Jimmy Westlake: Return to Mercury

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Jimmy Westlake

Jimmy Westlake's Celestial News column appears Tuesdays in the Steamboat Today.

Find more columns by Westlake here.

— On Aug. 3, 2004, a Boeing Delta II rocket left Cape Canaveral, Fla., riding a fiery plume and carrying a spacecraft named MESSENGER, the MErcury, Surface, Space ENvironment, GEoscience and Ranging spacecraft. Its mission: to study the innermost and smallest of the planets, Mercury, at close range.

Unlike Mars and Venus, which have been scrutinized and mapped by many spacecraft, Mercury has been visited by only one spacecraft before MESSENGER. That was the Mariner 10 spacecraft, which sailed past Mercury three times between 1974 and 1975, and mapped less than half of its surface.

On Monday, after a 41-month journey, MESSENGER arrived at its destination but only after looping around the Earth twice and Venus twice to alter its trajectory and nudge the spacecraft toward Mercury. And, the little spacecraft is not through yet. It will swing past Mercury two more times, in October 2008 and September 2009, before it settles into a permanent orbit around the planet in March 2011. Then, over a period of months, MESSENGER will map the entire surface of the planet and, we hope, answer some nagging questions about our neighboring world.

For example, why is Mercury so dense? It appears to be an iron planet topped with a thin, rocky crust. Such a planet should not exist according to our present understanding of planet formation. Another mystery: Does Mercury really have deposits of ice in the shadowed craters near its north and south poles? Some observations from Earth have suggested this is so, but it would be a very startling discovery on such a hot, foreboding world. What caused the long, meandering faults, called scarps, which cover Mercury's surface? No other world we've studied has such a network of faults.

Right now, giddy scientists are poring over the amazing photos returned by MESSENGER this week and viewing regions of the planet never seen before at close range. Already, one new mystery has emerged. The photos reveal many large-impact craters that are surrounded by dark rings, presumably some sort of dark-colored material excavated by the impacts and thrown out onto the surface. Bright rings and rays of material surround most impact craters.

We've much to learn about the space environment in the hot region near the sun, where Mercury orbits.

You can spot the planet Mercury in the sky for yourself this week, low in the western sky after sunset. It looks like a bright, yellowish-colored star. Catch it early, though, because the innermost planet sets by 6:45 p.m., only an hour and a half behind the sun.

Professor Jimmy Westlake teaches astronomy and physics at Colorado Mountain College's Alpine Campus. He is an avid astronomer whose photographs and articles have been published on the Web sites of CNN.com, NASA's "Astronomy Picture of the Day" Web site, Spaceweather.com, Space.com, Discover.com, MSNBC.com, NationalGeographic.com and in Sky & Telescope, Astronomy, Night Sky, Discover and WeatherWise magazines. His "Celestial News" article appears weekly in the Steamboat Pilot & Today. His "Cosmic Moment" radio spots can be heard on local radio station KFMU.

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