Venus Express will be ESA's first mission to Earth's nearest planetary neighbour, Venus. The mission was born after ESA asked for proposals, in March 2001, suggesting how to reuse the design of the Mars Express spacecraft.

The guidelines were extremely strict. The mission would have to run to a tight timeframe because it had to reuse the same design as Mars Express, and the same industrial teams that worked on that mission. It would have to be ready to fly in 2005.

Out of a number of promising proposals, ESA selected Venus Express. What made the mission especially attractive was that many of the spare instruments developed for ESA's Mars Express and Rosetta missions could be used to achieve Venus Express's science objectives, which were to study the atmosphere in great detail.

Venus is the Earth's nearest planetary neighbour. It draws twice as close to our planet as Mars ever does. In terms of size and mass, Venus is Earth's twin and yet it has evolved in a radically different manner, with a surface temperature hotter than a kitchen oven and a choking mixture of noxious gases for an atmosphere. Venus Express will make unique studies of this atmosphere.

In the past, both the Russians and Americans have sent spacecraft to Venus. Being the closest planet to the Earth, it was a natural target. These studies revealed details about the surface of the planet, mainly from NASA's Magellan radar mapper. However, Venus was out of the limelight during the last decade, despite several scientific puzzles. For example, how does the atmosphere 'work'? To find the answer, scientists break down the big question into smaller questions:

• "What are the global characteristics of the atmosphere?"
• "How does it circulate?"
• "How does the composition of the atmosphere change with depth?"
• "How does the atmosphere interact with the surface?"
• "How does the upper atmosphere interact with the solar wind?"

Experts have designed Venus Express to be the first spaceprobe to perform a global investigation of the Venusian atmosphere, of the plasma environment. World of mysteries

You cannot understand the Venusian weather and atmosphere by comparing them to Earth's. Scientists are unable to explain some of the more extreme atmospheric phenomena that take place on Venus. For example, the planet only rotates once every 243 Earth days. However, in the upper atmosphere, hurricane-force winds sweep around Venus, taking just 4 Earth days to circumnavigate the planet.

The surface of Venus also baffles scientists. The oldest craters seem to be only 500 million years old, which may indicate that the planet behaves like a volcanic pressure cooker. On Earth, the constant, steady eruption of volcanoes and the shifting of the Earth's surface, causing earthquakes, ensures that the energy released in the Earth is dissipated gradually. This probably does not happen on Venus. Instead, pressure builds up inside the planet until the whole world is engulfed in a global eruption, resurfacing the planet and destroying any craters that have formed. This probably happened last, 500 million years ago and so accounts for the lack of older craters. Today, there is a strong relationship between the surface and the atmosphere. Is there any similarity between the ocean-atmosphere relationship on Earth and the surface-atmosphere relationship on Venus? Venus Express will supply scientific data that could shed light on both of these mysteries.

Source: European Space Agency (ESA)