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.

Why Venus?

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 and of the plasma environment.

World of mysteries

The greatest mystery of Venus may be why its climate has evolved as it has. Venus is almost the same size as the Earth, and was created at a similar time from similar materials, yet it has evolved very differently. Both Venus and Earth probably once had atmospheres rich in water and carbon dioxide. On Earth, much of the carbon dioxide has been removed from the atmosphere and locked in carbonate rocks. On Venus, on the other hand, most of the water seems to have been lost to space, which leaves an atmosphere composed almost entirely of carbon dioxide. This acts as a powerful greenhouse warming agent, which heats the surface up to over 450 degrees Centigrade. Venus is completely enveloped in a thick layer of clouds, composed mainly of sulphuric acid but whose composition is still not fully known.

Another mystery of Venus is the atmospheric super-rotation. The surface of Venus, revealed through radar imagery, is known to rotate very slowly, with a period of 243 Earth days. However, the cloud tops, which are at roughly 65 km above the surface, rotate 60 times faster than the planet below, circumnavigating the planet in roughly 4 days. The causes of this extreme super-rotation are still unknown. Venus Express is the first satellite able to study the atmosphere below the cloud tops, by using near-infrared 'spectral window' wavelengths. (At most wavelengths, outgoing radiation is blocked either by the thick Venusian atmosphere or by the thick cloud deck; at a few wavelengths, called 'spectral window' wavelengths, infrared light from the lower atmosphere of Venus escapes through the clouds to space, where it can be detected by spacecraft.) It is hoped that this will yield clues as to what causes the super-rotation.