NASA Gives Official Nod To First Mercury Orbiter; Messenger Mission Moves Into Final Development Stages

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. MESSENGER’s seven scientific instruments -will provide the first images of the entire planet and collect detailed 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.

"MESSENGER is the most complex and challenging Discovery-class mission we have ever attempted, and our goal is to do something never before attempted," says 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 yearlong 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. The Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md., manages the mission for NASA 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.

"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 says. "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 says unlocking Mercury’s secrets will help us understand the forces that shaped Earth and the other terrestrial (rocky) planets.

Studying Mercury hasn’t been easy. Its average distance from the sun is 36 million miles ­ about two-thirds closer than Earth. The planet is only visible from Earth just after sunset or before sunrise, and astronomers have trouble observing it with telescopes through the haze of Earth’s atmosphere. Even the Hubble Space Telescope can’t view it, lest it risk frying its sensitive cameras.

"This is not a simple mission, but this team has a lot of very clever people who have devised a practical way to get a spacecraft to Mercury and protect it while it works in an extremely harsh environment," says MESSENGER Project Manager Max R. Peterson of the Applied Physics Laboratory. "We believe we have a very good plan in place."

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 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 highly successful Near Earth Asteroid Rendezvous (NEAR) ended in February 2001 after the NEAR Shoemaker spacecraft orbited and landed on asteroid 433 Eros. The Comet Nucleus Tour (CONTOUR) spacecraft, now under construction at APL, will take close-up images and analyze dust from at least two comets after launching in July 2002.

The MESSENGER science team taps expertise from APL; University of Colorado, Boulder; University of Arizona, Tucson; Southwest Research Institute; Boulder, Colo.; NASA’s Goddard Space Flight Center, Greenbelt, Md.; University of Michigan, Ann Arbor; Washington University, St. Louis; University of California, Santa Barbara; Brown University, Providence, R.I.; Northwestern University, Evanston, Ill.; and Massachusetts Institute of Technology in Cambridge. GenCorp Aerojet, Sacramento, Calif., and Composite Optics Inc., San Diego, are working with APL to build the spacecraft.

For more information visit the MESSENGER Web site at http://messenger.jhuapl.edu. Animation of MESSENGER’s journey to Mercury is available at http://messenger.jhuapl.edu/animations.html.