Investigating the Atmospheres at Venus and Mars through Aerobraking

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.