NASAs MESSENGER short for MErcury Surface, Space ENvironment, GEochemistry, and Ranging spacecraft will be the first robot space mission sent to orbit Mercury, the small metallic and silicate planet that orbits closest to our Sun. On Oct. 6, at roughly 4:40 a.m., MESSENGER flew by Mercury for the second time in a year. Swinging a mere 125 miles (200 km.) above the pockmarked and mountainous surface of Mercury, MESSENGERS onboard camera and instruments began recording thousands of data bits for transmission back to Earth. Eager astronomers awaited the results. While NASAs Mariner-10 spacecraft passed by Mercury in the 1970s, MESSENGER will be the first artificial satellite to go into orbit around the scorching planet in March 2011 (it made one of several flybys, Oct. 6, before it will settle into an elliptical orbit). Among MESSENGERs many images of Mercurys surface, none is more spectacular than one photograph showing a large crater named Kuiper (after U.S. astronomer Gerard Kuiper). This crater, located in the planets southern hemisphere, was first photographed by Mariner 10. MESSENGERs images also revealed large crater rays, like those seen on the Moon, splattered across the surface of Mercury. Most astronomers think these bright crater rays were formed by immense meteor impacts the force of the impacts thus splattered pulverized rock and dust in neat ray patterns across the surface. But a more fascinating, albeit controversial, theory suggests that explosive, hydrogen-driven volcanic eruptions may be the origin of some of the giant ray craters seen on Mercury and the Moon. Except for a tenuous exosphere containing wispy traces of hydrogen, helium, oxygen, sodium, calcium and potassium, Mercury is airless. Its year is very brief only 88 days while its surface temperatures run in the extreme from minus-297 F to plus-801 F. At Mercurys subsolar point, the thermometer reaches its zenith and at the dark bottoms of deep craters located near the poles, the thermometer plunges to the lowest temperatures on any terrestrial planet. Its possible that large quantities of ice are buried in these polar places of eternal darkness. Whats in the Sky: You can observe tiny Mercury, the planet closest to the Sun, at dawn in the eastern sky this week. If you rise before the Sun and look just above the horizon you will see a small, but decidedly bright star this object is Mercury. But since the planet is so low in the sky barely above the horizon you will need a flat, unobstructed horizon before you in order to observe this distant planet. You can use standard binoculars to better resolve Mercurys disk which exhibits phases much like our Moon. Lou Varricchio, M.Sc., was a NASA senior science writer. He is currently involved with the NASA/JPL Solar System Ambassador program in Vermont.