While many people are taking it easy for the holiday season, ESA is gearing up for a very busy New Year. On January 19, the space agency will begin an unprecedented and complicated year-long maneuver to radically alter the orbit of the ExoMars 2016 Trace Gas Orbiter (TGO) around Mars using the Red Planet's atmosphere as a giant brake.
Since it arrived in Martian orbit on October 19, the TGO has been in a highly eccentric orbit between an altitude of 250 km (155 mi) and 98,000 km (60.895 mi) with a period of four Earth days. The problem is ESA wants it in a circular orbit of 400 km (249 mi) and a period two hours.
The TGO doesn't carry nearly enough propellant on board for such a maneuver, so mission control will use a technique known as aerobraking to alter the orbit. In the 1984 sci fi feature 2010: The Year We Make Contact, this was depicted as a spectacular feat where a manned spacecraft from Earth goes into Jupiter orbit by making a fiery passage through the giant planet's atmosphere.
The TGO isn't going to do anything so drastic, for no other reason than it would never survive. Instead, it will make a series of dips into the rarefied upper reaches of the Martian atmosphere that will slow the spacecraft down by a tiny amount. After 13 months of such dips, the TGO will be in the planned orbit while only firing its main engine a few times for course corrections.
According to ESA, the tricky thing about such a long maneuver is that it requires constant attention by mission control in Darmstadt, Germany. Because the Martian atmosphere is only hundredth the density of Earth's, sunlight, seasonal changes, and dust storms can cause it to expand and contract by a surprising amount. This means each passage by the TGO must be carefully controlled and monitored, so it doesn't burn up, skip off into space, or miss the atmosphere entirely.
The space agency says the first aerobrake maneuver will happen on March 15 with thrusters being fired seven times. But to arrange that, the TGO must alter its orbit to 74º of the Martian equator. On February 3 and 9, its orbit will be reduced, so it can start skimming the atmosphere. Then by March it will be in a position to start braking properly.
"Then the atmosphere can start its work, pulling us down," says spacecraft operations manager Peter Schmitz. "If all goes as planned, very little fuel will then be needed until the end of aerobraking early in 2018, when final firings will circularize the 400 km orbit."
The video below shows the TGO's expected path around Mars during its first year.