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On July 1st, 2023, the European Space Agency (ESA) launched the Euclid Observatory<\/em>, a mission that will spend the next six years investigating the composition and evolution of the Universe. In particular, Euclid <\/em>will observe how the Universe has expanded over the past 10 billion years to test theories about Dark Energy. While fine-tuning and calibrating the telescope\u2019s instruments in preparation for the mission\u2019s first survey, the mission team noticed that a few layers of water ice formed on its mirrors after it entered the freezing cold of space. <\/p>\n While common, this is a problem for a highly sensitive mission like Euclid, which requires remarkable precision to investigate cosmic expansion. After months of research, the Euclid team tested a newly designed procedure to de-ice the mission\u2019s optics. On March 20th, the ESA announced that the team\u2019s de-icing approach worked (so far) and that Euclid\u2019s vision has been restored. If the method proves successful, it will have validated the mission team\u2019s plan to keep Euclid<\/em>\u2018s optical system working for the rest of its mission.<\/p>\n <\/p>\n The problem first became evident as science operations experts noticed a small but progressive decrease in light measured by the telescope\u2019s VISible instrument (VIS). This instrument is responsible for measuring visible light from distant galaxies to determine how the light\u2019s trajectory is affected by gravity fields. Mischa Schirmer, a calibration scientist for the Euclid consortium and one of the main designers of the new de-icing plan, explained in an ESA press release:<\/p>\n \u201cWe compared the starlight coming in through the VIS instrument with the recorded brightness of the same stars at earlier times, seen by both Euclid and ESA\u2019s Gaia <\/em>mission. Some stars in the Universe vary in their luminosity, but the majority are stable for many millions of years. So, when our instruments detected a faint, gradual decline in photons coming in, we knew it wasn\u2019t them \u2013 it was us.\u201d<\/p>\n<\/blockquote>\n It was always expected that there would be some water contamination with Euclid<\/em>, which is why there was an \u201coutgassing campaign\u201d shortly after launch. This consisted of the telescope being warmed up by onboard heaters and also partially exposed to the Sun, sublimating most of the water brought from Earth. However, a considerable amount remained after being absorbed in the telescope\u2019s multi-layer insulation, which slowly began building up on the VIS intrument\u2019s mirror surfaces. After months of research, lab studies, and calibrations, the team determined the source and began working on a solution. <\/p>\n The obvious solution was to heat Euclid <\/em>again by running all its internal heaters for days. However, this ran the risk of deforming the mechanical structure of the spacecraft, which could alter Euclid\u2019s optical alignment. Said Andreas Rudolph, Euclid Flight Director at ESA\u2019s mission control:<\/p>\n \u201cMost other space missions don\u2019t have such demanding requirements on \u2018thermo-optical stability\u2019 as Euclid. To fulfil\u00a0Euclid\u2019s scientific goals\u00a0of making a 3D map of the Universe by observing billions of galaxies out to 10 billion light-years, across more than a third of the sky, means we have to keep the mission incredibly stable \u2013 and that includes its temperature. Switching on the heaters in the payload module therefore needs to be done with extreme care.\u201d<\/p>\n<\/blockquote>\n The team began by individually heating two of Euclid\u2019s mirrors independently, a low-risk approach since they are located in areas where water vapor was not likely to contaminate other instruments. After analyzing the initial results, the team found that Euclid\u2019s vision was restored to its previous accuracy. However, this was a temporary solution, and a long-term strategy for regular de-icing is still being sought. In the meantime, the ESA promises to continue to monitor the telescope for changes and share any new findings publicly. <\/p>\n \n\n
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