‘Could a piece from a distant galaxy be a key to unlocking worlds that lie beyond our solar system?’ This is what is driving astronomers to give their full attention to 3I/ATLAS, only the third confirmed ‘alien visitor,’ or interstellar object, that has passed through our solar system to date. Discovered on July 1, 2025, by NASA’s Asteroid Terrestrial-impact Last Alert System in Chile, this comet is a ‘rarity because it carries material that is produced in regions far removed from our cosmic back yard.’ Its path is a ‘hyperbola, indicating that it is not gravitationally bound to our star,’ or in other words, this comet ‘comes from a region of our galaxy millions of light-years older in stellar evolution than our galaxy.’ According to University of Oxford astronomer Matthew Hopkins, ‘its age is estimated at up to 7.6 billion years, likely to be the oldest comet ever seen.’
The comet’s odyssey in the inner Solar System culminated in a pivotal event on October 29, 2025, when it passed through perihelion, an incredible distance of only 1.36astronomical units from our star’s center. It was here that the comet’s activity unexpectedly differed markedly from what has been seen in other Oort cloud comets in the past. Normally, as solar radiation gently causes surface ice to sublime, gas and dust are ejected to form an impressive glowing coma and tail. However, in this case, the brightness augmentation was observed to vary in proportion to the inverse seventh power of the distance to our star an unreasonably high value, much more spectacular than what has been recorded in other Oort cloud comets in the past.
These data were recorded not by ground stations since the comet was not visible when it was behind our star but by solar observation satellites, specifically the imagers on the SECCHI instruments aboard the STEREO-A spacecraft, the LASCO coronagraphs on the SOHO satellite, as well as the CCOR-1 instrument on the GOES-19 spacecraft. Also apparent were a coma with dimensions of some 300,000 kilometers, in addition to what was surprisingly an optical transition from red to strong blues, suggesting gaseous rather than dust scattering on sunlight.
Spectroscopy data indicate that compounds such as cyanogen and carbon dioxide could be the cause of this color transition; the latter has been seen fluorescing green when illuminated by solar radiation. In fact, observation by the Gemini North telescope at the University of Hawaii’s observatory revealed that the comet was emitting green light after its perihelion passage; this was attributed to the comet’s volatile materials being disturbed by the penetration of solar heat on its surface. This chemical signature provides insight into the composition of these frozen bodies that were formed in the vicinity of ancient stars, which could be abundant with water and carbon dioxide; the latter has been revealed to dominate its mass fraction by recent observation.
Kinematic analyses of the ESA mission Gaia have related the previously unknown orbit of 3I/ATLAS for millions of years, pinpointing 93 stellar flybys, which lacked the potential to seriously change its course. Its large excess velocity and large galactic inclination support the hypothesis that the object has been moving unchanged through the interstellar medium for millennia. Researchers such as Shokhruz Kakharov and Abraham Loeb have compared the orbital evolution of interstellar objects to enter the solar system, suggesting each comes from separate stellar populations. Thus, the thick disk origins of 3I/ATLAS suggest an ancient object of the galactic formation era, which differs from the younger thin disk population of 1I/‘Oumuamua and 2I/Borisov.
From an engineering stand point, the comet’s brief presence in our solar system emphasizes the difficulty in intercepting objects traveling at such high velocities. With speeds of over 60 km/s relative to our solar system, 3I/ATLAS will leave our solar system never to return, presenting the last opportunity through observation at closest approach to our planet, at 270 million kilometers on December 19, 2025. Future projects, such as the ESA’s Comet Interceptor, which will remain ready to launch when a comet is identified, might make way for laboratory analysis to address the current gap between laboratory testing and remote observation. The occurrence of these objects is not as uncommon as it would seem, as computer simulations show dozens of interstellar objects are apt to cross the path of the orbit of Jupiter on a yearly basis, with most of them being either too small, too faint, or both to be detected.
The coming era of observatories with the kind of wide field imaging and fast scanning, as seen with the Vera C. Rubin Observatory, will make detection soar, with as many as 70 interstellar objects per year becoming potential finds. For the present, however, 3I/ATLAS offers a unique opportunity that of studying this gigantic, ancient, and unusual body, whose brightness defies the physics of cometary phenomena. As new information from Hubble, the James Webb Space Telescope, and other observatories pours in, astronomers hope that it will be possible to distinguish between its unusual properties being merely extreme natural ones versus those indicative of unknown processes within the Solar System. Once it’s gone, all that will be left is data but this, like any sample, has already proven highly valuable.
