The rocket lifted off at 9:33 am (0333 GMT) and the probe was to separate from it two hours later to embark on its 163-day journey to Venus.

"I'm extremely happy", European Space Agency (ESA) scientific programme director David Southwood said 10 minutes after the launch, when all systems were normal.

The launch of the probe, originally scheduled for October 26, was delayed due to "contamination" detected inside the fairing -- the bullet-shaped hood that covers the payload on the top of the rocket -- in final checks at Baikonur.

Venus Express, the ESA's first probe to the planet, will explore its unusual stormy atmosphere and runaway global warming 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.

After its long journey, 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.

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