Change in Venus Express pericentre altitude (28 April to 18 August 2012)
This plot shows the change in Venus Express's pericentre altitude (Y-axis) as the mission progresses, from 28 April to 18 August 2012 (MTP (medium term plan) #79 through MTP #82). 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 2012, the pericentre altitude was allowed to decrease below the nominal range, down to 165 kilometres. The pericentre plateau is clearly visible to the left in the figure.
After a series of drag measurements, the pericentre altitude was raised again by firing the main spacecraft engine, bringing the pericentre altitude back in the nominal range on 6 May 2012 in orbit 2207.