Posted on 16 December 2016 by Daniel
Skimming an alien atmosphere
After the textbook-perfect arrival of ESA’s ExoMars orbiter, mission controllers are preparing for the ultimate challenge: using drag from the Red Planet’s atmosphere to lower the spacecraft into its final Mars orbit.
Editor's note: Today's post provides a few additional details for the news article "Skimming an alien atmosphere" published today in the main ESA website.
Aerobraking is an innovative technique in which the craft will be commanded to skim the wispy tops of the martian atmosphere, causing a tiny amount of drag that will slow TGO and steadily lower it onto the required two-hour, 400-km orbit.
After approximately thirteen months, which includes a two-month pause during a solar conjunction, this will cumulatively be enough to lower TGO’s orbit to the planned 400-km altitude while firing the engine only a few times, eliminating the need to have brought along a large quantity of fuel.
Optimising the aerobraking campaign
It may be possible to optimise the aerobraking campaign by making a clever adaptation of the use of TGO’s high-gain antenna, a steerable 2.2m-diameter dish used to communicate with Earth.
The current concept is to keep the antenna stowed during the campaign, in particular during each aerobraking skim through the atmosphere, with TGO oriented (‘slewed’) into a special attitude.
ExoMars/TGO first year in orbit
Later in each orbit, the spacecraft must be rotated back to point to Earth and communicate – and this process takes a lot of time.
“We are currently studying whether it’s feasible to have the antenna unstowed part of the time and use its steering mechanism to simply turn it toward Earth rather than slewing the entire spacecraft, which would save time and increase the communication and spacecraft tracking windows, which are required for the very frequent orbit determinations that will have to be done,” says TGO Spacecraft Operations Manager Peter Schmitz.
Uncertainty in how the Mars atmospheric density will slow the craft means that the expected time slots when the spacecraft will be able to communicate with Earth may vary from what’s planned – and this will affect the necessary orbit determination work, needed to allow TGO to continue on to the next orbits.
“The process of skimming the atmosphere, re-establishing communication, checking the resulting orbit and then continuing on to the next aerobraking pass will be a navigational ‘cat-and-mouse’ game on a planetary scale, lasting many months,” says TGO Flight Director Michel Denis.
ESA mission controllers have some previous experience with aerobraking using Venus Express, although that was done at the end of the mission as a technology demonstration. There were also significant differences in the two atmospheres.
"The atmosphere at the surface of Venus indeed is much higher than that of Mars, but in the region where Venus Express did aerobraking, the density of the atmosphere of Venus is less than where ExoMars will do it at Mars. The reason is that the velocity of VEX was much higher as Venus is a much more massive planet," says Project Scientist Håkan Svedhem.
NASA also used aerobraking to bring the Mars Reconnaissance Orbiter and other spacecraft into low orbit at Mars.