









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.


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.















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.



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.