A first for exploration at Venus, ESA's Venus Express satellite has conducted a series of low passes through the planet's upper polar atmosphere to find it is much thinner than expected.

The low passes were made in July and August 2008, October 2009 and February and April 2010 and ten measurements taken in order to measure the density of the atmosphere above the planet's north polar region. The planet's atmosphere extends to an altitude of 250 kilometres above the surface, but for these experiments, Venus Express dived down to just 175 kilometres, with an even lower pass planned in the coming weeks.

"Near 185 kilometres on Venus the density is around 2x10^-12 kg/m³, which is equivalent to the density on Earth near 350-400 kilometres altitude," explains team leader Ingo Mueller-Wodarg of Imperial College London. "This is roughly the density that the International Space Station experiences on average, so you could call it a near-vaccum by our every day definition."

The atmosphere in this polar region was some 60 percent thinner than predicted, suggesting some unanticipated natural processes are at work. "The message from this is that the atmosphere is not purely driven by solar radiation, but that the structure is also affected by other processes," Mueller-Wodarg tells Astronomy Now. "We are still investigating what could cause this collapse at the poles, but one idea is that winds play their role."

He adds, "It is extraordinary that we managed to measure these very small densities – the key was to use the entire spacecraft as our instrument." The spacecraft does not have instruments to directly sample the atmosphere so the measurements were obtained by radio tracking the satellite from Earth, watching for the drag on the spacecraft as it was decelerated by the changing density of the atmosphere. The engineering team also oriented Venus Express's solar wings such that one solar wing was edge-on and the other face-on, so the air resistance would twist the spacecraft, a technique that revealed a sharp density change from the dayside to nightside of the planet.

The results of the low passes will eventually be used to alter the orbit of the spacecraft such that the time it takes to make one complete orbit is halved – its current 24 hour loop takes it from an altitude of 250 kilometres to 66,000 kilometres. Mueller-Wodarg cautions that this maneuver is still being investigated so is not yet guaranteed to happen, but says that the change in orbit period will allow a new set of observations to be made. "The orbit will become more circular, so some instruments can monitor the planet for longer than they used to be able to do during each orbit."

“If our experiments show we can carry out these maneuvers safely, then we may be able to lower the orbit in early 2012,” adds ESA Project Scientist Hakan Svedhem.