Taking the economy route, NASA's MESSENGER spacecraft will spend six-and-a-half years just getting to its target for at least one year of scientific observations. The 4.9-billion-mile marathon will require 15 trips around the sun, six trajectory-warping planetary flybys and another half-dozen critical rocket firings.

That's what it will take to get the one-ton MESSENGER into orbit around hellish Mercury, an extraordinarily difficult feat even though the closest planet to the sun is just a stone's throw from Earth as astronomical distances go.

But Mercury orbits deep within the sun's gravity well and MESSENGER's $427 million mission hinges on the use of compact scientific instruments, lightweight materials and an innovative flight plan utilizing the gravity of Earth, Venus and Mercury itself to slow and shape the probe's descent into the inner solar system.

The MESSENGER poster artwork shows the Delta launch vehicle and the spacecraft headed to orbit Mercury. Credit: Boeing

Liftoff atop a Boeing Delta 2 booster from the Cape Canaveral Air Force Station is scheduled for Aug. 2 at 2:16:11 a.m. EDT (0616:11 GMT), the opening of a 12-second launch window. If the spacecraft is not off the ground by Aug. 14, the flight will be delayed for a year while the inner planets return to favorable flyby positions.

But if all goes well, MESSENGER will brake into orbit around Mercury on March 18, 2011, kicking off a planned one-year mission to study the hot, heavily-cratered and surprisingly dense world in unprecedented detail.

"MESSENGER will help us understand the forces that have shaped the least explored and innermost of the terrestrial planets, Mercury," said Orlando Figueroa, director of the solar system exploration division at NASA headquarters. "Understanding Mercury, how it formed and how it evolved is essential to the understanding of the terrestrial planets: Venus, Earth and Mars." Only one other spacecraft - NASA's Mariner 10 - ever visited the innermost planet, flying past the scorched world three times in 1974 and 1975. But Mariner 10 was strictly a flyby mission, photographing just one side of the planet while collecting valuable but limited scientific data.

"Mariner 10 left us with even more questions than it answered," Figueroa said. "Now, 30 years later, advances in technology, mission design and materials have enabled us to go back with a much more capable mission, which can help us understand and unravel the mysteries of the closest planet to the sun.

"While at Mercury, MESSENGER will collect images of the entire planet and gather highly detailed information on Mercury's geological history, the nature of its atmosphere and magnetosphere, the makeup of its core and the character of the core materials. ... What we learn from MESSENGER about Mercury will teach us a great deal more about the nature and structure of the inner planets in our solar system."

Operating in a 12-hour orbit that will carry it within 125 miles of Mercury's stark surface, MESSENGER will beam back eight hours of scientific data per day, helping scientists figure out how the planet ended up with such a huge iron core, whether part of the core remains fluid and whether it helps drive Mercury's magnetic field.

Scientists also are eager to understand the nature of the planet's ultra-thin atmosphere and whether radar-reflecting deposits in permanently shadowed craters near Mercury's poles are made up of water ice.

"The question we're going after that seems most fundamental to me is how Mercury got put together," said Principal Investigator Sean Solomon of the Carnegie Institution in Washington. "What were the processes that contributed to the inner planets turning out so differently?

"They formed by common processes, the inner planets are all litter mates, if you will, products of a single early stage in the evolution of a star and a planetary system," he said. "And yet the siblings turned out very differently. In order to understand what processes most control the differences in outcomes, we really have to study and learn about the most extreme of those outcomes, and that's Mercury. So we think Mercury has a lot to tell us."

The solar system was born some 4.5 billion years ago when a vast cloud of rotating gas and dusty debris left over from ancient supernovas reached a critical density and collapsed under its own gravity to form a flattened disk. At its heart, pressures and temperatures eventually became extreme enough to ignite fusion reactions and the sun flashed to life.

Material in the outer disk, through complex, little-understood chemical and physical processes, ultimately formed the planets, asteroids and comets known today. Four dense terrestrial worlds - Mercury, Venus, Earth and Mars - formed close in to the sun while four gas giants - Jupiter, Saturn, Uranus and Neptune - formed much farther out. Remote Pluto likely is the largest member of a vast cloud of icy bodies that make up what is known as the Kuiper belt and as such, is not a planet in the traditional sense.

Just how the solar nebula differentiated, with rocky terrestrial worlds forming close to the sun and gas giants farther out, is not fully understood. Scientists hope Mercury will provide at least some of the answers.

With a diameter of 3,031 miles, Mercury is only slightly larger than Earth's moon and smaller than Saturn's moon, Titan. It is the densest planet in the solar system and circles the sun in a highly elliptical orbit with a high point of 43 million miles and a low point of 29 million miles. Earth orbits the sun at an average distance of 92 million miles.

The aptly named Mercury is the fastest planet in the solar system, completing an orbit around the sun every 88 Earth days and moving at an average speed of 108,000 mph. It rotates on its axis every 59 days but because of its high orbital speed and slow rotation, a solar day - sunrise to sunrise at the same point on the surface - lasts 176 Earth days.

"At certain latitudes, an observer on the surface could watch the sun rise, move directly overhead and stop, then retrograde back a bit," according to a National Oceanic and Atmospheric Administration web site. "After retrograding, the sun would then proceed back on its westward track. In addition, since Mercury has virtually no atmosphere to scatter light, the sky would be black, even though the sun disk itself would be over twice as large as what we observe from the Earth."

On the planet's sunlit side, temperatures can exceed 800 degrees Fahrenheit - lead melts at 621 degrees - while the nightside can reach 350 degrees below zero. It is the only terrestrial planet other than Earth with a global magnetic field.

"Mercury is an unusual planet even by the standards of the inner solar system," Solomon said. "It is, of course, the planet closest to the sun, it is a planet with an unusual rotation rate such that the solar day on Mercury lasts two Mercury years, it has the highest intrinsic density of any planet, the density is so high that two thirds of the planet must be iron metal."

The question is, how did it get that way?

MISSION PREVIEW
   PART 1: MESSENGER MISSION READY TO LAUNCH
   PART 2: THE LONG AND WINDING ROAD
   PART 3: THE 'HEAT' IS ON
   PART 4: A COMPLEX SCIENCE AGENDA