The European Space Agency's (ESA) eight-year Venus Express mission has come to an end. Having already extended its lifespan to four times that originally planned, the unmanned orbiter has exhausted its fuel during a final attempt to further prolong its usefulness. According to ESA, the spacecraft can no longer hold the correct attitude to maintain communications with Earth and will soon burn up in the Venusian atmosphere.
Venus Express entered Venus orbit in April 2006 as part of a mission to make a detailed study of the planet’s atmosphere. Originally scheduled to operate for two years, its mission was eventually extended to eight. However, as its rocket propellant ran low, its elliptical orbit was in danger of decaying into a death plunge that would have ended with the orbiter burning up in the Venusian atmosphere earlier this year.
To save the spacecraft from a fiery death, ESA carried out an experimental technique called aerobraking between June 18 and July 11, where the spacecraft was sent on a new course skimming the upper reaches of the Venusian atmosphere at typical altitudes of 131 to 135 km (81 to 85 mi). With each pass, the spacecraft slowed down and its orbit became more circular. A series of 15 thruster burns then stabilized the orbit. ESA hopes that experience in conducting the aerobraking maneuver may one day allow spacecraft to enter planetary orbits without expending as much fuel as at present.
Unfortunately, the aerobraking maneuver, while successful, was only a temporary solution. In a bid to prolong the Venus Express mission into 2015, ESA ordered the craft to carry out a new series of engine firings from November 23 to 30. However, the space agency says that the maneuvers were not sufficient and full contact was lost with the probe on November 28. Though telemetry and telecommunication links have since been re-established, the link is only intermittent and control of the spacecraft is no longer possible.
ESA says that the exact amount of fuel left on the spacecraft is unknown because the orbiter has no way of measuring this and its weightless condition means that some propellant could be floating in the tanks as globules away from the engine intakes.
"The available information provides evidence of the spacecraft losing attitude control most likely due to thrust problems during the raising maneuvers," says Patrick Martin, ESA’s Venus Express mission manager. "It seems likely, therefore, that Venus Express exhausted its remaining propellant about half way through the planned maneuvers last month."
Venus Express is now at the mercy of orbital decay and will eventually burn up in the Venusian atmosphere in the next few weeks.