MESSENGER will carry seven scientific instruments, including a camera, laser altimeter, magnetometer and several spectrometers. The spacecraft will globally image Mercury for the first time. 

The spacecraft will is scheduled for March 2004 and will orbit Mercury for one Earth-year beginning in April 2009. MESSENGER's five-year transit flight will include two flybys of Venus and two flybys of Mercury, "gravity assists" that will help the spacecraft refine its orbit to match Mercury's quick, elliptical orbit around the Sun.

MESSENGER’s orbit about Mercury will be highly elliptical, 200 kilometers (108 nmi) above the surface at its lowest point and more than 15,000 kilometers (8100 nmi) at its highest. The plane of the orbit is inclined 80° to Mercury's rotation axis, and the low point in the orbit is reached at latitude of 60°N. The low northern hemisphere altitude will allow for detailed measurements of Mercury's libration and the geology and composition of the giant Caloris impact basin. MESSENGER's 12 months in orbit cover 2 Mercury solar days. (The Mercury solar day, from sunrise to sunrise, is equal to 176 Earth days.) The first solar day is focused on obtaining global map products from the different instruments, and the second focuses on targeted science investigations.

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

The Principal Investigator is Dr. Sean Solomon of the Carnegie Institution of Washington. The Johns Hopkins University Applied Physics Laboratory (JHU/APL), Laurel, Md, will design, build and operate the spacecraft for NASA, in collaboration with industrial partners GenCorp Aerojet (propulsion system) and Composite Optics, Inc (integrated structure). Instruments and instrument subsystems in the science payload are being supplied by JHU/APL, NASA/Goddard Space Flight Center, the University of Colorado, and the University of Michigan. The US$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.