The 14-member consortium said it was earmarking 542 million euros (466 million dollars) for this project, as well as a similar sum for a space observatory, GAIA, to be launched in 2012, that will measure the distance of up to a billion stars.

The Mercury mission will comprise two orbiters and a lander, Roger Bonnet, director of ESA's science programme, told journalists.

One of the orbiters will loop over the poles at a height of 400 to 1,500 kilometers (250 miles to 950 miles), carrying two cameras and spectrometers to map the planet's surface and monitoring its gravity and rotation.

The other will observe Mercury's magnetic field and its interaction with the solar wind, the blast of ionised particles that radiate from the nearby Sun.

The lander will be a fixed probe that will be sent down to a relatively mild polar area, carrying a camera, seismometers and instruments to measure the temperature and hardness of Mercury's surface.

The orbiters will be designed to operate for a year, while the lander will operate for "at least" a week, according to the mission's specification.

Mercury is the closest, smallest and least understood of the four "inner" planets near the Sun.

The little planet -- the smallest of the Solar System except for Pluto -- is just over a third of the size of the Earth. Its diameter of 4,880 kilometers (3,050 miles) is about the size of our Moon.

Mercury's orbit around the Sun, just 58 million kilometers (36 million miles) away, is a breathtaking 87.7 days, an appropriate speed for a planet named after the wing-footed messenger of the Roman Gods.

But its rotational spin is so slow that the planet experiences a "day," a complete turn on its axis, every 59 Earth Days. Combined with the speedy orbit, it means that the time from one sunrise to the next on Mercury is 176 Earth days.

Mercury is so close to the Sun that its sunlit surface reaches 400 degrees Celsius (750 deg. Fahrenheit) at the hottest point, while its nighttime side plunges to a freezing minus 160 degrees Celcius (minus 256 Fahrenheit).

That makes conditions so hostile that every aspect of the mission's design and construction will be affected, Bonnet said.

Solar cells made from special gallium arsenide compounds will be made as a power source for the instruments in order to cope with high light intensities as well as high temperatures.

The mission has been named Bepi Colombo, after an Italian astronomer whose calculations enabled a flyby of Mercury by the US space probe Mariner 10, 25 years ago, that revealed a flinty-surfaced planet pocked with innumerable small craters.

The big questions exercising astronomers is why Mercury is so dense, how it acquired its magnetic field and the effects of the space collisions that smashed out these craters.

Answers to these could shed light on the origin of the Earth itself, it is hoped.

Bepi Colombo will be launched in 2009, but will take two and a half years to get into orbit around Mercury after a swingby of Venus.

The journey time will be reduced from an originally-planned four years, thanks to a small, powerful engine that will eject heavy xenon ions at high speed in order to build up acceleration over a period of months.

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