Coinciding with JWST's stunning first images, its operating team has published a report on its commissioning. Most things are going spectacularly well and repeatedly exceeding expectations, as the image's quality makes clear – nevertheless, such near-perfection has a time limit. The report notes; “The mirrors and sunshield are expected to slowly degrade from micrometeoroid impacts; the detectors are expected to experience cumulative slow damage from charged particles; the sunshield and multilayer insulation will degrade from space weathering.”
The report also raises the risk of more sudden failures, pointing out that “The science instruments include many moving parts at cryogenic temperatures.”
These threats were all known long before launch and incorporated into the expectation the telescope would still be performing after five years of operation, probably considerably longer. So far, JWST is performing above requirements on all measures, and above prelaunch expectations on most, raising hopes it should continue to operate even longer than planned.
However, the one fly in the ointment can be seen in the photograph below.
The bright spot at the lower right represents the previously reported “larger than anticipated” micrometeoroid impact in May.
Being hit by flying specks of dust is an occupational hazard for any space instrument, and JWST's operators certainly planned for such events. The report acknowledges the impact that occurred between May 22 and 24 “exceeded prelaunch expectations” based on our knowledge of micrometeoroid populations.
Indeed, JWST's engineers anticipated each of the 18 mirror segments would suffer 16 nanometers of cumulative damage over a six-year operating period. This strike alone caused ten times that to the C3 segment – the report states "The impact raised the wavefront error of segment C3 from 56 to 280 nm rms. Mirror commanding to adjust segment position and curvature reduced this error to 178 nm rms."
.For some functions, this has made effectively no difference – but for others, it's causing noticeable performance degradation.
Five other strikes were detected at the time the report was written last week, at a rate similar to expectations. Aside from the late May impact, all were of anticipated size.
“Some of the resulting wavefront degradation is correctable through regular wavefront control; some of it comprises high spatial frequency terms that cannot be corrected,” the report notes. There have also been even smaller impacts, too minor to be detected using wavefront sensing – but whose damage can build up with time.
Even the C3 impact didn't prevent the staggering quality of JWST's first images. The question remains whether it was simply bad luck that an unusually large impact occurred while the telescope was still being commissioned, or if the frequency of impacts this large was underestimated. The telescope's operators are investigating, although probably only time will give a definitive answer. Meanwhile, the value of minimizing time spent looking in the direction of orbit, which is accompanied by more (and more damaging) micrometeorite strikes, is being explored.
None of this should take away from the report's key conclusion; “JWST is fully capable of achieving the discoveries for which it was built," the report notes. "The telescope and instrument suite have demonstrated the sensitivity, stability, image quality, and spectral range that are necessary to transform our understanding of the cosmos through observations spanning from near-earth asteroids to the most distant galaxies. ”