As NASA’s James Webb Space Telescope prepares to release its first science images on July 12, engineers are keeping an eye on a small but potentially impressive future threat: micrometeoroids. Although the mission’s scientists had expected the telescope to be hit by these tiny pieces of space dust during its expected 20-year lifespan, a fairly large hit in May made them rethink what they thought they knew about the frequency with which Webb would be thrown. The performance of the telescope is unharmed for now. But understanding future impact risk is critical because Webb is an $11 billion investment for NASA, the European Space Agency and the Canadian Space Agency — and researchers hope it will change astronomy. “Time will tell if that last impact was just an anomaly,” Mike Menzel, Webb’s chief systems engineer at Goddard Space Flight Center in Greenbelt, Md., said at a June 29 news conference.
From its position in deep space, 1.5 million kilometers from Earth, the telescope looks into space with a 6.5 meter wide primary mirror – the largest ever launched into space. While the mirror makes Webb a highly capable telescope, its large size also makes the observatory vulnerable to being littered by fast-moving dust particles. So far, the telescope, launched on December 25, 2021, has been hit by five small micrometeoroids. All were of unknown size, but the researchers concluded that the fifth was larger than the first four and larger than they had anticipated.
Pelting predictions
Two decades ago, during Webb’s design phase, engineers knew it would be regularly showered with micrometeoroids. Unlike the Hubble Space Telescope’s mirror, which is smaller and placed inside a tube, Webb’s gold-plated beryllium mirror is completely exposed to the space environment. So the designers fired high-velocity particles into the mirror samples to see what kind of dimples they would make, and asked colleagues to calculate how many particles might be moving at Webb’s intended location — a region beyond the moon’s orbit called L2.The mission team “invested a lot of effort 20 years ago to try to get their meteoroid environment right,” says Bill Cooke, head of NASA’s meteoroid environment office at Marshall Space Flight Center in Huntsville, Alabama.
Engineers estimated that Webb would withstand about one impact per month, which could be large enough to shatter the mirror. And they decided it was a risk worth living with. They calculated that impact pitting would accumulate over time, but that dents would cover only 0.1% of the primary mirror after 10 years. Telescopes can still function if part of their primary mirror is damaged. In late May, the impact on Webb caught everyone’s attention. “I’ve spent the last six weeks answering questions about micrometeoroids,” Menzel said at a news conference. The impact left a tiny distortion in one of the 18 hexagonal segments that make up Webb’s primary mirror. Because the positions of Webb’s mirror segments can be adjusted with exquisite precision, engineers were able to tune the affected section to cancel out some, but not all, of the image degradation. (NASA says the telescope is still performing well beyond expectations.)Micrometeoroids are formed by collisions between asteroids and other planetary bodies. The particles are usually as small as a few tens of micrometers in diameter – the size of grains of sand – but can be as large as a bus. The Sun’s gravity pulls particles toward it, so dust generally flows from the outer regions of the solar system toward the inner parts.
Large micrometeoroids are much rarer than small particles, so it’s likely that Webb had the misfortune of encountering a large one relatively early in its life, says David Malaspina, a plasma physicist at the University of Colorado Boulder who studies the effects of cosmic dust on spacecraft . It is as if the card player drew a specific card from the deck in the first round of the game, as opposed to it coming later in the game. Scientists can only wait to see what happens next.Meanwhile, Webb engineers are taking a fresh look at their impact rate estimates, which come from a model that has been updated several times since Webb was designed1.
And they’re looking for meteor showers, which occur when Earth passes through the concentrated debris trail left by a passing comet. Dust from meteor showers represents only about a 5% risk of an impact on Webb, compared to a 95% risk of random or “sporadic” impacts from background dust streaming through the Solar System.Cooke’s office now generates its own meteor shower predictions for Webb’s team, so that mission controllers know when the telescope is about to pass through a strong plume of dust — and are able to reorient the instrument to block the particles from hitting its mirrors. This situation could occur in May 2023 and May 2024, when Webb could pass through debris from Halley’s Comet.
Source Journal Reference: Alexandra Witze, Surprising dust strike on Webb telescope has scientists on alert, Nature News (2022),https://www.nature.com/articles/d41586-022-01877-8
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