The European Space Agency’s Euclid space telescope has identified its first previously unknown comet, detected in images captured between 16 and 24 September 2026.

The find emerged from routine processing of the mission’s deep-field observations, which were designed to map distant galaxies rather than track fast-moving Solar System bodies. Researchers noted that confirming the object required only a brief interval between initial detection and independent validation, underscoring the telescope’s unexpected sensitivity to transient phenomena.

Mikko Pöntinen of the University of Helsinki, working with the Solar System Objects Science Working Group of the Euclid Consortium, first flagged the signal using custom software that scans for moving points of light in the telescope’s data. “I noticed a streak across several frames,” Pöntinen said. “By extrapolating its motion, I located it in additional exposures, confirming it appeared in five separate image sets.” Follow-up analysis revealed a faint, diffuse halo consistent with outgassing activity.

Cross-referencing with existing comet and asteroid catalogs yielded no match, prompting the team to notify the International Astronomical Union’s Minor Planet Center. The center verified the object as a new comet on 8 October, assigning it the designation C/2026 S1 (Euclid). Current estimates place the comet roughly 1.3 billion kilometres from Earth, a distance that situates it between the orbits of Jupiter and Saturn. Its trajectory is highly inclined relative to the planetary plane, appearing nearly overhead from the Sun’s perspective and following an orbit that will require thousands of years to complete.

Euclid’s design intentionally avoids the bright band of sky where most asteroids and comets reside, reducing glare that could interfere with its primary goal of high-resolution galaxy imaging. Moreover, the spacecraft’s “step-and-stare” observation mode typically surveys each sky region only once, limiting opportunities for follow-up. Despite these constraints, monitoring moving objects remains essential to distinguish genuine signals from artifacts in the telescope’s deep-field data.

“Discovering a comet under these conditions was unexpected,” said Bruno Altieri, Euclid Archive Scientist and member of the discovery team. “The object’s orbit and timing allowed it to appear in multiple frames, but such events are rare.” Valeria Pettorino, Euclid Project Scientist, added that while the comet does not qualify as a target for the upcoming Comet Interceptor mission, the episode provides a valuable test of coordination between Euclid’s alert system and external follow-up networks.

Comet C/2026 S1 (Euclid) represents the first new comet discovered by Euclid, though the mission has previously recorded known comets in its dataset. The detection highlights the telescope’s capacity to contribute to planetary science when its deep-field strategy intersects with transient Solar System activity.

The find reinforces Euclid’s broader scientific value beyond dark-energy and dark-matter mapping, demonstrating how large-scale sky surveys can yield unexpected discoveries across multiple domains of astronomy.