It's small, dense, hot, and swift, and it's hard to get at. But after 30 years of neglect, Mercury, the innermost planet in the solar system, is back on the itinerary of planetary scientists. Early this morning (EDT), NASA's $427 million MESSENGER spacecraft was successfully launched on a 6.5-year, 8-billion-kilometer voyage to explore the mysterious planet.
Mercury is a planetary enigma that bedevils solar system scientists. Compared with the other rocky planets (Venus, Earth, and Mars), 4878-kilometer-diameter Mercury has a relatively huge metal core. Indeed, Mercury is hardly more than a ball of iron covered with a thin layer of rock. Some researchers think the planet's rocky mantle used to be much thicker but was shattered long ago by a catastrophic cosmic impact. "The most exciting question is 'What can Mercury tell us about the assembly of the inner planets?' " says MESSENGER's principal investigator, Sean Solomon of the Carnegie Institution of Washington, D.C. Solomon hopes that MESSENGER's inventory of elements and minerals at Mercury's surface will help researchers decide among competing theories about the planet's weird interior.
In addition to probing Mercury's composition, MESSENGER will chart the planet's magnetic field, map its surface, and look for ice reservoirs in deep craters at the planet's poles. So far, only half of the planet has been mapped--by NASA's Mariner 10 spacecraft, which made three flybys in 1974 and 1975.
MESSENGER will use ingenuity and patience to reach orbit around the planet. The powerful rocket motor and huge fuel reserve needed to send a craft directly to Mercury would have made the project too expensive. So between August 2005 and September 2009, MESSENGER will make close flybys of Earth, Venus, and Mercury itself that will eventually place the craft in orbit around Mercury in March 2011 for a 1-year mission.
By then, the European Space Agency (ESA) will be readying its own flight to Mercury, BepiColombo, slated for a September 2012 launch. The scientific payload for this ambitious two-spacecraft mission will be selected this summer. Gerhard Schwehm, ESA's head of planetary science, says BepiColombo will have a broader set of goals than MESSENGER's, including a much more detailed study of the planet's magnetosphere and a test of general relativity. "BepiColombo will provide us with a much more global picture," says Schwehm.