The ESA’s Euclid Telescope Discovered a New Comet Completely by Accident

Approximately 40 to 70 new comets are discovered every year, most of which are found by astronomers actively looking for comets or at least new celestial bodies in general. But at least one comet discovered in 2026 doesn’t have that distinction. It was discovered entirely by accident by a telescope that was not actively looking for comets. 

Say hello to C/2026 S1 (Euclid), a brand new comet that was just discovered by the European Space Agency’s Euclid space telescope. The comet is located about 1.3 billion kilometers away from Earth, which is roughly the distance between the sun and the orbits of Jupiter and Saturn. It was discovered by Mikko Pöntinen of the University of Helsinki, together with the Solar System Objects Science Working Group, who were using the Euclid telescope at the time of discovery. 

What makes this one weird, among the thousands of other comets humans have named, is how they found it. The group was intentionally pointing the telescope away from the elliptical plane, a band in the sky where the solar system’s planets, most of its asteroids and a significant number of its comets reside. The idea was to avoid the cosmic dust and debris left behind by those celestial bodies, allowing the group to take clearer pictures of distant galaxies in deep space. 

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While doing this, a comet zipped through. It could have gone entirely unnoticed, but Pöntinen had written software to identify moving objects, which flagged images where the comet appeared. Pöntinen was then able to use its speed and trajectory to find it in other images and report it as a new comet. 

“I was excited to see my software flag a ‘streak’ smeared across several images,” Pöntinen said in a statement. “Further inspection revealed that it had a fuzzy cloud around it, indicative of an active object like a comet that is shedding dust and gas.”

A happy coincidence

The most interesting part of C/2026 S1 (Euclid) is undoubtedly that it was discovered by a telescope that wasn’t designed to find comets on a mission that also wasn’t designed to find comets. 

According to Bruno Altieri, archive scientist and member of the Euclid solar system team, this is because of the way the Euclid telescope works. It spends most of its time in a wide survey that will eventually map about one-third of the extragalactic sky, which encompasses all the planets, radiation clouds, debris clouds and other structures outside of the Milky Way galaxy. 

It also employs a “step-and-stare” method of mapping the cosmos, which means it “steps” to a spot, stares at the sky for about an hour and then moves on. That means much of what it looks at is only looked at once. Since comets are often found thanks to their movement, it’s hard to identify such an object with this observation method.

Euclid also conducts a deep field survey. These are zoomed-in glimpses of specific parts of space conducted periodically around the schedule of the wide survey. These parts are revisited only a handful of times per year, and that’s the only reason C/2026 S1 (Euclid) was discovered. 

“This discovery is indeed a happy coincidence as few comets are expected to be discovered by Euclid,” Altieri said. 

Straight up from the sun

Very little is known about C/2026 S1 (Euclid) so far, as it’s only been imaged a few times. There are a few things that astronomers have already gleaned from what they have seen. For starters, this is a long-period comet, which means its orbit around the sun lasts more than 200 years. The exact time hasn’t been calculated, but early estimates put it at thousands of years before it completes a full orbit.

According to Altieri, that means it almost certainly came from the Oort Cloud, which is a shell of icy bodies, rocky planetesimals and other debris that surrounds the entire solar system. Comets from the Oort Cloud are referred to as “nearly isotropic” comets and orbit the sun from all sorts of zany angles. C/2026 S1 (Euclid) is no different. 

Space doesn’t have an up or down, but if it did, C/2026 S1 (Euclid) would be straight up from the sun as viewed from Earth. Its sweeping orbit takes it far above the solar system before cutting between Saturn and Jupiter on its way back down. It’s expected to reach perihelion — the point where it’s closest to the sun — in 2027. After that, it won’t be back there for thousands of years.