Houston: NASA (National Aeronautics & Space Administration) has launched its Messenger spacecraft on the first mission to Mercury beginning a seven-year and 5 billion-mile journey to head to the sun's closest planet in 30 years.

The $286 million, 2,442-pound satellite was launched at about 2:15 local time aboard a Boeing Co. Delta II rocket from the Cape Canaver al Air Force Station in Florida on a seven-year journey to the closest planet to the Sun.

It will be the first mission to Mercury since Mariner 10 visited in 1974 and 1975. Today's liftoff followed a delay yesterday due to "weather constraints''.

'Messenger' is a scientific investigation of the planet Mercury. Understanding Mercury and the forces that have shaped it, is fundamental to understanding the terrestrial planets and their evolution.

The Messenger spacecraft will orbit Mercury following three flybys of that planet.

The orbital phase will use the flyby information as an initial guide to perform a focused scientific investigation of this mysterious world.

Among the questions scientists hope to answer is whether Mercury, just slightly larger than Earth's moon, was once Earth-sized itself but lost its rocky exterior either to some cataclysmic collision or to slow ablation by the solar winds.

Scientists also believe there may be frozen water there, trapped in shadowy craters at the planet's poles, never exposed to the sunlight that creates a 1100 degF difference between daytime and nighttime temperatures on the planet.

"The inner planets (Mercury, Venus, Earth and Mars) all formed from the disk of gas and dust, the solar nebula, that surrounded our young sun. They formed by the same processes, the same time, (but) their outcomes were extremely different. And Mercy is the most extreme of these four planets," said Sean Solomon, the mission's principle scientist.

Messenger will reach Mercury after a seven-year sojourn through the solar system that will take it 15 times around the sun, making near passes of Earth once, Venus twice and Mercury itself three times.

Each planetary pass will act as a gravitational tug to slow Messenger's speed so that it can eventually slip into Mercury's orbit for a yearlong study. The only other up-close look planetologists have had of Mercury came in the mid-1970s when NASA's Mariner 10 spacecraft made three fly-bys, photographing about 45 per cent of the planet and discovering that it had a strong magnetic field, an indication that scientists say, that Mercury is about two-thirds iron.

Developed by the Applied Physics Laboratory at the Johns Hopkins University of Baltimore, Messenger will spend at least a year in orbit around Mercury looking for similarities with Venus, the Earth and Mars.

"This is a super mission," said Robert Strom, a planetary geologist from the University of Arizona who also participated in the Mariner 10 mission. "Never in my wildest imagination did I think we would get a spacecraft like this. It's got all the instruments on board to answer the questions raised by Mariner 10."