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Venus Express to get final acid bath before crashing to surface

ESA uses probe to test aerobraking for future missions

The smart choice: opportunity from uncertainty

After eight years orbiting the second planet of our solar system, the Venus Express probe is going to die this year, but the European Space Agency has some interesting plans to get the most out of the pre-teen before it disintegrates.

Venus Express left our planet in 2005 and began orbiting its namesake planet four months later, observing the Morning and/or Evening Star using a suite of seven instruments. The spacecraft is in an elliptical polar orbit around the planet that takes it 66,000km over the Venusian South Pole and 250 km above the north.

The propellant that's used to keep Venus Express in position is now almost gone, and the ESA has announced that in June or July the craft will make one final plunge into the atmosphere, skimming as low as 130km above the surface and flying through the clouds of sulfuric acid that encircle the planet.

The purpose of this is two-fold. First, the probe can gather data on the planet's atmosphere and take a peek at ground features. But more importantly, the spacecraft will be used to test out how well the atmosphere can be used for aerobraking.

Aerobraking is a technique for slowing spacecraft by using the drag from a planet's atmosphere, allowing engineers to save large amounts of fuel that would otherwise have been used for deceleration. The technique was first used in 1991 by Japan's Hiten spacecraft, and has since become a standard technique in space travel.

"We have performed previous short 'aerodrag' campaigns where we've skimmed the thin upper layers of the atmosphere at about 165 km, but we want to go deeper, perhaps as deep as 130 km, maybe even lower," says Patrick Martin, Venus Express mission manager.

"It is only by carrying out daring operations like these that we can gain new insights, not only about usually inaccessible regions of the planet's atmosphere, but also how the spacecraft and its components respond to such a hostile environment."

ESA says that it's possible the probe won't survive its close encounter with Venus and will break up under the buffeting it will receive on its way down. The planet's dense carbon dioxide atmosphere is both hot and stormy, with windspeeds approaching 200mph in the region the probe will fly through.

If it does survive, the ESA will use the remaining propellant to park the spacecraft in an orbit around Venus at around 450km, where it will continue broadcasting data back to Earth. ESA estimates the spacecraft will fall onto the surface by the end of the year.

It will be a sad end for a probe that has told us so much about Venus. Over its eight years in orbit, it has given us the clearest picture yet of the most similar planet to ours in the Solar System, and – as is often the case – raised questions that need answering.

First, the probe picked up elements of sulfur in the upper atmosphere, indicating that the planet may have been geologically active in recent history. Radar maps of the surface have shown large, barely weathered plains of lava, and so the planet is now assumed to be prone to volcanism.

The probe has also discovered that Venus has the most unusual polar vortex yet found. Many planets, including Earth, have weather systems that blast around the poles. Saturn even has a hexagonal storm system swirling around its North Pole.

But Venus Express has cataloged the extraordinarily complex shapes formed around the Venusian South Pole, with a storm system that changes daily in chaotic patterns, but which is slightly offset from the true South Pole itself by several hundred kilometers.

When the probe arrived around Venus, the controllers also found something rather concerning: the landmarks that they were looking for weren't where they were supposed to be. When the Magellan probe visited Venus in 1990, it measured the planet's spin and it was calculated that one day on Venus was the equivalent to 243.0185 Earth days.

But when Venus Express began taking measurements this was no longer the case. In the 16 years since the first visit, Venus' days had become 6.5 minutes longer. That may not sound like much, barely enough time for a good cup of tea, but in planetary terms it's a massive discrepancy.

While Venus Express has provided the data that could answer some of these questions, it's clear that more study is needed. ESA and Japan will give the planet a flyby when they send the BepiColombo probe to Mercury next year, and the Indian space agency wants to send a dedicated Venus probe in 2015 as well.

It's something of a pity that NASA is concentrating so much effort on Mars these days. It's clear Venus has a lot of new science to offer, but it's being somewhat overshadowed by its cooler, redder cousin. ®

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