An unexpected measurement has been achieved with the Venus Express, a satellite currently studying the atmosphere of Venus. While the satellite was not fitted with instruments to directly measure atmosphere density, the scientists have discovered by measuring the drag as the Venus Express experiences air resistance that the atmosphere is 60 percent thinner than expected. This "working on the fly" approach could allow the scientists to extend the life of the craft allowing them to collect more data.

The Venus Express has been successfully operating in orbit around Venus since April 2006, studying the atmosphere from the surface right up to the ionosphere at an an altitude of 250km. It made atmospheric dives in July-August 2008, October 2009, and February and April 2010 when it briefly skimmed down to 175km (109 miles) above the surface. Next week it will dive to 165km (102 miles).

To measure the drag operators at the European Space Operations Center in Darmstadt, Germany angled the satellite's solar panels at 90 degrees to one another, one panel perpendicular to the incoming flow and the other seeing it edge on to create a maximum twist effect in the atmosphere known as the “torque technique”. On board instruments known as “reaction wheels” keep the spacecraft at the right altitude despite the spin and the readings from these are recorded and translated into a density estimate by ESA’s Europe Venus Express.

Ten measurements have been taken so far in the upper reaches of the poisonous atmosphere at the poles, and it has been discovered that the polar atmosphere is 60 percent thinner than previously thought, suggesting that there may be other unforeseen natural processes at work in the atmosphere. There is also a sharp density change from the day to the night side of the planet.

“We couldn’t see this region with our instruments because the atmosphere was too thin to register, but now we are sampling it directly,” says Dr Mueller-Wodarg of Imperial College, London.

With the new density information mission controllers may be able to adjust the satellite's orbit to extend the lifetime of the mission. Currently the elliptical orbit swings between 250km and 66,000km (41,000 miles) around Venus over a 24 hour period but at great distances the sun's gravity pulls it slightly off-course meaning the engines must be fired periodically to compensate. Currently fuel is set to run out in 2015 but this could be extended if lowering the Venus Express into the atmosphere increases drag sufficiently to slow the spacecraft. Extending the life of the mission would mean opportunities for additional scientific measurements.

“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 timetable is still open because a number of studies have yet to be completed,” says Håkan Svedhem, ESA Project Scientist Venus Express. “If our experiments show we can carry out these maneuvers safely, then we may be able to lower the orbit in early 2012.”