Spacecraft to Start 6½-Year Mercury Trip

NASA (search) is about to embark on its hottest mission ever, to Mercury. The Messenger spacecraft, to be launched next week, will be blasted by up to 700-degree heat as it orbits the tiny planet closest to the sun — so close that it would be as though 11 suns were beating down on Earth.

Remarkably, the only thing between the probe's room-temperature science instruments and the blistering sun and pizza-oven heat will be a handmade ceramic-cloth quilt just one-quarter of an inch thick.

"If it doesn't stay toward the sun, it will fry everything," said Neal Bachtell, mechanical technician and master quilter.

Bachtell used X-Acto blades to cut the 3M Nextel fabric (search ) and then — relying on sewing tips from his mother — used an industrial sewing machine to stitch the off-white pieces together into an 8-by-9-foot quilt, using Teflon-coated fiberglass thread. It was a nasty job; the itchy, ceramic-fiber cloth sheds and is bad to inhale.

"Neal, you're making history today, buddy," Jack Ercol, the project's lead thermal engineer, said during a mid-July spacecraft showing in an ultraclean room.

"It's cool, it's cool," Bachtell replied.

Messenger will be the first spacecraft ever to orbit Mercury and the first in more than 30 years to come close. Even at that, members of the Johns Hopkins University spacecraft team assembled in Cape Canaveral (search ) realize this mission can't compete with Mars and its rovers, or Saturn and its newly arrived sentry, Cassini.

But there are plenty of cool facts about this red-hot mission, besides the off-the-charts-SPF sunscreen that was baked for days in ground testing.

You can see yourself in Messenger's twin solar wings, made up of a couple thousand little mirrors to reflect the intense sunlight in Mercury's neighborhood. The wings are two-thirds mirrors and just one-third electricity-producing solar cells.

Diode heat pipes burrowed into the extraordinarily insulated spacecraft will radiate internal heat from all the electronics. When Messenger passes between the sun and Mercury and it gets really sweltering — not too often and not for long because of Messenger's cleverly conceived flight plan — these pipes will shut down and the boxy craft will be like a house with all the windows closed on a steamy afternoon.

"It's basically a Thermos bottle," Ercol explained.

"We're actually taking on a very brutal mission from the standpoint of the sun and then from the orbiting standpoint because the planet itself is very hot."

Even though Mercury is 50 million miles from Earth at closest approach, Messenger will travel 5 billion miles to get there. It's technologically infeasible to fly straight to Mercury, a trip of a few months, and so the spacecraft must swing once past Earth, twice past Venus and thrice past Mercury before slowing down enough to slip into orbit around Mercury.

Estimated arrival time: March 2011.

Mariner 10 was NASA's last Mercury lookout. Equipped with an umbrella for shade, it flew by Mercury three times in 1974 and 1975, providing the first up-close pictures of the planet. The pictures are poor by today's standards; Messenger's photos will be superior by far.

The $427 million Messenger mission is part of NASA's bargain-focused Discovery program. That includes the launch aboard an unmanned rocket in the wee hours of Aug. 2, and all the scientific analysis seven years from now.

So why Mercury? Why now?

The technology for designing a spacecraft capable of withstanding such harsh heat for prolonged periods was unavailable until recently. Then with computer modeling, engineers had to come up with a spacecraft choreography to keep the heat down as much as possible during the one year that Messenger circles Mercury, its seven scientific instruments collecting data.

That's an Earth year of study. A Mercury year lasts 88 days — Earth days, that is.

Mercury is an average 36 million miles from the sun, making for a fast elliptical loop and thus a fast year. Earth, by comparison, is 93 million miles from the sun and takes four times as long to circle it.

In this land of extremes, the surface temperature changes a radical 1,100 degrees from day to night, from 800 degrees to minus-300 degrees.

No wonder metal-heavy Mercury — a little bigger than Earth's moon, yet about as dense as Earth — is so bewitching.

Scientists want to know how the planet turned out the way it did, and whether the perpetually dark carters at the poles hold ice. Anything scientists can learn about how Mercury formed will shed light on the origins of the other inner rocky planets of the solar system: Venus, Earth and Mars, each one so very different.

Once its mission is accomplished in 2012, Messenger will keep orbiting Mercury until it eventually crashes onto the surface. It will go down with a pair of U.S. flags, decals solemnly placed on one of Messenger's most heat-resistant surfaces.

The spacecraft team wanted to leave a flag on Mercury to show, for all time, that Americans were there.