A surprising dust strike on the Webb Telescope puts scientists on alert

A surprising dust strike on the Webb Telescope puts scientists on alert

An engineer in a clean room suit stands on a pod next to the primary mirror of the James Webb Telescope

The Webb Telescope’s extraordinarily large primary mirror gives it extraordinary capabilities, but also makes it vulnerable to impacts from space dust.Credit: NASA/Chris Gunn

As NASA’s James Webb Space Telescope prepares to release its first science images on July 12, engineers are keeping tabs on a small but potentially impactful future threat: micrometeoroids. Although mission scientists expected the telescope to be hit by these tiny bits of space dust during its expected 20-year lifespan, a relatively large blow in May caused them to reassess what they thought they knew about how often Webb will be skinned.

For now, the telescope’s performance is intact. But understanding the risk of future impact is crucial because Webb represents an investment of US$11 billion for NASA, the European Space Agency and the Canadian Space Agency – and researchers hope it will transform astronomy. “Time will tell if this latest impact was just some kind of anomaly,” Mike Menzel, Webb’s principal systems engineer at the Goddard Space Flight Center in Greenbelt, Maryland, said at a press conference on May 29. June.

From its location in deep space, 1.5 million kilometers from Earth, the telescope peers out into the cosmos using a 6.5-metre-wide primary mirror – the largest ever launched into the world. ‘space. Although the mirror makes Webb a very capable telescope, its large size also makes the observatory vulnerable to bombardment 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 deduced that the fifth was larger than the first four, and larger than they had expected.

Coat Predictions

Two decades ago, during Webb’s design phase, engineers knew it would be regularly bombarded by micrometeoroids. Unlike the Hubble Space Telescope 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 sample mirrors to see what kind of pits they would produce, and asked their colleagues to calculate how many particles might move to the location Webb predicted – a region beyond the Moon’s orbit called L2.

The mission team “invested a lot of effort 20 years ago trying to get the 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 experience about one impact per month that could be large enough to bang into the mirror. And they decided it was a risk worth taking. They calculated that the impact pits would accumulate over time, but the bumps would only cover 0.1% of the primary mirror after 10 years. Telescopes can still work if part of their main mirror is damaged.

Micrometeoroids are created by collisions between asteroids and other planetary bodies. The particles are typically 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 towards it, so dust generally flows from the outer regions of the solar system to the inner parts.

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

All of this shows that space is a dusty place. “You’re going to take hits,” Cooke says. “Once in a while, there will be one that catches your eye.”

On alert

Late May’s impact on Webb caught everyone’s attention. “I’ve spent the last six weeks answering questions about micrometeoroids,” Menzel said at the press conference. The impact left a small distortion in one of the 18 hexagonal segments that make up Webb’s primary mirror. Since the positions of Webb’s mirror segments can be adjusted with exquisite precision, engineers were able to modify the affected part to undo some, but not all, of the image degradation. (NASA says the telescope still performs well above expectations.)

Large micrometeoroids are much rarer than small particles, so chances are Webb was unlucky enough to encounter a large one relatively early in his life, says University of Colorado plasma physicist David Malaspina. in Boulder who studies the impacts of cosmic dust on spacecraft. It is as if a card player had drawn a particular card from the deck on the first turn of the game, instead of bringing it out 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 hit rate estimates, which come from a model that has been updated several times since Webb was designed.1.

And they look for meteor showers, which occur when Earth passes through a concentrated trail of debris left by a passing comet. Dust from meteor showers represents only about 5% of impact risk for Webb, compared to 95% risk from random or “sporadic” hits caused by background dust passing through the solar system.

Cooke’s office now generates custom meteor shower forecasts for the Webb team so mission controllers know when the telescope is about to pass through a strong dust current – and can reorient the instrument to prevent particles from hitting its mirrors. This situation could arise in May 2023 and May 2024, when Webb could pass through the debris of Comet Halley.

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