission of making a detailed inventory of atmospheric trace gases. This inventory could provide evidence of methane and other gases that could be a sign of active biological or geological activity on Mars. On Earth, most of the methane in the atmosphere is produced by lifeforms. The main objective of the TGO will be to establish if this is also the case on Mars.
A suite of four science instruments will make complementary measurements of the atmosphere, surface and subsurface. Its camera will help to brand features on the surface that may be related to trace-gases sources, such as volcanoes. Finally, the spacecraft will also look for water-ice hidden just below the surface, which along with potential trace gas sources could guide the choice for future mission landing sites.