The European Space Agency's Venus Express probe has launched from the Baikonur cosmodrome in Kazakhstan.
More than 90 minutes after take-off on Wednesday morning, the probe separated successfully from the Fregat element of its Soyuz-Fregat carrier rocket.
This is first space mission in over 10 years to Earth's closest neighbour.
The head of Starsem, which was in charge of the launch, Jean-Yves Le Gall, expressed delight at the "successful" separation.
The rocket lifted off at 9.33am (0333 GMT) and the probe separated from it 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, originally scheduled for 26 October, 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 will explore the planet's 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.
Venus's clouds reflect back 80%
of the Sun's radiation
The planet's clouds reflect back 80% of the Sun's radiation and absorb 10%, leaving 10% to filter to the surface.
The clouds provide such effective insulation that 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 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 - 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.
Unlike Mars (above), conditions
on Venus cannot support life
After its long journey, the craft is scheduled to arrive off Venus in April, when it will be placed in an elliptical orbit.
The orbiter, whose total mission costs are 220 million euros ($264 million), has enough fuel to operate for 1000 Earth days, the ESA says.