The European space telescope Euclid was not searching for comets when a small, bright scratch-like mark appeared in its images among distant galaxies. Its original mission was to draw a wide map of the universe to understand dark matter and dark energy, but a computer program that tracks moving objects revealed that the mark belonged to a previously unknown visitor.
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On October 19, 2026, the European Space Agency announced the discovery of comet 2026 C/2026 S1, after observing it in images taken between September 16 and 24. It is the first new comet discovered by the telescope, although it has previously imaged known comets.
The object is about 1.3 billion kilometers from Earth, and it moves high above the plane in which most planets orbit, in a highly elongated path that opens a window onto the edges of the solar system.
How did a line of light reveal a hidden object?
Mikko Pontinen, a PhD student at the University of Helsinki, developed a program that searches Euclid's images for the lines left by moving objects. When it found an unusual signal, it predicted the object's location in other images and found it in five sets of observations.
Pontinen said he was excited when his program detected a streak of light running across several images. Subsequent review revealed a halo around the object, caused by gas and dust released from its surface—a characteristic that distinguishes comets from many asteroids.
The telescope observed the object in 16 exposures spread across five imaging fields in an area called the "Northern Deep Field." Three images taken on September 17 show its transit in front of distant stars and galaxies over a period of approximately 36 minutes.
Finding it is a rare coincidence; the telescope usually avoids the belt of the sky crowded with objects of the solar system, and does not easily return to the same location after photographing it; therefore, scientists expect that its new discoveries of comets will be few.
A journey that takes hundreds of thousands of years
Comets are icy and dusty remnants from the formation of the solar system about 4.6 billion years ago. A preliminary analysis presented by the Euclid Consortium suggests that the newly discovered comet could travel as far as 1.5 trillion kilometers from the Sun and not return for about 345,000 years. These estimates are still subject to revision as its orbital measurements improve.
Such objects give scientists the opportunity to study ancient materials that have retained some of their properties since the formation of the planets, and the discovery also proves that automated searches of galaxy images can turn into an early warning system for distant comets.
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