04-10-2006, 05:30 PM
By Paul Rincon Reply
BBC News science reporter
The first space mission to Venus in a decade is about to reach its target.
On Tuesday morning, a European robotic craft will perform a 50-minute-long engine burn to slow its speed enough to be captured by Venus' gravity.
Venus Express will orbit our nearest planetary neighbour for about 500 Earth days to study its atmosphere, which has undergone runaway greenhouse warming.
Scientists hope the mission will shed further light on the mechanisms of climate change on Earth.
Though it is closer to the Sun than our own planet, a thick, highly reflective cloud layer means Venus absorbs less solar radition than the Earth. One might therefore expect surface conditions on the two planets to be similar.
But Venus' dense, largely carbon dioxide, atmosphere acts as a blanket, trapping incoming solar radiation to heat the planet's surface to an average temperature of 467C (872F) - hot enough to melt lead.
The European Space Agency (Esa) mission aims to shed light on how this world, so close to our own in size, mass, and composition, has evolved so differently over the last 4.6 billion years.
The main engine burn, beginning at 0817 BST, will be initiated by controllers at the European Space Agency's control centre in Darmstadt, Germany.
The critical manoeuvre, which needs to be performed perfectly for the mission to be a success, will reduce the spacecraft's velocity relative to Venus by 15%.
We want to see how the system works there because there are still many mysteries about Venus
Gerhard Schwehm, Esa
"It loses about 70% of its propellant, just on this one burn," Jerry Bolter of EADS Astrium, the spacecraft's main contractor, told the BBC News website.
"We loaded as much propellant as we could within the limits of the launcher, the tank and the structure's strength limits.
"That should give us the opportunity to extend the mission a bit further."
At 0845 BST, with its engine still firing, Venus Express will disappear behind the planet for about 10 minutes, severing contact between the craft and Earth.
Controllers will be waiting to hear the spacecraft's signal after this "occultation period" is up as an initial sign the manoeuvre has worked.
If all goes to plan, Venus Express will slip into an tight, elliptical orbit which will bring it to within 400km (250 miles) of the poles.
Speaking about the rationale for the mission, David Southwood, director of science at the European Space Agency (Esa), has said it is "as important to look at the failures of the Solar System as its successes".
Although Earth is unlikely to ever end up as hot, a build-up of greenhouse gases in the atmosphere could result in our own planet taking some uncomfortable steps toward the situation on Venus.
"Our prime objective is to study the processes that are going on inside Venus' atmosphere," Gerhard Schwehm, Esa's head of planetary missions told the BBC News website.
"Nearly all the instruments give us information on either the composition of the atmosphere, temperature profile, or circulation. We want to see how the system works there because there are still many mysteries about Venus."
Mission scientists hope to learn what causes Venus' atmosphere to rotate much faster than the planet below. The craft will also study a swirling double vortex at the planet's north pole, to discover how it remains stable and where it gets its energy from.
Our planetary neighbour's hostile climate could also hold answers to how global warming will affect Earth in future decades, helping to constrain computer models of climate change.
Venus Express lifted off on a Russian Soyuz rocket from the Baikonur Cosmodrome in Kazakhstan on 9 November 2005.
Nasa's Magellan spacecraft was the last to visit Venus, carrying out radar mapping of the planet from 1989-1994.
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