The Mercury Surface, Space Environment, Geochemistry and Ranging mission, or Messenger, will carry seven instruments into orbit around the closest planet to the Sun. It will send back the first global images of Mercury and study its shape, interior and magnetic field. Dr. Sean Solomon of the Carnegie Institution, Washington, DC, will lead Messenger.

The Deep Impact mission will send a 1,100-pound (500- kilogram) copper projectile into comet P/Tempel 1, creating a crater as big as a football field and as deep as a seven-story building. A camera and infrared spectrometer on the spacecraft, along with ground-based observatories, will study the resulting icy debris and pristine interior material. Dr. Michael A'Hearn will lead Deep Impact from the University of Maryland in College Park.

"These low-cost missions are both fantastic examples of the creativity of the space science community," said Dr. Edward Weiler, associate administrator for space science at NASA Headquarters in Washington, DC. "Messenger is a flagship-quality effort that, in tandem with a separate Pluto mission, enables us to seize the opportunity to complete our historic initial reconnaissance of the Solar System. Deep Impact presents a special chance to do some truly unique science, and it is a direct complement to the other two comet missions already in the Discovery Program."

Messenger, to be launched in spring 2004, will be NASA's first mission to Mercury since the Mariner 10 flybys in 1974 and 1975, which provided information on only half the planet. Its challenging flight plan begins with two Venus flybys, then two Mercury flybys in January and October 2008 and a subsequent orbital tour of Mercury beginning in September 2009.

Among Messenger's goals will be to discover whether Mercury has water ice in its polar craters. The cost of Messenger to NASA is $286 million. It will be built and managed by the Johns Hopkins University's Applied Physics Laboratory, Laurel, MD.

Deep Impact will be launched in January 2004 toward an explosive July 4, 2005, encounter with P/Tempel 1. It will use a copper projectile because that material can be identified easily within the spectral observations of the material blasted off the comet by the impact, which will occur at an approximate speed of 22,300 mph (10 kilometers per second.) The total cost of Deep Impact to NASA is $240 million. Deep Impact will be managed by NASA's Jet Propulsion Laboratory in Pasadena, CA, and built by Ball Aerospace in Boulder, CO.

NASA selected these missions from 26 proposals made in early 1998. The missions must be ready for launch no later than Sept. 30, 2004, within the Discovery Program's development cost cap of $190 million in Fiscal 1999 dollars over 36 months and a total mission cost of $299 million.

The Discovery Program emphasizes lower-cost, highly focused scientific mission. NASA has developed six other Discovery Program missions. Two have completed their primary missions, two are operational and two more are under development:

-- The Lunar Prospector orbiter has mapped the Moon's composition and gravity field for the past 18 months. It will complete its highly successful mission on July 31, when it is sent on a controlled impact into a crater near the south lunar pole. Scientists hope to observe a resulting plume of water vapor that would help confirm the presence of water ice in some of the Moon's permanently shadowed craters. In 1997, the Mars Pathfinder lander, carrying a small robotic rover named Sojourner, landed successfully on Mars and returned hundreds of images and thousands of measurements of the Martian environment.

-- The Near Earth Asteroid Rendezvous (NEAR) spacecraft is scheduled to enter orbit around the asteroid Eros in February 2000, after a problem with its initial attempt to do so early this year. The Stardust mission to gather samples of comet dust and return them to Earth was launched in February 1999.

-- The Genesis mission to gather samples of the solar wind and return them to Earth and the Comet Nucleus Tour (CONTOUR) mission to fly closely by three comets are being prepared for launch in January 2001 and June 2002, respectively.