Euclid, the European space telescope built to map galaxies, has found a comet nobody knew about, and a piece of cleanup software is the reason.
The comet, named C/2026 S1 (Euclid), smeared five sets of images between September 16 and 24. Moving-object software developed by Mikko Pöntinen, a doctoral student at the University of Helsinki, flagged the errant streak. "By predicting [the object's] position, I could identify it in other images," he said in a statement. Closer inspection showed a fuzzy cloud around it, a sign of an active comet shedding dust and gas. The Minor Planet Center confirmed the detection on October 8, making it one of 13 comets found so far this year. More than 4,000 are known.
The comet sits about 800 million miles from Earth, roughly between Jupiter and Saturn in distance. It is not in the flat plane where the planets orbit. Relative to the sun, it loops overhead, and one full orbit likely takes thousands of years, according to the European Space Agency.
That is why this counts as luck rather than strategy. Euclid, launched in 2023 in partnership with NASA, takes long exposures of distant galaxies. It looks away from the strip of sky along the solar system's plane, where comets and asteroids are common and dust glare can spoil deep-sky images. One of the discovery team, Bruno Altieri, said very few comets are expected from Euclid. A reasonable reading: this is a bonus, not a second job.
Still, the find shows something useful. Euclid needs to detect and remove stray signals to photograph about one-third of the sky, and the same filtering that discards contamination can also catalog it. Catching a comet this far from the sun gives astronomers time to watch it switch on as it approaches, and to study what it is made of.
How bright it gets, and how close it comes, depends on an orbit that only further tracking can pin down.