You can see where the Webb Telescope took a direct hit from a small meteor on one of its mirrors

You can see where JWST took a direct hit from a small meteor on one of its mirrors

Webb mirror alignment comparison, taken from the report “JWST Scientific Performance Characterization of Commissioning” (July 12, 2022). Credit: NASA/ESA/CSA

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

Meanwhile, the JWST Collaboration has released a full report, “Characterizing JWST Scientific Performance from Commissioning,” examining everything Webb has accomplished so far and what they expect during the mission. This paper recently appeared online and covers everything from navigating a telescope and notes the performance of many of its instruments. An interesting piece of information, which has not been released before, is how Webb suffered a series of micro-meteorite impacts, one of which caused an “irreversible change” in a single 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 from the mission’s many partner agencies. These institutions include the Space Telescope Science Institute (STScI), the Niels Bohr Institute, the Max Planck Institute for Astronomy (MPIA), the UK Astronomy Technology Center (UK ATC), the National Research Council of Canada (NRCC), and the National Institute of Technology. de Técnica Aeroespacial (INTA), Centro de Astrobiología (CAB), and many airlines, 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 entering service on July 12, 2022. This consists of a description of the observatory’s in-orbit performance, JWST design and engineering, and expected performance prior to launch. It was then compared to the performance of spacecraft, telescopes, scientific instruments, and the Earth system. Section 4 of the report, Visual Performance, looks at how the various Webb tools worked during the commissioning period.

You can see where the Webb Telescope took a direct hit from a small meteor on one of its mirrors

Primary mirror piece for the James Webb Space Telescope, made of beryllium. Credit: NASA/MSFC/David Higginbotham/Emmett Given

The primary mirror of JWST consists of eighteen hexagonal segments arranged in a honeycomb configuration. Each part is made of gold-plated beryllium, and they are all lined up to ensure the highest accuracy and sensitivity possible. The overall performance is measured in terms of wavefront error (WFE), which indicates how the light collected by the telescope’s mirrors deviates from the expected wavelength of light. The total extent is determined by calculating the deviation of the collected light from the root-mean-square (RMS) error – the spherical average of the entire wavefront.

This is expressed mathematically using units of a particular wavelength, measured in nanometers (nanometers) when dealing with infrared wavelengths. Section 4.7 addresses the effects of micrometeorites and their potential impact on Webb’s long-term optical performance. The assessment begins by reminding readers that any spacecraft will inevitably encounter micrometeorites, and then lists the number of impacts expected during its run:

“During launch, wavefront sensing recorded six localized surface distortions on the primary mirror attributed to the impact of micrometeorites. These distortions occurred at a rate (approximately one per month) consistent with pre-launch predictions. Each micrometeorite caused a deterioration in the foreground The wave of micrometeorites. The affected mirror piece, as measured during normal wavefront sensing. Some of the resulting wavefront degradation can be corrected by uniform wavefront control; and some have uncorrectable high spatial frequency terms.”

They also point out that these micro-meteoroid impacts have so far been detected by sensing the wavefront. Five of the six detected effects had negligible effects, contributing to an overall total of less than 1 nm to the overall wavefront error. However, the residual effect, which occurred between May 22 and May 24, caused a “significant irreversible change” in the overall shape of segment C3. This clip is on the lower right side of Webb’s primary mirror (when viewed from the front), and the effect is illustrated in the report (see image above).

Fortunately, the overall effect was small since only a small portion of the telescope’s area was affected by it. The mission teams also performed two re-alignment steps to correct the effect, bringing the telescope alignment up to a minimum RMS of 59 nm, which is approximately 5 to 10 nm above the previous best wavefront error RMS values. The report’s authors also go on to note that telescope “drifts and levels of stability” typically result in a “telescope contribution” of 60 (minimum) to 80 nm RMS – at which point wavefront control is usually implemented.

They also explain that it is not known at this time if the impact of May 2022 on the C3 segment is rare or something that might be expected to occur more frequently during the JWST mission. They say, this is essential if JWST mission teams hope to determine whether the telescope will be more vulnerable to damage by micrometeorites than pre-launch modeling predicted:

“The project team is conducting additional investigations into micrometeorite assemblies, how impacts affect beryllium mirrors, and effectiveness and efficiency trade-offs for potential mitigations such as orientation constraints that would reduce time spent looking in the direction of orbital motion, which have statistically higher meteorite rates and energies.”

To summarize, the impact on the C3 segment raised concerns among mission observers. But the plus side was that it wasn’t something they couldn’t handle and wasn’t expected to affect Webb’s long-term scientific operations. As the report summarizes:

“The main result of six months into the commissioning is: JWST is fully capable of making the discoveries for which it was made. JWST is envisioned “to enable fundamental breakthroughs in our understanding of the formation and evolution of galaxies, stars, and planetary systems”… We now know for sure that it will be.” The telescope and instrument cluster have demonstrated the sensitivity, stability, image quality, and spectral range necessary to transform our understanding of the universe through observations extending from near-Earth asteroids to the most distant galaxies.”

Furthermore, the report’s authors concluded that JWST performed better than expected, almost across the board. In terms of the optical alignment of its mirrors, the point spread function, the imaging time stability, and the precise guidance system that guides the observatory, Webb has exceeded expectations. They also indicate that the mirrors are cleaner, and that the scientific instruments generally delivered higher overall system throughput than pre-launch forecasts. All this adds up to some optimistic assessments:

“Collectively, these factors translate into significantly better sensitivity for most hardware modes than was assumed in the exposure time calculator for first cycle monitoring planning, in many cases by tens of percent. In most cases, JWST will deepen faster than expected. Additionally, JWST has enough propellant on board to last at least 20 years.”

The JWST Collaboration reported that more details will be provided in a series of planned papers. These will appear in a special issue of the Astronomical Society of the Pacific (PASP) publication dedicated to JWST.

James Webb Telescope hit by micrometeorites: NASA

more information:
Jane Rigby et al, JWST scientific performance characterization from commissioning. arXiv: 2207.05632v1 [astro-ph.IM]

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