Europe's Euclid space telescope has found its first unknown comet, C/2026 S1 (Euclid), even though it was built for a different job: mapping galaxies. A program that searches for moving objects spotted it, and the scientists say luck played a part.
The comet is about 1.3bn km from Earth, between the orbits of Jupiter and Saturn. Its orbit runs high above the plane in which the planets circle the Sun. The Minor Planet Center (MPC), which keeps records of such bodies for the International Astronomical Union, confirmed the comet on 8 October 2026. The European Space Agency (ESA) announced the find on 19 October.
A streak across images of the Euclid Deep Field North
Euclid photographs a patch of sky called the Euclid Deep Field North, in the constellation Draco. The images taken from 16 to 24 September 2026 are meant to help study dark matter and dark energy, not objects in the Solar System.
Mikko Pöntinen, a Finnish doctoral student at the University of Helsinki, wrote a program that looks for objects that change position from one image to the next. Such objects leave a smudge or streak, not a dot like the stars. "It was exciting to see my program flag a 'streak' smeared across several images," he says. One image, taken on 20 September, has an exposure of more than nine minutes (560.52 seconds), so the comet shows up as a long streak.
Pöntinen worked out which way the object was moving and found it in more images. In all, the comet appears in five sets of images, across 16 exposures. A close look showed a hazy rim of dust and gas around it. That is how an active comet behaves, not a plain asteroid.
He then checked the object against catalogues. It matched no known comet or asteroid, so he sent it to the MPC as a new comet. Telescopes in Switzerland, Austria, Hungary, Romania, Canada, the US and Hawaii watched it in late September and early October. On 8 October, a circular confirming the find was issued.
The comet's brightness is about magnitude 20.7, far too faint to see with the naked eye.
A telescope that does not hunt comets
Euclid is not a comet hunter. It avoids the plane of the Solar System, because dust glow there spoils its deep images of galaxies. It usually photographs each spot only once, and comets move, so a single image rarely shows enough to tell them apart.
That is why Bruno Altieri, a scientist at the Euclid archive, calls the discovery a "happy coincidence". He says Euclid is expected to find very few comets. The comet's high orbit and the way the telescope sweeps the sky happened to line up across several images.
The comet is not suitable for the Comet Interceptor mission, whose spacecraft is meant to meet a comet. Valeria Petorino, a scientist on the Euclid project, says the case is still a test of how the two teams will pass on alerts about future finds.
An orbit of thousands to more than 300,000 years
ESA says only that one trip around the Sun takes thousands of years, and that more observations will pin down the period. Italy's National Institute for Astrophysics (INAF) gives "more than 300,000 years".
The distance and the confirmation show that the comet is far away, active, and comes from the outer parts of the Solar System. It is not clear how INAF reached its figure. The first orbit estimate, based on the first weeks of observations, has a large margin of error, especially for a comet with such a long path.