Euclid Comet Find Points to New Solar System Tracking Role

The European Space Agency's Euclid spacecraft has an unexpected first to its name: a comet nobody had seen before. The Euclid comet, now designated C/2026 S1 (Euclid), turned up in images taken between 16 and 24 September. Euclid was built to map distant galaxies, not to chase fast-moving bodies in the Solar System, which makes the find notable, according to the mission's announcement.

The discovery began with software. Mikko Pöntinen, a doctoral student at the University of Helsinki, built a program to scan Euclid's images for small Solar System objects, and it flagged a "streak" smeared across several frames. By predicting where the object would appear next, Pöntinen tracked it across five image sets in all. A closer look revealed a fuzzy cloud of dust and gas around the object, the signature of an active comet shedding material as it travels toward the Sun.

Checks against catalogues of known comets and asteroids found no match, so Pöntinen reported the sightings to the International Astronomical Union's Minor Planet Center, the global clearing house for such observations. Ground-based astronomers then contributed follow-up observations, and on 8 October the center confirmed the object as a new comet. The find was reported on behalf of the Solar System Objects Science Working Group of the Euclid Consortium.

An unlikely tool for the job

Detecting comets with Euclid is difficult by design. The telescope takes deep, steady images of distant galaxies and deliberately points away from the dust-rich band along the Solar System's plane, where stray glare would hinder its sensitive imaging. Its step-and-stare observing strategy also visits each patch of sky only once, so there is no easy way to swing back and recheck a candidate.

In the Euclid Deep Field North survey, the object appeared as a streak because it moved during a 560.52-second exposure captured on 20 September, with its faint coma of dust and gas visible in the frame. In total, the comet yielded sixteen exposures across several fields of view, between two and four per field.

Tracking such objects matters for the main mission anyway. Moving trails and extra light from Solar System bodies have to be identified and removed so they do not contaminate the precision galaxy mapping that is Euclid's core task, according to the mission team.

The comet itself sits far from Earth: roughly 1.3 billion kilometres away, about the distance from the Sun to the gap between the orbits of Jupiter and Saturn. It does not orbit in the same plane as the planets but arcs "straight up" over the Solar System on an orbit expected to take thousands of years to complete, astronomers said. The exact orbital period will be refined as more data are collected.

A useful rehearsal for the next comet mission

ESA treats the find as a dry run for Comet Interceptor, a future mission designed to visit a pristine comet still on its first approach to the Sun. The short path from software flag to Minor Planet Center confirmation shows Euclid can detect new comets while they are still relatively far from the Sun, potentially turning up a suitable target for that mission one day.

Valeria Pettorino, Euclid's project scientist, said the process of discovery and alert gives a "real test case" for how the Euclid and Comet Interceptor teams will coordinate in the future. She added that this particular comet is not itself a suitable candidate for Comet Interceptor, and that the team was pleased to have found a previously unknown comet "for the Solar System community to follow-up on".

"Very few comets are expected to be discovered by Euclid so it was a happy coincidence that the combination of the comet's orbit and Euclid's scanning over those days captured the object in multiple images," said Bruno Altieri, Euclid's archive scientist and part of the discovery team.

Why comets are worth chasing

Comets are icy, dusty leftovers from the birth of the Solar System about 4.6 billion years ago, mostly originating in the distant Kuiper Belt and Oort Cloud beyond the outer planets. Bodies native to the Solar System have changed very little since they formed, so studying them helps scientists understand the raw materials that built the planets, including clues about where Earth's water and the building blocks of life may have come from, according to the mission's background material.

Euclid's main survey continues regardless. Over a six-year programme it will record the shapes and motions of billions of galaxies in a search for the hidden influence of dark matter and dark energy. The mission is built and operated by ESA with contributions from its member states and NASA; the Euclid Consortium of more than 2000 scientists from 300 institutes is responsible for the scientific instruments and data analysis.

The new comet is now catalogued and available for ground-based observers to track as it continues its long arc across the sky. Each new observation will sharpen the still-rough estimate of its orbit. For a telescope designed to ignore the Solar System and stare into the deep Universe, that is a useful side effect. More detail on the discovery is available in the Nanowerk report on Euclid's comet find, which draws on the ESA announcement.