Hubble pins down the size of interstellar comet 3I/ATLAS, bolstering natural-origin case

Hubble measures the solid core of an interstellar comet

On a January night as Earth, the Sun and a faint smudge of light from another star lined up almost perfectly, the Hubble Space Telescope locked onto a rare target: the interstellar comet 3I/ATLAS, already on its way back out of the solar system. Behind the comet’s hazy coma and thin tail, Hubble’s cameras were after something that had never been clearly seen before — the solid heart of an object born around another star.

Using those images, a team of astronomers now reports the first robust measurement of an interstellar comet’s nucleus: a dark, elongated chunk of ice roughly 2.6 kilometers (1.6 miles) across that, in almost every respect, behaves like the comets that routinely visit from the outer reaches of our own solar system.

“We report the successful detection of the nucleus of interstellar object 3I/ATLAS, achieved by applying the nucleus extraction technique to our Hubble Space Telescope observations,” Man-To Hui of the Shanghai Astronomical Observatory and colleagues wrote in a preprint posted Jan. 29.

Their analysis, led by Hui and veteran comet researcher David Jewitt of the University of California, Los Angeles, uses Hubble images taken in December 2025 and January 2026, after the comet had swung past the Sun. The study offers the most detailed physical portrait yet of an interstellar visitor, and it sharply narrows the room for claims that 3I/ATLAS is anything other than a natural comet.

A ~2.6-kilometer nucleus, likely elongated

The team found that the brightness of the central point source corresponds to the product of the nucleus’s cross-section and its reflectivity — technically, the V-band geometric albedo — of 0.22 square kilometers. Assuming a typical comet reflectivity of about 4%, they calculate an effective radius of 1.3 ± 0.2 km, or a diameter of 2.6 kilometers, Hui’s team wrote.

Light variations in the Hubble data imply that the nucleus is stretched at least twice as long as it is wide. If those variations come from rotation rather than flickering jets, the axis ratio is “no less than 2:1,” the authors found, indicating a football-shaped body rather than a near-sphere. The data rule out extremely rapid spin, suggesting a rotation period longer than one hour.

Those figures are far below some of the largest early size estimates, including a widely reported claim that ATLAS might span about 7 miles across. But they align with independent estimates based on how much gas and dust the comet sheds and how its motion is affected by that outgassing.

A rare viewing geometry reveals dust behavior

The Hubble campaign was timed to take advantage of a rare geometry. On Jan. 22, 2026, Earth, the Sun and 3I/ATLAS were nearly in a straight line, with the comet just 0.69 degrees from the Sun–Earth axis. That near-perfect “opposition” produces a brightness spike known as an opposition surge, sensitive to the properties of dust grains.

Hui’s team reports an opposition surge of roughly 0.2 magnitudes, with a characteristic width of about 3 degrees. They also measure how quickly the coma’s brightness decreases as the angle between the Sun, comet and Earth grows, finding a linear phase slope of 0.026 magnitudes per degree. Those numbers fall in the same range as dust from ordinary comets.

The Hubble images also reveal structure in the dusty atmosphere. A prominent anti-tail — a feature that can appear to point toward the Sun due to viewing geometry — is accompanied by three smaller jets, spaced roughly 120 degrees apart. Astronomers interpret these as signatures of discrete active regions on the spinning nucleus, similar to what has been seen in many solar system comets.

By comparing the comet’s overall brightness before and after its closest approach to the Sun on Oct. 29, 2025, the team finds that 3I/ATLAS brightened and faded asymmetrically. On the outbound leg, they measure an activity index of 4.5, meaning the comet’s brightness dropped more steeply with distance than it had increased on the way in. The authors describe this behavior as “not unusual in the context of solar system comets,” which often respond to sunlight in delayed or seasonal ways as heat penetrates their surfaces.

All of that points in the same direction, researchers say: 3I/ATLAS looks and behaves like a comet.

Chemistry and orbit support an ordinary comet from another star

That conclusion reinforces earlier work from other observatories. Since the object’s discovery on July 1, 2025, by the ATLAS (Asteroid Terrestrial-impact Last Alert System) survey telescope in Chile, telescopes including the James Webb Space Telescope and the Very Large Telescope in Chile have detected common cometary gases in its coma.

Those observations showed that 3I/ATLAS is unusually rich in carbon dioxide, with additional water, carbon monoxide and carbonyl sulfide, along with cyanide and nickel atoms. The mix is distinct in details but comfortably within the range seen in comets that formed in the frigid outskirts of our own planetary system.

The interstellar origin of 3I/ATLAS is not in doubt. Its orbit is strongly hyperbolic, with an eccentricity of about 6.1 and a speed relative to the Sun of around 58 kilometers per second when far from the Sun’s influence. The comet followed a retrograde path inclined about 175 degrees to the direction of the planets’ orbits, approaching from the constellation Sagittarius near the galactic center. It passed safely, never coming closer than about 1.8 astronomical units — 1.8 times the Earth–Sun distance.

Speculation about an artificial origin narrows further

Still, the comet became a focus of speculation during the summer of 2025, when Harvard astronomer Avi Loeb and collaborators posted a paper arguing that 3I/ATLAS might be an extraterrestrial spacecraft. Loeb pointed to what he described as unusual aspects of the comet’s trajectory, estimated size and composition. That claim drew quick and public criticism from many astronomers, who said the analysis used flawed statistics and ignored evidence of normal cometary activity.

As more data came in, including detections of common ices and gases from Webb and ground-based telescopes, the case for a natural origin strengthened. Loeb later acknowledged in interviews that a natural comet is the most likely explanation.

The new Hubble results leave even less room for exotic scenarios. The nucleus’s measured size, modest elongation and ordinary activity curves, combined with its detected volatiles, fit within the known diversity of comets that form around Sun-like stars, cold red dwarfs and other common types of stars.

Why “ordinary” matters for planetary science

For planetary scientists, that ordinariness carries weight. Interstellar objects like 3I/ATLAS are considered fragments ejected from other planetary systems during their formation. Each one is a physical sample of another system’s building blocks.

When the first known interstellar object, 1I/ʻOumuamua, was spotted in 2017, it left as many questions as answers. ʻOumuamua showed no detectable coma but experienced a small, unexplained acceleration as it left the solar system. Its shape appeared extremely elongated or flattened, based on how its brightness changed as it spun, and no direct measurements of its composition were possible. That fueled a wave of papers invoking everything from exotic hydrogen ice to artificial light sails.

The second interstellar visitor, 2I/Borisov, discovered in 2019, looked much more like a conventional comet. Hubble and other telescopes documented its dusty coma and tail and found a mix of ices broadly similar to many solar system comets. But even in that case, the nucleus itself could not be cleanly seen behind the coma.

With 3I/ATLAS, astronomers finally have an interstellar comet whose core is constrained as well as its chemistry. Jewitt, who led Hubble campaigns on both Borisov and ATLAS and has studied comets for decades, previously described the contrast between the first two interstellar objects by saying, “‘Oumuamua looked like a bare rock, but Borisov is really active — more like a normal comet. It’s a puzzle why these two are so different.” The new work suggests that, at least in some respects, the interstellar population may have more in common with Borisov than with ʻOumuamua.

Hui and colleagues also draw a broader conclusion from the mere fact that three interstellar objects have been detected since 2017. Given the limited sensitivity and sky coverage of current surveys, they argue that many 3I-like comets must have passed through the solar system in previous decades and centuries without being noticed.

That inference aligns with models of planet formation that predict vast numbers of icy bodies are flung into interstellar space as young planets migrate and gravitationally stir their birth disks. Upcoming facilities such as the Vera C. Rubin Observatory in Chile, which will conduct a deep, wide survey of the sky later this decade, are expected to detect many more such visitors.

3I/ATLAS itself will not linger. After a modestly close pass by Jupiter in 2026, it will head back into interstellar space, its jets dwindling as its surface cools. Hubble, now more than three decades into its mission and slated for retirement around the early 2030s, will likely have only a few more opportunities to study interstellar debris.

For now, the telescope’s view of 3I/ATLAS offers a clear if fleeting glimpse of a worldlet from another star: a dark, elongated nucleus wrapped in a veil of familiar ices and dust, reminding researchers that the processes that built our own solar system’s comets are probably at work around countless other suns.