The first spacecraft to visit Mercury in more than 30 years passes the planet on Monday at a distance of just 200km.

The fly-by is the first of three the Messenger probe will make in the coming years as it slows itself to enter into orbit around the small world in 2011.

The spacecraft will collect more than 1,300 images and make other observations during the encounter.

No probe has viewed Mercury up close since the Mariner 10 mission's third and final fly-by in March 1975.

"[Messenger] is lined up and ready to go; the team is ready and really pumped," said Marilyn Lindstrom, the US space agency mission's (Nasa) programme scientist.

"Mercury, here comes Messenger!"

The researcher said the entire planetary science community was impatient to get back to the nearest planet to the Sun.

The moment of closest approach is 1404 EST (1904 GMT).

'Lap times'

Lindstrom added: "[Messenger's] goal is to understand the surface, the interior, the magnetosphere and the atmosphere of this innermost planet; but in the process of doing that we hope to apply that [knowledge] to understand how all four of the terrestrial, Earth-like, planets formed."

Messenger is half-way through what will be a seven-year tour of the inner Solar System.

It is not due in orbit around Mercury until the March of 2011. To get there, it must perform a series of fly-bys and engine firings to put it on a correct course and, crucially, slow its final approach.

This week's pass, which takes place some 33 million miles from Sun, will reduce the spacecraft's velocity by 5,000mph. Even so, it will still pass over the cratered surface at a relative speed of 16,000mph.

"Messenger's orbital period around the Sun will be decreased by 11 days thus setting up a planetary car race with Mercury," explained Eric Finnegan, mission systems engineer at Johns Hopkins University Applied Physics Laboratory.

"Using its engine and future gravity assists, the spacecraft after being lapped by Mercury many times in its race around the Sun will eventually match the 88-day orbital period of the innermost planet."

Strange world

Messenger has already begun its flyby observations. The probe's instruments are expected to gather about 700GB of data in total over a period of 55 hours.

The final look will be on Tuesday at about midday EST (1700GMT), after which the probe will turn its communications equipment towards Earth to download the data treasure.

Messenger is operating in an extremely harsh environment.

Its electronics and observational instruments are protected behind a shield that allows them to operate at "room temperature". The Sun-facing side of he shield, however, experiences temperatures in excess of 600F.

All the terrestrial planets are believed to have formed at the same time by common processes - but Mercury itself is a bit of an oddball.

It is so dense that more than two-thirds of it has to be of an iron-metal composition.

It so close to the Sun that the temperature variation between day and night at the equator is 1,100 degrees; and yet there may be water-ice at the poles in craters that are in permanent shadow.

Europeans to follow

"Mariner 10 showed us a surface that was so heavily crater that it looked like geological activity on Mercury ended very early in the history of the Solar System," said Sean Solomon, Messenger's principal investigator from the Carnegie Institution of Washington.

"And yet, Mercury is the only other inner planet which like Earth has a magnetic field that we believe means it must have a very dynamic molten iron core.

"So how to reconcile this ancient surface with this modern-day internal dynamic activity is one of the mysteries we hope to solve."

On Friday this week, the European Space Agency (Esa) will sign an industrial contract with EADS Astrium to build BepiColombo.

This mission will be launched to Mercury 2013. It consists of two spacecraft - an orbiter for planetary investigation, led by Esa, and one for magnetospheric studies, led by the Jaxa (Japan Aerospace Exploration Agency).

The satellite duo will reach Mercury in 2019 after a six-year journey towards the inner Solar System.

Jonathan.Amos-INTERNET@bbc.co.uk

(BBC)

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