Change in Venus Express pericentre altitude with time from 9 December 2012 to 30 March 2013
This plot shows the change in Venus Express's pericentre altitude (Y-axis) as the mission progresses, 9 December 2012 to 30 March 2013 (Medium Term Plan (MTP) 87 through Medium Term Plan (MTP) 90). The X-axis shows the orbit number; each orbit lasts 24 hours.
The pericentre altitude decreases due to predicted forces experienced by the spacecraft. The pericentre plateau, where the low pericentre altitude is used for the atmospheric drag experiments, is clearly visible in the beginning of the figure. After a series of drag measurements, the pericentre is raised by firing the main spacecraft engine.
For the purposes of the tenth Atmospheric Drag Experiment campaign, which ended on 15 December 2012, the pericentre altitude was allowed to decrease beyond nominal, down to 165 km. The sharp jump in the pericentre altitude was due to four pericentre-raising manoeuvres carried out at the end of the Atmospheric Drag Experiment campaign, bringing the pericentre altitude back to the nominal range on 20 December.