James Webb Space Telescope picture shows noticeable damage from micrometeoroid strike

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https://www.space.com/james-webb-space-telescope-micrometeoroid-damage

James Webb Space Telescope picture shows noticeable damage from 
micrometeoroid strike
By Elizabeth Howell published about 3 hours ago

NASA is unsure how much of an effect space rocks will have on Webb's 
lifetime after the event 'exceeded prelaunch expectations of damage.'


comparison shows bright spot from micrometeoroid impact
On the right, the state of the James Webb Space Telescope on June 21, 
2022, after an unexpectedly large micrometeoroid strike and subsequent 
responses, compared to the expected state on the left. (Image credit: 
NASA/STScI)
A small space rock has proven to have a big effect on NASA's newly 
operational deep-space telescope.

A micrometeoroid struck the James Webb Space Telescope between May 22 
and 24, impacting one of the observatory's 18 hexagonal golden mirrors. 
NASA had disclosed the micrometeoroid strike in June and noted that the 
debris was more sizeable than pre-launch modeling had accounted for. 
Now, scientists on the mission have shared an image that drives home the 
severity of the blow in a report(opens in new tab) released July 12 
describing what scientists on the mission learned about using the 
observatory during its first six months in space.

Happily, in this case the overall effect on Webb was small. That said, 
the report outlines the investigation and modeling that engineers are 
undertaking to assess the long-term effects of micrometeroids on Webb.

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Based on fuel usage, the telescope should last 20 years in space. But 
scientists aren't sure how much of an effect micrometeroid strikes will 
have upon its operations, the report authors stated.

Micrometeroids are a known danger of space operations, and facing them 
is by no means new to scientists; the International Space Station and 
the Hubble Space Telescope are among long-running programs that are 
still operational despite occasional space rock strikes. However, Webb's 
orbit at Lagrange Point 2 about 1 million miles (1.5 million kilometers) 
away from the Earth may change the risk profile considerably.

Webb engineers first detected deformations on the primary mirror during 
the commissioning period during the alignment (or wavefront sensing) 
phase, which put the 18 segments of the hexagonal mirror into the best 
position to capture light.

These first six strikes met pre-launch expectations of rate as they came 
in at a rate of once per month, the report stated. Moreover, some of the 
resulting deformations are correctable through mirror realignments. But 
it's the magnitude of one of these six strikes that caused more concern, 
the paper noted, as it caused a significant blemish to a segment known 
as C3. The strike in late May "caused significant uncorrectable change 
in the overall figure of that segment," the report stated.

In this case, however, the overall impact to the mission is small 
"because only a small portion of the telescope area was affected." 
Seventeen mirror segments remain unblemished and engineers were able to 
realign Webb's segments to account for most of the damage.

Jupiter and its moon Europa, left, are seen through the James Webb Space 
Telescope’s NIRCam instrument 2.12 micron filter.

Jupiter and its moon Europa, left, are seen through the James Webb Space 
Telescope’s NIRCam instrument 2.12 micron filter. (Image credit: NASA, 
ESA, CSA, and B. Holler and J. Stansberry (STScI))
Engineers are still modeling how frequently such events will occur. "It 
is not yet clear whether the May 2022 hit to segment C3 was a rare 
event," the team wrote. By "rare," they said it is possible that they 
happened to get a high-energy impact that should statistically happen 
only once every few years.

Alternatively, it may be that Webb is "more susceptible to damage by 
micrometeoroids than pre-launch modeling predicted," the team wrote. 
Modeling is ongoing to estimate the hazardous population of 
micrometeoroids and to figure out remedies, such as restricting pointing 
direction.

One remedy could be minimizing the amount of time Webb points directly 
into its orbital direction, "which statistically has higher 
micrometeoroid rates and energies," the team wrote.

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Main mirror performance is assessed by how much it deforms starlight, 
according to Astronomy magazine(opens in new tab), and measured using 
what scientists call wavefront error root mean square. When Webb's 
mission began, the affected C3 segment had a wavefront error of 56 
nanometers rms (root mean square), which was in line with the 17 other 
mirror portions.

Post-impact, however, the error increased to 258 nm rms, but 
realignments to the mirror segments as a whole reduced the overall 
impact to just 59 nm rms. For the time being, the team wrote Webb's 
alignment is well within performance limits, as the realigned mirror 
segments are "about 5-10 nm rms above the previous best wavefront error 
rms values."

For now, engineers are keeping an eye on potential future 
dust-generating events such as in 2023 and 2024, when Webb is expected 
to fly through particles left behind by Halley's Comet, according to 
Nature(opens in new tab).

NASA's meteoroid environment office at the Marshall Space Flight Center 
in Huntsville, Alabama is modeling the impact risk to Webb associated 
with Halley. NASA officials have also emphasized during recent media 
briefings that the micrometeroid issue has their full attention, Nature 
added.

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Elizabeth Howell
Elizabeth Howell