Late last month, the European Space Agency's (ESA) unmanned orbiter Venus Express slipped into the incredibly dense atmosphere of Earth's "sister planet," never to be heard from again. Now experts have confirmed from telemetry just before its disappearance that the probe is likely out of fuel and will be unable to make contact with Earth ever again.

It's important to note that Venus Express has spent about eight years orbiting and observing Venus and completed its primary mission long ago.

"During its mission at Venus, the spacecraft provided a comprehensive study of the planet's ionosphere and atmosphere, and has enabled us to draw important conclusions about its surface," Håkan Svedhem, ESA's Venus Express project scientist, explained in a statement.

While exceptionally similar to Earth as a terrestrial planet - given its similar size, gravity, and bulk composition - we actually know very little about Venus' surface, as it cannot be seen through a highly reflective sulfuric acid cloud layer that shrouds the planet. This same layer is one of the reasons that Venus is the brightest natural object in the night sky.

The ESA probe was thus tasked with measuring both the thick atmosphere and mysterious surface of the planet with various instruments.

However, "after over eight years in orbit around Venus, we knew that our spacecraft was running on fumes," added Adam Williams, ESA's acting Venus Express spacecraft operations manager.

Unlike cars and aircraft, spacecraft don't come with fuel gauges, as such equipment is inaccurate in the absence of gravity. However, a team can make guesses as to when a craft's supply is running low. When this time came for Venus Express, the probe's team decided to run one final experiment, testing "aerobraking" in a daring campaign that sent the probe skimming through Venus' thick atmosphere over and over. (Scroll to read on...)

The campaign not only tested a concept that can help craft enter orbit without having to carry quite so much propellant, but also took the probe deeper into Venus' atmosphere than ever before.

However, following its last experimental pass, Earth lost contact with the Venus Express. Last minute telemetry indicates that the probe is now slowly spiraling towards the planet, and is very likely out of fuel.

"It was to be expected that the remaining propellant would be exhausted during this period," Williams said, "but we are pleased to have been pushing the boundaries right down to the last drop."

Still, without even a goodbye, it's a rather sad way to end things with the probe. NASA's Cassini team has bigger plans for their decade-old Saturn orbiter, plunging it straight towards the heart of its host planet in a fiery "Grand Finale" in 2016.

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