PhD student built software to catch streaks in Euclid images, and it flagged the telescope's first comet discovery

On September 29, 2026, Mikko Pöntinen, a PhD student at the University of Helsinki, reported a moving object in Euclid space telescope images. His own tracking program had flagged it as a streak. On October 8 the Minor Planet Center (MPC) listed it as a new comet.

The European Space Agency (ESA) built Euclid to photograph galaxies. It points away from the band of sky where most comets move.

ESA announced on October 19 that C/2026 S1 (Euclid) is the first previously unknown comet the telescope has found. The agency also said that this comet is not a suitable target for its Comet Interceptor mission.

A streak in Deep Field North

Euclid took the images between September 16 and 24 in a northern survey area called Euclid Deep Field North. The comet appears in frames from the VIS camera, which records visible light.

Three of the VIS frames were released by ESA as an animation. They were taken at 00:41:07, 00:58:53 and 01:16:39 UTC on September 17. Each exposure lasts 560.52 seconds, long enough for the comet to leave a short dash.

From the first detections, Pöntinen predicted where the object would be in later frames and checked them. He found it in five fields of view over several days. That came to 16 exposures, two to four per field.

In ESA's release he said he was "excited to see my software flag a 'streak'" in the data. Further checks showed "a fuzzy cloud around it", which pointed to dust and gas coming off a comet.

The team then made a cleaner image from eight shorter exposures of 89.5 seconds. They aligned the frames on the comet and combined them. Background stars and galaxies were then removed, leaving the core and its coma.

Report to the Minor Planet Center

The object matched no entry in the catalogs of known comets and asteroids. Pöntinen reported for the Solar System Objects Science Working Group of the Euclid Consortium. The MPC, which works for the International Astronomical Union, then alerted other observers.

In his report, Pöntinen gave the coma a width of 10 arcseconds, a tiny angle next to the full Moon's roughly 1,800. The MPC circular lists eight Euclid measurers, from M. Pöntinen to B. Y. Irureta-Goyena. Euclid itself is observatory code 273, a 3.9-foot (1.2-meter) reflector.

Ye measured positions from the Zwicky Transient Facility in California, dated September 22 to October 8. On October 5, M. Jaeger's team in Martinsberg, Austria, used a 16-inch (0.40-meter) reflector. They measured an 11-arcsecond coma.

H. Sato reported a coma of 8 arcseconds on October 6 with a remote iTelescope instrument at Beryl Junction, Utah. Other positions came from Romania, Switzerland, Pennsylvania, Hungary and the National Research Council of Canada.

On October 7, R. Weryk and R. Wainscoat used the 11.8-foot (3.6-meter) Canada-France-Hawaii Telescope. They reported a tail about 12 arcseconds long. MPC circular 2026-T146 came out at 15:40 UT on October 8 with an orbit from 54 observations.

Nearly upright over the planets

ESA gives the distance from Earth as about 810 million miles (1.3 billion kilometers). From the Sun, that is roughly the zone between the orbits of Jupiter and Saturn.

The MPC elements give an inclination of about 85 degrees to the plane of the planets. ESA describes the comet as currently "straight up" from the Sun's point of view. Its orbit will take thousands of years, and the agency's orbit visualization tool shows the path.

Perihelion, the closest point to the Sun, is listed for September 2, 2027, at 8.75 times the Earth-Sun distance. On October 7 it was about 8.96 times that distance away, so it gets only slightly closer.

Comets have been caught active this far out before. ALMA, a radio array in Chile, picked up carbon monoxide from the largest Oort Cloud comet ever detected beyond a billion miles out. A team led from Florida found a frozen object beyond Jupiter venting carbon dioxide.

Euclid steers clear of comets

ESA says Euclid avoids the band along the Solar System's plane because dust there adds glare to deep images. That band holds most comets and asteroids.

In its "step-and-stare" pattern, the telescope usually covers each section of sky once. As a result, a comet cannot simply be revisited on demand to confirm what it is, the agency says.

Animation of how Euclid scans the sky with its 'step-and-stare' method, imaging one patch at a time before shifting to the next. Video: European Space Agency, ESA

Euclid's main task is to study dark matter and dark energy. Over six years it will record the shapes and motions of billions of galaxies. Deep Field North was one of three deep fields in a first data release that mapped 26 million galaxies.

Comets and asteroids leave trails and extra light in those images that have to be removed. In 2023 Pöntinen led a paper in Astronomy & Astrophysics on spotting asteroid streaks in simulated Euclid images with deep learning. It estimated that up to 150,000 asteroids would show up in Euclid's data.

Four of his co-authors on that paper also signed the comet report. Earlier Euclid side results include seven new rogue planets in the Orion Nebula.

Practice for Comet Interceptor

Bruno Altieri, Euclid archive scientist and a member of the discovery team, said very few comets are expected from Euclid. He described this one as "a happy coincidence" of the comet's orbit and the telescope's scanning on those days.

Comet Interceptor is an ESA mission with Japan's JAXA, due to launch in late 2028 or early 2029. It will be parked in space, then sent to a pristine comet, where the main craft will release two probes.

ESA wrote that the quick turnaround shows Euclid can find comets while they are still far from the Sun. Valeria Pettorino, Euclid project scientist, called the discovery and alert "a real test case" for the two mission teams.

Period waits on more data

The first orbit uses positions from September 16 to October 8. ESA says the MPC will refine the orbital period once more data are collected. No period has been published yet.

There is also no published size for the nucleus. Euclid's measurements give a brightness of about magnitude 20.6, far too faint for the naked eye or binoculars.

Further tracking now depends on telescopes on the ground, since Euclid cannot revisit the comet on demand. The 16 Euclid exposures from September 16 to 24 hold the streak that Pöntinen's software first flagged.

The information was obtained from the European Space Agency and the Minor Planet Center.

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