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Mercury Rising: Mission Lifts Planet's Status

Posted on: Tuesday, 3 August 2004, 06:00 CDT

Mercury in a minute Average distance from sun: 36 million miles Size: About 3,000 miles across (a bit larger than Earths moon) Surface temperature: Varies from plus-850 degrees to minus-300 degrees Fahrenheit, the greatest range of any planet Year (time taken to travel around the sun): 88 Earth days Solar day (sunrise to sunset): 176 Earth days, making Mercurys day longer than its year Source: Encyclopedia of the Solar System

The Dallas Morning News

DALLAS - Poor, poor pitiful Mercury. For 29 years, scientists have let it be.

As the closest planet to the sun, Mercury often disappears in the glare. Without the majestic rings of Saturn or the dramatic dunes of Mars, Mercury suffers from planetary neglect. It never even garnered the lovable loner reputation given to Pluto, the farthest planet from the sun.

But now science is giving Mercury its due: the first mission there since 1975. The Messenger spacecraft, to be launched Monday, is scheduled to take a leisurely 6 1/2-year trip. In 2011, if all goes well, it will become the first mission ever to orbit the innermost planet.

Mercury-loving scientists are eager to study the elusive planet, which they say harbors secrets not only about its own past, but also about Venus, Earth and Mars.

"We think Mercury has a lot to tell us about how the Earth was formed, how the inner planets were formed, and how they all turned out so differently," says planetary scientist Sean Solomon, principal investigator for the mission.

Mercury is a planetary oddball in many ways. Much of it is solid metal. The nearby sun bakes part of it to searing temperatures while other regions remain in a permanent deep-freeze. And because of its peculiar orbital motions, an observer at some spots on Mercury would watch the sun rise, stop, then appear to retrace its tracks slightly before resuming its course and setting 176 Earth days later.

Mysteries that Messenger will investigate include how the planet acquired its powerful magnetic field, what gases make up its tenuous atmosphere, and whether water ice remains frozen in shadowy craters near its south pole. (The spacecraft's name is a messy acronym for MErcury Surface, Space ENvironment, GEochemistry and Ranging.)

Most of what scientists know about Mercury comes from the Mariner 10 spacecraft, which flew past the planet twice in 1974 and once in 1975. That mission photographed only 45 percent of Mercury's surface; the other half remains essentially unknown, said Solomon, of the Carnegie Institution of Washington, D.C.

Astronomers sometimes use large telescopes to try to photograph Mercury. But because the planet is so close to the sun, it appears only at twilight or dawn, very low on the horizon - right in the spot where atmospheric turbulence blurs any pictures taken through big scopes.

Many new findings about Mercury come from amateur astronomers, whose smaller scopes are less affected by the atmosphere's capriciousness.

"They can do better imaging of the surface with their 10- and 14- inch telescopes than we can do with the big ones," said Ann Sprague, a planetary scientist at the University of Arizona's Lunar and Planetary Laboratory.

Other discoveries come from radio telescopes, which shoot radio waves toward Mercury and measure how they bounce off the surface. Using such telescopes in Puerto Rico, California and New Mexico, scientists have spotted odd reflections from craters near Mercury's south pole.

The reflections could indicate the presence of water ice - or sulfur, or a rocky silicate-type material, or some other unknown substance frozen in the craters since the birth of the solar system, said Sprague.

"That's a very exciting discovery, and we're hoping Messenger will be able to shed some real knowledge on that subject," she said.

Similar deposits seen on Earth's moon have triggered speculation about a possible water resource for future lunar astronauts.

With all Mercury's mysteries, it might seem strange that scientists haven't been back for so long. But there are two major problems: how to get there, and how to survive the heat once you do.

Getting to Mercury is essentially a question of cost, says Robert Farquhar, mission manager at the Johns Hopkins University Applied Physics Laboratory in Laurel, Md.

Zooming straight from Earth to Mercury, a spacecraft could get there in 3 1/2 months, he says. But that would require carrying enough fuel to slow down the craft so that Mercury's gravitational pull could capture it directly into orbit. More fuel means a bigger launch vehicle and much greater costs.

As it stands, Messenger weighs 1,100 pounds when empty and carries 1,300 pounds of propellant - making it almost 55 percent fuel. The spacecraft will launch on a Delta 2 rocket, at a total cost of $426 million. Had it carried enough fuel to get to Mercury directly, it would have been 85 percent fuel, said Farquhar.

But as a tradeoff, the mission won't get to Mercury until 2008 and won't start orbiting it until 2011. Messenger is designed to fly by Earth once, Venus twice and Mercury three times, with each pass acting as a gravitational brake to place the spacecraft on the proper trajectory. By the time Messenger finally enters orbit around Mercury, it will have looped around the sun more than 15 times and traveled nearly 5 billion miles.

Once there, the spacecraft will have to cope with the sun's intense heat, as temperatures vary more than 1,100 degrees from night to day in some places.

To keep its cool, Messenger is made entirely of a composite material that radiates heat away quickly. It also features a large ceramic-fiber sunshade that can get as hot as a pizza oven while keeping the electronics behind it near room temperature, says mission engineer James Leary.

Messenger's seven scientific instruments have a wide range of tasks. A magnetometer will measure Mercury's magnetic field, attempting to determine if it is created by the sloshing motions of a liquid outer core, as Earth's magnetic field is. Cameras will photograph the surface in great detail, checking for volcanic flows, meteorite impacts and other geological signs. Other instruments will map the chemical makeup of Mercury's crust, as well as measure gases and charged particles in the atmosphere.

Mission controllers hope Messenger will spend an entire Earth year orbiting Mercury. Eventually, it will crash into the surface, taking with it a U.S. flag, said Farquhar.

Other countries might not be far behind the United States. The European Space Agency is collaborating with Japan on a two-pronged Mercury mission called BepiColombo, which could be launched as early as 2012.

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