A small space rock has proven to have a big effect on NASA’s new operational deep space telescope.
A micrometoid struck the James Webb Space Telescope between May 22 and 24, slamming into one of the observatory’s 18 hexagonal gold mirrors. NASA revealed the micrometeorite impact in June and noted that the debris was larger than pre-launch modeling had predicted. Now, mission scientists have shared an image that shows the severity of the impact in a report (opens in a new tab) published July 12 detailing what mission scientists have learned about using the observatory during its first six months in space.
Fortunately, in this case the overall effect on Webb was small. However, the report outlines the investigation and modeling that engineers are undertaking to assess the long-term effects of the micrometroids on Webb.
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Based on fuel usage, the telescope should last 20 years in space. But scientists aren’t sure what effect the micrometroid impacts will have on its operations, the report’s authors said.
Micrometoids are a known hazard of space operations, and confronting them is by no means new to scientists; The International Space Station and the Hubble Space Telescope are among the long-term programs still operating despite the occasional impact of space rocks. However, Webb’s orbit at Lagrange Point 2 about 1 million miles (1.5 million kilometers) from Earth could significantly change the risk profile.
Webb engineers first discovered deformations of the primary mirror during the commissioning period during the alignment (or wavefront sensing) phase, which placed the 18 segments of the hexagonal mirror in the best position to capture light .
These first six hits meet pre-launch frequency expectations as they come at a frequency of once a month, the report said. Furthermore, some of the resulting deformations can be corrected by mirror realignments. But the scale of one of those six impacts caused more concern, the paper noted, because it caused a significant flaw in a segment known as C3. The strike at the end of May “caused a significant uncorrectable change in the overall figure for this segment,” the report said.
In this case, however, the overall impact on the mission was small, “because only a small portion of the telescope area was affected.” Seventeen mirror segments remain intact, and engineers were able to reconfigure Webb’s segments to account for most of the damage.
Jupiter and its moon Europa, left, seen through the 2.12-micron filter of James Webb’s NIRCam space telescope. (Image: NASA, ESA, CSA and B. Holler and J. Stansberry (STScI))
Engineers are still modeling how often such events will occur. “It remains unclear whether the May 2022 impact in segment C3 was a rare event,” the team wrote. By “rare,” they said it’s possible to get a high-energy impact by chance, which statistically should only happen once every few years.
Alternatively, it could be that Webb is “more susceptible to micrometeoroid damage than pre-launch modeling predicts,” the team wrote. Modeling is ongoing to assess the hazardous population of micrometeoroids and devise countermeasures, such as limiting the direction of aim.
One remedy could be to minimize the time Webb points directly in its orbital direction, “which statistically has higher micrometeorite and energy levels,” the team wrote.
The efficiency of the primary mirror is judged by how much it distorts the starlight, according to the journal Astronomy (opens in new tab), and is measured using what scientists call the root mean square error of the wavefront. When the Webb mission began, the affected C3 segment had a wavefront error of 56 nanometers rms (root mean square), which was consistent with the 17 other mirror parts.
After the impact, however, the error increased to 258 nm rms, but readjustments of the mirror segments overall reduced the total impact to only 59 nm rms. For now, the team writes, the Webb alignment is at the performance limits, as the realigned mirror segments are “about 5-10 nm rms above the previous best rms wavefront error 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 by Halley’s Comet, according to Nature (opens in new tab).
NASA’s Meteorite Environment Office at the Marshall Space Flight Center in Huntsville, Alabama, is modeling the impact risk on the Halley-related Webb. NASA officials also emphasized during recent media briefings that the micrometroid problem is their full attention, Nature added.
Follow Elizabeth Howell on Twitter @howellspace (opens in new tab). Follow us on Twitter @Spacedotcom (opens in new tab) and on Facebook (opens in new tab).
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