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 European Space Operations Centre 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 may 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.
The current elliptical orbit takes 24 hours to complete and loops from 250 km to 66 000 km. When Venus Express is far away from the planet, it is pulled off course slightly by the Sun's gravity. So, every 40-50 days, its engines must be fired to compensate. The fuel to do this will run out in 2015 unless the orbit can be lowered using the drag of Venus' atmosphere to slow the spacecraft. It is a delicate, potentially dangerous operation and cannot be rushed.
"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 manoeuvres safely, then we may be able to lower the orbit in early 2012."
In the meantime, Venus Express may be feeling it's all a bit of a drag, but the science teams involved are happier than ever with their new data. "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.
This animation demonstrates the orbital perturbations to Venus Express caused by the atmospheric drag experienced as the spacecraft skims the atmosphere of Venus. To experience the drag, the spacecraft must pass the planet at altitudes below 200 km.
The orbital perturbation results in a shrinking of the elliptical orbit of Venus Express. The farthest point from the planet lowers and the orbit becomes slightly more circular. This effect has been exaggerated in the animation.
(Photo Credit: Animation courtesy of P. Rosenblatt, Royal Observatory of Belgium)
This image, of the "eye of the hurricane" on Venus was taken by the Visible and Infrared Thermal Imaging Spectrometer on board Venus Express.
This picture shows a region in the venusian atmosphere about 60 km from the surface, at a wavelength of about 5 micrometers. In this figure, the dipole assumes an eye-like shape and from here until the last image, it is possible to see how its shape evolves rapidly in a span of only 24 hours.
The yellow dot in the image indicates the location of the south pole.
(Photo Credit: ESA/VIRTIS/INAF-IASF/Obs. de Paris-LESIA/Univ. of Oxford)
Source: European Space Agency