Venus Upper Atmosphere – Results from the Venus Express Aerobraking Campaign

Abstract:

During the last year of operations Venus Express was conducting an experimental aerobraking campaign. The objectives were twofold; firstly, to exercise for the first time the techniques of orbit modification by aerobraking with a European spacecraft, in order to prepare for future scientific ESA missions, and secondly, to collect information on atmospheric density by direct measurements in an environment difficult to sample with other means. Several of the scientific instruments on board Venus Express measured gas abundances of various gasses, including CO2, up to an altitude of 130km, but no reliable measurements could be made of total density and no remote measurements could be done above this altitude.

The on board accelerometers gave direct measurements of the deceleration which in turn is directly proportional to the local atmospheric density. This provided an excellent way to study both the total density profile throughout the orbital arc in the atmosphere and small scale density variations in the region of the pericentre. The spacecraft behaved perfectly well throughout the whole campaign and provided a wealth of data both on the atmosphere and on the response of the spacecraft to the harsh environment with strong heat loads and some dynamic stress. At the time of the campaign the pericentre was located near the terminator at about 75 degrees Northern latitude.

The so called "walk-in" phase started at an altitude of 190 km on 17 May 2014 and the campaign ended on 11 July, after having reached a lowest altitude of 129.2 km. Subsequently, a series of orbit control manoeuvres lifted up the pericentre to 460 km altitude and the science activities were resumed after a thorough check-out of the spacecraft.

We have detected a highly variable atmosphere, both on a day to day basis and within the individual pericentre passes. The duration of each pass was approximately 100 s and the maximum dynamic pressure achieved was more than 0.75 N/m2. The orbital period was reduced over the duration of the campaign changing from 24 hours to 22 hours 20 minutes. A simple atmospheric model has been constructed that connects at the lower end to the measurements by the SpicaV and SOIR data and at the higher end to the results from previously derived data from drag campaigns at altitudes between 165 and 200 km. The valid range for this new model is 130-200km.