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© NasaArtist's concept of MESSENGER in orbit around Mercury.
He's really hot, really cold, maybe even a bit icy. He is the planet Mercury and this month he is ready for his extended close-up.

On Wednesday, NASA showed the first pictures taken by its Mercury Messenger spacecraft, which entered the planet's orbit on March 17.

Messenger will spend at least a year photographing, measuring and studying Mercury, which for now is the last frontier of planetary exploration.

''This is the last of the classical planets, the planets known to the astronomers of Egypt and Greece and Rome and the Far East,'' the principal investigator, Sean Solomon of the Carnegie Institution of Washington, said.

It ''captivated the imagination and the attention of astronomers for millennia'', Dr Solomon said, but science had never had such a front-row seat. ''We're there now.''

The space agency has sent orbiters to five planets - Mercury, Venus, Mars, Jupiter and Saturn - but it probably will take a decade or two before a spacecraft orbits Uranus or Neptune. (In 2015, NASA's New Horizons spacecraft will zoom past Pluto - which is no longer regarded as a planet.)

Half a dozen fly-bys by NASA probes - three by Mariner 10 in the 1970s and three by Messenger in the past three years - have seen Mercury close up, though briefly.

But now that Messenger has pulled into an orbit of Mercury, planetary scientists will be able to get their first long look at the smallest of the eight planets. The day side of Mercury can sear at 430 degrees; the night side drops to minus 185 degrees.

Particularly intriguing, scientists say, are the shadows in craters near Mercury's poles. There, the sun never shines and in the frigidity some scientists expect that Messenger will find frozen water.

On its arrival, engineers checked the orbiter's systems and gear. Finding everything in working order, they turned on the instruments, including the camera.

The first photo, of a bright crater called Debussy, was taken early on Tuesday. By the end of Thursday, it was expected to take 1500 photographs. More than 75,000 are planned over the next year.

Seven instruments began measuring emissions of neutrons, X-rays and gamma rays, which will allow scientists to deduce many of the minerals of the surface.

The extended observation will help them understand how Mercury, which is half the mass of Mars, still has a magnetic field - which is presumably generated by a molten outer core - while Mars does not.

James Head III, a geology professor at Brown University and an investigator on the Messenger mission, said Mercury could even give clues about how plate tectonics - the motion of pieces of crust - started on Earth. On other bodies, Mars and the Earth's moon among them, there is no sign of plate tectonics, with the crust remaining in one piece.

On Mercury, however, there are long tectonic ridges extending thousands of kilometres, which could have represented the first stage of the crust breaking up into plates, although the pieces never started moving as on Earth.

The spacecraft will officially begin its scientific measurements on Monday.