The May micrometeoroid impact on Webb’s mirror
In a detailed report [pdf] of Webb’s overall excellent operational status following its in-space commissioning, the science team also included an analysis of the May 2022 micrometeoriod impact on one segment of Webb’s mirror.
The image to the right, taken from figure 3 of the report, shows the remaining alignment error of Webb’s entire mirror, after alignment. Except for that one bright spot in the segment to the lower right, all of the segments show excellent alignment, well within the range predicted before launch. The bright spot however is from the impact, and suggests that one mirror segment is significantly damaged. From the report:
The micrometeoroid which hit segment C3 in the period 22—24 May 2022 UT caused significant uncorrectable change in the overall figure of that segment. However, the effect was small at the full telescope level because only a small portion of the telescope area was affected. After two subsequent realignment steps, the telescope was aligned to a minimum of 59 nm rms, which is about 5-10 nm rms above the previous best wavefront error rms values 7 . It should be noted that the drifts and stability levels of the telescope mean that science observations will typically see telescope contribution between 60 nm rms (minimum) and 80 nm rms (where WF control will typically be performed). Further, the telescope WFE combines with the science instrument WFE to yield total observatory levels in the range 70-130 nm (see Table 2), so the slight increase to telescope WFE from this strike has a relatively smaller effect on total observatory WFE.
In plain English, the impact while damaging has not seriously reduced the telescope’s predicted capabilities.
However, to be hit with this size impact so soon after launch is very worrisome, especially because Webb’s mirror is not housed in any protective tube like Hubble or most telescopes. From the report:
It is not yet clear whether the May 2022 hit to segment C3 was a rare event (i.e. an unlucky early strike by a high kinetic energy micrometeoroid that statistically might occur only once in several years), or whether the telescope may be more susceptible to damage by micrometeoroids than pre-launch modeling predicted.
The science team is presently trying to anticipate what might happen if the impact rate turns out to be much higher than expected, and what can be done to mitigate the degradation of the mirror should more impacts occur.
On Christmas Eve 1968 three Americans became the first humans to visit another world. What they did to celebrate was unexpected and profound, and will be remembered throughout all human history. Genesis: the Story of Apollo 8, Robert Zimmerman's classic history of humanity's first journey to another world, tells that story, and it is now available as both an ebook and an audiobook, both with a foreword by Valerie Anders and a new introduction by Robert Zimmerman.
The ebook is available everywhere for $5.99 (before discount) at amazon, or direct from my ebook publisher, ebookit. If you buy it from ebookit you don't support the big tech companies and the author gets a bigger cut much sooner.
The audiobook is also available at all these vendors, and is also free with a 30-day trial membership to Audible.
"Not simply about one mission, [Genesis] is also the history of America's quest for the moon... Zimmerman has done a masterful job of tying disparate events together into a solid account of one of America's greatest human triumphs."--San Antonio Express-News
In a detailed report [pdf] of Webb’s overall excellent operational status following its in-space commissioning, the science team also included an analysis of the May 2022 micrometeoriod impact on one segment of Webb’s mirror.
The image to the right, taken from figure 3 of the report, shows the remaining alignment error of Webb’s entire mirror, after alignment. Except for that one bright spot in the segment to the lower right, all of the segments show excellent alignment, well within the range predicted before launch. The bright spot however is from the impact, and suggests that one mirror segment is significantly damaged. From the report:
The micrometeoroid which hit segment C3 in the period 22—24 May 2022 UT caused significant uncorrectable change in the overall figure of that segment. However, the effect was small at the full telescope level because only a small portion of the telescope area was affected. After two subsequent realignment steps, the telescope was aligned to a minimum of 59 nm rms, which is about 5-10 nm rms above the previous best wavefront error rms values 7 . It should be noted that the drifts and stability levels of the telescope mean that science observations will typically see telescope contribution between 60 nm rms (minimum) and 80 nm rms (where WF control will typically be performed). Further, the telescope WFE combines with the science instrument WFE to yield total observatory levels in the range 70-130 nm (see Table 2), so the slight increase to telescope WFE from this strike has a relatively smaller effect on total observatory WFE.
In plain English, the impact while damaging has not seriously reduced the telescope’s predicted capabilities.
However, to be hit with this size impact so soon after launch is very worrisome, especially because Webb’s mirror is not housed in any protective tube like Hubble or most telescopes. From the report:
It is not yet clear whether the May 2022 hit to segment C3 was a rare event (i.e. an unlucky early strike by a high kinetic energy micrometeoroid that statistically might occur only once in several years), or whether the telescope may be more susceptible to damage by micrometeoroids than pre-launch modeling predicted.
The science team is presently trying to anticipate what might happen if the impact rate turns out to be much higher than expected, and what can be done to mitigate the degradation of the mirror should more impacts occur.
On Christmas Eve 1968 three Americans became the first humans to visit another world. What they did to celebrate was unexpected and profound, and will be remembered throughout all human history. Genesis: the Story of Apollo 8, Robert Zimmerman's classic history of humanity's first journey to another world, tells that story, and it is now available as both an ebook and an audiobook, both with a foreword by Valerie Anders and a new introduction by Robert Zimmerman.
The ebook is available everywhere for $5.99 (before discount) at amazon, or direct from my ebook publisher, ebookit. If you buy it from ebookit you don't support the big tech companies and the author gets a bigger cut much sooner.
The audiobook is also available at all these vendors, and is also free with a 30-day trial membership to Audible.
"Not simply about one mission, [Genesis] is also the history of America's quest for the moon... Zimmerman has done a masterful job of tying disparate events together into a solid account of one of America's greatest human triumphs."--San Antonio Express-News
Which begs a question or two: Is there a higher incidence of micrometeoroids at Lagrange points? Might stable L4/L5 have even more?
I would expect most debris in the solar system to all be orbiting in the same direction generally, so maybe just don’t point in certain directions?
Icepilot, you bring up a good point, are Lagrange points filled with debris? Our L4 does have a trojan asteroid and I do not know if L5 has had any observations of objects.
Icepilot asked: “Which begs a question or two: Is there a higher incidence of micrometeoroids at Lagrange points? Might stable L4/L5 have even more?”
This was called a “high kinetic energy micrometeoroid,” which means that it was traveling at high speed (momentum increases directly with speed, kinetic energy increases with the square of the speed). Objects “orbiting” the Lagrange points would not be traveling very fast relative to each other, otherwise they would quickly drift away from these points. At the fastest, the micrometeoroid could have been “orbiting” the Lagrange point in the opposite direction as Webb, but it still couldn’t have been a high energy relative to Webb.
Webb is in the second Sun-Earth Lagrange point. This point revolves around the Sun with the same period as the Earth, one year. This means that Webb also revolved around the L2 point in a year — it is not at the exact point but is a little way off — so its speed relative to the L2 point is very low, perhaps several feet per second.
In contrast, the Earth-Moon Lagrange points revolve around the Earth with the Moon, about once a month.
To answer the question: no, there should not be a greater incidence of micrometeoroids at Lagrange points.
Ryan Lawson wrote: “I would expect most debris in the solar system to all be orbiting in the same direction generally, so maybe just don’t point in certain directions?”
This is a good expectation for things that have settled into the plane of the ecliptic (the plane that the planets are in or nearly in (less Pluto, which is somewhat out of plane)). However, there are still plenty of orbiting objects that are well out of this plane. Also, even things within the plane can have highly eccentric elliptical orbits around the Sun, which would give them the possibility of traveling as much as 3 miles per second relative to Earth (and the Moon, and the Lagrange points). Now, that could give a dust particle a high amount of energy. Falling to Earth would add another couple of miles per second to a particle, and this is what we see when we see a shooting star. The regular meteor showers are of this type, too.
Since the Earth revolves around the Sun at about 20 miles per second, anything orbiting the Sun in the opposite direction would have much, much more speed on it. So, which direction would we not want to point a telescope? The direction that Earth orbits the sun. Fortunately, if we want an image from that direction, we can just wait six months, and the Earth would be going in the opposite direction.
After all this, who among you is still interested in being an astronaut?
Edward, you suggest not pointing in the direction of Earth’s orbit, and waiting 6 months to view such targets looking in the opposite direction. That makes sense for sure if impacts are only a concern to the face of the mirror, but I wonder how susceptible it is from impacts to the rear? It is certainly lower, but a ding inside one of the pockets on the back of a mirror segment (which backs directly to the reflective surface) would likely be pretty near as bad as the soze of the affected area in the image looks pretty sizable so was a wide area distortion.
That makes me think most imaging should be programmed for when the mirror can be pointed essentially 90 degrees to the Earths orbital trajectory, thus always presenting a “profile” vs “face on” or “back on” cross section. That would minimize the mirror area presented to this risk considerably, and ensure that any strikes that are suffered would be “glancing” blows imparting much lower kinetic energy “into” the mirror surface this minimizing the distortion potential..
If this makes sense then the timing should shift by 3 months, not 6, putting the observatory at 90 degrees from the risk axis.
For what it’s worth .
Umpteen billions spent on the scope – who was the idjit who decided to save a few shekels by not including a shield as insurance. I’m a qualified Army paratrooper. At Basic Airborne at Ft Eleanor Roosevelt (or whatever they’re gonna rename Benning) you are given the statistics of how rare parachute malfunctions are and how you are more likely to die walking across the street than from a chute gone bad. None the less, ever student is taught how to cope with malfunctions and every jumper goes out the door wearing a reserve chute. “if my chute don’t open wide, I’ve got my reserve by my side. SOUND OFF One Two LOUDER NOW! THREE FOUR” as you run morning PT.
The JWST is a much bigger target than Hubble, shield or no shield. NASA should have been amassing impact data on all earth return space capsules, the shuttle, Apollo, Gemini, even Mercury. I can’t imagine a richer data set to use for risk analysis from impact damage. The distribution of particles, from grains of sand and all the larger, is not homogonous. Witness the meteor showers we see each year annually. So, it becomes a race between mission success and hardware damage. Let’s hope the JWST doesn’t end up looking like Jello hit a window screen!
MDN,
Good thinking. I was thinking that since the damage was slight, although over a relatively wide area, a strike from the back would not much matter to the front, but the chances that the strike was from a 40 mile per second counter-directional micrometeoroid is small, so damage the rear could result in damage to the mirror.
Col Beausabre,
I have no insight on this, but it could be that a proper shield would have blown the weight budget. Additional possible problems include limiting the directional capability, as a shield increases the likelihood that I may be shined upon by the Sun, causing too much warming.
Spectrum Shift,
This may be why NASA was so surprised that this happened so early in the flight. It is my understanding that they took all that into account when planning the mission. ISS data, gathered since Webb’s mission was designed, didn’t seem to make them change their mind about the risks.
In the meantime, there hasn’t been another strike, yet, so it may have been an aberration (if anyone sees a pun, here, it was not intended).
Looking at the big picture (that pun is intended), NASA expects a much shorter lifespan for Webb than it has had for Hubble. Considering the cost, this reduces the benefit to cost ratio, meaning that the money could have possibly been better spent on other astronomical spacecraft for an overall better science to cost ratio. The real difference is: which science gives us (would have given us) a better understanding of the universe and world around us? If Webb helps to solve dark energy, dark matter, or both, then it may have been worth the high price of admission, but this does not seem to be one of its objectives (oh, geez; is that another pun?). It is sort of like Disney’s “Pirates of the Caribbean” ride; the “E” ticket portion is over and now it is time for the show.