Euclid was not built to hunt comets. ESA's space telescope launched in July 2023 to map about 1.5 billion galaxies, reaching back to a redshift of about 2, and to chart the large-scale structure of the universe. Comets are a long way down its list of priorities. Even so, the telescope has now found one that nobody had seen before.
The object, now officially designated C/2026 S1 (Euclid), appeared in images Euclid took between 16 and 24 September 2026. ESA announced the discovery on 19 October. Euclid has photographed known comets before, but this is the first new one credited to it.
A streak in the data
The find came from Mikko Pöntinen of the University of Helsinki, who works with the Euclid Consortium's Solar System Objects Science Working Group. His software searches Euclid's images for things that move. In a survey designed around galaxies that do not move at all, it flagged a streak.
Pöntinen then found the same object in five separate sets of images. It was not a sharp point of light. It had a fuzzy cloud around it, the sign of an active object shedding dust and gas. As Universe Today notes, the discovery relied on new software that Pöntinen designed.
Checks against catalogues of known comets and asteroids found no match, so he reported the observations to the International Astronomical Union's Minor Planet Center. The Minor Planet Center confirmed on 8 October that the object is a comet and gave it a name that credits the telescope. The report also alerted ground-based astronomers, who could then contribute follow-up observations.
What the orbit tells us
The Minor Planet Center's object page for C/2026 S1 gives the comet's orbital elements, based on 58 observations made between 16 September and 13 October 2026. Euclid's observation on 16 September is listed as the first reported one.
- Inclination: 85.2 degrees. The orbit is tilted almost at right angles to the plane the planets orbit in. ESA says the comet does not lie in the same plane as the planets and arches over the Solar System.
- Eccentricity: 0.9982. An eccentricity of 1 would mean an open, unbound path. This orbit is closed, but only just. It is a very long, thin ellipse, which matches ESA's statement that one orbit takes thousands of years.
- Perihelion: 8.74 AU, on 18 September 2027. That will be the comet's closest approach to the Sun, and it is still very distant, well beyond the asteroid belt.
- Brightness: about magnitude 20.5 in the early Euclid observations listed by the Minor Planet Center. That is very faint.
ESA puts the comet at about 1.3 billion kilometres from Earth, roughly the distance from the Sun to a point between the orbits of Jupiter and Saturn. At that range the comet already shows a fuzzy coma, which ESA reads as the mark of an active object.
Why a galaxy survey is a strange place to find one
Euclid's survey design works against finding Solar System objects. Euclid deliberately avoids the asteroid- and comet-rich band of sky along the Solar System's plane, to reduce unwanted glare from dust that would hinder sensitive deep-sky imaging. It also uses a "step-and-stare" strategy that typically visits each section of the sky only once, so a comet cannot simply be revisited on demand to confirm its characteristics.
Those choices make sense for a cosmology mission. They also help explain why this comet was found where it was. Because its orbit is so steep, it does not follow the planetary plane, and it passed through one of Euclid's fields. The comet turned up in the Euclid Deep Field North, and Universe Today reports it was seen in five sub-fields with two to four exposures each. As Bruno Altieri, Euclid Archive Scientist, put it in ESA's release, it was a happy coincidence that the comet's orbit and Euclid's scanning over those days captured the object in multiple images.
Why It Matters
ESA describes comets as icy, dusty leftovers from the birth of the Solar System about 4.6 billion years ago, and notes that those originating in our Solar System have changed very little since they formed. Studying them helps scientists understand the raw ingredients and conditions that shaped the planets. Universe Today adds that comets may be especially relevant as deliverers of water to the inner Solar System, including to Earth.
The find also says something about Euclid's archive. Moving-object software applied to a dataset meant for something else turned up a comet. ESA says the quick turnaround between identification and validation shows that Euclid can detect new comets while they are still relatively far from the Sun, potentially leading to a suitable target for its future Comet Interceptor mission.
C/2026 S1 itself is not a suitable candidate for Comet Interceptor, according to Altieri. Valeria Pettorino, Euclid Project Scientist, says the process of discovery and alert gives the two mission teams a real test case for how they will coordinate in the future.
For now, C/2026 S1 (Euclid) is a faint object on a long, steep orbit. It will reach its closest point to the Sun in September 2027, far beyond the asteroid belt, and then head back out on an orbit that takes thousands of years to complete.
