Space is a tough environment for any spacecraft, with intense radiation, hard vacuum, and wild temperature swings. The ESA's Euclid space telescope has encountered a different problem—its mirrors are accumulating ice. It's not unheard of for trapped moisture to form ice on a spacecraft, but even a thin layer of ice on the telescope's mirrors threatens to throw off its sensitive infrared observations. So, the team is beginning a de-icing procedure from a million miles away.
The ESA launched Euclid last summer on a mission to study the oldest and faintest galaxies in the universe. Like the James Webb Space Telescope, Euclid conducts its infrared observations from the far side of the moon at the Earth-Sun L2 Lagrange point. After reaching its destination last year, the ESA used onboard heaters to evaporate any excess moisture inside the spacecraft to avoid the exact problem it is currently experiencing. Unfortunately, it didn't work perfectly.
According to the ESA, the telescope's mirror has developed a layer of ice no thicker than a strand of DNA. The instruments still work, but mission managers have noted a "small but progressive decrease" in the amount of light detected. The team measured this by observing the same objects repeatedly with its visible light camera. "Some stars in the Universe vary in their luminosity, but the majority are stable for many millions of years," said the ESA's Mischa Schirmer. "So, when our instruments detected a faint, gradual decline in photons coming in, we knew it wasn’t them – it was us."
The ice buildup is probably due to internal components (like insulation) absorbing water during assembly. In the vacuum of space, these water molecules occasionally sublimate and then freeze on other components—in this case, the mirrors. The team spent several months running simulations and laboratory studies to understand the problem, eventually concluding there are several ultra-thin layers of ice on the mirror, measuring just tens of nanometers thick. The ESA notes this is a "remarkable testament" to the power of Euclid that it can detect such a small obstruction.
The team will again run the telescope's onboard heaters to clear the ice, but it's going about it very carefully. The quickest and most effective way to remove the ice would be to do a decontamination cycle, which runs all the heaters for several days, raising the internal temperature from about -140 degrees Celsius to -3 degrees Celsius. However, this would heat up all parts of the spacecraft, causing some components to expand more than others. There's no guarantee all the parts will fit together correctly after it cools down. Instead, the ESA is heating "low-risk" sections of the telescope's optics, starting with two mirrors that can be independently heated. After each heating cycle, the team will check how many photons they get back.
As the impact of the ice accumulation is currently very low, the spacecraft will continue science operations while the ESA begins the de-icing process. The telescope will continue releasing absorbed water throughout its life, so the ESA may need to devise a long-term de-icing solution.
