WASHINGTON — The telescope for a major NASA astrophysics mission has passed a key review that the agency says keeps it on track for launch in the mid-2020s despite uncertainty about its funding for 2020 and beyond.

NASA announced Aug. 28 that the telescope assembly for the Wide Field Infrared Survey Telescope (WFIRST) mission passed its preliminary design review (PDR), allowing the project to continue work to finalize its design.

WFIRST, scheduled for launch in the mid-2020s, is NASA’s next flagship astrophysics mission after the James Webb Space Telescope. The mission will use a 2.4-meter mirror provided to NASA by the National Reconnaissance Office. That mirror is currently being modified by L3 Harris Technologies for use on WFIRST.

The telescope will later be equipped with both a wide-field imager instrument and a coronagraph, which blocks light from stars to allow direct imaging of planets and dust disks orbiting them. Both will have their own preliminary design reviews, along with one for the entire mission.

“The science enabled by our telescope is extraordinary,” Jeff Kruk, project scientist for WFIRST, said in a statement. “We are asking, ‘what is the fate of the universe?’ by looking at how the expansion of the universe is accelerating, and we are asking, ‘are we alone?’ by looking for exoplanets in neighboring planetary systems.”

Despite its compelling scientific rationale, which made it the top-ranked large mission in the 2010 astrophysics decadal survey, WFIRST has faced problems in its early development, including threats of cancellation. In 2017, NASA directed changes to the mission’s design to reduce its overall cost to no more than $3.2 billion, including converting the coronagraph from a full-fledged science instrument to a technology demonstration.

NASA’s fiscal year 2019 budget proposal, released in February 2018, sought to cancel WFIRST, citing the mission’s cost and “competing priorities” at the agency. Congress, though, rejected that proposal, providing $312 million for the mission in the final fiscal year 2019 omnibus spending bill.

NASA’s fiscal year 2020 budget proposal again proposed cancelling WFIRST. The House version of a commerce, justice and science appropriations bill also rejected that proposal, including $510.7 million for WFIRST. The Senate has yet to release its version of a spending bill, and the agency will likely start the 2020 fiscal year in one month on a continuing resolution that continues spending at 2019 levels.

Despite the fiscal uncertainty, NASA has kept up work on WFIRST without interruption. “The work continues because we have been funded,” Paul Hertz, director of NASA’s astrophysics division, said at a town hall meeting during an American Astronomical Society conference in June. “The team has been directed to stay on plan, stay on schedule, stay on budget, and let the politics of whether it’s funded in [fiscal year] ’20 take care of itself.”

Hertz said at that meeting that the WFIRST mission PDR will likely take place in October. That will be followed by an independent cost and schedule review required for a confirmation review, which he estimated would take in late 2019 or early 2020.