By Paul Rincon
BBC News science reporter, Baikonur
Europe is all set to send a probe to Venus, the first mission to our nearest planetary neighbour in a decade.
Venus Express will launch on a Russian Soyuz rocket from Baikonur Cosmodrome in Kazakhstan at 0333 GMT on Wednesday.
The robotic craft will orbit the planet for about 500 Earth days to study its atmosphere, which has experienced runaway greenhouse warming.
Scientists hope the mission will shed further light on the mechanisms of climate change on our own world.
Venus is almost identical in size to Earth, and is thought to have a similar composition. But there the resemblance ends.
A dense, largely carbon dioxide, atmosphere acts as a blanket, trapping incoming solar radiation to heat the planet's surface to an average temperature of 467C (872F).
Surface pressure is about 90 times that on Earth. Several Soviet probes sent to land on Venus in the '60s were crushed before they could touch down.
"Venus is, I feel, Earth's little sister. But she went very badly wrong," David Southwood, director of science for the European Space Agency (Esa), told the BBC News website.
Speaking from launch pad six at Baikonur, where the spacecraft will blast off on Wednesday, Dr Southwood added: "Back at the beginning of the Solar System, Venus and Earth were really very similar and you wouldn't have known which one to bet on being habitable.
"It's as important to look at the failures of the Solar System as its successes."
Venus has turned out to be far less Earth-like than was expected. The planet's cloud layer is very reflective, absorbing much less solar radiation than Earth even though it is closer to the Sun.
"The $64,000 question about the climate is why is it so different? Why is it so hot and why is the pressure so high?" proffered Fred Taylor of the University of Oxford and an interdisciplinary scientist on the mission.
By studying this hostile world, scientists hope to understand better how a warming future on our own planet might evolve.
Professor Southwood explained: "Venus had a catastrophic growth in its temperature, killing all possibility for life on the surface. We should worry about that.
"Our atmosphere has a fragile balance - it could turn around in any direction and life as we know it could become impossible."
Venus Express copies the basic structure of the highly successful Mars Express spacecraft, launched in 2003, and many of its science instruments. The mission has had a fast turnaround, taking just three years to put together.
"This is the fastest mission, we've ever done," said Professor Southwood, "There were people who said we couldn't make it by 2005, we'd have to wait until 2007.
"I invited one or two of them to the launch just to prove to them, it isn't faked for TV."
The Soyuz launcher is operated by the Starsem company, and Marc Grosheitsch is its vice-president.
"Launching a spacecraft is one thing, but when you know that it is going to explore somewhere like Venus, it is something else entirely," he said.
Lift-off from Baikonur was originally scheduled for 26 October, but was postponed after insulating material from inside the rocket's fairing was found to have contaminated the orbiter.
This has been cleaned up and Venus Express is now back on the launch pad.
After launch at 0333 GMT, the Soyuz rocket's three lower stages separate. Then, the upper "Fregat" booster stage of the rocket with the probe mounted on top enters a sub-orbital trajectory.
With the aid of two burns - at 0343 GMT and then at 0456 GMT - the Fregat stage will hurl the spacecraft into an escape trajectory that takes it directly to Venus.
The first possible signal from the spacecraft letting ground controllers know that all went well will come at 0530 GMT.
Venus Express should reach its target in about five months and enter an elliptical polar orbit.