Space. Damaged by a micrometeoroid, should James Webb fear for his future?

Less than six months after its insertion, the James Webb Space Telescope was hit by a micrometeoroid that irreversibly damaged part of its primary mirror. More precisely segment C3, one of the 18 hexagonal mirrors responsible for returning the light rays to the secondary mirror, which in turn sends them to the measuring instruments.

However, researchers want to be reassuring: Even with a rejection of the windshield, JWST continues to exceed all expectations. “The effect on the level of the telescope as a whole is minimal in terms of only a small part of the mirror,” details a preliminary report published last week by NASA, ESA and the Canadian Space Agency. , project partners. A recalibration of the 17 other mirrors also made it possible to largely compensate for the deformation of the C3 segment without being able to completely correct it.

For the first time, a report also shows the extent of the damage to the image: the impact on the mirror, bottom right, is clearly visible in the form of a white spot. In a single event, the C3 segment suffered ten times greater degradation than researchers expected over the next six years. Revealed in early June, the impact would have taken place between May 22 and May 24.

Six hits found

Is it due to an accident? This is one of the roads that space agencies prefer. But the latter also does not rule out the possibility of a misjudgment of the threat. “The project team is conducting further studies of the micrometeoroid population, how influences affect mirrors of berylliumand the effectiveness of certain risk mitigation measures, “the report states.

During the telescope’s six months of commissioning, five other collisions with the mirror were recorded, with negligible effect on image quality. These figures remain in line with the researchers’ predictions, which expected a micrometeoroid impact per month that could distort the mirror. No fewer than 19 other harmless collisions were also detected within three months.

A known threat

Micrometeoroids have long been identified as one of the major threats to space exploration. These small fragments of metal or stone, the mass of which is less than a gram, are ubiquitous in the solar system: every year more than 30,000 tons of incognito are deposited on our planet (we then talk about micrometeorites), which are sometimes found buried in the poles. But size is not everything. This small waste can reach several kilometers per second: at these speeds, the smallest collision with a waste the size of a semolina grain can produce energy comparable to a large-caliber shot. And microscopic debris exposes spacecraft to a phenomenon of wear and tear over time.

Without this being fully proven, some satellites have already succumbed to such influences. The habitable modules in the ISS all have a Kevlar shield that is responsible for stopping or braking micrometeoroids or other space debris that may cross its path. And for larger objects, the station is able to perform evasive maneuvers.

A mirror exposed to the room

Not so for the particularly vulnerable James Webb Space Telescope. Unlike its cousin Hubble, whose mirror is protected at the bottom by a metal tube, the JWST’s mirror sits outside. A rock passes, and the collision with its giant 6.5-meter mirror is almost certain. But the risk was considered acceptable: according to the researchers’ calculations, the deformation of the mirror should not exceed more than 0.1% of its total surface after ten years … provided that the effects like that of the month must not multiply.

Over the next few months, therefore, the file should be the subject of special attention from the JSWT teams. And roads are already being considered to limit the risk of collision, for example by reorienting the telescope to reduce the surface of the mirror, which is exposed when crossing a meteoroid rain. A phenomenon that may occur in May 2023 and May 2024 is reminiscent of the scientific journal Nature, when the telescope will cross the wake of Halley’s comet.

A time machine

The culmination of a project that lasted more than 25 years and had seen its cost rise to ten billion dollars, the James Webb Space Telescope was launched on December 25 from the Kourou Space Center in French Guiana. Often presented as the successor to Hubble, it is above all complementary: by operating in the infrared, JWST can go back in time by examining the distant universe, as it was only a few hundred million years after the Big Bang.

The size of a tennis court, JWST was sent 1.5 million kilometers from Earth at the Lagrange L2 point, which allows it to accompany our planet in its orbit around the Sun.

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