NASA's Messenger spacecraft this afternoon will whiz past Mercury at 141,000 mph and snap an estimated 1,200 detailed photos of the planet's surface from a mere 124 miles up. It will be the first of three passes before the craft starts orbiting the planet closest to the sun in 2011. What scientists find could provide clues about how the Earth was formed.
Some astronomers speculate that Mercury may have formed elsewhere and was knocked into its current orbit by a collision. In a previous expedition, NASA obtained photos of about 45% of Mercury's surface, and the newest fly-by will begin to fill in the rest. Instruments will also collect data on what minerals and chemicals are present on the planet's surface, and measure the planet's gravitational and magnetic fields. (Read more Mercury stories.)