Venus Express, Atmospheric Surprise, and Orbits

"Venus' Atmosphere Proves a Real Drag, Leading to a Discovery "
Space.com (October 7, 2010)

"A spacecraft using a bold new method to study Venus — flying directly through the planet's atmosphere - has found that the atmosphere at Venus' poles is thinner than expected.

"The European Space Agency's Venus Express probe made the discovery during a series of dives through the atmosphere of Venus over the past two years. Scientists measured the drag on the spacecraft during these atmospheric dips to determine Venusian air density....

"...Venus Express has no instruments capable of measuring this density directly, researchers said. So mission planners have improvised. They sent the probe skimming down into the alien atmosphere on exploratory dives - at altitudes of about 110 miles (180 km) - in July-August 2008, October 2009, February 2010 and April 2010.

"During these jaunts, radio tracking stations on Earth watched for the drag exerted on the spacecraft. In addition, operators turned one of Venus Express' solar wings edge-on and the other face-on so that air resistance twisted the spacecraft, researchers said."

The Venusian atmosphere is 60% thinner than expected at the poles. Why? Good question: one that researchers don't have an answer for yet. It's not what they expected to find.

Which, as the Lemming has mentioned before, is at least as exciting as having predicted conditions confirmed by observations. This way, there's something new to learn.

There's something besides research going on with these dips into the atmosphere. The European Space Agency's Venus Express has been in an orbit that takes it quite a distance out from Venus. Far enough so that it has to fire its engines to correct for what the sun's gravity does to the orbit.

That takes fuel, which will run out in 2015 - according to the article. The mission planners are using these passages through the atmosphere to bring the orbit inward - reducing its orbital period from 24 hours to 12 hours.

They figure they'll have that done by 2012.