A Mars airplane soaring over wild and windy terrain. An orbiter scanning for evidence of Martian microbes. A lander to probe for water deposits on the red planet, and a novel way to scoop up Mars samples for return to Earth.

For its 2007 Mars Scout program last week NASA selected four innovative candidate missions for further investigation. Each of the proposed spacecraft missions is capable of returning a treasure trove of impressive science data.

NASA's Mars exploration program has taken a bold step to help unravel the enigmatic planet's past and present - including its potential as a distant abode for life.

The selected proposals were judged to have the highest science value among 25 proposals submitted to NASA earlier this year - a step by step process that will lead to the selection next year of a Scout craft that heads for Mars in 2007.

The Mars Scout competition is dedicated to complement and augment the overall NASA Mars Exploration Program. Once one of the four Scout missions gets the nod, it must be ready for liftoff before December 31, 2007, within a total mission cost cap of $325 million.

Last week's Scout selections are "wonderfully diverse" in how they view Mars - all from the broad point of view of Mars habitability, said James Garvin, NASA's lead scientist for Mars Exploration in Washington, D.C.

"Each takes a new vantage point, interesting measurement methods, and exploits them to make major headway on some of our prioritized goals," Garvin said. "Each offers its own 'scientific risk', given the current state of imperfect knowledge about Mars," he told SPACE.com.

"We have missions that all examine Mars in new ways," Garvin said.

One of four finalists in the Mars Scout program is the Sample Collection for Investigation of Mars - or SCIM for simplicity.

SCIM, however, is far from being a simple mission. It would perform a "hit and run" fly-over of Mars, snagging samples of Martian atmospheric dust for return to Earth.

Laurie Leshin is leading the SCIM effort at Arizona State University in Tempe.

Central to the SCIM proposal is carrying out the first return of a Martian sample at far less cost, lower risk and in a shorter period than billion-dollar-plus Mars return sample spacecraft previously advocated. NASA is still hungry to return specimens directly from the surface, but not until next decade.

Meanwhile, SCIM would make a "high pass" of Mars, going within 25 miles (40 kilometers) of the surface of the planet. For about one minute, the probe would zip through the Martian atmosphere, then swing toward a return to Earth.

During this high-dive act, dust particles would be gathered onboard SCIM, trapped and preserved by a lightweight and porous cushion of aerogel. This high-tech substance is now being used on the Stardust mission to collect dust streaming off comet Wild-2 in 2004.

Leshin believes that the aerogel would catch about 1,000 fine dust particles measuring 10 microns (1/100 of a millimeter) or larger. Once back on Earth, each grain would be carefully studied, with each grain like a little rock from Mars, she adds.

"We know that at any given time the Martian atmosphere contains 10 to 400 billion metric tons of dust. We have tuned SCIM's trajectory so that it reaches Mars in southern spring, a season that is always dusty. So, we are going at the absolute right time to collect the most dust," Leshin told SPACE.com.

During the plunge, SCIM will also gather up about a quart of Mars' atmosphere.

Like an Olympic diver, what is the degree of difficulty for SCIM performing its maneuver through the Martian atmosphere?

"For SCIM, we have run 2,000 simulations of the SCIM entry at Mars through sophisticated computer programs," Leshin reports. In all cases, she added, the simulations point to success, with the probe diving in and out of the Martian atmosphere, within the dust plume, and easily handled by the spacecraft.

"We're confident that we will hit the mark and collect plenty of dust," she said.

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