Probe preparing for Mercury orbit

21 February 2011 Last updated at 14:50 GMT By Pallab Ghosh Science correspondent, BBC News, Washington DC Eric Finnegan explains some of Mercury Messenger's special features

The arrival of a Nasa probe in orbit around Mercury will transform our understanding of the innermost planet.

Messenger will be the first probe to orbit Mercury; it was previously visited by the Mariner 10 spacecraft, which made several passes in the 1970s.

The team behind the mission were outlining details to scientists at a conference in Washington DC.

Messenger has already made three flybys of Mercury and is set to enter orbit around the rocky world on 17 March.

When Mariner 10 visited the planet in 1974, it sent back pictures of what seemed to some at the time an uninteresting planet compared with Venus, Mars and the Solar System's gas giants.

But Dr Nancy Chabot who is Messenger's instrument scientist, said Mercury was "under-appreciated".

But observations from Earth began to show that far from being boring, Mercury may well be unique.

It is a planet of extremes. The world is the closest to the Sun, yet it could have ice at its poles.

And it has a giant metal core unlike any of the other inner Solar System planets. Planetary scientists began to think that understanding Mercury might be the key to understanding how all the inner rocky planets formed. And so the Messenger mission was born.

According to Dr Chabot, "what you can learn when you are in orbit is so different from when you are just flying past by gathering data as you go. This is really going to revolutionise what we know about this planet."

Messenger will be the first spacecraft to study Mercury's geology in detail. In particular, scientists will be interested in data from the planet's giant core.

There are three theories as to how the planet came to have such an inner structure: It was created that way; it used to be much larger and a giant impact ripped off much of the rocky crust; or, most intriguingly, that Mercury was once much larger - but an early solar event partially vaporised its surface.

The Messenger mission should help determine which of these theories is correct.

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