There are been recent news about a larger-than-expected micrometeoroid impact on the James Webb Space Telescope's optics. Apparently this was one of several impacts that have already been observed.

The question is: how are these measured and analysed? Do we know exactly when these events occur because they are logged on accelerometers or strain gauges or gyros or other 'microphones'? Does star-tracking (FGCS) respond quickly enough? Or is the impact only known about later when a dark- or light-field 'selfie' of the primary mirrors is taken?

JWST dark- and light-field image + splat

Do enough instruments respond with sufficient accuracy so we can figure out exactly the momentum and energy (or speed, direction, and mass) of the particle and where/what it hit? Can we tell if the particle traversed the heat shields or other structures before it hit the mirror?

The blog posting also says "By adjusting the position of the affected segment, engineers can cancel out a portion of the distortion". That certainly can't be true for a point-defect on the primary mirror. Do these impacts lead to global distortions to a mirror segment that can be corrected with the seven actuators?

There are several nice youtube videos posted (e.g. Becky, Anton) but they really don't address this question. I do see there is a related question already posted but it really asks about the effects of the impact in degrading observations rather than the detection and analysis of the events.

I see now that the NASA blog says "Between May 23 and 25, NASA’s James Webb Space Telescope sustained an impact to one of its primary mirror segments". The fact that they know where but not when suggests that the information comes only from a routine selfie of the mirror. That is probably the most sensitive measurement.

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    $\begingroup$ It would be much better to ask this question at the Space Exploration StackExchange. How spacecraft operate is off-topic here. $\endgroup$ Jun 12, 2022 at 20:34
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    $\begingroup$ @DavidHammen once one reads the question one sees it goes on to ask about the telescope's optics, and not just "how spacecraft operate". I've added "optics" in the title to reflect the nature of the question's text but perhaps this can be divided into two questions, one for each site. The bit about accelerometers and detection may get better answers in Space SE, but how the space telescope's optics are actuated and used to compensate "normal wear and tear" to achieve the best performance is fine in either site and could certainly be left here. $\endgroup$
    – uhoh
    Jun 12, 2022 at 21:44
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    $\begingroup$ Too bad we don't have the Optics and Photonics SE site yet. $\endgroup$
    – uhoh
    Jun 12, 2022 at 21:49
  • $\begingroup$ Well then, :-) , does "Astronomy" qualify as one kind of "Space exploration" ? Or would we be better off rewriting the charter of both groups so the delimiter is more explicit? (and would anyone actually read the charters [very sad face] $\endgroup$ Jun 13, 2022 at 13:59

1 Answer 1


It has become clearer that these impacts are detected and quantified through the NIRCam wavefront analysis ('mirror selfie') that is conducted routinely. This is detailed in this July 12th report (Fig 3 shown below) and further discussed in Discover magazine. There is no mention of these impacts being detected by any other means.

NIRCam wavefront analysis

The damage is on segment C3, just behind the shadow of the strut at the lower right. The impact was unexpectedly large:

"Pre-launch projections ... predicted that on average each segment would receive a cumulative total of 16 nm added [waveform error] over six years. The May impact resulted in one segment receiving more than 10 times that average in a single event."

The waveform error on segment C3 was initially 56 nm rms and increased to 258 nm after the impact and then reduced to 178 nm rms after tweaking by the segment's shape and position with the hexapod actuator. For reference, this error is much larger than lambda/2pi for the shortest wavelength of 600 nm making that particular segment not very helpful at that wavelength. However, this error is greatly mitigated by the fact that this is just one of 18 segments and most of the interest is at much longer wavelengths.


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