Venus Express starts 26m-mile journey to discover why Earth's twin turned bad

· Relief at successful launch after probe failure
· Scientists hope for insight into greenhouse effect

Venus Express, Europe's first probe to Earth's mysterious sister planet, blasted into space from the Baikonur cosmodrome in Kazakhstan yesterday, with scientists hoping the mission will help them better understand the development of the greenhouse effect on Earth.

The probe's successful launch on a Soyuz rocket was greeted with intense relief by European and Russian space officials gathered in the freezing cold just after dawn. Last month a European probe Cryosat was lost when its Russian launcher failed. A second successive failure would have put a severe strain on the fledgling partnership between Russia and Europe.

But the Russian Soyuz performed perfectly and Venus Express began its five-month, 26m-mile journey to the planet many scientists call Earth's evil twin. It will be the first probe to visit Venus since Magellan in the early 1990s.

Scientists think the Earth and Venus had a similar start 4.6bn years ago during the solar system's birth. But while one planet went bad, the other nurtures millions of species. One is a searing cauldron beset by a runaway greenhouse effect, the other a watery paradise. Venus Express will help scientists to understand why.

Two hours after liftoff, delighted European Space Agency officials announced that Venus Express, a box-shaped probe the size of a small car, had been successfully fired on its journey. Twenty minutes later the first signal from the probe was picked up. Venus Express will reach its target in April and begin surveying the planet with its range of detectors. It will measure the chemical composition of the atmosphere, take high-resolution photographs, and assess its magnetic field.

"Essentially we want to know why Venus went bad," said David Southwood, head of science at ESA.

Although Venus is Earth's nearest planetary neighbour, previous space probes have revealed it to be a hellish world. The surface temperature is 465C (869F), the atmosphere is a crushing 90 times more dense than ours, and the planet is shrouded in sulphuric acid droplets.

"Earth could easily have turned out like that," Professor Southwood said. "It is important we find out why, and Venus Express should do that."

The probe is scheduled to orbit the planet for two Venusian days, the equivalent of 486 days on Earth, and hopes to discover the fate of the planet's water. If Earth's ocean was spread evenly over the globe, it would have a depth of almost two miles. If Venus's water vapour was allowed to liquefy - tricky given the planet's searing heat - it would produce a global puddle only 30cm (12in) deep. One theory is that Venus's water began to boil off almost as soon as the planet formed. Its proximity to the sun, an average distance of 67m miles compared with Earth's 93m miles, gave Venus a hotter and more deadly headstart in life.

The water vapour might have disassociated into hydrogen and oxygen, with the atoms of the former slowly seeping out into the cold depths of space. A suite of instruments on Venus Express will monitor the planet's upper atmosphere for signs that hydrogen is leaking away.

"We want to know what particles and atoms are leaving Venus and how its atmosphere is interacting with the sun," said Don McCoy, the mission's project manager.

Venus also has a distinctive geological structure. While Earth's crust lies on a series of plates that bend and buckle to produce earthquakes and volcanic eruptions, Venus has a single uniform plate covering the whole planet, which is relatively young at 500m years old. Scientists speculate that while Earth's floating plates let the planet's internal energy dissipate gradually, on Venus it may build up and engulf the whole planet in a global eruption.

Researchers also want to find out about the origins of the planet's greenhouse effect. Vast amounts of atmospheric carbon dioxide have created its scorching temperatures: hot enough to melt lead. But why did this happen on Venus and not on Earth?

For the ESA, the launch marks a new phase. "Europe now has a probe in orbit round the moon and Mars and will soon have one in orbit round Venus - that's the big three as far as we are concerned," Prof Southwood said. "We have shown the world we are a major space power. We have arrived."

FAQ: Burning questions

Scientists hope Venus Express will help them answer these questions:

Why is Venus so hot?

The planet is the ultimate greenhouse. The atmosphere is 96% carbon dioxide, an important reason for the 465C (869F) surface temperatures, and measurements there will shed light on the greenhouse effect on Earth. But scientists say there is more to it: could heat-emitting chemical reactions be taking place?

What happened to its magnetic field? Unlike the Earth, Venus has no magnetic field. This means the atmosphere is being slowly siphoned off by the barrage of charged particles arriving from the sun.

Is it still volcanically active?

It was in the past but whether there is any activity now is unclear. If the activity has stopped or reduced, scientists will want to know why.

Is there life on Venus?

Unlike Earth, Venus has no liquid water. There might have been oceans in the past but the intense heat is one possible reason why all the water disappeared. However, there is some water vapour in the clouds which has lead to some far-fetched ideas that simple forms of life, such as microbes that photosynthesise the sun's light, might exist on the planet.

What are the icecaps made of?

Venus is so hot that only certain compounds would remain solid at the poles. They seem to be metallic but what are they? They could be giant pools of exotic metals such as tellurium or more common compounds of metals such as as iron, which would stay solid at the temperatures involved.

Contributor

Robin McKie in Baikonur

The GuardianTramp

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