It is not every ten years or so that a traveler from outside the Sun’s realm enshrouds itself in a glowing mist, but comet 3I/ATLAS has done precisely that and in the process, it has become an emotive testing ground for planetary science. Found on 1 July 2025 by the ATLAS survey telescope in Chile, the object is the third confirmed interstellar object to travel through the Solar System, after 1I/ʻOumuamua in 2017 and 2I/Borisov in 2019. Unlike its mysterious predecessors, 3I/ATLAS is displaying a more conventional cometary show, with an expanding tail and brightening coma now revealed in dramatic detail by the Gemini South telescope high up in the Chilean Andes.
Gemini observations, with the Multi-Object Spectrograph, show a dusty and gaseous cloud, diffuse, enveloping the icy nucleus the coma and a tail that has become considerably longer since previous imaging. Astronomer Karen Meech of the University of Hawai‘i Institute for Astronomy explained, “The primary objectives of the observations were to look at the colours of the comet, which provide clues to the composition and sizes of the dust particles in the coma, and to take spectra for a direct measure of the chemistry. We were excited to see the growth of the tail, suggesting a change in the particles from the previous Gemini images, and we got our first glimpse of the chemistry from the spectrum.”
Spectroscopy, the method of breaking light into its constituent wavelengths, is the master key to unraveling the comet’s composition. In the 3I/ATLAS case, the spectral information indicates a dust-and-ice composition that is surprisingly similar to comets that are formed in our own Solar System. This result implies common planetary formation processes within stellar systems a hint that planet-building raw material might be universal in type. Unsurprisingly, 3I/ATLAS’s activity started unusually far from the Sun, outside of Jupiter’s orbit, by virtue of rich carbon dioxide ice, which sublimates at lower temperatures than water ice.
Its orbit is as informative as its chemistry. With an eccentricity of 6.2, 3I/ATLAS is on a steeply hyperbolic trajectory, guaranteeing that it will not be gravitationally captured by the Sun. Shokhruz Kakharov and Abraham Loeb’s galactic dynamics analysis followed its course back billions of years, arriving at the conclusion that it began life in the thick disk of the Milky Way where the older, lower-metal stars live and is approximately 4.6 billion years old, the oldest of the three known interstellar visitors. ʻOumuamua, on the other hand, is some 1 billion years old, and Borisov some 1.7 billion.
Its passage through the Solar System provides infrequent alignments for observation. On October 3, 2025, 3I/ATLAS will come within 30 million kilometers of Mars, a closeness that has encouraged the European Space Agency to have its Mars Express and ExoMars Trace Gas Orbiter ready to photograph and spectrally study the comet. Tools like the High Resolution Stereo Camera and NOMAD spectrometer would be able to detect molecular tracers water vapor, organics in the coma, though brightness limits are a hurdle. NASA’s Mars Reconnaissance Orbiter, China’s Tianwen-1, and even ESA’s JUICE mission on its way to Jupiter are also contemplating opportunistic observations.
Its physics of tail formation is just as intriguing. Comets usually develop two tails: a dust tail sculpted by solar radiation pressure and an ion tail developed when solar wind removes electrons from gas molecules. Both are starting to form in 3I/ATLAS, with the structure of the dust tail suggesting developing particle sizes as solar heating increases. Polarimetric observations with the Very Large Telescope have shown a peculiar deep negative polarization at small phase angles, a signature not observed among other comets or asteroids, which may connect it to some trans-Neptunian objects and the Centaur Pholus.
From an engineering standpoint, catching up with such a high-speed interstellar target 3I/ATLAS will be traveling at around 68 kilometers per second at perihelion is still out of the reach of present mission readiness. Even ESA’s Comet Interceptor, due for launch in 2029, may have trouble with such speeds. However, the scientific reward of visiting an interstellar comet is immense: as planetary scientist Martin Barstow said, “They undoubtedly carry chemical signatures from outside the solar system, so gaining observations tells us a lot about the possibility of material traveling between planetary systems.”
As 3I/ATLAS moves toward its near-approach to the Sun on October 29, 2025, before returning to interstellar space, scientists are scrambling to gather all the photons of information. Each spectrum, picture, and polarization curve contributes to an expanding image of a body that started out before Earth was even formed, and within it holds the history of worlds long since frozen.
