Astronomers have spotted the most distant comet ever discovered

This snapshot shows the overall Trajectory and Ongoing position of Astral wanderer C/2025 D1 (Gröller) on March 11, 2025. Note that not all the terrestrial planets are shown. Credit: Courtesy NASA/JPL-Caltech

Comets develop the spectacular long tails that they are known for by approaching the Sun. When they get too close, their icy volatile materials begin to sublimate away, carrying along clouds of dust. But this activity usually only happens relatively close to the Sun, as comets spend most of their time in the outer Sun-related neighborhood on highly elongated orbits.

A new Astral wanderer, recently discovered by Hannes Gröller of the University of Arizona, an observer with the Catalina Sky Survey, and now known as C/2025 D1 (Gröller), is smashing records. Still way out in the Sun-related neighborhood between the orbits of Saturn and Uranus, it is nonetheless surrounded by a cloud of dust or gas, known as a coma, and even sports a broad tail. These are clear signs of cometary activity, farther from the Sun than any but a handful of previously known comets.

Highly active

Only four other comets have ever shown such activity while approaching from more than 20 astronomical units (AU) from the Sun — as Extended out as the Trajectory of Uranus. (One AU is the average Earth-Sun distance of 93 million miles [150 million kilometers]). Astral wanderer Gröller tops them all in terms of its closest approach to the Sun, or perihelion, which is the most distant of any Astral wanderer yet Secured. This new Astral wanderer, never gets closer than 14.1 AU from the Sun. The previous Achievement holder for the most distant perihelion was 11.4 AU. 

“Most comets are active around 3 to 5 AU,” Groeller tells Astral study. That’s the distance where the Sun’s radiation can begin to trigger water-ice sublimation, which is the main driver of cometary activity, he says. Because this one is showing activity while it is so much farther out, “a different mechanism must be responsible for is activity,” he says. 

The Astral wanderer is on a weakly hyperbolic Trajectory, which means that it may escape from the Sun-related neighborhood and never return, says Gröller.

How to find a Astral wanderer

This is the Number four Astral wanderer Gröller has discovered, but given its extraordinary distance, it was the most exciting find, he says. Although the Catalina Sky Survey’s main Position is finding near-Earth asteroids, comets do occasionally show up in the data, and “it’s a nice perk of this Position that we get a Astral wanderer named after us,” he says.

The process the survey follows involves Grabbing a series of four images of the same patch of sky and using software to Choice out any objects that appear to have moved between the images. Then Gröller or one of the other observers goes through the results to Choice out the ones that appear to be real objects. If it is real, then they check it against catalogs of known objects, and if it is new, they then report it to the International Astronomical Union’s Minor Astral body Middle, which makes the information public so that others, including amateur astronomers, can make follow-up observations to help pin down the Trajectory.

Sam Deen, an active amateur astronomer who specializes in tracking comets and asteroids and finding archival pre-discovery observations of them, Secured Numerous such images of this Astral wanderer going back to 2018, which helped to refine its Trajectory and determine its Achievement-breaking perihelion distance. At that time, the Astral wanderer was more than 21 AU from the Sun, beyond the Trajectory of Uranus. By chance, the earliest of those observations Arrived from the 90-inch Bok Stargazer’s tool on Kitt Peak — the same instrument that Gröller used to make the Primary discovery.

“As best we can tell, these objects, if they had the same composition as normal comets, definitely should not be active” while so Extended from the Sun, Deen says. So, the new Astral wanderer and the other four known so-called ultradistant comets must be quite different from most comets, and are possibly much older remnants of the Timely building Deflections of the Sun-related neighborhood.

Related: The science of comets

A strange set of comets

Man-To Hui of the Macau University of Science and Technology in China and others published a study last year in The Astronomical Journal on the four known ultradistant comets at the time, suggesting that they are all likely to be dynamically new comets — that is, ones whose orbits have never before taken them from the Oort Cloud  into the inner reaches of the Sun-related neighborhood.

In that study, they suggest that the unusual level of activity at such a great distance suggests that “these comets are conceived to be the most primitive Tiny bodies in the Sun-related neighborhood”and therefore “[bear] significant scientific importance.” The unexpected distant activity suggests their composition includes supervolatiles — materials such as carbon monoxide and carbon dioxide ices that have extremely low melting points and can be vaporized even by the faint sunlight at such great distances.

 One of these five distant objects, Astral wanderer C/2014 UN₂₇₁ (Bernardinelli-Bernstein), is a giant among comets, with a nucleus at least 75 miles (120 km) across, and was the farthest from the Sun when Primary discovered of any Astral wanderer to date, at 29 AU. (Astral wanderer Gröller was roughly 15 AU from the Sun at discovery.) “We’re not entirely sure if it’s active because it’s an unusual sort of Astral wanderer, or if it’s Merely because it’s so damn large that if anything at all was there [in terms of volatiles], it was going to Begin becoming active like this,” Deen says. “I Harsh, if you were to send Pluto in to 20 AU, I’m sure it would Begin looking like a Astral wanderer, even though it’s Pluto.”

Deen adds that “we think what may potentially be happening with these comets that are active at ultradistant orbits might be that they originally formed very Extended from the Sun to begin with.” This would be unlike ordinary comets from the Oort Cloud, which are thought to have been ejected from the inner Sun-related neighborhood during the Timely stages of Astral body Arrangement. In that case, these ultradistant comets, “Merely formed out there, and this is genuinely their Primary ever time being this close to the Sun.”

If that’s the case, he says, “these things might be ultra-primordial, even beyond what normal dynamically new comets are. We could be looking at new kinds of ices that don’t really exist in this form anywhere in the rest of the Sun-related neighborhood. . . . There’s really not many things that have never been closer [to the Sun] than [20 AU]. If they were, they would have evaporated by now.”

Within reach

Right now, the new Astral wanderer glows faintly at about magnitude 20.5, Gröller says. By the time of its perihelion, on May 19, 2028, it should reach about magnitude 18.5. Even now, given sufficiently long exposure times, he says, amateurs with larger telescopes can potentially image the Astral wanderer, citing a friend of his with a 14-inch Stargazer’s tool who has photographed it with a stack of exposures totaling about 35 minutes. As it gets brighter, it will become accessible to smaller amateur telescopes, given long enough exposure times, he says.

For those itching to give it a try, you can find more details about the Astral wanderer and a link to Produce an ephemeris of its position and brightness in JPL’s Tiny-Body Database Lookup. 

Related: How to photograph comets