A small space rock had a big impact on NASA’s newly operational deep space telescope.
A micrometeoroid struck the James Webb Space Telescope between May 22 and May 24, hitting one of the observatory’s 18 hexagonal golden mirrors. NASA reported a micrometeoroid impact in June and noted that the debris was larger than predicted in pre-launch simulations. Now scientists on the mission have shared an image that shows the severity of the impact in the report. (will open in a new tab) released July 12 and describes what the scientists involved in the mission learned about the use of the observatory in its first six months in space.
Fortunately, in this case, the overall impact on Webb was small. However, the report describes the research and modeling that engineers are doing to evaluate the long-term effects of micrometroids on Webb.
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Based on fuel consumption, the telescope should work in space for 20 years. But scientists aren’t sure what impact the micrometroid’s strikes will have on its performance, the report’s authors said.
Micrometeoroids are a known hazard in space operations, and their impact is by no means new to scientists; The International Space Station and the Hubble Space Telescope are among the long-term programs that are still running despite occasional impacts on space rocks. However, Webb’s orbit at Lagrange Point 2, about 1 million miles (1.5 million kilometers) from Earth, could change the risk profile significantly.
Webb engineers first discovered deformations of the primary mirror during commissioning during the alignment (or wavefront measurement) phase, which allowed the 18 segments of the hexagonal mirror to be positioned in the best position to capture light.
The report says that these first six warnings lived up to speed expectations as they came once a month. Moreover, some of the resulting deformations can be corrected by retuning the mirror. But more worrying was the magnitude of one of those six strikes, the newspaper notes, as it caused significant damage to a segment known as C3. The strike at the end of May “resulted in significant uncorrectable changes in the segment’s total figure,” the report said.
In this case, however, the overall impact on the mission is small “because only a small portion of the telescope area was affected.” Seventeen of the mirror segments remain intact, and engineers were able to rebuild Webb’s segments to account for most of the damage.
Engineers are still modeling how often such events will occur. “It is not yet clear if entering the C3 segment in May 2022 was a rare event,” the team wrote. Under “rare,” they said that they might have happened to have a high-energy exposure that statistically should only happen once every few years.
Alternatively, Webb may be “more prone to micrometeoroid damage than preliminary simulations predicted,” the team writes. Simulation continues to assess the dangerous population of micrometeoroids and find remedies such as limiting the guidance direction.
One remedy could be to minimize the amount of time Webb points directly to his orbital direction, “which statistically has higher micrometeoroid velocities and energies,” the team writes.
According to Astronomy magazine, the performance of a primary mirror is measured by how much it distorts starlight. (will open in a new tab), and is measured using what scientists call the root-mean-square error of the wavefront. When the Webb mission began, the wavefront error of the affected C3 segment was 56 nanometers (rms), which was consistent with 17 other mirror parts.
However, after impact, the error increased to 258 nm (rms), but retuning the mirror segments as a whole reduced the total impact value to 59 nm (rms). The team currently writes that Webb alignment is within performance limits as the retuned mirror segments are “about 5-10 nmrms above previous best wavefront error rms”.
For now, engineers are monitoring potential future dust events, such as those in 2023 and 2024, when Webb is expected to fly through particles left behind by Halley’s comet, Nature reports. (will open in a new tab).
NASA’s Meteoroid Environment Office at the Marshall Space Flight Center in Huntsville, Alabama is modeling the Webb impact risk associated with Halley. NASA officials also stressed during recent media briefings that the issue of micrometroids is at the center of their attention, Nature added.
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