Euclid detected its first previously unknown comet, C/2026 S1 (Euclid), in observations made from September 16 to 24, 2026. The Minor Planet Center (MPC) issued a circular identifying the comet on October 8; the European Space Agency (ESA) reported the discovery on October 19. ESA’s account and the MPC circular document the discovery and its published orbit.
How Euclid’s images revealed the comet
Moving-object software developed by Mikko Pöntinen flagged a streak across Euclid images. Pöntinen predicted where the object would appear next and found it in five image sets. Follow-up inspection revealed a diffuse coma, the cloud of dust and gas surrounding an active comet, according to ESA.
ESA says the comet appeared across five fields of view, with two to four exposures per field and 16 exposures in total. Euclid is designed to image distant galaxies and avoids the band along the Solar System’s plane, where comets and asteroids are more common, to reduce glare from dust. The discovery came from the comet’s path crossing Euclid’s observations, rather than from a survey aimed at finding comets.
What the published orbit tells us
The MPC’s orbital solution gives C/2026 S1 (Euclid) an eccentricity of 0.9956082 and an inclination of 85.22516 degrees. Eccentricity describes how stretched an orbit is: a value close to 1 corresponds to a highly elongated path. Inclination describes the orbit’s tilt relative to the reference plane. Together, these values describe a steeply tilted, elongated orbit.
The solution uses 54 observations made from September 16 through October 8, 2026, and lists a mean residual of 0.5 arcseconds. The MPC lists the comet’s perihelion, its closest point to the Sun, for September 2, 2027, at 2.07900 TT. ESA describes the orbital period as thousands of years.
Why the discovery matters to Comet Interceptor
ESA says C/2026 S1 (Euclid) is not a suitable target for Comet Interceptor. The discovery and alert process nevertheless gave the Euclid and Comet Interceptor teams a test case for coordinating their work in the future.
