Venus Express Finds Planet's Atmosphere a Drag

Venus Express has flown through the upper reaches of the planet's poisonous atmosphere.

Venus Express went diving into the alien atmosphere during a series of low passes in July-August 2008, October 2009, and February and April 2010.

The aim was to measure the density of the upper polar atmosphere, an experiment that had never been attempted before at Venus.

The campaign has returned 10 measurements so far and shown that the atmosphere high above the poles is a surprising 60% thinner than predicted.

This could indicate that unanticipated natural processes are at work in the atmosphere. A team led by Ingo Mueller-Wodarg of Imperial College, London, are currently investigating.

The density is critical information for mission controllers, who are investigating the possibility of driving the spacecraft even lower into the atmosphere in order to change its orbit and extend the lifetime of the mission.

"It would be dangerous to send the spacecraft deep into the atmosphere before we understand the density," says team member Pascal Rosenblatt, Royal Observatory of Belgium.

The fact that Venus Express can make these measurements at all is remarkable. The spacecraft was not designed for it and so does not have instruments capable of directly sampling the atmosphere. Instead, radio tracking stations on Earth watch for the drag on the spacecraft as it dips into the atmosphere and is decelerated by the Venusian equivalent of air resistance.

In addition, operators at ESA's ESOC in Darmstadt, Germany, turned one solar wing edge-on and the other face-on so that air resistance would twist the spacecraft.

Venus' atmosphere extends from the surface up to an altitude of around 250 km. During April, Venus Express briefly skimmed down to 175 km above the planetary surface.

As well as the surprisingly low density overall, the twisting of the spacecraft has also registered a sharp density change from the day to the night side of the planet. Next week, Venus Express will go diving again, this time lowering itself to 165 km.

These measurements will be used eventually to help make changes to the orbit of Venus Express, halving the time it takes to circle the planet and providing new opportunities for additional scientific measurements.