Baring secrets of 'evil twin sister'

Venus is sometimes known as Earth's evil twin sister.

Its atmosphere is a suffocating mix of carbon dioxide and sulphuric acid, its temperatures soar above 465C - enough to melt lead - and the crushing pressure at its surface is 90 times greater than on Earth.

Yet Venus is in many ways just like Earth. It is a relatively small, rocky planet orbiting at roughly the same distance from the Sun.

Like Earth, Venus is within the "Goldilocks zone" of the Solar System, which in theory should be neither too hot nor too cold for liquid water, and hence life, to exist.

But something happened to Venus that turned it into the hellish place it is today.

Rather than possessing oceans of liquid water and a life-sustaining atmosphere, Venus is a dry, hot place with a runaway greenhouse effect that traps heat tightly to its surface.

For all its similarities to Earth, Venus is in fact a very different type of planet to its nearest neighbour.

The Venus Express space probe, which on Tuesday manoeuvred itself successfully into an orbit around Venus, may help to explain the planet's history.

Using a space equivalent of the handbrake turn, it fired its engine for a crucial 50 minutes to slow it down from 29,000km/h to 25,000km/h.

This allowed it to be captured by Venus's gravity field, and so enter into an elliptical orbit around its poles.

Andrew Coates, of the Mullard Space Science Laboratory in Dorking, Surrey, said: "We'll be able to study the lower atmosphere of Venus for the first time. Venus has evolved into our twisted sister or evil twin and we'll be able to try to see why."

Professor Fred Taylor of the University of Oxford, a guiding light behind the £140 million ($398 million) mission, expressed the relief of many at the European Space Agency who saw the handbrake manoeuvre as the most dangerous moment in the probe's 153-day, 400-million-kilometre journey.

Speaking via a video link from ESA's operations centre in Darmstadt, Germany, he said: "The tension in the control room was just wonderful. It felt like you were in the actual spacecraft."

For the next four weeks the probe will further manoeuvre itself to adjust its orbit to carry out the analysis of the dense, swirling atmosphere. It will move from its current, nine-day orbit to a symmetrical 24-hour polar orbit.

Its instruments will conduct an in-depth investigation of the structure, chemistry and dynamics of the Venusian atmosphere.

Over the next two years, Venus Express should help to explain why our close neighbour has turned from a potentially habitable haven to a place of hellish activity. 

- INDEPENDENT