The European space telescope Euclid has a very specific mission: photographing a vast portion of the sky to better understand how the Universe has evolved. It mainly observes very distant galaxies, billions of light-years away. But in September, a much closer object appeared in its images.
It all began when a computer program spotted a small streak in several photographs. Mikko Pöntinen, a doctoral student at the University of Helsinki, developed this tool to find objects moving in front of galaxies. When examining the images, he noticed that a sort of blurry cloud surrounded the streak.
Official illustration of the Euclid space mission dedicated to mapping galaxies and the distant Universe.
Credit: ESA, via Wikimedia Commons.
Wikimedia image
This is a characteristic sign of comets. These small bodies contain ice and dust, among other materials. When the ice turns into gas, it can carry dust with it and form a more or less pronounced halo around the nucleus. The researchers then checked that no previously catalogued comet or asteroid matched this object.
The discovery was confirmed on October 8, 2026, by the Minor Planet Center, the international organization responsible for collecting observations of small bodies in the Solar System. The newcomer was named C/2026 S1 (Euclid). It appears in 16 images taken between September 16 and 24.
At the time of the observations, the comet was about 1.3 billion kilometers from Earth. It follows a highly inclined trajectory relative to the plane in which the planets orbit. Its journey around the Sun would take several thousand years, although astronomers still need to refine its orbit.
That is precisely what makes the discovery surprising. Euclid avoids the regions of the sky where planets, asteroids, and comets are concentrated as much as possible. These nearby objects can interfere with its observations of galaxies. The telescope also does not regularly return to the same location to track their movements.
Yet this comet appeared in its field of view several times. This fortunate coincidence shows that Euclid can also help discover objects in the Solar System. Ground-based astronomers can then observe them to better determine their trajectories.
The ESA sees this in particular as a way to prepare for future missions to comets, such as Comet Interceptor, designed to encounter an object that has not yet been studied extensively. This new comet is not suitable for that mission. Nevertheless, it proves that a telescope aimed at the far reaches of the Universe can also make discoveries much closer to home.