Final resting place of the Messenger probe on Mercury, where it crash landed on April 30 while traveling at 8,700 miles per hour. Credit: NASA/APL/CIW Through most of its life, NASA's scrappy Messenger probe was something of a unsung hero. The first spacecraft ever to orbit Mercury didn't have the you-are-there immediacy of a Mars rover, the daredevil appeal of landing on a comet, or the romance of visiting a beautiful ringed planet. But with today's death--the result of a long-anticipated crash into the planet it studied--we can clearly see what an incredibly successful explorer Messenger was. Mercury has long been a solar-system enigma. It is not particularly small (roughly halfway in size between Mars and the moon), and it is not particularly far away (third closest planet to Earth after Mars and Venus), but the first planet from the sun is devilishly hard to study. Seen from Earth it hangs low in the sky; from space it hugs so close to the solar glare that the Hubble telescope cannot aim at it. Astronomers were so stymied that they didn't even know how quickly Mercury rotated until 1965, when they found out, not by looking but by bouncing radar signals off its surface. Getting Messenger to Mercury wasn't easy, either. After the 2004 launch, it took nearly 7 years of flight--including one Earth flyby, two Venus flybys, and three passes by Mercury--before the probe was able to enter orbit. Compare that to the 9-month travel time to Mars. The obstacle, once again, is the sun: Its gravitational pull tends to speed up any inward-traveling space probe, but getting into orbit requires a slow approach, making for a tricky maneuver. When Messenger finally arrived for good on March 18, 2011, it encountered a planet that we barely understood. That quickly changed. Within two years, Messenger mapped the full surface of Mercury, half of which had never been seen before. As Mercury came into sharp focus for the first time, scientists have begun to appreciate the truly wonderful, exotic strangeness of the innermost planet. [
For more science updates, follow me on Twitter: @coreyspowell
