Artist’s impression of the ExoMars 2016 Trace Gas Orbiter at Mars. Credit: ESA/ATG medialab
Skimming an alien atmosphere
After the smooth arrival of ESA’s latest Mars or-
biter, mission controllers are now preparing it for
the ultimate challenge: dipping into the Red Plan-
et’s atmosphere to reach its final orbit.
The ExoMars Trace Gas Orbiter is on a multiyear
mission to understand the tiny amounts of meth-
ane and other gases in Mars’ atmosphere that
could be evidence for possible biological or geo-
logical activity.
Following its long journey from Earth, the orbiter
fired its main engine on 19 October 2016 to brake
sufficiently for capture by the planet’s gravity.
It entered a highly elliptical orbit where its alti-
tude varies between about 250 km and 98 000 km,
with each circuit taking about four Earth days.
Ultimately, however, the science goals and its
role as a data relay for surface rovers mean the
craft must lower itself into a near-circular orbit at
just 400 km altitude, with each orbit taking about
two hours.
Mission controllers will use ‘aerobraking’ to
achieve this, commanding the craft to skim the
wispy top of the atmosphere for the faint drag to
steadily pull it down.
“The amount of drag is very tiny,” says space-
craft operations manager Peter Schmitz, “but after
about 13 months this will be enough to reach the
planned 400 km altitude while firing the engine
only a few times, saving on fuel.”
During aerobraking, the team at ESA’s mission
control in Darmstadt, Germany, must carefully
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monitor the craft during each orbit to ensure it is
not exposed to too much friction heating or pres-
sure.
The drag is expected to vary from orbit to orbit
because of the changing atmospheric, dust storms
and solar activity. This means ESA’s flight dynamics
teams will have to measure the orbit repeatedly to
ensure it does not drop too low, too quickly.
The aerobraking campaign is set to begin on 15
March, when Mars will be just over 300 million km
from Earth, and will run until early 2018.
Mission controllers are now working intensively to
prepare the craft, the flight plan and ground sys-
tems for the campaign.
Late last month, they adjusted the angle of the
orbit with respect to the Mars equator to 74º so
that science observations can cover most of the
planet.
Next, to get into an orbit from where to start
aerobraking, the high point will be reduced on 3
and 9 February, leaving the craft in a 200 x 33 475
km orbit that it completes every 24 hours.
ESA mission controllers have some previous ex-
perience with aerobraking using Venus Express, al-
though that was done at the end of the mission as
a demonstration. NASA also used aerobraking to
take the Mars Reconnaissance Orbiter and other
spacecraft into low orbits at Mars.
“This will be our first time to use aerobraking to
achieve an operational orbit, so we’re taking the
extra time available now to ensure our plans are
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