Theo ❁

.@ESA_Euclid was built to investigate dark matter and dark energy by mapping billions of distant galaxies. While carrying out that survey, ESA’s space telescope has also discovered something much closer to home: its first previously unknown comet, C/2026 S1 (Euclid), recorded in images taken between 16 and 24 September 2026. The discovery came through software developed by Mikko Pöntinen, a doctoral researcher at the University of Helsinki @helsinkiuni working with the Euclid Consortium’s Solar System Objects Science Working Group. His program searches for moving objects against the background of stars and galaxies. During a long exposure, a comet changes position enough to leave a small streak. Around this one, the images also revealed a diffuse cloud, or coma, indicating that it was releasing dust and gas. The streak itself records its movement during the exposure; it should not be confused with a comet’s physical tail. With no matching object in existing catalogues, Pöntinen reported it to the Minor Planet Center. Ground-based follow-up helped confirm the discovery on 8 October. The comet lies approximately 1.3 billion kilometres from Earth, high above the planets’ orbital plane. Its journey around the Sun takes many thousands of years, although its orbital period still needs refinement. Finding it required a fortunate alignment of circumstances. Euclid avoids regions close to the Solar System’s plane, where sunlight scattered by interplanetary dust interferes with its sensitive observations. This comet’s steeply inclined trajectory carried it into the telescope’s Deep Field North, where it appeared in 16 exposures across five fields of view. Those repeated sightings allowed the team to trace its motion and investigate its nature. Comets contain ice and dust inherited from the formation of the Solar System around 4.6 billion years ago. Studying them helps astronomers investigate the materials from which the planets formed. Euclid’s observations already show activity around this distant object, giving researchers an opportunity to follow how it develops as it continues along its orbit. The discovery also provides practical experience for ESA’s future Comet Interceptor mission, which will wait in space before travelling towards a suitable comet. C/2026 S1 is unsuitable for that mission, but finding and confirming it has tested the discovery and alert process that could help identify a future target.