Surprising dust strike on Webb telescope has scientists on alert

As NASA’s James Webb Space Telescope gears up to release its first scientific images on 12 July, engineers are keeping an eye on a small, but potentially impactful, future threat: micrometeoroids. Although mission scientists expected the telescope to be dinged by these tiny bits of space dust over its anticipated 20-year lifetime, a relatively large hit in May has caused them to re-evaluate what they thought they knew about the frequency with which Webb will be pelted.

For now, the telescope’s performance is unharmed. But understanding the future impact risk is crucial because Webb is a US$11-billion investment for NASA, the European Space Agency and the Canadian Space Agency — and researchers hope it will transform astronomy. “Time will tell whether that last impact was just kind of an anomaly,” said Mike Menzel, Webb’s lead systems engineer at the Goddard Space Flight Center in Greenbelt, Maryland, at a news briefing on 29 June.

From its location in deep space, 1.5 million kilometres from Earth, the telescope gazes into the cosmos using a 6.5-metre-wide primary mirror — the largest ever launched into space. Although the mirror makes Webb a highly capable telescope, its large size also leaves the observatory vulnerable to being pelted by fast-moving dust particles. So far, the telescope, launched on 25 December 2021, has been hit by five small micrometeoroids. All were of unknown size, but researchers have deduced that the fifth was larger than the first four, and bigger than what they had anticipated.

Pelting predictions

Two decades ago, during Webb’s design phase, engineers knew that it would regularly be pelted by micrometeoroids. Unlike the Hubble Space Telescope’s mirror, which is smaller and contained inside a tube, Webb’s gold-coated beryllium mirror is completely exposed to the space environment. So the design engineers fired high-speed particles into mirror samples to see what kind of pits they would produce, and asked colleagues to calculate how many particles might be zipping around at Webb’s intended location — a region beyond the Moon’s orbit called L2.

The mission team “invested a great deal of effort 20 years ago, to try to get their meteoroid environment right”, says Bill Cooke, head of NASA’s meteoroid environment office at the Marshall Space Flight Center in Huntsville, Alabama.

Engineers estimated that Webb would endure about one impact per month that could be large enough to ding the mirror. And they decided that it was a risk worth living with. They calculated that impact pits would accumulate over time, but that the dents would cover only 0.1% of the primary mirror after 10 years. Telescopes can still work if part of their primary mirror is damaged.

Micrometeoroids are created by collisions between asteroids and other planetary bodies. The particles are usually as small as a few tens of micrometres across — the size of sand grains — but could be as big as a bus. The Sun’s gravity pulls particles towards it, so dust generally flows from the outer regions of the Solar System towards the inner parts.

Even tiny particles can cause physical damage to spacecraft when they hit as fast as a speeding bullet — the velocities reached in space. The International Space Station is pitted with tiny holes left by micrometeoroids, for instance. And, in 2013, a micrometeoroid temporarily knocked out a US weather satellite.

All of this shows that space is a dusty place. “You’re gonna take hits,” Cooke says. “Occasionally there will be one that gets your attention.”

On alert

The late-May impact on Webb caught everyone’s attention. “I’ve spent the last six weeks answering micrometeoroid questions,” Menzel said at the news briefing. 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 accuracy, engineers were able to tweak the affected part to cancel out some, but not all, of the image degradation. (NASA says that the telescope is still performing well above expectations.)

Large micrometeoroids are much rarer than small particles, so the odds are that Webb was just unlucky enough to encounter a big one relatively early in its lifetime, says David Malaspina, a plasma physicist at the University of Colorado Boulder who studies cosmic-dust impacts on spacecraft. It’s as if a card player had drawn a particular card from the deck on the first round of play, as opposed to it coming up later in the game. Scientists can only wait to see what happens next.

In the meantime, 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 are looking out for meteor showers, which happen when Earth passes through a concentrated trail of debris left by a passing comet. Dust from meteor showers constitutes only about 5% of the impact risk to Webb, compared with the 95% risk from the random, or ‘sporadic’, hits caused by background dust flowing through the Solar System.

Cooke’s office is now generating custom meteor-shower forecasts for the Webb team so that mission controllers will know when the telescope is about to pass through a heavy stream of dust — and will be able to reorient the instrument to block particles from hitting its mirrors. This situation might arise in May 2023 and May 2024, when Webb could pass through debris from Comet Halley.