WASHINGTON, Aug. 4 (UPI) -- It is a dead hunk of rock orbiting so close to the sun its surface temperature is hot enough to fry lead. Its atmosphere is so tenuous it almost is not worth mentioning. Among its few distinctions, it is the densest of the terrestrial planets, with the oldest and most pristine surface, the highest surface temperature, the greatest temperature variance, the shortest orbital period and the longest day.
Given everything known about planetary geology, atmospheric chemistry and biology, there is almost no chance the planet Mercury harbors any kind of life.
Yet on Tuesday, NASA launched -- within a window that lasted only 12 seconds -- its MESSENGER spacecraft on a voyage that will take seven years to end up in orbit around the innermost, second-smallest and arguably least interesting member of the solar system.
The reason, space agency scientists said, is little Mercury harbors mysteries they would very much like to unravel.
MESSENGER, which stands for MErcury Surface, Space ENvironment, GEochemistry and Ranging, is only the second U.S. spacecraft sent to the the planet, and it will be the first to establish an orbit. It will take so long to reach its destination because its trajectory must use a series of gravity assists from Earth, Venus -- and Mercury -- to lower its speed enough to fall into orbit. In such a maneuver, the spacecraft flies close by a planet and either picks up or loses speed due to gravitational action.
Each of the six planned gravity assists will change the shape, size, speed and tilt of MESSENGER's flightpath until the spacecraft only needs its onboard propellant to insert itself into Mercurial orbit, on March 18, 2011.
MESSENGER's first close flyby actually involves a return to Earth in August 2005. Then it will fly past Venus twice, in October 2006 and June 2007. After that come three Mercury flybys, each followed about two months later by a course correction maneuver, to put MESSENGER in position to enter its final orbit, where it will operate for at least a year.
During the Mercury flybys, in January 2008, October 2008 and September 2009, MESSENGER will map nearly the entire planet in color and image much of the area unseen by Mariner 10 -- which visited Mercury 25 years ago and snapped more than 1,000 photographs -- and measure the composition of the surface, atmosphere and magnetosphere.
Then, following orbital insertion, the craft will begin to explore why Mercury, 65 percent of which is composed of a metal-rich core, is twice as dense as Earth, Venus or Mars.
It will also map the planet's surface, stereoscopically, so 3D images can be constructed. Most of what was imaged by Mariner 10 is cratered and ancient, like the surface of Earth's moon -- except Mercury's surface is older. NASA scientists are wondering why the planet contains huge, rounded escarpments nearly a mile in height and hundreds of miles long, something that is common on Mercury but rare on Mars. To date, there is no known geological mechanism for them.
One of Mariner 10's more surprising discoveries was Mercury's global magnetic field -- the only one on a terrestrial planet other than Earth. The Earth's field is thought to be generated by swirling motions in the molten liquid outer portions of its core. But Mercury is so much smaller -- 3,030 miles in diameter (4,878 kilometers), or about the size of Saturn's moon Titan -- that its core should long ago have cooled and solidified, rendering it incapable of generating magnetism on a planetary scale.
Understanding the nature of Mercury's core could explain much about how Earth generates its magnetic field.
Earth's field is highly dynamic. It changes constantly in response to solar wind and flares. The effects of these phenomena are seen as they black out power grids and electronics, and interfere with radios and telephones.
Mercury's field experiences similar dynamics, so understanding them will help scientists understand the interaction of the sun with Earth's field. Though Mercury's field is thought to be a miniature version of Earth's, scientists do not know for sure because Mariner 10 did not measure Mercury's field long enough to determine, for example, how strong it is.
Earth's field is a called a dipole field, meaning Earth acts like a giant bar magnet, with a positive and negative side. Mercury's field also is a dipole, but it is not certain whether it is generated as a single field or is an agglomeration of multiple, smaller fields -- a phenomenon that exists on the moon and Mars.
Mercury's axis of rotation is nearly perpendicular to the planet's orbit, so sunlight in the polar regions strikes the surface at a constant angle. There are large craters at the poles, the interiors of which are permanently shadowed and remain perpetually cold -- below minus 350 degrees Fahrenheit (minus 212 degrees Celsius).
Earth-satellite-based radar images of Mercury's polar regions show the large crater interiors are highly reflective. The most common material that could explain this characteristic behavior is water ice. Scientists think comets and meteorites falling on the Mercurial surface at the poles could have deposited ice over billions of years, or water vapor might have migrated from the planet's interior and frozen at the poles.
It also is possible the polar reflective areas consist of some different material, such as sulfur. In any event, MESSENGER's instruments will be directed to investigate and see if what is lurking in Mercury's polar craters can tell scientists something about the prevalence of water in the solar system.
As Sean Solomon, MESSENGER's principal mission scientist said recently, in order to understand why the various planets of the solar system formed the way it did, it is important "to study the most extreme of those outcomes, and that's Mercury."
Phil Berardelli is UPI's Science & Technology Editor. E-mail email@example.com