Venus, the Earth's nearest planetary neighbour, is remarkably similar in size and mass to our own world. However, its atmosphere and climate could hardly be more different. The reasons for these contrasts are proving difficult to understand. Scientists still do not know, for example, the details of the greenhouse effect on Venus, which keeps the surface hot enough for molten metal to flow, despite the fact that Venus absorbs less heat from the Sun than the Earth does.

Venus and Earth have also evolved quite differently. Venus has vast, smooth plains, no continents and extensive volcanic activity that produces dense cloud layers with an exotic, sulphur-rich composition.

Most puzzling of all is the atmospheric circulation which features hurricane force winds at high levels that sweep around Venus in just four days - remarkably rapid for a planet that only rotates once every 243 Earth days.

"The planet's weather systems and climate characteristics cannot be understood by comparison with Earth," said Professor Taylor. "The failure of extrapolated terrestrial models to account for Venus' behaviour has wide implications in fields ranging from solar system evolution to climate forecasting on Earth."

Venus Express is proposed to be launched on a direct trajectory to Venus with a Soyuz-Fregat rocket from Baikonur in November 2005. After a flight of about 150 days, it will brake into a highly elliptical 5-day orbit around Venus.

The spacecraft will then be manoeuvred to its operational polar orbit between 250 km and 45,000 km above the planet where, for two Venus years - equivalent to 450 Earth days - it will study the atmosphere, the surface and the plasma environment of Venus.

In order to lower costs, Venus Express is to be based on the European Space Agency's Mars Express spacecraft (which is scheduled for launch in summer

2003) and it will use seven flight spare experiments from Mars Express and the Rosetta comet chaser.

"Venus Express is a strong candidate to be part of the next wave of Venus exploration, including Japanese and probably American space missions, which will probe the environment of this mysterious planet," concluded Professor Taylor.

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