WASHINGTON — NASA is continuing to examine various, potentially less expensive options for a mission to land on Jupiter’s moon Europa even after completing a recent review, postponing a call for instruments for the spacecraft.

At a meeting of the Outer Planets Assessment Group (OPAG) Sept. 6 in La Jolla, California, Curt Niebur, a program scientist in the planetary science division at NASA Headquarters, said mission planners are continuing to examine several factors, including mission cost and science return, as they evaluate the design of the mission.

The lander mission, he said, successfully passed an early-stage review called a mission concept review in June. However, he said the agency had not settled on a specific, single concept for the mission.

“As a result of that mission concept review, what we want to do is essentially continue exploring the different options we have for a Europa lander mission,” he said. “We want to continue balancing the trade amongst risk, cost and science return.”

Concepts for the lander presented earlier this year call for a spacecraft launched on a Space Launch System rocket no earlier than late 2025, arriving at Jupiter in mid-2030 and landing on Europa in late 2031. Most of the spacecraft’s estimated mass of 16.6 metric tons is propellant needed to first enter orbit around Jupiter and, later, landing on Europa itself.

A report released in February by a NASA-chartered science definition team (SDT) identified the science goals of the lander, as well as a notional suite of five instruments that could be used to achieve those goals and fit within a payload accommodation on the lander of 42.5 kilograms. The battery-powered spacecraft would operate for about 20 days after landing.

“The SDT report is the starting point,” Niebur said later in the meeting. “But accomplishing everything in the SDT report is a very expensive mission. What we’d like to see are some less expensive options.”

That could, he acknowledged, mean looking at missions that perform less science but at a lower cost. “I firmly believe that you can’t make substantial cost reductions and maintain the full science return of this mission,” he said. “If you really want to see a more streamlined mission concept, you’re going to have to be willing to give up some science. So, yes, science is on the table.”

Kevin Hand, a JPL scientist who was one of the leaders of the SDT, said that could mean changes to both the instrument complement on the lander as well as its operations. “There are the primary things to consider — instruments, et cetera — and there are the secondary items that are influenced by science,” he said.

One option, Hand said, might be to reduce the number of samples the lander collects from Europa’s icy surface for analysis. “Samples take energy, they take time,” he said.

While cost estimates, both done internally by NASA and externally by the Aerospace Corporation, are being developed, Niebur and others at the OPAG meeting did not disclose what ranges those estimates span. Many in the planetary science community, though, estimate that a full-fledged lander mission may cost several billion dollars.

Because that analysis of potential lander options is ongoing, NASA is holding off issuing an announcement of opportunity (AO) for instruments that would go on the lander. “Until we finish that exploration, it’s premature to release an AO,” Niebur said.

He suggested that no decisions were imminent on an alternative design for the lander. “We are reconsidering the trade space. That’s not something you do over the span of a week or two,” he said.

A further complication for a Europa lander mission is funding. The administration requested no funding for the mission in its fiscal year 2018 budget request. While the Senate version of an appropriations bill that funds NASA is silent on the mission, the House version included $495 million for both the lander mission and the Europa Clipper multiple-flyby mission that is further along in development.

“We’ll wait to see what happens with the FY18 budget to come up with a longer-term plan beyond the next month or so for how we’ll proceed with this potential mission,” Niebur said.

Progress is going well on Europa Clipper, Niebur and others said at the OPAG meeting. The mission passed a major project milestone called Key Decision Point B in February, allowing it to enter a preliminary design phase. Robert Pappalardo, project scientist for the mission, said at the OPAG meeting that the mission is on schedule to complete a series of preliminary design reviews by next August.

One issue with Europa Clipper that Niebur raised is the growth in resources in the spacecraft’s instrument payload. “It wasn’t so much that the resources grew, but it was the amount that they grew,” he said. The mission, he said, has put into place a “resource monitoring plan” to track that growth, and understand what instruments are having issues as early as possible.

Power demands from those instruments, though, have led to a design change in the spacecraft. The spacecraft’s two solar arrays now consist of four and a half panels each, up from four panels from earlier designs. “We needed to increase the total size of the arrays slightly in order to accommodate the energy demands of the payload,” Pappalardo said.