damage to the James Webb Telescope’s primary mirror from a micrometeorite strike in May is worse than first thought, according to new images revealed in a new report.
A paper published Tuesday on the academic preprint server arxiv.org detailing Webb’s performance during the commissioning of the telescope revealed that most of the micrometeorite strikes on Webb’s big mirror resulted in negligible damage, but a strike that occured in mid-May even left the telescope with permanent damage.
“The single micrometeorite impact that occurred between 22 — 24 May 2022 UT exceeded prelaunch expectations of damage for a single micrometeoroid triggering further investigation and modeling by the JWST Project,” the report read.
Unlike the Hubble Space Telescope, which encloses the primary mirror the telescope uses to collect light and focus light on scientific instruments in a cylindrical housing, Webb’s 6.5-metre diameter segmented mirror is exposed to space. But given Webb’s orbit around Lagrangian point 2, or L2, a region of space about 1 million miles from Earth, scientists only expected Webb to encounter potentially hazardous micrometeorites about once per month.
During the commissioning period from late January into June, as ground controllers calibrated, aligned, and tested Webb’s mirrors and instruments, the primary mirror sustained six total micrometeorite strikes.
Of those strikes, five did little damage, causing less than 1 nanometer of wavefront error root mean square (RMS), a technical way to describe how much Webb’s mirror distorts the starlight the mirror collects. Most of the distortion added by those five strikes can be corrected out of the mirror, since the 18 hexagonal segments that make up its face can be individually and finely adjusted.
But the sixth strike, which impacted a mirror segment labeled C3, did more damage that can be fully corrected for. That micrometeorite strike raised the wavefront error of the segment from 56 nanometers to 178 nanometers after correction by adjusting the segment.
Because every mirror segment is adjustable, however, the damage to the C3 segment could be compensated for and did not compromise the resolution of Webb’s primary mirror as a whole, according to the report. The total wavefront error for the entire mirror increased by around 9 nanometers due to the strike.
“It is not yet clear whether the May 2022 hit to segment C3 was a rare event (ie an unlucky early strike by a high kinetic energy micrometeoroid that statistically might occur only once in several years),” the report read, “or whether the telescope may be more susceptible to damage by micrometeoroids than pre-launch modeling predicted.”
The report goes on to note the Webb project team is considering actions to mitigate future micrometeorite strikes, such as limiting how long the telescope can be pointed in directions known to expose the mirror to a higher probability of micrometeorite strikes.
Preserving the long-term health of the Webb telescope is a high priority for Nasa and astronomers everywhere.
After more than 20 years and $10 billion spent in development, the space telescope was launched atop an Ariane 5 rocket on Christmas Day. That launch was more precise than expected, saving Webb considerable propellant it would have used to correct its course after launch, and nearly doubling the observatory’s projected operational lifespan — so long as space rocks don’t spoil its optics.
“Before launch, JWST was required to carry propellant for at least 10.5 years of mission lifetime,” the report read. “Now that JWST is in orbit around L2, it is clear that the remaining propellant will last for more than 20 years of mission lifetime.”