WASHINGTON — A NASA decision last month to replace an instrument on the Europa Clipper mission with a less expensive, but less capable, alternative is leaving scientists concerned about the ability of the mission to meet some of its objectives.

NASA announced March 5 that it would end development of a magnetometer called the Interior Characterization of Europa Using Magnetometry (ICEMAG) for the mission, citing cost growth and risk of further overruns. At the time of the decision, NASA said the cost of the instrument had grown to $45.6 million, three times its original estimate.

In its place, NASA will fly a “facility magnetometer” that will collect some of the same magnetic field data as ICEMAG in the vicinity of Europa, an icy moon of Jupiter. The agency subsequently said Margaret Kivelson, a planetary scientist at UCLA who also is the new chair of the Space Studies Board, will lead the development of the magnetometer.

The facility magnetometer will lack components known as scalar vector helium sensors, whose development problems led to ICEMAG’s cost overruns. Robert Pappalardo, project scientist for the mission, said in an April 23 presentation at an Outer Planets Assessment Group (OPAG) meeting here that challenges with the sensors’ fiber optic cables, which are sensitive to the temperature and radiation conditions at Jupiter, “essentially brought on its downfall.”

Instead, the facility magnetometer will rely on more conventional fluxgate magnetometers. Those devices are very precise, Pappalardo noted, but suffer offset errors over time. He estimated the original ICEMAG scalar vector helium sensors would have provided an order of magnitude improved accuracy.

The increased errors of the fluxgate magnetometers, he said, “does put at risk” some of the key, or Level 1, science requirements of the mission, notably estimating the thickness of Europa’s ice shell as well as the depth and salinity of the liquid water ocean beneath the ice. Other Level 1 science requirements aren’t affected, he emphasized.

Pappalardo showed models of the difference between potential ICEMAG and facility magnetometer measurements. With the better performance of ICEMAG, he estimated the ocean depth could be known to an accuracy of 20 kilometers. With the facility magnetometer, “ocean depth would be very poorly known: somewhere between 20 to 100 kilometers,” he said of one particular case. “That’s not a lot of information over what we have now.”

The performance of the magnetometer will also be dependent on the conductivity of that ocean. “In a high-conductivity case, we won’t know much,” he said, because of much higher errors. “We’re going to have to hope that Europa cooperates or that we can beat down this error to closer to the ICEMAG requirement.”

Pappalardo said the magnetometer team is looking for ways to improve its accuracy. That could include periodic rolls of the spacecraft to measure the ambient magnetic field and better calibrate the magnetometer.

Alternative launch plans

Overall, though, the Europa Clipper mission is moving ahead largely as planned. Lori Glaze, director of NASA’s planetary science division, said at the OPAG meeting that the mission is on track for a confirmation review this summer, at which point the agency will formally set a cost estimate.

Europa Clipper is now aiming for a launch in 2023, one year later than prior plans, a decision that project officials said was driven by workforce availability for the mission. Both NASA and Congress now support a 2023 launch date after years of debate where NASA sought to launch the mission later in the 2020s, if at all.

“I think this is one of the big wins from the president’s 2020 budget,” Glaze said. “We’re finally in sync between the president’s budget and Congress and we’re all in line with that advanced launch date of 2023.”

There’s still disagreement, though, on how to launch Europa Clipper. The fiscal year 2019 appropriations bill, as in previous years, mandated the use of the Space Launch System for the mission. SLS would allow the spacecraft to fly directly to Jupiter without the need for any gravity assists, arriving less than two and a half years after launch.

NASA’s fiscal year 2020 budget proposal, though, calls for using a commercially procured launch vehicle. That would require use gravity assist maneuvers and increase the travel time of the mission by several years, but NASA argued in its budget request that it would save “over $700 million” versus SLS.

The project has been looking at a number of options for the non-SLS option. Speaking at a National Academies committee meeting in March, Barry Goldstein, Europa Clipper project manager, said one option under consideration would be a launch on a SpaceX Falcon Heavy equipped with a Star 48BV kick stage. That trajectory, known formally as Delta-V Earth Gravity Assist 3-Minus, involves a launch in November 2023 and an Earth flyby in October 2025 prior to arrival at Jupiter in September 2029.

The travel time of a little less than six years is only slightly shorter than some other alternatives previously studied. However, it has the advantage of not requiring any gravity assist flybys of Venus, with the spacecraft getting only slightly closer to the sun on its trajectory than the Earth. “That solves a world of problems on thermal management,” Goldstein said. “We no longer have the challenge of the thermal problems that we had getting close to Venus.”

A second advantage, he said, is that it offers a backup launch window roughly a year later, whereas with the Venus flyby trajectory the mission would have to wait until 2025 if it can’t launch in 2023. “We’re not 100 percent there yet, but things are looking very positive” for the new trajectory, he said.

Lander uncertainty

The outlook is less positive for a follow-on lander mission to Europa. The 2019 appropriations bill included $195 million to work on the lander, directing NASA to launch it by 2025.

The fiscal year 2020 budget request, though, included no funding for the mission, just as had been the case in previous years. The budget proposal noted that the agency estimated a lander mission to cost as much as $5 billion, and that a midterm review published last year of the most recent planetary science decadal survey recommended that a lander mission “be assessed in the context of other planetary priorities in the next decadal survey.”

In previous years, Congress has added funding to the budget for a Europa lander. However, the mission’s most influential proponent, Rep. John Culberson (R-Texas), who had been chairman of the House appropriations subcommittee that funds NASA, lost reelection last November.

At the OPAG meeting, NASA’s Glaze said that the agency will use the funding appropriated for work on a Europa lander in 2019 to support key technology development that could support such a mission in the future.

“I can assure you that the funding that we do have is being directed towards lots of early technology development and risk mitigation to prepare for a future Europa lander,” she said. “What I always say is, it’s not if, it’s when.”