NASA has set the target launch date for the Mars Science Laboratory next fall, but the Curiosity rover's landing site is still being debated by a team of researchers walking a tightrope between science and safety.

Artist's concept of the Mars Science Laboratory's Curiosity rover, which will analyze rocks with its ChemCam instrument. Credit: NASA/JPL-Caltech

The nuclear-powered rover, which is about the size of a Mini Cooper, will launch on an Atlas 5 rocket from Cape Canaveral between Nov. 25 and Dec. 18, 2011, NASA announced last week. Earth and Mars will be properly aligned during that period to allow the spacecraft to reach Mars.

Landing will occur between Aug. 6 and Aug. 20, 2012.

NASA officials selected the final launch and landing periods based on the optimum positions of two probes circling Mars. During the dangerous entry and landing phase of the mission, the rover will be sending data back to Earth through the Odyssey orbiter passing overhead. Odyssey will immediately transmit the information back to controllers on Earth using a bent-pipe technique.

Passing data through an orbiter will allow scientists on Earth access to much more real-time information than would be possible if the rover was broadcasting directly home.

"It is important to capture high-quality telemetry to allow us to learn what happens during the entry, descent and landing, which is arguably the most challenging part of the mission," said Fuk Li, manager of NASA's Mars Exploration Program at at the Jet Propulsion Laboratory. "The trajectory we have selected maximizes the amount of information we will learn to mitigate any problems."

But scientists are still deciding the ultimate destination of the Curiosity rover.

Matt Golombek, co-chair of the rover's landing site steering committee, said the group plans a series of open workshops and closed meetings during the next year to gauge the opinion of the science community.

"It's always going to be a risk-reward trade between science and safety of landing," Golombek told Spaceflight Now. "They're each going to have their pluses and minuses, and NASA will decide where the sweet spot is."

Mars Science Laboratory will test out a complex new landing mechanism called a sky crane, which will lower the rover to the Martian surface on a tether from a rocket-powered descent stage.

The Pathfinder, Spirit and Opportunity rovers touched down on parachutes and giant airbags that bounced to a stop on Mars.

"Airbags are incredibly compliant to all sorts of things," Golombek said. "There are concerns with slopes, but airbags will go to the lowest point around anyway. We haven't found the sky crane any less capable. There's not a huge difference, even though they're obviously dramatically different designs."

Animation of the sky crane landing system as the descent stage releases the rover for touchdown. Credit: NASA/JPL-Caltech

The sky crane lander is more precise than the airbag system, allowing the rover to aim for a smaller target ellipse about 20 kilometers, or 12.4 miles, in diameter. Combined with the imaging prowess of MRO's HiRISE camera, the better landing accuracy means scientists can scout specific rock formations and terrain for scientific value and safety.

For the first time, scientists are analyzing stereo imagery and high-fidelity mineral maps showing individual boulders, ancient riverbeds and chemical concentrations when deciding on a landing site for a Mars mission.

After the Curiosity rover's launch was postponed two years by a flurry of technical problems, the landing site selection committee opened up a new round of discussions on adding more locations to the lists of four finalist sites.

The team converged on two new sites, NE Syrtis and East Margaritifer, and requested the Odyssey spacecraft and the Mars Reconnaissance Orbiter take detailed high-resolution stereo images and collect mineral maps of the regions.

"Our knowledge of the sites has improved dramatically from when they were added to be considered," Golombek said.

In a telecon earlier this month, the team agreed not to add either site to the list of four finalists. The East Margaritifer location was deemed not to be as scientifically compelling as the other candidates. The NE Syrtis site was intriguing due to volcanic flows and a variety of mineral indicators that its rocks were altered by water in ancient Martian history.

But high-resolution snapshots from MRO showed steep slopes, ripple fields and large rocks that could pose a hazard to a landing spacecraft. The steering committee unanimously agreed the NE Syrtis site should not be added as a formal candidate for the Curiosity rover, Golombek said.

Scientists are pleased with the four sites now under final consideration, Golombek said, and officials don't plan to scout any more locations.

"They are all extraordinarily interesting places," Golombek said. "These are not places that you have to settle to land at. These are places where we can really address some of the most important scientific questions about Mars."

The rover is tasked with studying the potential habitability of ancient watery environments on Mars.

Holden crater, one of the mission's four candidate landing sites, lies in the Martian southern hemisphere ands holds a stack of clay-bearing minerals deposited by a gently flowing stream or lake.

"Clay-bearing minerals tend to be very good preservers of organic material here on Earth," Golombek said. "They tend to be deposited in very still water or very weakly-running water. These are the finest particles that would fall out of the water column."

The target landing site in the 100-mile-wide Holden crater is one of the safest on Mars. Stereo mapping and topographic data show no large rocks and very low slopes.

But the Curiosity rover's expected range of more than 10 miles would be tested early on in a mission to Holden crater. That's because the robot would have to drive outside of the target ellipse to reach the suspected clay minerals.

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 crater, located near Holden in the southern hemisphere, is another finalist being put under the scientific microscope because of strong indications of an ancient river delta.

"Sitting within the edge of the crater, it has prima facie evidence of a delta, in which a stream or a river broke through the edge of the crater and deposited a series of beds out into this lake," Golombek said. "There's really no disagreement among the scientific community about the interpretation."

Eberswalde, which stretches about 40 miles across, contains more clay minerals likely formed in an environment of standing water, a potential breeding ground for ancient micro-organisms or organic material.

Mapping of Eberswalde from orbit shows the lander would be faced with steeper slopes, hills and more rock fields than any of the other finalists, according to scientists.

Mawrth Vallis, one of the oldest valleys in the Martian northern hemisphere, 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.

But scientists say the scientific case for significant amounts of ancient liquid water at the site, plus its uncertain potential of being a habitable location in the past, makes Mawrth Vallis not as compelling as other regions being considered for the one-shot landing.

The last finalist, Gale crater, is distinguished by a 3-mile-high central peak of unknown origin.

"It's a large crater with a mound of material in it that actually extends above edge of the rim of the crater," Golombek said. "You can't deposit it by having a lake there because the mound extends higher than the material around it.

Gale is located near the equator on the opposite side of Mars from the other three candidate sites.

Orbiting instruments have detected sulfate layers at Gale similar to materials encountered by the Opportunity rover during its six-year treak across the Red Planet. The sulfates were probably left over as liquid water evaporated on the Martian surface.

But the Curiosity rover would have to touch down inside the crater next to the sulfate deposits, which lie on the lower part of the central mound. That is a disadvantage of the Gale crater site.

"You land next to the land and you actually have to climb up into the mound," Golombek said.

NASA's associate administator for science will make the final call on a touchdown point in May or June of 2011, about six months before launch.