One of the telescope’s large mirrors was hit by a micrometeoroid that was larger than expected and bigger than engineers were able to test on the ground, the space agency said.
Though assessments are still continuing and the telescope appears to be operating well enough, the collision did have a “marginally detectable effect in the data”, Nasa said in its announcement.
The object hit one of the pieces of mirror that allow the telescope to work between 23 and 25 May, the space agency said.
Nasa said that the telescope had been built to withstand such impacts, even if the piece of rock was larger than expected.
During the building process, researchers used a mix of simulations and actual impacts on pieces of mirror to see how the telescope would be able to withstand impacts with particles flying at very fast speeds across space, it said.
“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,” said Paul Geithner, Webb deputy project manager (technical) at Nasa’s Goddard Space Flight Center in Greenbelt, Maryland.
“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.”
The space telescope is also able to deal with any collisions by moving its mirrors around to correct any problems with specific segments. It has already been moved around to deal with the collision, and further fine adjustments are still required, Nasa added.
The telescope is also able to move around in space, so that engineers can move its more sensitive pieces out of the way in advance of known meteor showers. The particle that hit the telescope was not part of such a known shower, however, and pending further investigation, Nasa believes that it was unavoidable.
Over time, however, the telescope will inevitably be hit by more solid objects in space, degrading its performance. That process is expected, and Nasa simply hopes that the telescope’s performance is high enough that even with the degradation it will be able to prove useful for years to come.
“With Webb’s mirrors exposed to space, we expected that occasional micrometeoroid impacts would gradually degrade telescope performance over time,” said Lee Feinberg, Webb optical telescope element manager at Nasa Goddard.
“Since launch, we have had four smaller measurable micrometeoroid strikes that were consistent with expectations and this one more recently that is larger than our degradation predictions assumed. We will use this flight data to update our analysis of performance over time and also develop operational approaches to assure we maximise the imaging performance of Webb to the best extent possible for many years to come.”