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After 14 agonizing years of delays and surviving the white-knuckle tension of launch and deployment, the world's most powerful space telescope faces only one remaining threat: space rocks. NASA has announced the JWST's primary mirror was hit by a micrometeorite in May. It's the fifth impact large enough to be measurable since the telescope was launched, but the first larger than what had been allowed for in planning. Fortunately, however, the impact was still small enough to have only a minor effect on image quality.
“Micrometeoroid strikes are an unavoidable aspect of operating any spacecraft, which routinely sustain many impacts over the course of long and productive science missions in space,” NASA noted in a statement. The larger the mirror, the more impacts will occur, so the JWST's designers were fully aware of the threat. Tolerance for very small impacts was engineered into the telescope's design, for example by adjusting the position of underperforming segments. Beyond a certain size impact, however, the damage will affect image quality, even if only slightly.
It would have been nice if the first such impact didn't occur until after scientific work began so that the first images at least were truly pristine, but it was not to be. Between May 23 and May 25, NASA reports, JWST's C3 primary mirror segment sustained impact damage beyond the tolerance threshold.
Subsequent testing showed, however, “The telescope is still performing at a level that exceeds all mission requirements despite a marginally detectable effect in the data,” in NASA's words.
Consequently, we are still on track to all be dazzled by the first scientific images on July 12, even if they will be slightly less spectacular than might have been.
“We always knew that Webb would have to weather the space environment, which includes harsh ultraviolet light and charged particles from the Sun, cosmic rays from exotic sources in the galaxy, and occasional strikes by micrometeoroids within our solar system,” Paul Geithner of Goddard Space Flight Center said. “We designed and built Webb with performance margin – optical, thermal, electrical, mechanical – to ensure it can perform its ambitious science mission even after many years in space.”
Having beaten expectations on how clean the JWST's optics could be kept on the ground, this impact will still leave the telescope's overall performance above expectations, NASA claims.
The Internet is not entirely convinced, however.
The risk of impacts is reduced by turning the JWST's mirrors away from incoming meteor showers, at the cost of using its fuel up more quickly, but little can be done about sporadic micrometeorites like this one.
Ever keen to find a lining as silver as a telescope mirror's backing, NASA notes the primary's large area and close appraisal make it “A highly sensitive detector of micrometeorites”. Tracking impacts will improve our knowledge of their frequency and size profile.
