A small space rock has proven to have a big effect on NASA's newly
operational deep-space telescope.
A micrometeroid 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 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.
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.
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.
Main mirror performance is assessed by how much it deforms starlight,
according to Astronomy magazine, 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.
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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Space & Astrophysics