WASHINGTON — The head of NASA’s science directorate has requested modifications to the design of its next flagship astrophysics mission based on the recommendations of an independent review.

The proposed changes to the Wide Field Infrared Survey Telescope (WFIRST) mission are intended to reduce the spacecraft’s projected cost by at least $400 million and address issues about the technical maturity and risk of some elements of the space observatory while the mission is still in its earliest phases of development.

In an Oct. 19 memo to Chris Scolese, director of the Goddard Space Flight Center, which hosts the WFIRST program, NASA Associate Administrator Thomas Zurbuchen said he was acting to make changes to the WFIRST program based on recommendations from the WFIRST Independent External Technical/Management/Cost Review, or WIETR. NASA carried out that review this summer based on a recommendation from a 2016 report by the National Academies, which worried that potential cost growth in WFIRST could affect the balance of other astrophysics missions funded by the agency.

“This report is as thorough and thoughtful as we hoped,” Zurbuchen said in a statement. “We are taking the report’s findings and recommendations very seriously as we think about the future of this exciting mission.”

In the memo, Zurbuchen directed Scolese to make a number of changes to the design of WFIRST, including cost reductions to both its main “widefield” instrument as well as a separate coronagraph instrument. Those reductions are intended to bring the mission’s estimated total cost down from its latest estimate of $3.6 billion to an earlier target of $3.2 billion.

“I am directing the Goddard Space Flight Center to study modifying the current WFIRST design, the design that was reviewed by the WIETR, to reduce cost and complexity sufficient to have a cost estimate consistent with the $3.2B cost target set at the beginning of Phase A,” Zurbuchen wrote in the memo.

In addition to seeking cost reductions from the two instruments, Zurbuchen said that the coronagraph instrument should be treated as a “technology demonstration” instrument. Zurbuchen noted in the memo that the WIETR report concluded incorporating that advanced instrument, designed to block light from individual stars to detect planets and debris disks orbiting them, “has been one of the mission system design and programmatic drivers” that “is certain to present risks to the primary mission” as development continues.

The independent review also raised questions about the risk classification of the mission. WFIRST is considered a “Class B” risk mission by NASA, which means it is high priority but only medium to high cost and with a medium mission lifetime. That is less stringent than the Class A assignments usually given to “strategically important missions with comparable levels of investment and risks,” Zurbuchen wrote.

The review, the memo noted, suggested NASA add more engineering development and spare hardware, as well as additional analysis, “to provide a more robust program” than its existing Class B risk classification. It also called for a “top-to-bottom cost-benefit assessment to balance scope, complexity, and the available resources” for WFIRST.

Zurbuchen, in his memo, requested that the review be completed in time for a system requirements review and mission design review scheduled for February 2018, which will support a review known as Key Decision Point B in March or April 2018. An independent cost review will also be carried out by this review to confirm that the revised WFIRST design fits into the $3.2 billion cost estimate.

That revised design will continue to make use of a 2.4-meter telescope, one of two obtained by NASA from the National Reconnaissance Office in 2012. That telescope, much larger than the 1.3-meter telescope originally proposed for WFIRST when astronomers identified the mission as its top priority flagship mission in the 2010 decadal survey, was intended to reduce costs while improving the scientific performance of the mission.

Zurbuchen, though, left open the possibility of revisiting that decision should this latest redesign fall short. “If the result of this study is the conclusion that WIFRST cannot be developed using the current 2.4m telescope architecture within the $3.2B cost target,” he wrote, “I will direct a follow-on study of a WFIRST mission consistent with the architecture described by the Decadal Survey.”

NASA, while releasing the Zurbuchen memo responding to the WIETR report, did not release the report itself. Zurbuchen and the two co-chairs of the WIETR panel, Orlando Figueroa and Peter Michelson, are scheduled to discuss the report at an Oct. 25 meeting of the Space Studies Board’s Committee on Astronomy and Astrophysics in Irvine, California.