With Messenger safely nestled in its payload bay, a massive Delta II rocket roared off from its launching pad at about 2:16 am (0616 GMT), turning in a matter of seconds from a fire-breathing giant into a tiny speck of light in the sky.

The launch was delayed by one day due to strong winds created by a tropical storm churning in the Atlantic Ocean.

But "Tropical Storm Alex is no longer an influence on weather in the Cape Canaveral vicinity," George Diller, spokesman for the Kennedy Space Center, told reporters shortly before launch.

Minutes after launch, the Delta rocket released the refrigerator-sized probe on the first leg of its 7.9-billion-kilometer (4.9 billion mile) odyssey through the solar system.

One of the most enigmatic planets of the solar system, Mercury endures more solar radiation than any other planet and has one of the densest crusts of all, scientists say.

Its daytime temperatures could reach 450 degrees Celsius, or 840 degrees Fahrenheit, which is enough to melt lead.

However, at night, Mercury plunges into a deep freeze that could be enough to liquefy oxygen: the temperature plummets to minus 212 Celsius, or minus 350 Fahrenheit.

Despite these climatic quirks, researchers, using radar, detected in the early 1990s what appeared to be a glint of ice inside the planet's massive polar craters.

Scientist at the National Aeronautics and Space Administration are also puzzled why Mercury has a disproportionately large iron core, which takes up more than 60 percent of its total mass.

"The answers to these questions will not only tell us more about Mercury, but illuminate processes that affect all the terrestrial planets," said Sean Solomon, a lead scientist on the Messenger project.

The probe will try to tackle questions left unanswered by a similar mission undertaken by NASA three decades ago. Probe Mariner 10 sailed past Mercury three times in 1974 and 1975 but, given the level of that era's equipment, was able to gather data on less than half of the planet's surface.

By contrast, Messenger is packed with state-of-the-art technological marvels, including a color imager and a vast array of spectrometers capable of bombarding the Mercury surface with gamma- and X-rays, neutrons and laser beams, and measure its magnetic fields.

The spectrometers could help determine with more certainty if indeed there is ice on Mercury, measure the structure of surface rocks and its thin atmosphere, NASA officials said.

The plan calls for three flybys of Mercury in 2008 and 2009 and a yearlong orbit of the planet starting in March 2011.

Messenger project manager David Grant called the 427-million-dollar mission that will include 15 loops around the Sun "risky and difficult," but said there was every reason to believe in its successful outcome.

"The team is confident that the spacecraft they designed, built and tested is ready for the journey and its mission to Mercury," he said.

The successful launch is also seen as a political boost for NASA, which has been targeted for severe budget cuts by congressional appropriators eager to reduce the skyrocketing budget deficit fueled by massive spending on the wars in Iraq and Afghanistan.

Last month, a House subcommittee cut President George W. Bush's 2005 request for the space agency by 1.1 billion dollars to a total 15.1 billion, which is more than a quarter-billion less than NASA's budget for the current fiscal year.

The White House has threatened to veto the bill if the funds are not restored.

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