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


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Comparison of Webb mirror alignment taken from the report “Characterization of scientific results of JWST since commissioning” (July 12, 2022). Credit & Copyright: NASA/ESA/CSA

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

Meanwhile, the JWST collaboration released a full report titled “Characterization of JWST Scientific Results Since Commissioning” in which they reviewed everything Webb has achieved so far and what they expect throughout the mission. This document has recently appeared on the Internet and covers everything from navigating the telescope to the operation of its many instruments. An interesting fact that has not been previously published is how Webb survived a series of micrometeoroid impacts, one of which caused “uncorrectable changes” in one segment of the mirror.

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) — as well as from many of the mission’s partner agencies. These include the Space Telescope Science Institute (STScI), the Niels Bohr Institute, the Max Planck Institute for Astronomy (MPIA), the UK Astronomical Technology Center (UK ATC), the National Research Council of Canada (NRCC), the National Institute de Técnica Aeroespacial (INTA), Center for Astrobiology (CAB) and many aerospace companies, universities, research institutes and agencies around the world.

Their paper evaluates the performance of the JWST over a six-month commissioning period before it enters service on July 12, 2022. It consisted of a characterization of the observatory’s in-orbit performance, the design and architecture of the JWST, and pre-launch performance predictions. They were then compared to those of the spacecraft, telescopes, scientific instruments, and ground system. Section 4 of the Optical Performance report looks at how the various Webb instruments performed during the commissioning period.

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

A segment of the main mirror of the James Webb Space Telescope made of beryllium. Image Credit & Copyright: NASA/MSFC/David Higginbotham/Emmett Given.

The main JWST mirror consists of eighteen hexagonal segments arranged in a honeycomb pattern. Each segment is made of gold-plated beryllium, and they are all aligned to provide 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 the light. The total extent is determined by calculating the deviation of the collected light from the root mean square error (RMS) – the spherical average of the entire wavefront.

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

“During commissioning, wavefront sensors recorded six local deformations of the surface of the main mirror, which are explained by micrometeoroid impacts. They happened at a rate (about once a month) that was in line with pre-launch expectations. Each micrometeoroid caused wavefront deterioration in the impact mirror segment measured during conventional wavefront soundings. Some of the resulting wavefront degradation can be corrected by conventional wavefront control;

They also indicate that these micrometeoroid strikes have been detected so far by wavefront sounding. Five of the six impacts detected had minor effects, resulting in a total wavefront error of less than 1 nanometer. However, the remaining impact, which occurred between May 22 and May 24, caused a “significant irreversible change” in the C3 segment total. This segment is located on the lower right side of the primary Webb mirror (when viewed from the front) and the effect is shown in the report (see image above).

Fortunately, the overall effect was small, since it affected only a small part of the telescope’s area. The mission teams also performed two retuning steps to correct for the impact, bringing the telescope alignment to a minimum rms of 59 nautical miles, about 5 to 10 nautical miles above previous best wavefront error rms. The authors of the report further note that “levels of drift and stability” in a telescope typically result in a “telescope contribution” in the range of 60 (minimum) to 80 nm RMS, at which point wavefront control is typically performed.

They also explain that it is currently unknown if the impact on the C3 segment in May 2022 was rare or something that can be expected frequently throughout the JWST mission. This is important, they say, if the JWST mission teams hope to determine if the telescope will be more susceptible to micrometeoroid damage than preliminary simulations predicted:

“The project team is conducting additional research into the micrometeoroid population, how impacts affect beryllium mirrors, and the effectiveness and trade-offs between the effectiveness and efficiency of potential mitigation measures, such as pointing restrictions that minimize the time spent searching in the direction of orbital motion. which statistically has a higher micrometeoroid nature. speed and energy.

In summary, the impact on the C3 segment was a concern for mission controllers. But the upside is that they couldn’t decide anything, and it’s not expected to affect Webb’s long-term scientific work. The report summarizes:

“The key result of six months of commissioning is that the JWST is fully capable of making the discoveries for which it was built. JWST was conceived to “provide fundamental breakthroughs in our understanding of the formation and evolution of galaxies, stars, and planetary systems. “…we now know for sure that this will happen. The telescope and instrument suite have demonstrated the sensitivity, stability, image quality and spectral range needed to change our understanding of the cosmos through observations spanning from near-Earth asteroids to the most distant galaxies.”

Moreover, the authors of the Report concluded that the performance of JWST was better than expected in almost all areas. In terms of the optical alignment of its mirrors, the point spread function, the temporal stability of the image, and the precise pointing system that points the observatory, Webb has exceeded all expectations. They also indicate that the mirrors are cleaner and the science instruments in general are delivering higher overall system throughput than expected before launch. All this adds up to some optimistic estimates:

“Together, these factors result in significantly better sensitivity for most instrument modes than suggested by the cycle 1 observation planning exposure time calculator, in many cases by tens of percent. In most cases, JWST will deepen faster than expected. , JWST has enough fuel on board to last at least 20 years.”

The JWST collaboration stated that further details will be provided in a planned series of papers. They will appear in a special JWST issue of Publications of the Astronomical Society of the Pacific (PASP).

Micrometeoroid hit James Webb telescope: NASA

Additional Information:
Jane Rigby et al., Characterizing JWST scientific results since commissioning. archive: 2207.05632v1 [astro-ph.IM]

Courtesy of Universe Today

Quote: You can see where the Webb Telescope got a micrometeorite direct hit on one of its mirrors (July 21, 2022) obtained on July 21, 2022 from micrometeorite. -mirrors.html

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