James Webb Space Telescope Dinged By Micrometeoroid

The best way to start this is probably by saying, "Webb is fine.  They expected and designed for micrometeoroid impacts, knowing that the instrument is likely to be hit several times over its lifetime in deep space."  Still, when the word got out that between May 23 and 25th, the telescope sustained an impact to one of its primary mirror segments, there was a bit of nervousness in the community of people who know the instrument is out there and are hoping for Big Things from the infrared space telescope.  

After initial assessments, the team found the telescope is still performing at a level that exceeds all mission requirements despite a marginally detectable effect in the data. Thorough analysis and measurements are ongoing. Impacts will continue to occur throughout the entirety of Webb’s lifetime in space; such events were anticipated when building and testing the mirror on the ground. After a successful launch, deployment, and telescope alignment, Webb’s beginning-of-life performance is still well above expectations, and the observatory is fully capable of performing the science it was designed to achieve.

I need to remind everyone that the JWST isn't even fully certified as ready for operation, yet, and won't be certified until nearly mid-July.  

In the lead-up to the release of Webb’s first full-color images and spectroscopic data on July 12, the Webb team is now in the last phase of commissioning the science instruments. The first two instrument modes, NIRCam imaging and NIRISS imaging, have been declared ready for science; watch the “Where is Webb” page as the team works their way through the other 15 instrument modes. 

A little more on the design phase - designing for surviving micrometeorioid impacts  To begin with, Webb's primary mirrors are made from metallic beryllium because it is lightweight, stiff, strong and dimensionally stable at the telescope’s operating temperature.  Like most things, Beryllium changes dimensions with temperature, getting smaller as it gets colder, but it virtually stops shrinking once it goes below a temperature of 100 Kelvin (about minus 280 degrees Fahrenheit or minus 173 degrees Celsius).  Making the mirrors out metallic beryllium instead of the conventional approach of glass coated with a metal enabled performance improvements.

Webb’s mirror was engineered to withstand bombardment from the micrometeoroid environment at its orbit around Sun-Earth L2 of dust-sized particles flying at extreme velocities. While the telescope was being built, engineers used a mixture of simulations and actual test impacts on mirror samples to get a clearer idea of how to fortify the observatory for operation in orbit. This most recent impact was larger than was modeled, and beyond what the team could have tested on the ground.

“We always knew that Webb would have to weather the space environment, which includes harsh ultraviolet light and charged particles from the Sun, cosmic rays from exotic sources in the galaxy, and occasional strikes by micrometeoroids within our solar system,” said Paul Geithner, technical deputy project manager at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “We designed and built Webb with performance margin – optical, thermal, electrical, mechanical – to ensure it can perform its ambitious science mission even after many years in space.”

There are interesting aspects to this.  Consider the recent potential for a meteor storm at the end of May.  There are regular, predictable meteor showers all year long, and if observations show there's an increased chance of meteoroid impacts, the telescope can be repositioned with respect to the meteor trail to survive better.  Webb has the ability to sense and adjust mirror positions to at least partially correct for impacts. By adjusting the position of the affected segment, engineers can cancel out a portion of the distortion. This minimizes the effect of any impact, although not every distortion from every kind of impact can be cancelled out this way.

They already did an adjustment like this for this recent impact to correct some of the degradation, and may do more.   

Last words to Lee Feinberg, Webb optical telescope element manager at NASA Goddard, who said, 

“Since launch, we have had four smaller measurable micrometeoroid strikes that were consistent with expectations and this one more recently that is larger than our degradation predictions assumed. We will use this flight data to update our analysis of performance over time and also develop operational approaches to assure we maximize the imaging performance of Webb to the best extent possible for many years to come.”