Planetary experts say they're "jazzed up" for a potential scientific bonanza today as a spacecraft from Earth flies within 125 miles of the planet Mercury.
The Maryland-built Messenger spacecraft, launched by NASA in 2004, will buzz the planet nearest the sun about 2 p.m. today, providing mankind's first close-up look in 33 years.
Today's visit is only a prologue. In 2011, Messenger is to begin a year in orbit around Mercury. But researchers say they hope the craft will start today to answer some of the vexing questions that arose after Mariner 10 flew past the bizarre little world in 1974 and 1975.
One puzzler: Just how did tiny Mercury, with its big, dense metal core and strong magnetic field, evolve just 30 million miles from the sun? Some theorists say they believe the planet might have formed much farther from the sun and migrated later to its present orbit.
"I think we're in for some big surprises," said Faith Vilas, a scientist with the mission and director of the MMT Observatory at Mount Hopkins, Ariz. She has been studying Mercury since the days before Mariner 10 photographed 45 percent of its surface.
"The first thing most of us want to see is what the other 55 percent of Mercury's surface looks like," she said.
Marilyn Lindstrom, Messenger program scientist at NASA headquarters, said it was like waiting 30 years to decide a Super Bowl matchup.
"We've been waiting to go back to Mercury that long, and people are that jazzed up," she said. "Not just the science team, but the entire planetary science community."
Eight years in development, the $446 million Messenger mission was designed and is managed by the Johns Hopkins University's Applied Physics Lab near Laurel. Engineers control the craft from an operations center on the APL campus.
After a 3 1/2 -year, 4.9 billion-mile journey, Messenger will zip past Mercury today at a relative speed of 16,000 mph, passing within 124 miles of the planet's surface at 2:04 p.m.
During 55 hours of flyby operations, the spacecraft is taking 1,300 pre-programmed photos and spectroscopic observations. Its seven instruments will map Mercury's surface chemistry and mineralogy, its magnetic field and tenuous atmosphere - all clues to its evolution, structure and natural environment.
Mercury is currently in the Earth's daytime sky, just east of the sun. After sunset, it is faintly visible low in the western sky, setting about 5:30 p.m. Messenger is passing from west to east behind the planet as seen from Earth.
Out of contact with controllers during the flyby, it will turn its high-gain antenna back toward Earth after noon tomorrow and begin uploading its discoveries to APL.
Mission managers said it might take several weeks to process images of previously unseen regions. But Sean Solomon, the mission's principal investigator, promised, "We'll try to get something out quick, so stay tuned." A formal release is scheduled for Jan. 30.
A second Mercury flyby is planned for October, with a third in September 2009 as controllers continue to use the planet's gravity to slow Messenger down enough to enter orbit in March 2011.
Mercury's proximity to the sun - just one-third of Earth's distance - is the reason it has taken scientists so long to return and attempt an orbit. The sun is 11 times brighter there, and structures on the sunward side of the spacecraft will experience temperatures hotter than a kitchen broiler.
Mission designers had to wait for new technology, including the lightweight, heat-resistant ceramic cloth they're using as a sunshade, to keep scientific instruments at room temperature.
Second, flying to Mercury means accelerating down the sun's "gravity well" like a cyclist rolling downhill. Braking with rockets would require a prohibitively heavy supply of fuel.
It wasn't until the 1980s, Solomon said, that mission designers devised the looping trajectory that takes Messenger past Earth, Venus and Mercury multiple times, using each planet's gravity as a brake. By 2011, Messenger's speed must match Mercury's pace around the sun.
With enough time around Mercury, scientists say they hope Messenger can eliminate some of competing theories about the planet's origins.
Each of the inner, "terrestrial" planets - Mercury, Venus, Earth and Mars - has a dense, metallic inner core surrounded by a rocky mantle. But Mercury's core is by far the largest in proportion to the mantle, making up 75 percent of the planet's radius.
Scientists have several theories about this. The young sun may have blasted away the more volatile silicate grains in the solar nebula that would have condensed to form the planet's mantle.
Or, the silicates might have been blown away as the planet formed. Or perhaps a giant impact blasted away much of the planet's mantle.
Discovering the precise chemical and mineralogical makeup of Mercury's surface rocks might tell scientists which theory is correct. And that, they hope, will also shed light on the origins of the Earth.
"There is an absolute revolution going on in our understanding of the evolution and origins of the solar system," said Jim Green, who heads the planetary science division at NASA headquarters.
"It is such an exciting field right now, and Mercury holds a very special place because it is so close to the sun that we are bound to learn much more about the history of the inner part of the solar system."