For the past 8.5 years, Venus Express has been orbiting our nearest neighbor, quietly unveiling the shrouded planet's secrets.
"The spacecraft is expected to drop below 150 kilometers [93 miles] in early January 2015, and plunge much deeper into the atmosphere of Venus by early February," says Patrick Martin, Venus Express mission manager. "It will be destroyed in the atmosphere most likely toward the end of January or early February."
The craft will probably burn up as it makes its final descent.
During its time in orbit nearly 155 million miles (250 million kilometers) away from Earth, Venus Express revealed many surprises about Earth's sister planet. It found some evidence for both past water on the surface and for recent volcanic activity, and spied on the mammoth, shape-shifting vortexes that swirl at the planet's poles. It studied the planet's odd, super-rotating atmosphere, which whips around the Earth-size world at 250 miles (400 kilometers) per hour. And it even detected evidence for lightning—and ozone—in the thick, mostly-carbon-dioxide atmosphere.
"Both geologically and from the point of view of the atmospheric circulation, Venus Express has just been an amazing mission," says Ellen Stofan, NASA's chief scientist. "And it's really helped to refine the question of what you want to do next at Venus."
Spacecraft's Final Moves
Launched in 2005, Venus Express was slated to orbit Venus for just two years. But it refused to quit gathering data, and nearly a decade later, the spacecraft was still soldiering on—even performing a daring, experimental aerobraking maneuver earlier this year in an attempt to raise the craft's orbiting altitude. Just last month, mission scientists presented data from the maneuver at the American Astronomical Society's Division for Planetary Sciences meeting in Tucson, Arizona.
But two weeks after that meeting, scientists lost contact with the craft.
They'd been trying to raise the spacecraft's orbiting altitude, which consisted of a series of thruster burns and carried the inherent risk of burning through whatever fuel was left after years spent skimming the Venusian cloud tops.
On November 28, the flight control team was unable to contact Venus Express. By December 3, the team managed to regain intermittent contact with the spacecraft, which was then in "survival mode." Mission managers guessed that Venus Express had run out of propellant—an eventuality predicted to happen toward the end of this year.
Without gas in the tank to power the thrusters, Venus Express is now caught in the grip of the planet's gravity. The team is still getting a carrier signal from the craft, but is no longer getting any data from it.
"We are all sad to see this great mission end, but at the same time we are very proud of its many achievements and successes," Martin says. "And we will certainly celebrate this fantastic, once-in-a-lifetime adventure in the near future."
Exploring Earth's Hellish Sister
Venus's extreme atmospheric pressure will crumple and scorch the spacecraft before it hits the surface of a world with the hottest recorded temperatures in the solar system (aside from the sun, of course). Surface pressures are 90 times higher than on Earth, and visitors are not tolerated for long. In the 1970s and '80s, Venus made quick work of the half dozen robotic landers that formed the backbone of the Soviet Venera program; yet, some of the spacecraft managed to survive long enough to send back photos and data from the surface.
Though brief, these short-lived victories marked the few decades when the spotlight of space exploration pointed at Venus. After NASA's last Venus probe, Magellan, burned up in the planet's atmosphere in 1994, it would be more than a decade before Venus Express was launched.
Now, with only a single, already crippled Japanese spacecraft heading to Venus, Earth's sister is again retreating into the planetary shadows.
But it's a world that could teach us a few things. Scientists suspect now-hellish Venus was once very much like the world we're standing on. Data suggest it was watery and more temperate then, and possibly capable of sustaining life. Billions of years ago, though, something went dreadfully wrong. Greenhouse gases overwhelmed the planet, shattering a fragile equilibrium and transforming the former oasis into a toxic, roasted world.
"Venus and Earth started out not that differently," Stofan says. "It really is our twin sister gone bad. To understand why that is, and then what the implications of that are for how habitable worlds evolve around other stars, is really critical."
Correction: An earlier version of this story misstated the year the Magellan probe blew up as 1990. It was 1994.