The Delta II rocket carrying Messenger is scheduled to take off at 2:16 am (0616 GMT) Monday from Cape Canaveral in Florida for a journey of several years, the US space agency said.

Messenger will fly over Mercury three times in 2008 and 2009 before entering the planet's orbit in March 2011. It will travel a total of 7.9 billion kilometers (4.9 billion miles), even though Mercury is 91 million kilometers (56 million miles) from Earth.

"This is an ambitious discovery mission. Understanding Mercury, how it formed and (how) it evolved is essential to the understanding of other terrestrial planets, Venus, Earth and Mars," said Orlando Figueroa, director of solar system exploration at NASA.

Thirty years ago, the Mariner 10 spacecraft flew over Mercury three times but was able to photograph just 45 percent of the planet's surface and left many questions unanswered. It did not enable scientists to learn how its surface formed.

"Now, 30 years later, advances in technology, mission design and materials have enabled us to go back with a much more capable mission, which can help us to understand" the solar neighbor, Figueroa said.

For example, Messenger has been designed to withstand extreme temperatures on Mercury thanks to a ceramic-material sunshade.

Scientists hope that the spacecraft will provide the first images of the entire planet and collect detailed information on its geological history, the nature of its atmosphere and its magnetosphere.

Messenger, which weighs 1.2 tonnes, will also measure the structure and state of Mercury's core and the origin of its magnetic field.

Since Mariner 10, scientists have known that Mercury has a density similar to that of Earth, though it is scarcely larger than the moon. It has the thinnest atmosphere of any of the terrestrial, or rocky, planets in the solar system -- Mercury, Venus, Earth and Mars.

Its temperature variations are so extreme that despite its proximity to the sun it might have ice in permanently shadowed craters at its poles, where the temperature is lower than minus 108 degrees C (minus 162 degrees F).

"One of the most bizarre questions is whether the planet closest to the sun might really have ice at the poles," said Sean Solomon of the Carnegie Institution of Washington, principal investigator for the mission.

"The question that seems most fundamental to me is how Mercury got put together, what were the processes that contributed to the inner planets turning out so differently," Solomon said.

The spacecraft will carry seven scientific instruments: an imaging system, an altimeter to measure the planet's topography, a magnetometer to explore its magnetic field and four spectrometers to study its crust, magnetic field, magnetosphere and atmosphere.

Questions about Mercury abound: Why is the densest planet in the solar system made primarily of iron? Why does it have a magnetic field? How is it that the planet closest to the sun, with daytime temperatures close to 450 degrees C (840 degrees F), has what appears to be ice at its poles?

Scientists are hoping Messenger will bring answers, but they will have to wait several years for them.

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