Venus' Atmosphere Proves a Real Drag, Leading to a Discovery

Starting its scientific surveying in July 2006, the European Space Agency's(ESA) Venus Express has been carrying out the most detailed study of the planet's thick and complex atmosphere to date. CREDIT: ESA

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.

"It's really a very accurate and precise method," Venus Express project scientist Hakan Svedhem said at a news briefing Wednesday (Oct. 6). Svedhem and his colleagues presented their findings at the American Astronomical Society's Division of Planetary Sciences meeting in Pasadena, Calif.

The observations will help prepare Venus Express for one final, deep dive that will alter its orbit and extend its operational life, the researchers said.

Thinner than they thought

Venus Express' atmospheric dives revealed that the planet's polar atmosphere is about 60 percent thinner than predicted, researchers said. The team is trying to figure out what accounts for this surprising result.

The spacecraft's twisting motion during its dives also registered a sharp density change from Venus' day side to its night side, scientists said.

The Venus Express team is gearing up for another dive next week, when the probe will make its deepest foray yet into Venus' atmosphere — down to about 102 miles (165 kilometers) above the planet's surface.

This is getting closer to the aerobraking altitude that would adjust the probe's orbit, which Svedhem estimated to be about 87 miles (140 km) above the surface. But the team isn't ready to make this move yet.

"The timetable is still open because a number of studies have yet to be completed," Svedhem said. "If our experiments show we can carry out these maneuvers safely, then we may be able to lower the orbit in early 2012."

Changing course

Venus Express arrived at Venus in April 2006 and has been studying the planet's atmosphere ever since. The probe currently has a highly elliptical polar orbit that loops from 155 to 41,000 miles (250 to 66,000 km) above the Venusian surface.

When Venus Express is far away from the planet, the sun's gravity pulls it slightly off course. As a result, the spacecraft's engines must be fired every 40-50 days to compensate. The fuel to do this will run out in 2015.

Mission planners want to give the probe a chance to keep operating beyond 2015. So they're considering shaking things up and bringing Venus Express much closer to the planet, reducing its orbital period from 24 hours to 12.

To do this, scientists would dip Venus Express deep into the planet's atmosphere. The resulting drag would slow the spacecraft down and change its orbit. This maneuver is delicate and potentially risky, according to researchers.

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

However, 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.

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