Investigating the Atmospheres at Venus and Mars through Aerobraking

Abstract

Aerobraking is an effective technique to save significant
amounts of fuel while lowering an orbit around a planet. It was first
applied by NASA at Venus, with Magellan during its extended mission, in
1993. The first aerobraking at Mars was done, also by NASA, with Mars
Global Surveyor, in 1999. ESA did its first experimental aerobraking
with Venus Express, near the end of its mission, at Venus in 2014. ESA’s
ExoMars Trace Gas Orbiter, TGO, was the first ESA spacecraft to do
aerobraking as a part of its nominal mission. TGO reduced its apocentre
attitude to near 400 km, to achieve a near circular orbit, during one
full year of aerobraking, finishing in April 2018.
Remote measurements of atmospheric parameters are difficult at
high altitudes due to the tenuous medium. However, atmospheric drag
experienced during aerobraking is directly proportional to the local
atmospheric density and this drag can be measured accurately by on board
accelerometers and so provide precise data on the local atmospheric
density along the s/c trajectory. Additional, even more sensitive data,
averaged along the trajectory, can be acquired by precise tracking of
the s/c orbit. Large data sets have been collected from both the Venus
Express and the TGO aerobraking campaigns. Both these data sets show
larger than expected day to day variations of the atmospheric densities
of the two planets throughout these campaigns. In spite of the large
differences in the surface pressure/density at the two planets, the
upper atmospheres are surprisingly similar, albeit highly variable.
This talk will focus these atmospheric differences and similarities,
and comparisons with existing models will be made. Potential
improvements in the method and optimisation of measurements will be
evaluated, and applications to ESA’s next Venus mission, ENVISION, will
be discussed.