False-colour image shows a Moon-like surface

The American space agency (Nasa) has given the first Mercury orbiter mission the go-ahead, for what will be the first trip to the Sun's closest neighbour in 35 years.

Messenger - short for MErcury Surface, Space ENvironment, GEochemistry, and Ranging - will be launched in March 2004 and will reach Mercury in April 2009.

"Our goal is to do something never before attempted," said Dr Jay Bergstralh of Nasa. "Conducting a year-long mission to orbit a planet only 36 million miles (58 million km) from the Sun for relatively low cost is an amazing concept."

Iron planet

Tiny Mercury, the closest world to the Sun, is among the least explored of the planets, yet astronomers say it is fundamental to our understanding of the inner Solar System.

Our goal is to do something never before attempted

Dr Jay Bergstralh, Nasa

So far, it has been visited by only one spacecraft, Mariner 10 in 1974. It is a planet of extremes. Its density is the highest of any planet. It is believed to be made mostly of iron.

In the distant past, Mercury may have been a larger planet with an iron core. A giant impact appears to have stripped it of its lighter outer layers leaving the small dense world we see today.

Its ancient, Moon-like, surface records events from the earliest periods of the Solar System's history.

Hot and cold

It is the only rocky planet besides the Earth to possess a global magnetic field and temperatures on the planet vary from nearly the highest in the Solar System, at the equator, to among the coldest, in the permanently shadowed poles where ice deposits lurk.

Following two flybys, Messenger will go into orbit around the planet seeking answers to Mercury's puzzles:

• What is the origin of Mercury's high density?
• What is the composition and structure of its crust?
• What is the nature of the mysterious polar caps?