Facing the alternative of a fiery death, the ESA’s Venus Express orbiter has completed a daring maneuver that extended the life of the unmanned explorer by several months. Under command from Earth, the spacecraft spent a month skimming the outer edge of the Venusian atmosphere to alter its velocity and send it into a new orbit that will keep it operating until perhaps the end of the year.
Venus Express, whose main objective is the long term observation of the Venusian atmosphere, was reaching the end of its service life after arriving at Venus in April 2006. It’s propellant was nearly spent and its 24-hour elliptical orbit that took it from an altitude of 250 km (155 mi) at the north pole to 66,000 km (41,000 mi) over the south pole was in danger of decaying into a death plunge that would have ended with the orbiter burning up in the Venusian atmosphere.
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To save the spacecraft from a fiery death, the ESA carried out an experimental technique called "aerobraking," 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) for a few minutes at a time over the period of a month. With each pass, the spacecraft slowed down and its orbit became more circular.
During the skimming, ESA discovered that the density of the upper Venusian atmosphere is more variable in density than first thought, increasing by a factor of around 1,000 at altitudes between 130 and 165 km (81 and 103 mi). As a result, the spacecraft heated up by over 100º C (212⁰ F) during several 100 second-long passages through the atmosphere. However, the ESA says that the orbiter came through in better shape than expected.
At the end of the skimming maneuver, the Venus Express was ordered to fire its thrusters 15 times with each thruster firing more than 8,000 pulses to burn a total of 5.2 kg (11.4 lb) of propellant. The result was a new orbit between 460 km (285 mi) and 63,000 km (39,000 mi) that extends the life of the craft for a few more months.
Now that the Venus Express has completed its maneuver, the orbiter will resume its observations as its instruments are reactivated. However, its propellant is almost entirely exhausted, so no more course corrections can be made. The ESA says that it expects the craft to burn up in the atmosphere sometime in December.
The ESA hopes to use the information gathered by the aerobraking maneuver on future missions as a way of sending arriving spacecraft into planetary orbit with less fuel.
The video below shows the aerobraking maneuver in action.