WASHINGTON — An underwater mission to the lakes of Titan, a touch-and-go visit to the surface of a comet and a three-pronged Mars sample-return expedition are among more than two dozen candidate missions a National Research Council (NRC) committee studied over the past year as it drafts NASA’s next 10-year plan for robotic exploration of the solar system.

Steve Squyres, the Cornell University professor who chairs the NRC planetary decadal committee, highlighted these and other mission proposals during a daylong astrobiology seminar here Oct. 14. Squyres said the panel identified 28 finalist candidates culled from more than 200 proposals submitted by the science community over the past year.

Squyres would not say which of the 28 finalist proposals found their way into a draft of the survey recently submitted to the NRC for review, though he recapped some half-dozen of the missions the panel studied in the process.

“These are not necessarily in the decadal plan,” Squyres said, adding that the final survey report is slated for issue in early March. “They are some of the ones that have the greatest potential significance for astrobiology.”

Squyres, who led NASA’s overachieving Mars Exploration Rover mission, highlighted three candidates for voyages to the red planet, including a trace gas orbiter that would look for a source of methane below the surface, a spacecraft that could characterize ice deposits at the planet’s poles and a three-part Mars sample-return mission that would spread over many years the cost of developing and flying what he termed the “holy grail” of robotic space science missions.

The last planetary decadal survey, issued in 2003, concluded a Mars sample-return mission was technologically within reach and recommended NASA begin planning for implementation early this decade. In February, a Mars sample-return planning team led by NASA’s Jet Propulsion Laboratory, Pasadena, Calif., presented to the NRC survey committee a three-part Mars sample-return mission with a total life-cycle cost estimate of $6 billion to $7 billion, according to briefing slides from the Feb. 23 presentation.

 During the first leg of the mission, NASA would launch a dust- and rock-gathering rover dubbed the Mars Astrobiology Explorer-Cacher, or MAX-C, to the red planet in 2018 on the same rocket with Europe’s astrobiology-focused ExoMars rover. A second rover, sent on a subsequent mission with a Mars Ascent Vehicle (MAV), would fetch MAX-C’s cached soil samples, stuff them into a coconut-sized return capsule and launch them into orbit aboard the MAV.

Once in orbit, the sample container would rendezvous with a Mars orbiter that would capture the sample and transfer it to an Earth entry vehicle for the return trip home.

Squyres said breaking the mission into three legs spreads its considerable cost over time.

“The idea is do the stuff that we know how to do now and then invest right away into a technology program that can bring those other elements of the program to fruition in a reasonably timely fashion,” he said.

NASA is already revamping its Mars exploration strategy, announcing earlier this year plans to join the European Space Agency on a set of missions starting in 2016 that would culminate with the return of Mars surface samples sometime after 2020. To afford the new campaign, NASA announced this summer plans to discontinue its line of competitively selected Mars Scout missions created following the recommendations of the 2003 decadal survey.

Other candidate missions for the NRC’s forthcoming planetary science decadal survey, Squyres said, include sending a spacecraft to rendezvous with the nucleus of a comet, where an instrument suite would use a thermal infrared camera and other imaging systems to characterize the surface while a rotary brush wheel gathers dust and rock for return to Earth.

Squyres said the “touch-and-go sampling” mission would load the collected samples into an Earth return vehicle for the voyage home, during which sensors would monitor the temperature and pressure of the cargo to give scientists an idea of changes the sample may undergo on the way back.

While not as sophisticated as a cryogenic sample-return mission, which could freeze particles in a pristine state during the return voyage, Squyres said the proposed sample-monitoring approach is technologically more feasible in the near-term. He also said the astrobiology community is considering a cryogenic sample return visit to a comet in “some subsequent decade,” but in the meantime, “this one is going to be a big, big step forward.”

The survey team also considered an orbital tour of Europa that would use an ice-penetrating radar to find thin spots in the frozen surface of Jupiter’s liquid moon and potentially unlock clues to its watery underworld.

Missions to Saturn’s moon system also could yield insight into distant underwater environments, including an orbiter proposed for the gas giant’s sixth-largest moon, Enceladus, where tidal fluctuations produce plumes near the south pole thought to be similar to those found at the bottom of Earth’s oceans. The Enceladus orbiter would gauge the source of the plumes using near-infrared and thermal imaging, and a mass spectrometer would help determine the composition of the matter they emit.

Two proposed missions to Saturn’s largest moon, Titan, include an orbiter that would look through the planet-like moon’s dense atmosphere to image ice on the surface, as well as methane and ethane pools where Squyres said some form of life “very unfamiliar to us” could thrive.

A separate mission focused solely on Titan’s liquid-hydrocarbon lakes would probe these methane and ethane environments found principally in the polar regions of the moon. A Titan orbiter would deploy a floating instrument suite comprising a boat with cameras, an echosounder, meteorology package and a rain gauge to make measurements at the lakes’ surface. A separate submersible vehicle and instrument suite would drop below the liquid gas surface and make measurements before resurfacing to relay data to the orbiter.

While not all of the mission candidates highlighted will make the decadal’s final list of robotic exploration priorities, Squyres said all are important to astrobiology and would be considered as part of subsequent decadal reviews.

But as the science community prepares to look ahead at the next 10 years of solar system exploration, NASA is still very much focused on the pressing challenges of the current decade. The agency, for example, expects to spend upward of $2 billion on the Mars Science Laboratory mission by the time it launches in November 2011, two years later than planned. The 2003 decadal survey estimated the high-priority mission could be accomplished for around $650 million.

Aware of the need to refine decadal survey cost estimates with independent analysis, the NRC determined the forthcoming study would place a much greater emphasis on evaluating technical maturity and probable costs of proposed missions. NASA, for its part, has agreed to pay for what agency officials hope will prove to be more accurate cost estimates for a limited number of high-priority candidates.

Melissa McGrath, chief scientist of science and mission systems at NASA’s Marshall Space Flight Center in Huntsville, Ala., and chairwoman of the American Astronomical Society’s Division for Planetary Sciences, said it would cost billions of dollars for NASA to do all of the top-ranked missions recommended in the decadal survey. As a result, she said, NASA generally sticks to the missions ranked at the top of the priority list.

“And even then sometimes those don’t get implemented,” she wrote in an e-mail, citing Europa, the top-ranked large mission from the 2003 decadal. “The Decadal Survey is recommendations. And although NASA takes it as gospel, they have to do what they can with the cards — money, politics, technological readiness — they are dealt.”