Scientists have narrowed a list of candidate landing sites for the Philae lander, a piggyback probe on Europe's Rosetta mission that will attempt the first ever descent to a comet this fall and latch onto its jagged surface.

A view of comet 67P/Churyumov-Gerasimenko from Rosetta's navigation camera. Credit: ESA/Rosetta/NAVCAM
 
If it works, Philae will spend at least two days -- perhaps up to several months if its batteries can be recharged -- drilling into the comet, measuring its structure and chemical make-up, taking pictures, and searching for a link between the comet's ice deposits and the water in Earth's oceans.

Some scientists theorize Earth was seeded with water and the building blocks of life by comets billions of years ago.

"Unlocking these time capsules, looking at the gas that come off them, the dust, and in particular the ice and the water they're made of give us great clues about the origin not only of the solar system and the planets, but potentially even life because we know comets also contain organic molecules -- the building blocks of DNA and RNA," said Mark McCaughrean, senior scientific advisor in the European Space Agency's science and robotic exploration directorate.

Since Rosetta's arrival at comet 67P/Churyumov-Gerasimenko on Aug. 6, the craft's cameras have mapped its surface and started an exploration campaign expected to continue until at least the end of 2015.

The Rosetta spacecraft, with solar panels stretching 105 feet (32 meters) tip-to-tip, has maneuvered closer to the comet since its rendezvous earlier this month. This week, Rosetta is about 50 kilometers, or 31 miles, from 67P/Churyumov-Gerasimenko flying in a three-legged triangular trajectory with periodic direction changes to stay near the comet.

Officials met in Toulouse, France, last weekend to go over Rosetta's initial findings and whittle down the list of potential destinations for Philae.

The sites were assigned a letter from an original pre-selection of 10 possible sites. Three sites (B, I and J) are located on the smaller of the two lobes of the comet and two sites (A and C) are located on the larger lobe, according to an ESA press release.

Five landing sites, seen in these views of the comet, were selected during an Aug. 23-24 meeting in Toulouse, France. See a larger image. Credit: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA
 
Check out photos of the shortlist of possible landing locations.

Photos taken by Rosetta's cameras will help scientists identify a primary and backup landing site for Philae. Officials are scheduled to meet Sept. 14 to decide on the top two landing locations.

Rosetta will fly within 12 miles (20 kilometers) of the comet at the end of September to get even better views of the top two candidates before a final decision on Philae's landing site in mid-October.

Philae's landing is scheduled for mid-November, when Rosetta will spring-eject the automated robot for glacial descent to the comet's nucleus. Scientists estimate it will take several hours for Philae to complete the journey, during which it will extend three landing legs fitted with shock absorbers to dampen the impact with the comet.

The probe's landing can only be targeted with a precision of 500 meters, or 1,640 feet.

Once it contacts the surface, Philae will fire a harpoon into the ground to prevent the lander from bouncing off. Cognizant of the comet's tenuous gravity field, designers also equipped Philae with ice screws to affix itself to the nucleus.

The landing is scheduled when the comet is approximately 280 million miles from Earth.

None of the sites considered by the Philae lander team are without risk, and scientists say the final decision will be driven primarily by the likelihood of a successful landing.

"It is clear that we have to compromise," said Stephan Ulamec, project manager for the Philae lander at the German Aerospace Center, or DLR.

Germany leads the Philae project with a consortium of institutions across Europe.

Comet 67P/Churyumov-Gerasimenko's unique shape -- with a large and smaller lobe separated by a collar-like neck -- is rife with hazards for Philae. Slopes, boulders, fissures and loose soil with the consistency of quicksand could await the lander.

No place on the comet is free of such hazards, so the challenge for Philae's earthbound team is finding the least dangerous site that still promises a big scientific payoff.

Ground teams have also studied the comet's rotation to find potential landing sites that have near-equal amounts of sunlight and darkness. Another consideration at the top of engineers' minds is the lander's ability to communicate with the Rosetta mothership, which will act as a conduit for telemetry and scientific data between Philae and Earth.

Artist's concept of the Philae lander touching down on comet 67P/Churyumov-Gerasimenko. Credit: ESA/ATG medialab
 
"Based on the particular shape and the global topography of Comet 67P/ Churyumov-Gerasimenko, it is probably no surprise that many locations had to be ruled out," Ulamec said.

"The candidate sites that we want to follow up for further analysis are thought to be technically feasible on the basis of a preliminary analysis of flight dynamics and other key issues," Ulamec said in a statement. "For example, they all provide at least six hours of daylight per comet rotation and offer some flat terrain."

The 220-pound (100-kilogram) lander's 10 scientific instruments will work for at least 64 hours after arriving on the comet's surface. That is the amount of charge in Philae's on-board battery.

With some careful planning and luck, Philae could run on bonus time if it receives enough sunlight to recharge its battery. But too much sunlight would cause the lander to overheat, posing another risk.

"The comet is very different to anything we've seen before, and exhibits spectacular features still to be understood," said Jean-Pierre Bibring, a lead lander scientist and principal investigator for Philae's camera system. "The five chosen sites offer us the best chance to land and study the composition, internal structure and activity of the comet with the ten lander experiments."

Follow Stephen Clark on Twitter: @StephenClark1.