In about a month's time, the European Space Agency's (ESA) Venus Express spacecraft will adjust its orbit and dip into the outer venusian atmosphere. This hypervelocity skimming will allow scientists to not only obtain a little more data on Venus's atmosphere, but to also learn more about the seat-of-your-pants technique called aerobraking. It's also the mission's swan song after an incredible array of discoveries, from snow to an unexpected planetary spindown.
An aerobraking procedure is one in which the atmosphere of a planet is used to slow a spacecraft, either for full atmospheric entry, or as a means to adjust the orbit without expending precious fuel. For example, it's a way to circularize an elliptical orbit, by 'braking' a craft during its lowest pass - bleeding off orbital velocity via the drag of the atmosphere. The Mars Global Surveyor, Mars Odyssey, and Mars Reconnaissance Orbiter missions all made use of aerobraking to reach their nominal orbital configurations. The Magellan mission to Venus did this too - circularizing its orbit.
But it's a tricky business. The top of a planetary atmosphere is a somewhat fickle beast, strongly affected by solar radiation and planetary conditions. Small variations in the local density of gas can have major effects on a spacecraft plowing through at well over 10,000 mph.
So why risk Venus Express? Its science mission is effectively completed, and its fuel is almost gone, there's little to lose by playing chicken. If the spacecraft doesn't survive this dip into the ionosphere (at an altitude of some 130 km, more than 30 km deeper than the spacecraft has gone before) it will simply burn up. If it makes it through, it'll live to play for another few months, gathering a little more data, before taking the ultimate dive.
This all seems more reasonable when you consider the incredible scientific bounty from the mission. After arriving at Venus back in 2006 it has gazed at this cloud-covered world in infrared, visible, and ultraviolet light, measured magnetic fields, the interaction of the solar wind with the atmosphere, and studied the atmospheric temperature structure and composition. One of many major discoveries came when the spacecraft spotted a crazy double vortex at Venus's southern pole - shown here in the infrared.
It also discovered an unexpected long-term climate trend. When it arrived at Venus the atmosphere at 70 kilometers altitude was circulating with an average speed of about 300 km/h. But now, some 8 years later, that powerful 'super-rotation' (faster than the solid body of the planet itself spins) has shot up, hitting 400 km/h.
Why is this? We don't know.
Even more surprising was the discovery that the solid surface has altered its spin rate in the years since the 1990s when the Magellan spacecraft made measurements. Venus is a ponderous beast, it takes some 243 Earth days to spin around once (compared to 4 days for its bulk atmosphere). But 16 years ago it was spinning a little faster, by some 6.5 minutes each rotation.
That's an incredible change, this is an entire rocky planet, with enormous inertia. One possible explanation is linked to the thick atmosphere, which can exert significant frictional drag on the surface. It may also mean that Venus has a molten core, allowing its outer crust to respond more readily to these atmospheric forces.
The list of Venus Express discoveries goes on, from an ozone layer, to the possible existence of an atmospheric 'snow line' at about 125 km altitude, where temperatures plummet to 175 Kelvin - allowing for carbon dioxide to freeze out.
All in all Venus Express has earned the right to one last tango, even if it's a dangerous one.