A Soyuz rocket carrying the European Space Agency's (ESA) Venus Express will launch from the Baikonur cosmodrome in Kazakhstan at 0333GMT on Wednesday.

 

After a journey of 162 days, the probe will orbit the planet for 16 months studying its dense atmosphere which is thought to be the result of an extreme greenhouse effect.

 

ESA scientists hope the mission will study the Venus clouds in unprecedented detail and give them valuable data about the mechanisms of climate change here on Earth.

 

Venus is almost identical in size and mass to the Earth, and is thought to have a similar geological composition. But while Earth has a life-sustaining atmosphere, its twin is far from hospitable.

 

The planet's surface is cloaked in a dense atmosphere of carbon dioxide which traps heat from the sun, warming the surface to around 477 C (890 F).

 

The atmospheric pressure at the surface is more than 90 times that of Earth, there is no surface water and the ground is covered by toxic clouds of sulphuric acid.

 

There have been several previous missions to Venus, but early probes in the 1960s were crushed by the pressure before they landed.

 

Twenty years later, the Russian Venera landers touched down and survived long enough to send back images of a barren, rocky world before they too succumbed to the hostile environment.

 

Surveys

 

More recent missions have concentrated on mapping the planet's surface using radar, leading scientists to conclude that the planet was regularly resurfaced with lava until 400 million years ago.

 

The fact that so much is still unknown is exciting mission scientists such as Haken Svedham: "Venus was once quite similar to Earth, but now the planets are very different. Why are they so different now and when did this change start?"

 

"It would be great to discover an active volcano and to prove that the planet is still active geologically," he said.

 

The spacecraft is based on the successful Mars Express which launched in 2003 which has allowed the Venusian version to be built in just three years.