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Spacecraft Set to Swing by Mercury

Mercury temperature swings are the greatest in the solar system, despite being the closest planet to the Sun. There is not enough atmosphere on the planet to entrap heat.
(Image: © NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington)

A space probe is headed for a second swing past Mercury topick up a gravitational boost and eventually become the first spacecraft to orbitthe closest planet to our sun.

Scientists expect to get more than 1,200 pictures when NASA'sMESSENGER spacecraft zips past Mercury early Monday, which would help reveal mostof planet?s remaining unmapped terrain. The flyby should also provide a gravityassist that will prepare MESSENGER to enter orbit around Mercury in March 2011.

"For needles with smaller and smaller eyes, this teamis getting better and better," said Sean Solomon, MESSENGER's principalinvestigator at the Carnegie Institution of Washington, during a Wednesdayteleconference. He described the maneuvers as a "threading exercise"requiring the highest precision.

MESSENGER, short for the bulky name MErcury Surface, Space ENvironment,GEochemistry, and Ranging, should pass 125 miles (200 km) above Mercury, orroughly the same as the separation distance during afirst flyby on Jan. 14. Its cameras and instruments will cover 30 percentof the planet surface, including never-before-seen areas on the western side ofthe planet opposite to the first flyby's coverage.

No spacecraft has caught such close looks at the planetsince NASA's Mariner 10 probe, which zipped by Mercury three times in 1974 and1975.

The earlier Mariner probe managed to map just 45 percent ofthe planet's surface during its threeflybys, while MESSENGER scoped out half of the planet's unchartedsurface during its first flyby.

However, MESSENGER still needs to perform an intricate dancewith Mercury before it can turn full-time photographer. Each flyby requiresprecise earlier adjustments to the spacecraft's course that normally use upprecious onboard propellant.

Mission planners skirted this issue and saved up morepropellant for the spacecraft's later mission by taking advantage of the solarwind.

"MESSENGER is first interplanetary mission to use solarsailing as a means to control its trajectory," said Daniel O'Shaughnessy, thelead MESSENGER navigator at Johns Hopkins University Applied Physics Laboratoryin Laurel, Md. He added that they achieved accuracy within a third of a mile (1km) using "only the subtle push of sunlight and without a single drop ofpropellant in over six months."

The spacecraft will begin its 15,000 mph (24,140 kph) flybyin the early morning hours of U.S. EDT on Monday, Oct. 6. Its team willoccasionally lose contact as MESSENGER turns this way and that to take picturesand compile seven large image mosaics of the planet surface. The closest approachduring the roughly 30-hour encounter is set for about 4:45 a.m. EDT (0845 GMT).

A 17-minute power outage will occur as MESSENGER passes intoMercury's shadow, requiring the spacecraft to rely on internal batteriesinstead of its solar panels.

The first MESSENGER flyby found evidence that volcanoesand not impacts had created Mercury's flat, smooth plains. It also showedthat Mercury's magnetic field is elongated like a tear drop, with the solarwind pushing against the side closest to the sun and pressing it close to thesurface.

MESSENGER approaches its upcoming flyby as an experiencedplanetary hopper, having revisitedEarth once and swung by Venus twice since its August 2004 launch. A thirdflyby of Mercury is also scheduled for Sept. 2009, before the spacecraft entersorbit in 2011.

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