Micrometeoroid strikes are an unavoidable part of operating a spacecraft. But after the James Webb Space Telescope (JWST) was hit with a larger than expected piece of space dust earlier this year, engineers are making changes to the way the telescope will be pointed in an attempt to avoid excess or larger impacts from space dust.
“We have experienced 14 measurable micrometeoroid hits on our primary mirror, and are averaging one to two per month, as anticipated,” said Mike Menzel, Webb lead mission systems engineer at NASA’s Goddard Space Flight Center, in a JWST blog post. “The resulting optical errors from all but one of these were well within what we had budgeted and expected when building the observatory. One of these was higher than our expectations and prelaunch models; however, even after this event our current optical performance is still twice as good as our requirements.”
Lee Feinberg, JWST optical telescope element manager at NASA Goddard, said earlier this year that the primary mirror segments have been engineered to withstand bombardment from dust-sized particles. And JWST’s mirrors exposed to space, it was expected that occasional micrometeoroid impacts would “gracefully degrade telescope performance over time.” But the damage from an impact that occurred sometime between May 23-25 was larger than any of the team’s predictions.
Therefore, a working group was convened to analyze the issue. The group consisted of optics and micrometeoroid experts from NASA Goddard‘s team, the telescope’s mirror manufacturer, the Space Telescope Science Institute, and the NASA Meteoroid Environment Office at NASA’s Marshall Space Flight Center. The goal was to ensure all parts of the observatory continue to perform at their best.
NASA said the team concluded the higher-energy impact observed in May was a rare statistical event both in terms of energy, and in hitting a particularly sensitive location on JWST’s primary mirror. To minimize future impacts of this magnitude, the team decided that future observations will be planned to face away from what is called the “micrometeoroid avoidance zone.”
The size of these micrometeoroids are small — they are small bits of dust at best, not rocks. But velocity plays a huge factor.
“Micrometeoroids that strike the mirror head on (moving opposite the direction the telescope is moving) have twice the relative velocity and four times the kinetic energy, so avoiding this direction when feasible will help extend the exquisite optical performance for decades,” Feinberg explained.
This new method of operating the telescope does not mean that these areas of the sky cannot be observed, NASA said, but only that observations of those objects will be more safely made at a different time in the year when JWST is in a different location in its orbit. Observations that are time critical, such as solar system targets, will still be done in the micrometeoroid avoidance zone if required. This adjustment to how observations are scheduled will have a long-term statistical benefit.
JWST’s 25-square-meter-primary mirror – as well as its tennis-court-sized sunshield — provides huge targets for micrometeoroid hits. As opposed to the Hubble Space Telescope’s mirror, JWST’s mirror is, always directly exposed to space, as HST’s is smaller and has a protective cover.
“Knowing this, we built Webb with large margins,” Feinberg said on Twitter earlier this year.
The mirror segments were engineered to withstand bombardment from the known micrometeoroid environment (other spacecraft have been in orbit at the second Lagrange point L2), with dust particles flying at extreme velocities. During the telescope’s construction, engineers used a mixture of simulations and actual test impacts on mirror samples to get data on how to fortify the observatory for operation in space.
Additionally, JWST has one thing other space telescopes don’t have: the ability to adjust the mirror segments to within microns of precision. The team said that with the capability to sense and adjust mirror positions enables partial correction for the result of impacts.
“By adjusting the position of the affected segment, engineers can cancel out a portion of the distortion,” wrote JWST media team member Thaddeus Cesari, earlier this year. “This minimizes the effect of any impact, although not all of the degradation can be cancelled out this way.”
Engineers have already performed adjustments for the C3 segment that was hit C3 with the largest piece of space dust. The corrections will be repeated when needed in response to future events as part of the monitoring and maintenance of the telescope throughout the mission.
But the team continues to reassure that JWST’s performance is still well above expectations, and the observatory is fully capable of performing the science it was designed to achieve.