May the oldest comet on record also be one of the most bizarre? The interstellar visitor 3I/ATLAS, found on July 1, 2025, is rewriting the book on what a comet from outside our Solar System will do. Barreling through space at 58 kilometres per second on a hyperbolic trajectory with a record-breaking eccentricity of 6.2, it is only the third confirmed interstellar object, following 1I/ʻOumuamua and 2I/Borisov. Its path and velocity indicate an origin in the Milky Way’s thick disk a group of ancient, low-metallicity stars so it might have been en route for billions of years before a random collision with our Sun.
Early ground-based observations suggested a big, shining object, but the high-resolution imaging by the Hubble Space Telescope on July 21 indicated a dense nucleus no larger than 2.7 kilometres in radius, surrounded by a fuzzy coma. Such a small core was concealed in earlier estimates of size by brightness from surrounding dust. Unlike other Solar System comets, Hubble saw no dust or ion tail, evidence of its anomalous behavior.
True breakthroughs, however, emerged from spaceborne infrared spectroscopy. NASA’s SPHEREx mission, conducted on the period between August 8 and 12, leveraged its sensitivity to volatile ices to investigate the composition of the coma. It saw a prevalent carbon dioxide signal at 4.1–4.4 microns, with hardly any absorption by water-ice between 2.6 and 3.5 microns and nothing at 4.6–4.8 microns for carbon monoxide features. The observations are reported to produce 99% or more of the measured continuum flux, measured to result from CO₂-rich dust, probably in centimetre-to-decimeter-sized pieces. The composition is echoed by evolved Kuiper Belt comets such as 103P/Hartley 2 that have rich CO₂, little CO, and water emission only if warmed enough near the Sun.
The James Webb Space Telescope’s August 6 observations with the NIRSpec Integral Field Unit validated SPHEREx’s results and provided nuance. JWST spotted low-level solid water-ice and trace carbon monoxide in addition to CO₂ dominating the spectrum, as well as carbonyl sulfide. The most dramatic finding was the highest CO₂-to-water ratio ever recorded in a comet. This implies that 3I/ATLAS formed close to the “carbon dioxide ice line” in its birth protoplanetary disk where temperatures were cool enough to cause CO₂ to condense but not water, the JWST team said. Alternatively, it could have been subjected to high radiation doses that dried out the water with respect to CO₂, or preserved internal structures that block heat penetration, restricting water sublimation.
Infrared spectroscopy played the crucial role: by breaking light up into its constituent wavelengths, both SPHEREx and JWST were able to detect molecular fingerprints in the coma. SPHEREx’s wavelength coverage differentiated between CO₂, CO, and H₂O, whereas the greater sensitivity of JWST was able to resolve weak features and measure abundance ratios. These are particularly useful for interstellar objects, whose composition can tell us about the thermal and chemical conditions of their parent systems.
Orbital mechanics also limit 3I/ATLAS’s tale. With Monte Carlo simulations and the GalPot galactic potential model, scientists retrograded its path over a billion years, with a median age of 4.6 billion years and a thick-disk origin. Its high approach velocity and shallow hyperbolic path suggest ejection by gravitational encounters, maybe with a giant planet or through stellar mass loss that gave it enough energy to leave its native system in its entirety.
As it heads towards perihelion around Halloween 2025, Earth will be on the opposite side of the Sun, restricting direct observation until December. By that time, the comet could have experienced additional outgassing, possibly emitting water vapor if solar heating is sufficient to reach the requisite ~200 K level. Whether it remains intact is questionable; interstellar comet 2I/Borisov disintegrated following perihelion, and analogous thermal stresses may fragment 3I/ATLAS.
For planetary scientists, 3I/ATLAS is an unusual laboratory. Its mature volatile inventory, CO₂-rich, CO-poor, with leftover water-ice provides a snapshot of deep-space weathering on cometary bodies. Comparing it with ʻOumuamua’s volatile-poor state and Borisov’s volatile-rich youth establishes an evolutionary sequence for interstellar objects. As future surveys such as the Vera C. Rubin Observatory’s LSST raise ISO detection rates, these compositional standards will be essential to chart the diversity of planetesimal formation throughout the galaxy.
In the meantime, 3I/ATLAS maintains its sole journey through our Solar System, a chilled messenger from a vanished star system, bringing with it in its ices the chemical imprint of a distant place and time removed from our own.
