Hot as a pizza oven on one side and colder than an Antarctic winter on the other, the planet Mercury isn't a place most people would spend good money to visit.
But scientists and engineers in Maryland and across the country are as happy as tourists queued for a flight to the tropics as they await the planned Aug. 2 launch of the first mission in 31 years to the sun's nearest neighbor.
Called Messenger, the unmanned, desk-sized spacecraft was designed and built at the Johns Hopkins University's Applied Physics Laboratory near Laurel. Its scientific instruments were built at NASA's Goddard Space Flight Center in Greenbelt, the University of Colorado and the University of Michigan.
Messenger's 7 1/2 -year mission: to go where only NASA's Mariner 10 has gone before, to answer questions that have lingered since Mariner's flybys in 1973 and 1974.
"Messenger will help us understand the forces that have shaped the least-explored and innermost of the terrestrial planets," said Orlando Figueroa, NASA's director of solar system exploration. "Mariner left us with even more questions than answers."
The most fundamental is "how Mercury got put together," said the Carnegie Institution's Sean C. Solomon, principal investigator on the Mariner team.
"The [four] inner planets are litter-mates, if you will, products of a single early stage in the evolution of a star and its planetary system," he said. "And yet the siblings turned out very different."
Composed mostly of metal rather than lighter rocks, Mercury is denser than Venus, Earth and Mars. It's also the only one of the four with no significant atmosphere, and one of two with a magnetic field. (Earth is the other.)
"In order to understand what processes most control the differences in outcome, we really have to study and learn about the most extreme of these outcomes," Solomon said.
$426 million mission
The $426 million Messenger mission is set to begin shortly after 2 a.m. Aug. 2 with liftoff atop a Delta 2 rocket from the Cape Canaveral Air Force Station in Florida. If there are delays, the launch window reopens each night until Aug. 14.
If all goes well, the 1.2-ton craft will make 15 loops around the sun. It will swing by Earth again in August 2005, past Venus twice (October 2006 and June 2007) and by Mercury three times in 2008 and 2009.
In March 2011, after a circuitous journey of 4.9 billion miles, it will slip into a nearly polar orbit of Mercury, swooping as low as 125 miles from the surface.
Tiny Mercury, only half again as big as our moon, presents two challenges that have discouraged a return since Mariner 10, scientists said - the heat in Mercury's neighborhood and the difficulty of getting there.
If the sun is hot enough to render steering wheels and beach sand untouchable from 93 million miles away, imagine what it's like on Mercury - just 36 million miles from the same heater.
On Mercury, the sun appears more than twice as big as it looks from Earth, and it's 11 times brighter and hotter. In tests, Messenger's components had to withstand temperature swings from 800 degrees Fahrenheit to 240 degrees below zero, said James Leary, the mission systems engineer at APL.
To hold Messenger's internal temperatures close to room temperature, its instruments and electronics are shaded by a high-tech white parasol, made of the same ceramic that shields the space shuttle from the heat of re-entry.
Solar radiation at Mercury's orbit will shove Messenger so hard that controllers will have to adjust its orbit periodically to keep it on track.
NASA's return to Mercury has also been delayed by the difficulty of spiraling a spacecraft in toward the planet and then slowing enough to go into orbit.
Mariner 10 didn't bother to slow down. It simply whizzed by with cameras rolling. "We wouldn't have to slow down [either] if we didn't want to go into orbit," said APL's Robert W. Farquhar, the mission manager. Messenger could have made that trip in 3 1/2 months.
But slowing down and orbiting has costs. Braking solely with a rocket would have required designers to reduce its science payload and allocate 85 percent of Messenger's weight allowance to fuel. Or, they would have needed a much bigger, budget-busting launch rocket.
Instead, APL's planners elected to use the gravity of three planets to apply the brakes, extending the trip to 6 1/2 years.
Using a trajectory devised in 1985 by Chen-wan Yen, of NASA's Jet Propulsion Laboratory, they'll steer Messenger through six close flybys of Earth, Venus and Mercury. Like a billiard ball, Messenger will use each planet's gravity (and five rocket burns) to slow the craft and guide it toward orbit around Mercury.
There, Messenger will spend at least a year circling the planet - twice every 24 hours.
The scope of Messenger's mission is suggested by its name. Mercury was, of course, the Roman messenger god, but the spacecraft's name is officially a strained acronym for "MErcury Surface, Space ENvironment, GEochemistry and Ranging."
The mission's planners hope to answer several key questions.
The first is why the planet is so dense. One theory suggests that early in the evolution of the solar system, the fierce heat of the young sun, or perhaps heavy bombardment by asteroids, blew away much of the planet's rocky outer layers, leaving a higher percentage of denser metals behind.
Messenger's remote sensing instruments - X-ray, gamma-ray and infrared spectrometers - will measure the mineral composition of the surface.
Mariner photographed only 45 percent of Mercury, so Messenger's cameras and instruments will provide the first detailed, global image of the planet. A laser altimeter will map its craters, plains and mile-high cliffs. Precise radio tracking of Messenger's orbit will yield gravity measurements and maps of the thickness of its crust.
The data might also reveal whether Mercury has a molten iron outer core, which would explain its magnetic field.
"One of the most bizarre questions," Solomon added, "is whether the planet closest to the sun, with these extreme variations in temperature, might really have ice lurking in shadowed regions of the poles."
The craft will also study the sparse gas molecules that drift around the planet. Too thin to be called an atmosphere, the stuff might come from the solar wind, from comets that fall into the sun, and from the planet's own rocks. Scientists want Messenger to help discover its origins.
But those answers are years and billions of miles away. And "exploration remains an inherently high-risk business," Figueroa said.
For now, he said, "we must only celebrate success one step at a time. ... We look forward to a successful launch and a seven-year journey toward our destination."