NASA yesterday added Mercury to the list of planets that will be in the scientific spotlight early next century, announcing approval of the Mercury Messenger, a mission to the Suns closest neighbor.

The spacecraft will carry a comprehensive array of instruments to map the planet and its magnetic field, and to analyze the chemical composition of the surface. A novel experiment will even use a precise laser altimeter to determine whether the planets iron core is solid or liquid.

"There have been no chemical measurements of Mercury even by Mariner 10 so were really breaking new ground in terms of the global composition of the surface," said Sean Soloman, principal investigator on the Messenger project.

Mariner 10, the only previous mission to Mercury, flew past the tiny planet three times in 1974 and 1975, gathering images of about 40 percent of its surface and information about its magnetic field.

Mercury is by far the densest of the planets in the solar system, a fact that puzzles scientists. The planet must be very high in metal compared to rocky components, but researchers dont understand why. Hypotheses include a suggestion that a giant impact early in the planets history blew off much of its rocky crust, and an idea that a very hot early sun vaporized much of this material.

Soloman, who is director of the Department of Terrestrial Magnetism at the Carnegie Institute of Washington, said Messenger will give scientists the information they need to determine which ideas are correct, and learn which processes affected the evolution of the inner solar system.