By Paul Rincon
BBC News science reporter, Baikonur
The first space mission in more than a decade to Earth's closest neighbour, Venus, has lifted off from the Baikonur Cosmodrome in Kazakhstan.
A Soyuz-Fregat rocket launched the probe. Image: European Space Agency
Europe's Venus Express probe blasted off on a Russian rocket at 0333 GMT on Wednesday.
The robotic craft will orbit the planet 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.
"It's just such an exciting mission," said Prof Manuel Grande from the Rutherford Appleton Laboratory in Oxfordshire, UK.
"Earth is halfway between Venus and Mars, and when we put this together with Mars Express, it will tell us about the evolution of all three planets."
The probe was launched on a Soyuz rocket topped by a Fregat booster designed to propel the spacecraft on a direct course to Venus.
Mission controllers in Darmstadt, Germany, confirmed that the probe had left Earth's orbit and started its journey to Venus about 90 minutes after launch.
They picked up the first signal from the spacecraft a few minutes later.
Venus Express should reach its target in about five months and enter an elliptical polar orbit.
Jerry Bolter of EADS Astrium, which built the spacecraft's propulsion system, said the mission was put together in record time.
"It's the fastest turnaround for any spacecraft in Europe," he told the BBC News website.
"We now look forward to next year when we have a very challenging burn for the main engine to do."
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), said.
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.
Venus Express on launch pad (ESA/STARSEM-S. CORVAJA)
"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.
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.