Euclid finds its first new comet: what C/2026 S1 is

ESA's Euclid space telescope was built to map billions of faraway galaxies, not to chase objects in our own Solar System. Yet it has just found something new close to home. On 19 October, ESA announced that Euclid has discovered its first previously unknown comet, now officially called C/2026 S1 (Euclid).

How did a galaxy telescope find a comet?

Euclid takes long, steady exposures of the deep sky. Anything that moves during an exposure leaves a smear. Mikko Pöntinen of the University of Helsinki, working with the Euclid Consortium's Solar System team, wrote software that looks for these moving objects. It flagged a streak across several images. By predicting where the object would be next, he found it in five sets of images in total. A closer look showed a fuzzy cloud around it, a sign of an active comet shedding dust and gas.

How was it confirmed?

The object did not match any known comet or asteroid, so it was reported to the International Astronomical Union's Minor Planet Center (MPC). That alerted astronomers on the ground, who added their own observations. The MPC confirmed it as a comet on 8 October. Euclid had seen known comets before. This is the first one it has discovered.

Where is the comet now?

ESA puts it about 1.3 billion kilometres from Earth, roughly the distance from the Sun to somewhere between the orbits of Jupiter and Saturn. It is not in the flat plane of the planets, though. Seen from the Sun, it is almost "straight up", arching over the Solar System on an orbit that takes thousands of years to complete. It is far too faint for backyard observing.

Why was this discovery unlikely?

Two reasons. Euclid deliberately avoids the band of sky along the plane of the Solar System, where most comets and asteroids are, because dust there adds glare. And its step-and-stare survey usually visits each patch of sky only once, so it cannot simply go back for another look. "Very few comets are expected to be discovered by Euclid," said Bruno Altieri, Euclid Archive Scientist. The comet's orbit and Euclid's scanning just happened to line up.

Why does it matter?

Tracking moving objects also helps Euclid clean its galaxy maps of trails and stray light. The bigger prize is ESA's future Comet Interceptor mission, which needs a fresh comet spotted while it is still far from the Sun. This one is not a suitable target, but the quick discovery-and-alert process was a real test of how the two teams could work together.

Want something you can actually see? The Orionid meteor shower, made of debris from Halley's Comet, peaks on the night of 21–22 October, and our guides to stargazing binoculars and a first telescope explain what to use.