The root cause of a problem affecting imaging sensors inside three of the James Webb Space Telescope’s (JWST) primary instruments continues to elude NASA, but under the worst-case scenario the U.S. space agency expects to spend about $30 million to remanufacture the faulty parts, the program’s director said March 3.

During testing in December, NASA discovered the performance of multiple detector arrays had degraded since they were tested two years ago and put in storage. The arrays, built by Teledyne Imaging Sensors of Camarillo, Calif., are planned to fly on JWST’s Near Infrared Camera, Near Infrared Spectrograph and Fine Guidance Sensor-Tunable Filter Imager. Teledyne spokeswoman Robyn McGowan declined to comment on the issue.

NASA in February established a Failure Review Board to investigate the problem and issue a report in April. If it is determined that a full set of new flight and backup detectors is needed, it would cost about $30 million and take about a year for delivery for integration with the three instruments, JWST Program Director Rick Howard told the NASA Advisory Council during a meeting here.

“Right now we still don’t have a root cause,” Howard said.

During testing of five detector arrays two years ago, about 0.5 percent of the pixels were found to be out of specification. When one of these arrays was tested in December, the number — to the dismay of scientists — had grown to about 2 percent. Subsequent testing revealed three of the other four arrays had experienced a similar degree of unexpected pixel degradation. Astronomers fear the problem — if not corrected — could grow worse with time and compromise the quality of JWST’s imagery.

NASA is in the process of establishing new cost and schedule estimates for the JWST program. The spacecraft most recently had a $5 billion price tag and was scheduled for launch in June 2014. An independent review last year estimated the program is 15 months behind schedule and would cost another $1.5 billion. Howard said NASA will submit a revised budget and schedule for JWST to the White House this fall as it prepares a 2013 spending proposal that will be sent to Congress in February.