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Mercury Probe Prepares for Close Encounter

Mercury gets a close look Monday, when NASA's Messenger spacecraft slings 142 miles over the puny planet closest to the sun. For mission scientists, it's a festive occasion.

"A planetary flyby is very much like Christmas morning for the science team. We know there are presents under the tree," says Messenger principal investigator Sean Solomon of the Carnegie Institution of Washington. "We expect to be surprised and we expect to be delighted."

The fly-by, aimed at mapping the last uncharted 5-10% of the planet, will be the third one by the spacecraft. Messenger's engineers have plotted those past flybys, and this final one, to brake the spacecraft's speed enough so that Mercury will capture the orbiter in a final March 2011 rendezvous that will set the spacecraft permanently circling the planet.

"We must hit the flyby of Mercury at the critical point so that our trajectory is absolutely right for the orbital insertion in 2011," Solomon says.

Such orbital gymnastics have become almost commonplace for space engineers in recent decades. Voyager 2 flew by Jupiter, Saturn and Uranus for speed boosts on its way out of the solar system. The Cassini spacecraft swung by Venus twice, Earth once and then Jupiter before threading a daringly-close flyby of Saturn's rings that allowed the planet to capture it in orbit in 2004.

Aerospace engineers can make such complicated stuff look easy, almost easy enough to forget this is actual "rocket science." But the mechanics of modern rockets in many ways still present daunting challenges, some 83 years after engineer Robert Goddard fired off the first liquid-fueled rocket. (His announcement was: "The first flight with a rocket using liquid propellants was made yesterday at Aunt Effie's farm in Auburn (Mass.)" )

Consider the still mysterious problem of fuel sloshing around inside spacecraft.

"Anything that creates a lot of liquid spinning really stands in the way of spacecraft stability," says Sathya Gangadharan of Embry-Riddle Aeronautical University in Daytona Beach, Fla. Shifting liquids might set the spacecraft pointing the wrong way, for example, or choke the combustion of fuel.In 1969, for example, fuel slosh sent NASA's ATS-5 spacecraft into a flat spin, ruining its experiments on gravity. And in 2007, a SpaceX Falcon 1 rocket engine shut down due to a fuel slosh 180 miles high, wrecking its launch.

In talks this year at three American Institute of Aeronautics and Astronautics conferences, Gangadharan has discussed the latest effort to keep fuel slosh under control. Big problems come when liquid fuel in the microgravity of space starts zipping around fuel tanks in an oscillating fashion that begins to feed on itself, reinforcing the strength of its swings, a problem called "resonance" seen in many engineering applications.

Engineers usually put plates called baffles into fuel tanks to stop the sloshing, but those add costly weight, and don't always stop sloshing. Spacecraft designers would rather design fuel tanks that are slosh-proof and save the weight and the worry. And they would rather not have to run expensive ground tests on fuel tanks to make sure they are slosh-safe before launches.

With his student, Brandon Marselle, and NASA's James Sudermann, Gangadharan has focused on two approaches to stopping sloshing:

•Creating simple models of fuel tanks and sloshes that might warn engineers when sloshing resonances may occur.

•Full-bore computer models of fuels slopping around inside spacecraft tanks that reveal all the ways that fuel slosh might destabilize a spacecraft.

These last "computational fluid dynamics" (CFD) efforts are the state of the art for aerospace engineering, but they are expensive to create and need to be automated, Gangadharan says. He's confident that the newer models in coming years will lick the problem of spacecraft fuel slosh.

"I have been impressed by current state-of-the-art CFD slosh results which indeed agree closely with experiment(s) and can even compute baffle damping," says Frank Bugg of NASA's Marshall Space Flight Center. Bugg calls Gangadharan's expectation of a solution to the long-running problem of fuel slosh coming in a few years, "reasonable," given the progress of the field. "We're planning tests to verify analytical results," he adds, by e-mail.

So that's the story of space science — the scientists get to open the Christmas presents, and the engineers get to worry about Santa getting stuck in the chimney.

"We've known about this problem for 50 years, it's not a new concern," Gangadharan says. "We just need to find a solution for it."

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