NASA has given the first Mercury orbiter mission the go-ahead to move into full-scale spacecraft development -- setting up the first trip to the Sun's closest neighbor in more than a generation.

MESSENGER, short for MErcury Surface, Space ENvironment, GEochemistry, and Ranging, will launch in March 2004 and orbit Mercury for one Earth-year beginning in April 2009.

Artist's concept of the MErcury Surface, Space ENvironment, GEochemistry and Ranging (MESSENGER) spacecraft, which will conduct the first orbit study of the planet Mercury. A ceramic-material sunshield protects the spacecraft and its scientific instruments from the intense heat and solar radiation near the planet. Photo: JHU-APL

"MESSENGER is the most complex and challenging Discovery-class mission we have ever attempted, and our goal is to do something never before attempted," said Dr. Jay Bergstralh, chief scientist for NASA's Solar System Exploration Division in NASA's Office of Space Science in Washington, D.C. "Conducting a year-long mission to orbit a planet only 36 million miles from the Sun for relatively low cost is an amazing concept, and we have selected a top-flight team to build and fly this mission."

Dr. Sean C. Solomon of the Carnegie Institution of Washington (D.C.) is the mission's principal investigator and lead scientist. The Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, MD, manages the mission for NASA's Office of Space Science and will design, build and operate the MESSENGER spacecraft. Preliminary work on the mission began 18 months ago.

MESSENGER will be only the second spacecraft to visit Mercury. Mariner 10 flew past it three times in 1974 and 1975 but gathered data on less than half the planet.

MESSENGER's seven scientific instruments -- including a camera, laser altimeter, magnetometer and several spectrometers -- will globally image Mercury for the first time. It also will collect unprecedented information on the composition and structure of Mercury's crust, its geologic history, the nature of its thin atmosphere and active magnetosphere, and the makeup of its core and polar materials.

"This is an opportunity to complete the detailed exploration of the inner solar system, on a planet where we've never even seen half the surface," Solomon said. "We've had many exciting missions to Mars and Venus that yielded new theories about the processes that shaped the inner planets, and for 25 years now Mercury has clearly stood out as a place where major questions remain to be answered. Mercury is that last piece of the puzzle."

Among questions MESSENGER's science team will investigate: Why is Mercury -- the densest planet in the solar system -- mostly made of iron metal? Why is it the only inner planet besides Earth with a global magnetic field? How can the planet closest to the sun, with daytime temperatures soaring past 850 degrees Fahrenheit at its equator, have what appears to be ice in its polar craters? Solomon said unlocking Mercury's secrets will help us understand the forces that shaped Earth and the other terrestrial (rocky) planets.

MESSENGER's five-year voyage includes two flybys of Venus and two flybys of Mercury, "gravity assists" that will help the spacecraft tune its path and match Mercury's quick, elliptical orbit around the sun. The mission team will also use pictures and data from the Mercury flybys to refine the orbit study.

Once in orbit MESSENGER has to deal with the intense heat at Mercury, where the sun is up to 11 times brighter than on Earth. But MESSENGER's instruments will operate at room temperature behind a sunshield made of the same ceramic material that protects parts of the space shuttle. The spacecraft will also pass only briefly over the hottest parts of the planet's surface, limiting the instruments' exposure to reflected heat.

The $256 million MESSENGER mission is the seventh in NASA's Discovery Program of lower-cost, scientifically focused space flights and the third Discovery project managed by APL. The mission cost figure does not include the launch vehicle and mission operations.

The MESSENGER science team taps expertise from APL; University of Colorado, Boulder; University of Arizona, Tucson; Southwest Research Institute, Boulder, CO; NASA's Goddard Space Flight Center, Greenbelt, MD; University of Michigan, Ann Arbor; Washington University, St. Louis, MO; University of California, Santa Barbara; Brown University, Providence, RI; Northwestern University, Evanston, IL; and Massachusetts Institute of Technology, Cambridge. GenCorp Aerojet, Sacramento, CA, and Composite Optics Inc., San Diego, CA, are working with APL to build the spacecraft. NASA's Jet Propulsion Laboratory, Pasadena, CA, will provide navigation support for the mission.