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Heading for Mercury{News} Published online 10 January 2008 Among the planets in the solar system, Mercury holds many superlatives: innermost, smallest, fastest and nearly the most dense (after Earth). On Monday, when the NASA Messenger probe cruises past, Mercury can shed its most dubious distinction: least explored. It will be the first visit by a spacecraft since Mariner 10 left in 1975. “There are all these big question marks left by Mariner 10,” says Messenger project scientist Ralph McNutt, a planetary scientist at Applied Physics Laboratory in Laurel, Maryland. In particular, Mercury’s huge iron core and weak magnetic field remind scientists that there are many unanswered questions about the planet’s formation. “To answer them, you need an orbiter, and that’s where Messenger comes in.” Launched in 2004, Messenger took a roundabout route to Mercury, looping around the Earth once and Venus twice. On Thursday, NASA held a press conference previewing the flyby, and released a picture from the spacecraft's approach. The flyby is more than just a milepost. NASA scientists say that, with its arsenal of geochemical instruments, Messenger may on first pass crack some of the mysteries of this oddball planet. This includes capturing pictures of Mercury’s unseen face. Mariner 10 captured only 45% of the surface: Messenger's flyby will allow coverage of half of the remainder. It will also complete the portrait of Caloris Basin, a 1,300 km–wide crater created from an impact so powerful that scientists say its shock waves were responsible for weird surface features on the opposite side of the planet. By tallying the craters within the crater, scientists can date this ancient impact.
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