The spacecraft Messenger (search) rocketed away Tuesday on a long quest to reveal the secrets of mysterious, superhot Mercury, the sun's nearest planet.
"A voyage of mythological proportions," a NASA (search) flight controller announced as soon as Messenger shed its final rocket stage hours before dawn.
The journey will take 61/2 years, covering nearly 5 billion miles on a roundabout ramble through the inner solar system. The probe should reach Mercury (search) by March 2011, then spend a year gathering data.
Scientists want to know how the planet turned out the way it did, and whether the perpetually dark craters at the poles hold ice. Anything scientists can learn about how Mercury formed will shed light on the origins of Venus, Earth and Mars, each one very different.
Tuesday's launch came one day later than planned; Monday's attempt had to be scrubbed because of bad weather.
It was a busy night for the U.S. space agency with the Mercury send-off coming less than an hour before another NASA unit directed a spacewalk by the astronaut and cosmonaut aboard the international space station.
The Messenger mission is part of NASA's bargain-focused Discovery program — $427 million for the launch and all the scientific analysis years later in a mission devised by Johns Hopkins University.
Scientists have been yearning to study Mercury up close ever since Mariner 10 zoomed by three times in the mid-1970s. If all goes well, Messenger will be the first spacecraft to orbit that planet.
Because it can't carry enough fuel, Messenger cannot fly straight to Mercury. So it will fly once past Earth, twice past Venus and three times past Mercury for gravity assists — and make 15 loops around the sun — before slowing enough to slip into orbit around the small, hot planet.
The heat encountered once in orbit will be the equivalent of 11 suns beating down on Earth, about 700 degrees. But its instruments will operate at room temperature, protected by a custom-built ceramic-fabric sunshade just one-quarter of an inch thick. All Mariner 10 had was a quaintly old-fashioned umbrella.
That's why, in large part, it's taken so long to return to Mercury. Scientists had to figure out how to beat the heat.
Mariner 10 provided "a glimpse of this planet of extremes," said Orlando Figueroa, director of NASA's solar system exploration division. Because it only flew by Mercury and did not circle the planet, Mariner 10 observed less than half the orb.
Messenger will view Mercury from all sides.
"I say we are long overdue for another visit with some permanence to help us unveil the secrets of this planet, the innermost and least understood of the terrestrial planets," Figueroa said.