Over the last month, the Venus Express spacecraft has gently fallen through Venus's thick, carbon dioxide atmosphere, reaching a cloud-skimming altitude of 80 miles (130 kilometers).

For three weeks, the spacecraft will continue to fly low over the planet's sulfuric shroud, trying to sniff out any faint magnetic field that might be present, collecting information about the planet's clouds at these low altitudes and testing a technique that future robotic explorers might use. The maneuver, known as aerobraking, uses the planet's atmosphere to slow a spacecraft and adjust its orbit.

It isn't meant to be the final plunge for Venus Express. But it could be, depending on how the spacecraft fares and how much fuel is left. If all goes according to plan, though, Venus Express will again rise from the clouds, collect a few more months' worth of data, and beam its final message to Earth sometime later this year.

"The spacecraft is quite robust, and I think it will come out without any major damage from this campaign. But there are always risks," says Venus Express project scientist Håkan Svedhem of the European Space Agency in Noordwijk, the Netherlands.

Launched in late 2005, the European Space Agency's probe slipped into a lopsided polar orbit around Venus in April 2006. Traveling from 41,000 miles (66,000 kilometers) above the planet's south pole, to just 155 miles (250 kilometers) above its north pole, the spacecraft has been tirelessly collecting data about Earth's nearest neighbor.

And after eight years, Venus Express has gathered a wealth of observations, and raised a host of new questions, about a sizzling planet that might once have been very Earthlike.

An Active World

Some of the mission's discoveries, such as finding evidence for past oceans, have been fundamental for understanding the history of Earth's sister planet. Venus Express has also revealed that the planet may have been volcanic in the past, meaning there's another geologically active world nearby that scientists can study.

The spacecraft provided solid evidence of volcanic activity on the planet, including Venus's young-looking surface and indications of subterranean hot spots. Venus Express also detected bursts of sulfur dioxide in the atmosphere, suggesting that something like volcanoes could be replenishing the supply.

"Venus Express was definitely the first mission to give us evidence of geologically recent volcanism," says Suzanne Smrekar, a planetary scientist at NASA's Jet Propulsion Laboratory in Pasadena, California. The planet "could be active today, but we will need new data to confirm that."

Shortly after the probe arrived, it began finding new mysteries: The planet's super-rotating atmosphere, which whips around Venus in just a few days, was moving at 250 miles (400 kilometers) per hour—nearly 62 miles (100 kilometers) per hour faster than when NASA's Magellan probe visited in the 1990s.

"That's very peculiar," Svedhem says. "Nobody could predict it before, and we don't really understand what's happening, either."

Conversely, the planet itself seems to have slowed down. It already takes about 243 Earth-days to complete one rotation—but Venus Express determined that the length of a day on the planet is now 6.5 minutes longer than when Magellan visited. Scientists don't know why.

And the probe's photos of the mammoth, shape-shifting vortex at the planet's south pole revealed a phenomenon that's nothing short of spectacular. Similar to a massive hurricane, the vortex is the size of Europe with an eye that's several miles across. Sometimes it has a single eye. Occasionally it has two or even three.

The only other place in the solar system that scientists have seen such a thing is at Venus's north pole, where a similarly chaotic maelstrom churns away. Scientists aren't yet sure what's driving the vortices, but they suspect it has something to do with Venus's super-rotating atmosphere.

Twisted History

Earth's sister planet is very similar to our home in many ways. It's roughly the same size, has the same gravitational pull, and sits in an area where, under the right circumstances, water could be liquid on the surface. In fact, for much of the solar system's youth, the two planets could have been more or less like twins.

But something happened to Venus. Instead of the planet maintaining a temperate climate, the scales tipped and the planet roasted. A runaway greenhouse effect parched the surface, stripping it of any lakes or oceans (scientists are still seeing water molecules escaping from the atmosphere). Miles-thick, heat-trapping, dense carbon dioxide enveloped the planet. Now the surface temperature hovers near 450 degrees Celsius, and surface pressures are about 90 times greater than on Earth.

Scientists hope to one day tell the full tale of how Venus evolved from something rather Earthlike into today's toxic world. Venus Express has helped fill in the end of that story, but the pages at the beginning are still missing. And with few spacecraft planned to visit Venus in the foreseeable future, it seems that history will remain a mystery.

"We know very little about the surface of the planet, and we know very little about the early history of Venus," Svedhem says. "We can see Venus as a laboratory for the Earth—the Earth in a different condition—and try to understand why that's the case."