Start the countdown clock at one year: the effort to assemble and test the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft, set to embark next March on an historic voyage to the innermost planet, is well under way.

On Feb. 3, MESSENGER's integrated propulsion system and structure arrived at The Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md., which is building the remainder of the spacecraft and manages the Discovery-class mission for NASA. Having already completed the first round of vibration tests and a thermal "bake out" to clean the structure, the MESSENGER team will start installing electronic components on the craft in April.

The frame for MESSENGER's signature sunshade - which will protect the craft and its instruments from the intense heat at Mercury - is due to arrive this week from GenCorp Aerojet. Layers of ceramic fabric will be added to the frame at APL over the next two months.

MESSENGER's seven scientific instruments - being provided by APL, NASA's Goddard Space Flight Center in Greenbelt, Md., the University of Colorado, Boulder, and the University of Michigan, Ann Arbor - are expected to arrive later this spring. Integration and testing will continue at APL through early September, and then MESSENGER heads to Goddard Space Flight Center for additional prelaunch space-environment tests.

In early January, MESSENGER is scheduled to leave Goddard for Kennedy Space Center, Fla., in final preparation for its March 2004 launch aboard a Boeing Delta II rocket.

After launch and a 5-year journey through the inner solar system, MESSENGER will orbit Mercury for one Earth year, providing the first images of the entire planet and collecting 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 mysterious polar materials. While cruising to Mercury the spacecraft will fly past the planet twice - in 2007 and 2008 - snapping pictures and gathering data critical to planning the orbit study that begins in April 2009. MESSENGER will also fly by Venus in 2004 and 2006.