It has to be one of the slowest parking attempts ever made, but ESA's ExoMars Trace Gas Orbiter (TGO) has completed a daring maneuver that saw it surfing the outer layers of the Martian atmosphere for 11 months. The purpose of the exercise was to gradually lower the unmanned probe's trajectory to place it in a planet-hugging, near-circular orbit at an altitude of about 400 km (250 mi), allowing the spacecraft to begin its mission to study trace gases on Mars as well as act as a communications relay between Mars surface rovers and Earth.

Launched on March 14, 2016 atop a Proton-M rocket from the Baikonur Cosmodrome in Kazakhstan, the ExoMars Trace gas Orbiter arrived at Mars on October 19 of that year. However, the spacecraft was in a highly elliptical four-day orbit at an altitude ranging from 200 to 98,000 km (125 to 61,000 mi), which was completely unsuitable for its mission.

To alter this orbit in the most economical manner possible, in March 2017 mission control ordered the TGO to carry out a maneuver called aerobraking, which involves repeatedly hitting the very top layer of the Martian atmosphere to reduce its velocity and was first tried with ESA's Venus Express orbiter in 2014. Though to a human observer the atmosphere at about 103 km (64 mi) above Mars may seem like hard vacuum, there is enough air present to allow the solar panels on the TGO to act like the air brakes on an aircraft and slow it down.

According to ESA, this braking isn't much – only decelerating the spacecraft by 17 mm per second per second, which would bring a car traveling at 50 km/h (31 mph) to a stop over a distance of 6 km (3.7 mi). It's also a very tricky maneuver because with Mars at an average distance of 225 million km (140 million mi), it as to be carried out by the TGO under autonomous control. Worse, the Martian atmosphere expands greatly during the day and the summer months, so a single miscalculation could result in a probe burning up in an uncontrolled entry.

The braking took over 950 orbits of the TGO and was completed on February 20 at 17:20 GMT when the spacecraft fired its thrusters for 16 minutes, placing it an orbit ranging between 1,050 and 200 km (650 and 125 650 mi). This orbit will be fine tuned over the next month in a series of 10 thruster maneuvers to bring it into its final mission orbit in mid-April.

Once on station, the TGO will have completed a month of instrument tests and calibrations, after which it will begin its primary mission of making a detailed inventory of atmospheric trace gases that could provide key insights into geological activity and possible biological signatures. In addition, it will look for signs of subsurface ice as well as acting as a data-relay for surface lander missions beginning later this year.

The video below discusses the aerobraking maneuver and TGO's mission.

Source: ESA