For the decade 2013-2022, five separate panels of scientists and experts agreed on a suite of missions that would get the greatest scientific return from money spent, recognizing that even these projects could be budget busters.

"We have a long history in the planetary (exploration) program ... of generating cost numbers that are too optimistic," said astronomer Steven Squyers of Cornell University, who led the group that crafted the report and its recommendations.

"The people who truly believe in some project tend to be by nature optimistic and ... that comes back to bite us sometimes," Squyers said in a telephone interview.

This latest decadal survey of planetary science missions included input from an independent contractor to make sure the budgets were in line with what NASA has projected -- to be, as Squyers put it, "brutally realistic."

NASA's proposed budget for fiscal 2011 is $18.7 billion, but Congress is still wrangling over it.

Human space flight, which accounted for about $3.2 billion of the space agency budget in 2010, was not considered in this review.

President Barack Obama followed earlier administrations in recognizing the end of the space shuttle era -- the final mission is due to occur later this year.

He also canceled his predecessor's Constellation moon program, angering some lawmakers and former astronauts who argued that it would make the United States a second or third-rate power in space.

Planetary science, largely accomplished with robotic probes and Earthly laboratories, costs less than putting people in space. Obama's budget request for fiscal 2012 includes about $1.48 billion for planetary science.

BRINGING BACK SAMPLES FROM MARS

Squyers, who was principal investigator on NASA's Mars Exploration Rover mission, said the choice to send space missions to Mars and Europa was based on strong consensus among the five panels.

NASA's top priority, according to the survey's recommendations, should be the Mars Astrobiology Explorer Cacher, or MAX-C, which could help determine whether Mars ever supported life and offer insight on its geologic and climate history. It would also be the first step in an effort to get samples from Mars back to Earth.

However, the report said this mission should only be undertaken if NASA's cost is about $2.5 billion, which is $1 billion less than independent estimates provided to the panel. The mission would be run jointly by NASA and the European Space Agency, according to the survey.

A mission to Europa and its subsurface ocean -- which might support life -- should be the second priority mission, the experts said. But its estimated price tag of $4.7 billion may make it too expensive without an increase in NASA's planetary science budget or a paring of the mission's costs.

The third top priority for large missions should be a $2.7 billion mission to Uranus.

The survey recommended medium-size missions as part of the New Frontiers program that explores the solar system, and smaller low-cost missions like those conducted by the agency's Discovery program.

The National Research Council is part of the U.S. National Academy of Sciences, which are private, non-profit institutions that offer science, technology and health policy advice under a congressional charter.

(Editing by Paul Simao)