WASHINGTON — A NASA mission to a potentially habitable moon of Jupiter has cleared a major review despite uncertainty about when, or how, it will launch.

NASA announced Aug. 19 that it had formally confirmed the Europa Clipper mission to proceed into its next phase of development, known as Phase C. That will cover final design of the spacecraft, followed by assembly and testing.

“We are all excited about the decision that moves the Europa Clipper mission one key step closer to unlocking the mysteries of this ocean world,” Thomas Zurbuchen, NASA associate administrator for science, said in a statement announcing the milestone.

Europa Clipper will enter orbit around Jupiter and make dozens of close approaches to Europa, one of the planet’s largest moons. Europa has an icy surface, below which most scientists believe is a deep ocean of liquid water. Combined with the interior heat source that keeps the ocean from freezing, and the presence of organic compounds, Europa has the basic requirements to support life.

While the programmatic milestone Europa Clipper achieved, known in NASA parlance as Key Decision Point C, is the point where NASA sets the schedule and budget for the mission, exactly when Europa Clipper will launch is not yet clear. In the statement announcing the mission’s confirmation, NASA noted that current plans have the spacecraft ready for launch as soon as 2023. However, the mission has a formal launch readiness date of 2025.

That uncertainty is linked to how the mission will be launched. The mission’s preferred launch option is the Space Launch System, which will allow the spacecraft to travel directly to Jupiter without the need of gravity assists, arriving within three years of launch. Language in appropriations bills for fiscal year 2019 and prior years also directed NASA to use the SLS.

However, in its fiscal year 2020 budget request, NASA proposed using a commercial launch vehicle, such as a Delta 4 Heavy or Falcon Heavy, to launch Europa Clipper, saying that doing so would save several hundred million dollars even though the transit time to Jupiter would be about seven years. A report by NASA’s Office of Inspector General in May downplayed the cost savings by using an alternative launch vehicle, but noted it is not possible to launch Europa Clipper on an SLS in 2023 since there won’t be an available SLS for that mission then.

The announcement didn’t include a cost estimate for the mission, but agency spokesperson Alana Johnson said Aug. 21 that the agency baseline commitment for the mission is $4.25 billion, covering all costs for the entire mission. The NASA Office of Inspector General report cited an assessment last October by the mission’s standing review board that estimated the mission’s cost at between $3.5 billion and $4 billion.

Despite the uncertainty about launch vehicle and schedule, other aspects of the mission are going well. “Things are looking reasonably good,” Robert Pappalardo, project scientist for Europa Clipper at the Jet Propulsion Laboratory, said at an Aug. 21 meeting of the Outer Planets Assessment Group (OPAG) in Boulder, Colorado.

Earlier in the year, NASA decided to replace a planned magnetometer instrument on the spacecraft that had suffered extensive cost overruns with a less complex “facility” magnetometer. That had raised concerns about whether the new magnetometer would have the precision needed to achieve science goals such as measuring the depth of Europa’s subsurface ocean. Pappalardo and others at the OPAG meeting said that changes to the instrument’s operation, notable periodic rolls of the spacecraft to calibrate it, should recover sufficient precision to achieve those science goals.

Jeff Foust writes about space policy, commercial space, and related topics for SpaceNews. He earned a Ph.D. in planetary sciences from the Massachusetts Institute of Technology and a bachelor’s degree with honors in geophysics and planetary science...