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NASA Probe To Fly By Mercury

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By MARC KAUFMAN, The Washington Post

Published: January 14, 2008

WASHINGTON - Right around noon today, if all goes as planned, a spacecraft called Messenger will swoop past Mercury and begin two days of unprecedented picture-taking and data-collecting.

The flyby, the first visit to Mercury in more than 33 years by an emissary from Earth, will mark a key moment in a NASA mission that ultimately will place the first satellite into orbit around the tiny planet that sits closest to the sun.

The planetary science community is eagerly awaiting images and information that should shed light on some of the enduring mysteries about the planet, such as where in the solar system it was formed and why its hard metal core is so large and its outer rock crust so scant, compared with Earth and other rocky planets.

"Mercury is a difficult place to get to, and it's taken a long time to get back," said principal investigator Sean Solomon, who has worked on the mission for more than 11 years. "But now we're in place to learn things about one of our few sister rocky planets, and we're ready for some real surprises."

The desk-size spacecraft was launched in 2004 and has taken a circuitous path to Mercury, swinging twice by Venus and once by Earth for gravity assists. Messenger will make two more passes by Mercury to let the planet's gravity slow it down enough for it to swing into orbit in 2011.

Still, today's whisker-close flyby will be, NASA officials said, a high point of the mission. Not only will Messenger pass within a record 124 miles of Mercury's surface at a relative speed of more than 16,000 mph, it also will quickly send its first observations of Mercury's physical and magnetic makeup. The observations will be made by instruments that could answer some of the most basic questions about Mercury's character and history.

It will be the closest pass by Messenger in the entire mission, and the nearest to the planet's equator.

"The biggest mystery of Mercury is why it has so much heavy metal - a core very different in size from other planets," Solomon said. "We think we can begin to unravel the mystery once we know the chemical makeup of the planet's surface."

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