Euclid space telescope discovers first new comet by accident
Key takeaways
- Euclid discovered C/2026 S1 (Euclid) in September 2026, located approximately 1.3 billion kilometres from Earth between the orbits of Jupiter and Saturn
- A bespoke moving object detection software developed by Mikko Pöntinen at the University of Helsinki flagged the comet as a streak across multiple images
- The discovery demonstrates Euclid can detect new comets while still relatively far from the Sun, potentially aiding ESA's future Comet Interceptor mission
The European Space Agency's Euclid space telescope, launched in 2023 to study dark matter and dark energy across 10 billion light-years of space, has accomplished something it was never designed to do: discover a previously unknown comet. The International Astronomical Union's Minor Planet Center confirmed C/2026 S1 (Euclid) as a new comet on 8 October, making it the first discovery of its kind by the mission.
The comet was spotted in images captured between 16 and 24 September 2026 during the Euclid Deep Field North survey. It lies approximately 1.3 billion kilometres away, roughly between the orbits of Jupiter and Saturn, though not in the same plane as the planets. Instead, it arches over the Solar System on an orbit that will take thousands of years to complete.
How the discovery happened
The detection required both serendipity and human ingenuity. Mikko Pöntinen, a PhD student at the University of Helsinki, developed moving object detection software specifically to monitor small Solar System objects within Euclid's images. The software flagged a "streak" smeared across several image frames, the telltale sign of a fast-moving object. By predicting the comet's position across time, Pöntinen identified it in a total of five separate image sets, yielding 16 exposures in all.
"I was excited to see my software flag a 'streak' smeared across several images," Pöntinen said. Further inspection revealed a fuzzy cloud, or "coma" in astronomical terminology, surrounding the object. This halo of dust and gas indicated an active comet shedding material as it approaches the Sun.
Why this matters for Euclid's mission
The discovery is striking because Euclid deliberately avoids the asteroid- and comet-rich band of sky along the Solar System's plane. This positioning reduces glare from dust that would otherwise hinder the telescope's sensitive deep-sky imaging and compromise its primary mission: revealing the hidden influence of dark matter and dark energy on billions of galaxies.
Euclid also uses a "step-and-stare" imaging strategy, which means it typically visits each section of sky only once. a comet cannot simply be revisited on demand to confirm its characteristics, a constraint that makes Solar System discoveries particularly challenging.
Nonetheless, tracking Solar System objects is operationally essential. Without identifying comets and asteroids appearing in Euclid's field of view, their light trails would appear as "noise" in the data, corrupting the precision galaxy mapping that underpins the entire mission.
"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 Archive Scientist.
The Comet Interceptor connection
While C/2026 S1 (Euclid) itself does not meet the criteria for ESA's forthcoming Comet Interceptor mission, the discovery process gives us a real test case for how our two mission teams will coordinate in the future. The quick turnaround between initial detection and validation demonstrates that Euclid can identify comets while they remain relatively far from the Sun, potentially leading to a suitable target for ESA's future Comet Interceptor mission.
Valeria Pettorino, Euclid Project Scientist, emphasised this strategic benefit: "While this particular comet is not a suitable candidate for Comet Interceptor, the process of discovery and alert gives us a real test case for how our two mission teams will coordinate in the future. And we are of course happy to have discovered a previously unknown comet for the Solar System community to follow-up on."
The discovery protocol itself has broader implications. Comets are icy, dusty remnants from the Solar System's formation approximately 4.6 billion years ago, with most originating in the distant Kuiper Belt and Oort Cloud. Because they have changed very little since formation, they hold clues about the raw ingredients and conditions that shaped the planets, including the origins of Earth's water and the building blocks of life.
What this means for space missions
The Euclid discovery illustrates an underappreciated principle in space telescope operations: instruments designed for one purpose often generate unexpected scientific value. By equipping data analysis teams with the right software tools and workflows, missions can extract insights far beyond their primary objectives.
The detection of one unexpected comet along the way demonstrates that even the most precisely engineered scientific instruments can surprise their operators.
