Change in Venus Express pericentre altitude during the period 4 March through 7 May 2012
This plot shows the change in Venus Express's pericentre altitude (Y-axis) as the mission progresses, from Medium Term Plan 77 (3-31 March) to 79 (28 April to 26 May). The X-axis shows the orbit number; each orbit lasts 24 hours.
The pericentre altitude decreases due to predicted forces experienced by the spacecraft. For the purposes of the eighth Atmospheric Drag Experiment campaign, which started on 25 April, the pericentre altitude is allowed to decrease beyond nominal, down to 165 kilometres. The sharp jump in the pericentre altitude is due to the orbit-raising manoeuvre carried out at the end of the Atmospheric Drag Experiment campaign, bringing it back to nominal on 7 May 2012.
The pericentre plateau, where the low pericentre altitude is used for the atmospheric drag experiments, is clearly visible in the figure. After a series of drag measurements, the pericentre is raised by firing the main spacecraft engine.