You can see where the Webb Telescope received a direct hit from a micrometeorite on one of its mirrors

You can see where the Webb Telescope received a direct hit from a micrometeorite on one of its mirrors

You can see where JWST received a direct hit from a micrometeorite on one of its mirrors

Comparison of Webb mirror alignments, from the report “Characterizing JWST Scientific Performance from Commissioning” (July 12, 2022). Credit: NASA/ESA/CSA

The world is still reeling from the release of the first images from the James Webb Space Telescope (JWST). These provided a comprehensive overview of the type of science operations Webb will conduct during his 20-year mission. They included the most sensitive and detailed look at some iconic astronomical objects, spectra of an exoplanet atmosphere, and a deep-field view of some of the most distant galaxies in the universe. Since their release, we’ve also been treated to glimpses of solar system objects captured by Webb’s infrared instruments.

Meanwhile, the JWST Collaboration released a comprehensive report titled “Characterizing JWST Science Performance from Commissioning,” in which they reviewed everything Webb has accomplished so far and what they anticipate throughout the mission. This document recently appeared online and covers everything from the navigation of the telescope to the performance of its many instruments. An interesting tidbit, which hasn’t been published before, is how Webb suffered a series of micrometeoroid impacts, one of which caused an “uncorrectable change” in a mirror segment.

The team behind this study included researchers from the three participating space agencies – NASA, the European Space Agency (ESA) and the Canadian Space Agency (CSA) – and the mission’s many partner agencies. These include the Space Telescope Science Institute (STScI), Niels Bohr Institute, Max-Planck-Institut für Astronomie (MPIA), UK Astronomy Technology Center (UK ATC), National Research Council Canada (NRCC), from the Instituto Nacional de Técnica Aeroespacial (INTA), the Centro de Astrobiología (CAB) and many aerospace companies, universities, research institutes and agencies around the world.

The paper they compiled evaluates the performance of the JWST during the six-month commissioning period before it enters service on July 12, 2022. It was to characterize the observatory’s in-orbit performance, the design and architecture of the JWST, as well as the expected performance before launch. These were then compared to the performance of the spacecraft, telescopes, scientific instruments and ground system. Section 4 of the report, Optical Performance, discusses the operation of Webb’s various instruments during the commissioning period.

You can see where the Webb Telescope received a direct hit from a micrometeorite on one of its mirrors

A primary mirror segment of the James Webb Space Telescope, made of beryllium. Credit: NASA/MSFC/David Higginbotham/Emmett

The primary mirror of the JWST consists of eighteen hexagonal segments arranged in a honeycomb configuration. Each segment is comprised of gold-plated beryllium, and all are aligned to ensure the highest possible resolution and sensitivity. Overall performance is measured in terms of Wavefront Error (WFE), which refers to how the light collected by the telescope’s mirrors deviates from the expected wavelength of light. The overall extent is determined by calculating the deviation of the collected light from the Root-Mean-Square (RMS) error – the spherical mean of the entire wavefront.

This is expressed mathematically using the units of the particular wavelength, measured in nanometers (nm) when dealing with infrared wavelengths. Section 4.7 discusses micrometeoroid impacts and their potential effect on Webb’s long-term optical performance. The assessment begins by reminding readers that any spacecraft will inevitably encounter micrometeoroids, then lists how multiple impacts were expected during the commissioning period:

“During commissioning, wavefront detection recorded six localized surface deformations on the primary mirror that are attributed to micrometeoroid impact. These occurred at a rate (approximately one per month ) consistent with pre-launch expectations. Each micrometeoroid caused wavefront degradation of the impacted mirror segment, as measured during regular wavefront detection. Part of the wavefront degradation which results can be corrected by regular control of the wavefront; a part includes high spatial frequency terms which cannot be corrected.

They further indicate that these micrometeoroid impacts have so far been detected by wavefront sensing. Five of the six impacts detected had negligible effects, contributing a combined total of less than 1 nanometer to the overall wavefront error. However, the remaining impact, which occurred between May 22 and May 24, caused a “significant non-correctable change” in the overall C3 segment figure. This segment is located on the lower right side of Webb’s primary mirror (seen from the front), and the effect is shown in the report (see image above).

Fortunately, the overall effect was small as only a small portion of the telescope area was affected. Mission teams also performed two stages of realignment to correct for the impact, which brought the telescope’s alignment to a minimum of 59 nm RMS, about 5-10 nm above the best RMS values ​​of previous wavefront error. The report authors also go on to note that “drifts and stability levels” in the telescope typically result in a “telescope contribution” of between 60 (minimum) and 80 nm RMS – the point at which wavefront control is usually done.

They also explain that it is not known at this time whether the May 2022 impact on the C3 segment was rare or something that is expected to occur frequently throughout JWST’s mission. As they indicate, this is essential if the JWST mission teams hope to determine whether the telescope will be more susceptible to damage from micrometeoroids than predicted by pre-launch modeling:

“The project team is conducting further investigations into the micrometeoroid population, how impacts affect beryllium mirrors, and the effectiveness and efficiency trade-offs of potential mitigations such as pointing restrictions that would minimize time spent looking. in the direction of orbital motion, which statistically has higher micrometeoroid rates and energies.

In summary, the impact on the C3 segment has raised concerns among mission controllers. But the upside is that it wasn’t anything they couldn’t fix, and it shouldn’t affect Webb’s long-term science operations. As the report summarizes:

“The key result of six months of commissioning is: JWST is fully capable of achieving the discoveries for which it was built. JWST was envisioned” to enable fundamental breakthroughs in our understanding of formation and evolution of galaxies, stars and planetary systems. “…we now know for sure that it will. The telescope and suite of instruments have demonstrated the sensitivity, stability, image quality and spectral range that are needed to transform our understanding of the cosmos through to observations ranging from near-Earth asteroids to the most distant galaxies.”

Additionally, the report authors conclude that the JWST’s performance has been better than expected, almost across the board. In terms of the optical alignment of its mirrors, the point spread function, the temporal stability of its imagery, and the fine guidance system that points the observatory, Webb exceeded expectations. They also indicate that the mirrors are cleaner and that the science instruments generally delivered higher total system throughput than expected before launch. This all adds up to optimistic assessments:

“Collectively, these factors result in significantly better sensitivity for most instrument modes than was assumed in the cycle 1 observation planning exposure time calculator, in many cases tens of for hundred. In most cases, JWST will go further faster than expected. , JWST has enough propellant on board to last at least 20 years.”

The JWST collaboration said further details will be presented in a series of planned documents. These will appear in a special issue of Publications of the Pacific Astronomical Society (PASP) devoted to the JWST.

James Webb Telescope hit by micrometeoroid, NASA says

More information:
Jane Rigby et al, Characterization of JWST science performance since commissioning. arXiv:2207.05632v1 [astro-ph.IM]

Provided by Universe Today

Quote: You can see where the Webb Telescope was hit directly by a micrometeorite on one of its mirrors (2022, July 21) retrieved July 21, 2022 from telescope-micrometeorite-mirrors.html

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