-- The spacecraft Messenger rocketed away Tuesday on a long quest to reveal the secrets of mysterious, super-hot Mercury, the sun's nearest planet.

"A voyage of mythological proportions," a NASA 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 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 was scrubbed because of bad weather.

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.

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.

Russian Gennady Padalka and American Mike Fincke improved the parking situation at the space station, putting up the latest devices for guiding in a brand-new line of cargo ships.