The Venus Express probe separated from the Soyuz Fregat carrier around 90 minutes after the 0333 GMT blast-off on the first expedition to Earth's closest neighbour in over 10 years.

"A perfect mission," said Jean-Yves Le Gall, head of Starsem, which was in charge of the launch, after the take-off.

The probe soon emitted its first signal. "The baby cried. Venus Express has begun its operational mission," said Jean-Pierre Cau of EADS Astrium, which built the spacecraft's propulsion system.

Venus Express will explore the unusual stormy atmosphere and runaway global warming on Venus in the hope of better understanding Earth's greenhouse-gas problem.

Venus, the second planet from the Sun, is similar in size, mass and age to Earth but has a vastly different and ferociously hot weather system.

Also known as the Evening Star, thanks to the bright light it reflects from the Sun, the planet is blanketed by thick clouds of suffocating gas driven by often hurricane-force winds and a surface pressure and temperature high enough to crush and melt lead.

The planet's clouds reflect back 80 percent of the Sun's radiation and absorb another 10 percent, leaving just 10 percent to filter down to the surface.

But the clouds provide such effective insulation the surface zone becomes a pressure cooker capable of melting metal.

"Venus has no surface water, a toxic, heavy atmosphere made up almost entirely of carbon dioxide (CO2) with clouds of sulphuric acid, and at the surface the atmospheric pressure is over 90 times that of Earth at sea-level," the ESA notes.

The planet's searing surface temperature of 477 Celsius (890 Fahrenheit) -- the hottest in the Solar System -- and immense atmospheric pressure have caused many previous missions to fail or send data streams lasting only minutes before their instruments were crushed.

It is hoped, however, that the 1.27-tonne unmanned Venus Express orbiter will be able to use seven powerful instruments on board to map the planet's surface and weather system, looking at temperature variation, cloud formations, wind speeds and gas composition.

The craft is scheduled to arrive off Venus in April, when it will be placed in an elliptical orbit, swooping to as low as 250 kilometres (156 miles) above the surface to a height of 66,000 kilometres (41,250 miles).

The orbiter, whose total mission costs are 220 million euros (264 million dollars), has enough fuel to operate for 1,000 Earth days, the ESA says.

In the first mission to Venus in 1961, a Soviet-made probe lost contact with ground control about seven million kilometers (4.37 million miles) from Earth.

A year later the US Mariner 2 became the first successful interplanetary mission, finding the surface to be dry and scorchingly hot.

The first successful landing on Venus came in 1970 when Soviet probe Venara 7 parachuted a capsule of scientific instruments onto the Venusian surface.

The first TV pictures were provided by its successor, the Venera 9 probe, in 1975.

The last dedicated mission to Venus, other than a flyby, was the US craft Magellan, from 1989-94, which used high-quality radar to make detailed maps of 98 percent of Venus' topography.

Venus Express was to have been launched from Baikonur on October 26. Five days before, though, the ESA said final checks had detected "contamination" inside the fairing, the bullet-shaped hood that covers the payload on the top of the rocket.

That postponement followed the disastrous launch of the 140-million euro (170-million dollar) ESA ice-monitoring satellite CryoSat, which was lost on October 8 after the rocket's booster stage failed.

The craft's sister orbiter Mars Express is currently on a mission circling Mars.

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