PARIS -- NASA officials are in the final stages of deciding the destination for the agency's Curiosity rover, a complex mobile science laboratory scheduled to be shot toward Mars in November.

Artist's concept of the Mars Science Laboratory Curiosity rover on the Red Planet. Credit: NASA/JPL-Caltech
 
Scientists already whittled down a list of several dozen candidate landing sites to four finalists, and now it's up to project managers and top NASA officials to determine which of the landing sites Curiosity will study.

The spacecraft is scheduled to launch Nov. 25 and reach Mars as early as Aug. 6, 2012.

The four finalists: Eberswalde crater, Gale crater, Holden crater and Mawrth Vallis.

Matt Golombek, co-chair of the landing site steering committee, said all four locations hold great promise if Curiosity ends up there.

"In many cases, it boils down to people's preferences and not necessarily that one site is better than another," Golombek said. "They're all great places, but there's different science at different sites. And deciding amongst those could be very difficult."

The final decision will come from Ed Weiler, NASA's associate administrator for science in Washington. That's expected by the end of July, according to multiple mission officials.

The selection will help engineers tweak the spacecraft's flight path after it blasts off atop an Atlas 5 rocket. Theisinger said NASA committed final trajectory information to the launch team by August.

The process of collating data, preparing presentations and forming a recommendation has already started.

The Mars Science Laboratory project office could finalize their recommended landing site in meetings this week, but then the decision must be approved by managers at NASA's Jet Propulsion Laboratory in California and ultimately by Weiler, according to Pete Theisinger, the mission's project manager.

The rover's precision landing system allows engineers to carefully target its landing site to avoid craters, large boulder fields and mountains. High-resolution stereo imagery from NASA's Mars Reconnaissance Orbiter satellite also provides detailed maps of each landing site.

And unlike NASA's previous rover missions, Curiosity won't use airbags to land. Earlier rovers dropped to the surface cushioned by inflated airbags, bouncing and rolling to a stop up to a mile from the initial touchdown point.

Curiosity's rocket-assisted touchdown will use a sky crane vehicle to gently lower the rover to the surface on a bridle, so engineers don't have to worry about sharp rocks or craters along a lengthy bounce path away from the initial landing site.

Theisinger said all four candidate sites are equally safe in the view of engineers.

"There are extremely modest differences between the sites from an engineering standpoint, and none of those differences I think is sufficient for us to say from an engineering standpoint you can't go to one of them," Theisinger said.

Golombek, who also particpated in landing site decisions for previous rovers, said engineering concerns helped guide the decision on those missions. This time, he said, it's all about the science.

"What's happened in previous site selections is that the engineering constraints have helped, if you will, the scientists make a decision because there have been really clear places where things are safer," Golombek said.

Using high-resolution imagery and the latest spectroscopy data, engineers plotted hazards at each location and developed potential drive routes the rover would take to reach the most interesting scientific sites.

"We actually can see the rocks directly in those images and map them out. We provide all that data to the engineers to determine how difficult or slow it takes to drive in those different terrains," Golombek said.

According to Theisinger, the rover is targeting an ellipse measuring about 25 kilometers by 20 kilomters, or about 15 miles by 12 miles. Better navigation later this summer and actual trajectory projections after launch could refine the ellipse size.

"The end result of all that work, and this is the first time this has ever happened, is that it's very hard to distinguish any of the four sites based on safety or trafficability," Golombek said. "That means it boils down to science. The difficulty there is everybody likes different kinds of science better. It's almost like religion."

All four sites show strong evidence that water once existed there, potentially creating a habitable environment where ancient life could have thrived. Scientists hope organic molecules may still be there today.

Two of the landing sites are on the southern hemisphere, one in the northern hemisphere, and another is near the Martian equator.

Relief maps of the four MSL landing site finalists. Clockwise from upper left: Eberswalde crater, Gale crater, Mawrth Vallis and Holden crater. Credit: NASA/JPL-Caltech
 
Eberswalde is an ancient river delta including dried up stream channels and strong signs of clay deposits, another indicator of the former presence of water. Researchers believe the site's 40-mile-wide crater was once filled with water being supplied by a river.

With two basins separated by a central high mound, Eberswalde harbors tough driving conditions for Curiosity. But engineers say the rover could handle it.

Holden, a crater spanning nearly 100 miles in diameter, is part of a string of craters that look like they were once connected by a stream. Scientists propose landing on a relatively flat plateau, then driving to several nearby sites where exposed bedrock could hold clues about the location's history.

The crater was probably once filled with water, and a catastrophic flood may have emptied the lake more than 3 billion years ago.

Gale is another site once flowing with water. The crater's central mound reaches more than 3 miles above the floor, and researchers want Curiosity to touch down in a flat region just off the the mountain's flank. Then the rover would start climbing the mound, pausing along the way to study layers of clay, sulfur and oxygen-bearing minerals.

Engineers say the challenging drive isn't a problem for Curiosity. Scientists already mapped out a sample traverse path climbing nearly a mile in altitude up the mountain, and the scenery would spectacular, Golombek said.

Mawrth Vallis, one of the oldest and broadest valleys on Mars, offers one advantage over other sites. The rover would land directly on layered rocks containing clay materials that may have formed in a watery environment.

At the fifth landing site workshop held in May, scientists gathered for the last time to digest the latest data on each site.

"The data dribbles in slowly," Golombek said. "It's our understanding and interpretation that has changed. There were definitely some new results that helped improve the cases of some of the sites and their scientific desirability."

Now the decision is in the hands of NASA management.