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NASA Sending a 'Messenger' for a Closer Look at Mercury

Posted on: Monday, 2 August 2004, 06:00 CDT

Mercury gets a long-awaited place in the sun this month with the launch of the MESSENGER probe to the fiery, pint-size planet.

Today is the start of a two-week launch window for the $427 million spacecraft, NASA's first visit to Mercury in three decades. Weather forecasters say there's a good chance for a launch today.

MESSENGER stands for MErcury Surface, Space ENvironment, GEochemistry and Ranging, a long acronym reflecting the long list of the craft's chores. The probe aims to enter orbit around Mercury in 2011 after flybys in 2008 and 2009.

''It's an unusual planet, even by the standards of the inner solar system,'' says mission scientist Sean Solomon of the Carnegie Institution of Washington (D.C.).

About one-third the size of Earth, Mercury orbits the sun once every 88 days, but it spins so slowly that a day on Mercury lasts nearly six months on Earth. The planet endures temperatures above 840 degrees on its days and below minus-350 degrees at night.

Like its larger cousins, Earth, Venus and Mars, Mercury is regarded as a ''terrestrial'' planet, a rock with an iron core. And it is a bit of a mystery as well.

Visible from Earth for only several weeks each year, the planet orbits too close to the sun for safe viewing by the sensitive optics aboard the Hubble Space Telescope. That makes Mercury only slightly better understood than Pluto, the most distant planet from the sun.

''Understanding Mercury is vital to understanding the terrestrial planets,'' says NASA solar system exploration official Orlando Figueroa. The spacecraft will rely on an 8-foot-by-6-foot sunshade, heated to pizza-oven temperatures by the sun, to shield its instruments as they try to answer long-standing questions about Mercury:

* Its dense iron core makes up more than 60% of the small planet's mass and produces a magnetic field of surprising strength, which perplexes astronomers.

* NASA's last visit to Mercury, the Mariner 10 flybys of 1974-75, mapped only 45% of the planet's surface. MESSENGER will map the entire planet in great detail.

* Craters in Mercury's poles, which are shielded from the sun's rays, could contain ice. The spacecraft's instruments might be able to sense whatever minerals are hidden in the craters.

Like other interplanetary missions, MESSENGER will use ''gravity assists,'' flybys of Earth, Venus (twice) and Mercury (three times) to send the spacecraft on its 4.9-billion-mile, 6.5-year trip into orbit around Mercury.

''We have to gradually slow down the spacecraft relative to Mercury,'' says Robert Farquhar of the Johns Hopkins University Applied Physics Laboratory in Laurel, Md., the mission manager.

Without the gravity assists, the 1,100-pound instrument weight limit on the craft would have dropped to 360 pounds, which would have forced scientists to jettison several experiments.

By 2012, mission controllers might lower the probe's altitude to only 18 miles above Mercury. The craft eventually will crash-land on the planet, Farquhar says, bearing a U.S. flag to the surface.

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