Astronomers are tracking a rare interstellar visitor that is rewriting expectations about the kinds of objects drifting between the stars — and sparking fresh debate about what it really is.

The cometlike body, designated 3I/ATLAS, was first spotted in May 2025 by the ATLAS survey in Hawaii. It is only the third confirmed object from beyond the solar system, following the 2017 discovery of 1I/ʻOumuamua and the 2019 detection of comet 2I/Borisov. Each arrival is a once-in-a-lifetime chance for scientists to examine the building blocks of other planetary systems.

Observations from more than 200 observatories show that the interstellar object is massive — possibly more than five kilometers across and weighing tens of billions of tons. That makes it thousands of times larger than 1I/ʻOumuamua, which resembled an elongated rock, and still much bigger than Borisov, which looked like an ordinary comet.

Its sheer size is challenging long-held models of how debris is flung from young planetary systems. Astronomers did not expect objects this big to escape the gravity of their home stars in large numbers.

An unusual chemistry

Even more surprising is what the comet is made of. The James Webb Space Telescope found that its coma — the bright cloud of gas surrounding its nucleus — is dominated by carbon dioxide. The CO₂ levels are several times higher than the amount of water vapor detected, a stark contrast to most solar system comets, which typically release more water than CO₂ when they approach the Sun.

Ground-based telescopes have also spotted hints of carbon monoxide, carbonyl sulfide and faint traces of water ice. Together, the findings suggest 3I/ATLAS was formed in a much colder or chemically distinct region than comets familiar to us.

Teams in Europe studying how light scatters off its dust found another puzzle. The polarization pattern — essentially how light waves are aligned after bouncing off the comet — is unlike those seen in ordinary comets or asteroids. The results hint at dust grains or surface textures that may have been shaped in very different conditions, further underscoring the alien nature of this visitor.

Adding to the intrigue, the object shows no measurable “rocket effect.” Comets often accelerate slightly as gas jets push them along, but 3I/ATLAS is traveling strictly under the pull of gravity. The most straightforward explanation is its sheer mass: at this scale, any outgassing simply isn’t strong enough to budge it.

Still, the lack of non-gravitational acceleration has kept speculation alive. “This could be the most massive interstellar object we’ve ever detected,” said Avi Loeb, a Harvard astrophysicist who has previously suggested that anomalies in past visitors could be consistent with artificial origins. He has argued that 3I/ATLAS should not be ruled out as a candidate for something more unusual.

NASA officials quickly pushed back, stressing there is no evidence the object is anything other than natural. “All of our data so far support the conclusion that 3I/ATLAS is a comet,” the agency said in a recent briefing.

Watching closely

For now, astronomers are eager to collect as much data as possible before the object disappears back into interstellar space. It will swing closest to the Sun on 30 October, passing just inside the orbit of Mars. Its closest approach to Earth will be about 170 million miles, more than 700 times the distance to the Moon, so it poses no threat.

European Space Agency missions at Mars, including the ExoMars Trace Gas Orbiter, plan to attempt observations as 3I/ATLAS passes about 30 million kilometers from the planet in early October. There are also proposals to turn the newly launched JUICE mission’s instruments toward the comet for opportunistic measurements.

Meanwhile, Earth-based observatories and the Vera Rubin Observatory in Chile are continuing to monitor changes in brightness and chemistry, while the James Webb telescope will make further infrared measurements. Scientists are also watching how solar activity — including a predicted coronal mass ejection — might interact with its gas and dust.

A fleeting guest

By early 2026, 3I/ATLAS will fade from view as it continues its hyperbolic path out of the solar system. Unlike comets bound to the Sun, it will not return.

Still, astronomers say its brief visit is already reshaping understanding of what other planetary systems can produce. “Every interstellar object we’ve seen has been different,” said the University of Hawaii’s Institute for Astronomy. “That tells us there’s enormous diversity out there. We’ve only just begun to sample it.”

Whether regarded as an icy giant, a chemical outlier, or to some as something more exotic, 3I/ATLAS is giving scientists a rare window into the wider galaxy — a reminder that our solar system is not alone in sending out cosmic messengers.