SAN FRANCISCO — More than a year after a proposed flagship mission to Europa was scrapped due to its budget-busting $4.5 billion price tag, a clipper ship that would make 32 passes by the icy Jupiter moon to assess its suitability for life has emerged as scientists’ top choice for a pared-down expedition.

An in-depth study of Europa, which is believed to harbor an underground ocean, ranked just behind returning samples from Mars in the National Academy of Sciences’ priority list for major planetary science initiatives between 2013 and 2022. A study team at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, Calif., has been working since the so-called decadal survey’s release in March 2011 to design a Europa mission for $2 billion.

For now, NASA has no money and no authorization for starting new planetary science programs, though it has announced plans for a second Mars Science Laboratory-class rover, slated for launch in 2020, as part of its ongoing Mars exploration initiative.

“That’s very important,” John Grunsfeld, NASA associate administrator for science, said at a standing room-only town hall meeting at the American Geophysical Union conference here Dec. 4.

“This is not a new program. This is a continuation of our Mars exploration program and so we’re going to get started on it right away,” Grunsfeld said, adding that the mission fit within the five-year budget plan NASA submitted for the U.S. government’s 2013 fiscal year that began Oct. 1.

“To do a Europa mission requires a new start, and that’s something to work on over the next few years,” Grunsfeld said.

The Aerospace Corp., a Los Angeles-based federally funded research and development center, estimated a rover built with spare Mars Science Laboratory hardware and existing systems engineering and employees would cost about $1.5 billion, he said.

Unlike Curiosity, which landed on Mars on Aug. 6 to look for habitats that could have supported and preserved microbial life, the new rover may include a cache to store rock and soil samples for an eventual return to Earth.

The new Mars rover “may very well … be the tide that rises all ships,” David Senske, co-chairman of the JPL Europa mission study team, told SpaceNews. “They needed to make a decision, they made a decision and now we can focus on this.”

Although NASA currently is funded under a continuing resolution based on last year’s budget, 2013 appropriation bills proposed by both the House and Senate contain funds for work on a Europa mission, Senske said.

“The bills never got to conference, but both have Europa money in them. What the total dollar amount will be, I don’t know. But once we get over the fiscal cliff issues and into the next calendar year, much more discussion on this will be forthcoming, I believe,” Senske said.

Meanwhile, NASA is continuing with Europa mission studies and with developing technologies for instruments useful for any missions to the outer planets’ icy moons, Grunsfeld said.

The JPL study team, which presented its findings to NASA managers the week of Dec. 9, determined that by swapping pricy radioisotope power generators for a solar-powered system, engineers could add a high-resolution camera and other instruments to their top-ranked enhanced Europa Clipper mission, a 2.3-year, $2 billion mission featuring 32 flybys of the moon.

The probe’s travel time would depend on the launch vehicle used. A United Launch Alliance Atlas 5 rocket would need to send it on a Venus gravity-assist trajectory that would take six years to reach Jupiter’s orbit. NASA’s initial Space Launch System heavy-lift rocket, which is being designed to lift 70 metric tons, could send Europa Clipper on a more direct 2.8-year route with mass to spare.

The probe would be outfitted with ice-penetrating radar, which could spot pockets of water close to the crust; an infrared spectrometer to look for different types of terrain; a magnetometer to study the ocean’s thickness and salinity; a dedicated antenna for Doppler measurements of gravity fields and tides; and a topographical imager to map surface features.

The team also looked at two versions of an orbiter, which would do a more thorough job of assessing Europa’s ocean while circling the moon for about a month but would not address other science goals. In place of additional instruments, an orbiter would require much more shielding from radiation than the clipper ship, which would spend far less time overall in the harshest radiation regimes.

“Our conclusion was that if NASA wants to give us the $2 billion and wants an enhanced version of the mission, the clipper is the way to go,” Senske said.

The next step is a preliminary concept review, slated for April.

“After that, there would need to be additional funds to go forward,” Senske said.

In the meantime, the Juno probe NASA launched in August 2011 is en route to Jupiter and is expected to enter orbit around the gas giant in 2016. Thirty-three orbits are planned for the solar-powered spacecraft before it is deliberately crashed into Jupiter in October 2017.