A、nearest of all planets to the sun B、extremely hot C、little known to people D、biggest of all planets D

　　CAPE CANAVERAL, Fla.—The spacecraft Messenger 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 flight controller announced as soon as Messenger shed its final rocket stage hours before dawn.
　　The journey will take 6 1/2 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 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—costing＄427 million for the launch and all the scientific analysis years later in a mission devised by John Hopkins University.