The micrometeoroid that hit the James Webb Space Telescope in May caused significantly more damage than expected and will have a lasting impact on the telescope’s observations, according to a NASA report on the spacecraft’s performance. By contrast, other micrometeoroid impacts during the spacecraft’s first six months of operation have had a negligible effect.
The report contains an image showing the damage to one hexagonal segment of the observatory’s main mirror, called C3. “The single micrometeorite impact that occurred between 22—24 May 2022 exceeded prelaunch expectations of damage for a single micrometeoroid,” says the NASA report.
The performance of the main mirror is determined by how much it deforms incoming starlight and measured by a quantity called wavefront error rms (root mean square). At the beginning the mission, the C3 segment had a wavefront error of 56 nanometers rms, a level similar to the main mirror’s other 17 segments. The impact increased C3’s wavefront error to 258 nm rms.
Spacecraft engineers can change the position and curvature of each segment and in this way were able to reduce the error to 178 nm rms. This has a measurable effect on the error of the main mirror as a whole. “However, the effect was small at the full telescope level because only a small portion of the telescope area was affected,” says the report.
The JWST team say the impact increased the error associated with entire main mirror to about 59 nm rms. “About 5-10 nm rms above the previous best wavefront error rms values.” That’s well within the performance limits the team were hoping for.
Nevertheless, the impact raises questions about the nature of the space environment where the JWST operates. This is a point in space about a million kilometers from Earth where the gravitational fields of the Sun, Moon and Earth are in balance and so provide a relatively stable location.