When astronomers first noticed a weak, fast-moving object streaking across the sky in July 2025, few realized they were seeing one of the rarest events in modern astronomy. The newly spied body — ultimately identified as 3I/ATLAS, only the third known interstellar object to visit our Solar System — has since sparked a scientific frenzy, inspiring a worldwide campaign by NASA, ESA and researchers around the globe to monitor a cosmic wayfarer born in another solar system.
In the months since, 3I/ATLAS has evolved from a cryptic pinprick to one of the most well-studied interstellar visitors yet examined. Its brilliant and evolving coma; unusual chemical make-up; and dual-tail structure are providing scientists with an up close, detailed look at these cosmic building blocks from a far away planetary system.
And now, with the heliophysics data newly in hand, researchers are monitoring the comet even when it has vanished behind the Sun — piecing together one of the most continuous records of any interstellar object to date.
A Visitor on a One-Way Trip Out of the Solar System
3I/ATLAS — with the formal designation of C/2025 N1(CC /ATLAS) was first spotted on July 1, 2025 by Asteroid Terrestrial-impact Last Alert System in Chile. Astronomers soon realized that its was hyperbolic, indicating it is not gravitationally bound to the Sun. As with ʻOumuamua and 2I/Borisov before it, this object is from interstellar space and will eventually depart again to never return.
On late-October 3I/ATLAS arrived at perihelion, coming within approximately 1.36 AU of the Sun. In mid-December, it made its closest — though still very distant — approach to Earth at 1.8 AU, meaning zero threat to the planet.
But this distance has not prevented astronomers from recording spectacular detail of its appearance.
The Glow, the Colors and the Tails: A Comet Like No Other We’ve Seen
Sharp images from NASA’s Hubble Space Telescope in late November show that the comet, officially named C/2019 Q4 (Borisov), is wrapped in a glowing envelope of dust, and its tail — the telltale sign of a comet — stretches for more than 100,000 miles. The strange color has stumped scientists: gases such as cyanogen or ammonia could be rotting the surface, but its precise spectral signature is still being analyzed.
At roughly the same time, ESA’s JUICE spaceship — which is on its way to Jupiter — imaged 3I/ATLAS with two separate tails:
- an ion (plasma) tail, sculpted by the solar wind
- a dust tail produced by sunlight shoving particles from its surface
The double tail shape indicates that 3I/ATLAS is acting like a regular comet, releasing gas and dust as the Sun warms its surface.
More surprising still: Scientists believe the comet might be missing a dust mantle — a hardened veneer that tends to develop on long-lived comets in our Solar System. Without that insulating veneer, our solar system’s interstellar comets like 3I/ATLAS might have erupted in more spectacular fashion, revealing raw material untouched since the day it congealed around some faraway star.
A Chemical Cocktail From Another Star, Ready to Blend With the Milky Way
New spectroscopic findings have now unveiled that 3I/ATLAS harbors one of the most peculiar molecular inventories ever found in a comet.
Comet C/2016 R2 was imaged with the James Webb Space Telescope in 2017, with other telescopes including ground-based ones canvassing the comet using ALMA array of telescopes to reveal that its coma consists of an unusually high abundance of carbon dioxide (CO₂) — higher than any other known comet in the Solar System.
Other detected molecules include:
- water vapor (H₂O)
- carbon monoxide (CO)
- carbonyl sulfide (OCS)
- methanol, at unexpectedly high abundance
- (HCN), a prebiotically relevant molecule
- icy grains and dust particles
Methanol and HCN are also of special interest to astrobiologists because they can be precursors of more complex organic molecules. Their profusion implies that the environment where 3I/ATLAS was born might have been a rich one in terms of organics — perhaps akin to the protoplanetary disks where life-friendly chemistry arises.
Polarimetric or polarization measurements add another complication: the comet’s dust exhibits a strong negative polarisation branch, something not seen in Solar System comets. This implies that the grains in the comet have an anomalous shape, internal structure or chemical composition — perhaps shaped by conditions around its home star.
A Turning Point: Following the Comet Hidden Behind the Sun
Among the most groundbreaking advances was NASA’s (Polarimeter to Unify the Corona and Heliosphere) PUNCH mission. In late 2025, 3I/ATLAS entered an area of the sky near the sun that made it impossible to observe from ground-based or Earth-orbiting telescopes.
But PUNCH — and a few other heliophysics spacecraft — imaged the comet Sept. 20 through Oct. 3, spotting its faint tail as it swept through the solar corona’s overly bright glare.
It was the first time heliophysics instruments were used intentionally to observe an interstellar comet. The data provides key observational measurements that fill in what had been a critical gap in our understanding and shows how the comet’s tail responded to solar wind forces during its closest solar encounter.
The Closest Look Ever at an Interstellar Object
With data pouring in from:
- Hubble
- JWST
- ALMA
- JUICE
- ground observatories worldwide
- PUNCH*, SOHO, and STEREO heliophysics satellites
…scientists have a multi-faceted record of 3I/ATLAS that is unlike other interstellar visitors prior to it. Unlike 1I/ʻOumuamua (which lacked a tail) or 2I/Borisov (which comported itself more like a typical comet), 3I/ATLAS is showing chemical, physical, and polarimetric properties with no known precedent.
Yet with each new observation, the same conclusion is reached: this comet probably originated in a completely different protoplanetary environment and still bears the hallmarks of a remote star system.
What Scientists Still Don’t Know
Even as the data deluge, major mysteries persist:
- Why is the comet 36 per cent more active in CO₂ than an average comet?
- The chemicals that make it glow blue-green?
- Are its dust particles porous, metallic, icy or coated with exotic organic materials?
- 3I/ATLAS: Did 3I/ATLAS — or its ices, at least — originate closer to (or farther from) its parent star than comets here did?
- Is this object a fairly common kind of interstellar comet, or the one-in-a-million exception?
Since only three interstellar passersby have ever been identified, scientists also have no basis for what “normal” interstellar chemistry looks like.
A Short-Lived But Historic Encounter
3I/ATLAS will remain visible for a few more months, before disappearing forever into deep space. When the information does get there, it will carry secrets from a star system that might have billions of years under its belt — and be very different from our own.
For now, however, astronomers have a rare chance: to observe an interstellar comet in real time as it evolves, to follow its chemistry and to seek how far-off planetary systems cobble together the icy building blocks of the galaxy.
3I/ATLAS may visit only fleetingly — but the data it brings could transform our understanding of how worlds form across the galaxy.
