Mercury is the smallest of the inner solar system's "rocky planets," a nasty little place covered with moonlike craters, cooked by sunshine 11 times as bright as Earth's and imbued with enough mystery to fill a whodunit.

Why is Mercury basically a gigantic iron ball? Why does it have a magnetic field? Why does it have a cloud of sodium gas surrounding it? Is the shiny stuff that fills the polar craters ice, and if so, where did it come from?

"Mercury is very unusual," said the Carnegie Institution of Washington's Sean C. Solomon. "It belies conventional wisdom in a number of ways and cries out for further exploration."

To try to answer some of these questions, NASA developed the Messenger space probe, which was scheduled to blast off early today from Cape Canaveral Air Force Station to begin a seven-year trip to Mercury, the first time a spacecraft has headed that way since the 1970s. The 12-second launch window -- the first of 16 brief, daily launch opportunities ending Aug. 18 -- opened at 2:16 a.m.

On a direct flight, Messenger, which stands for MErcury Surface, Space ENvironment, GEochemistry and Ranging, would travel about 50 million miles and reach its destination in a few months. But because it is going to orbit Mercury, it will need to slow down so the planet can pluck it out of the ether.

Deceleration is hard for something traveling toward the sun, and Messenger will use an Earth flyby, two Venus flybys and three flybys of Mercury itself to refine its trajectory and put on the brakes before going into orbit in March 2011. By then the spacecraft will have flown 4.9 billion miles.

Messenger will then begin its year-long mission, circling Mercury twice a day in a near-polar orbit to collect data and images under conditions so extreme that new technologies in insulation and lightweight materials were needed before scientists could even contemplate the project.

Messenger's seven instruments are clustered behind a special ceramic sunshade that protects them from temperatures as high as 840 degrees Fahrenheit. Even so, the heat bouncing off the planet itself is so intense that engineers have given the spacecraft an elliptical orbit so that after it closes to within 124 miles of the planet's surface, it zooms 9,420 miles out into space twice a day to cool off.

"Going to the outer planets is a piece of cake compared to going to Mercury," said Robert G. Strom, a University of Arizona planetary geologist and member of the Messenger team. "We are orbiting hell, but in this case hell is very interesting."

Messenger is a child of NASA's low-budget Discovery Program and cost $427 million over the past eight years. Johns Hopkins University's Applied Physics Laboratory built it, engineering a graphite machine with a launch weight of 2,424 pounds, only 1,101 pounds of which was allotted to spacecraft and instruments.

The rest is fuel, needed for orbit insertion and periodic "burns" to hold Messenger in position. The task, said the APL's David Grant, project manager for the mission, is to launch with the heavens aligned properly for the eventual flybys. "We have to use an optimum trajectory [to save] energy and fuel," Grant said. "We don't have free choice on day, time or opportunity" for launch. If they miss the windows this month, they will have to wait a year to try again.

Mercury, with a diameter one-third Earth's, is about 36 million miles from the sun, and zips around it in 88 Earth days. But its rotisserie-slow rotation -- one Mercury day lasts two Mercury years -- causes the planet's day-night temperature to careen between 840 degrees Fahrenheit and minus-350 degrees Fahrenheit, the biggest differential of any planet in the solar system.

Most of what little is known about Mercury derives from 1974 and 1975 Mariner 10 flybys that mapped 45 percent of the surface and collected enough intriguing photographs and data to set researchers scratching their heads. Information gathered since then has only deepened the mystery.

What Mariner found was a heavily cratered planet with an ancient surface uncluttered by the extensive volcanism, erosion and faulting that made and remade the surfaces of its rocky planet cousins: Venus, Earth and Mars.

"These geological processes wiped out the information of what planets were like at the beginning of the solar system," said geophysicist Raymond Jeanloz of the University of California at Berkeley. "The only other example we have like this for a large object is the moon. We'd like to check our ideas on Mercury."

Seventy percent of Mercury's mass is an iron core, while Earth's core is proportionally only half as big. There is no obvious reason for this anomaly, but researchers have put forth several hypotheses.

Some suggest that when the cloud of dust and gas that surrounded the sun began to collapse as the solar system formed 4.5 billion years ago, lighter elements nearby were sucked away by gravity, leaving heavier iron to gather into the ball that became Mercury.

Others say the sun's heat was so great that it baked the crust off the young Mercury, leaving a metal cinder behind. And still others suggest a giant intruder -- asteroid or comet -- hit Mercury a glancing blow, peeling off most of the skin and sending it into space, where the sun vaporized and absorbed it.

Each theory requires the crust of Mercury be composed of different substances, and Messenger will study light emitted from the planet to deliver the answer. "We'll be looking big-time," said mission team member William McClintock, a senior research associate at the University of Colorado's Laboratory for Atmospheric and Space Physics. "Right now we have no measurements at all."

And is the core molten? For Mercury to have the magnetic field that Mariner 10 found, a fluid outer core would seem to be a requirement, Solomon said, except Mercury is too small to have a fluid outer core. It should have dried up.

To answer this question, Strom said, Messenger will measure whether Mercury "wobbles" as it turns on its axis, much the way a raw egg hesitates when spun. A solid core would spin uniformly -- like a hard-boiled egg.

Messenger also will take pictures of Mercury's surface to search for definitive evidence of past volcanic activity, to try to find the reservoir that feeds the sodium cloud that cloaks the planet and to analyze the deposits at the polar craters that researchers suspect may be water ice.

"Never in my wildest dreams did I believe they could do all this for the price, but they did," Strom said. "You're getting a Cadillac mission for a Volkswagen price."

Artist's impression of the MErcury Surface, Space ENvironment, GEochemistry and Ranging (Messenger) spacecraft in orbit around Mercury.