NASA has added two spectacular shows to Discovery, its program of quick and cheap space missions. In 2008 and 2009, a spacecraft will take a close look at Mercury, the space agency announced on Wednesday. And in 2005, another mission will shoot a massive copper cannonball into a comet to learn more about its innards. The scheduled date for these fireworks, which space enthusiasts can watch from Earth: 4 July.
The spacecraft Messenger, to be launched in spring 2004, will orbit Mercury for 1 year after two brief flybys. Loaded with cameras to image the planet's surface and spectrometers to analyze its surface and atmosphere, Messenger should shed light on how planets form and why some, like Mercury and Earth, have retained their magnetic fields while others, like Mars, have shed them, says planetary scientist Sean Solomon of the Carnegie Institution of Washington in Washington, D.C., who leads the $286 million mission. It should also reveal whether volcanoes have shaped Mercury's surface and if ice exists in the shadows of craters, says planetary scientist Faith Vilas of the Johnson Space Center in Houston.
In January 2004, a $240 million mission called "Deep Impact" will take off for comet Tempel 1, which circles the solar system every 5.5 years. A year and a half later, the observation module will release an "impactor"--essentially a 500-kilogram copper bullet--which will slam into the comet's surface at a speed of 10 kilometers per second. A camera onboard the bullet will transmit images as it hurtles toward its target; the hovering observer module will record both the crash and the size and shape of the resulting crater, and analyze solid and gaseous material released by the blast.
The crash may help answer questions about the composition of comets and their chemical histories, says Lucy McFadden of the University of Maryland, College Park, one of the project's scientists. Comets formed from primordial material condensing at the edge of the solar system, but their interiors may have heated and undergone chemical changes during their tours through the solar system. So far, scientists have only been able to model these processes. "This is an in situ experiment that will constrain these theories," says McFadden. Indeed, Deep Impact marks planetary science's graduation from classic, observational studies to active experimentation, notes Alan Stern of the Southwest Research Institute's Department of Space Studies in Boulder, Colorado.
Deep Impact has something to offer the Earth-bound, too: If skies are clear, the celestial collision will be visible with a pair of ordinary binoculars. But don't expect too much: The comet will look like a small smudge, and the impact will show up as a mere pinpoint of light.