“Each one of these discoveries is a rehearsal… next time, we’ll be ready to send a spacecraft,” noted Dr. Franck Marchis of SETI Institute during a presentation about interstellar comet 3I/ATLAS. The first optical image of this comet taken after perihelion, on October 31, 2025, is already a milestone in research on objects that originate from outside our solar system. This is an extremely rare object and only the third interstellar object that has crossed our cosmic neighborhood and is exhibiting a chemical and physical composition that is very different from that of solar system comets.
Discovered by the survey telescope of the ATLAS project in Chile on July 1, 2025, 3I/ATLAS is currently on a hyperbolic trajectory out of our solar system never to return. With an eccentricity of 6.14 and a velocity of over 130,000 miles per hour, there is no doubt that 3I/ATLAS is of extragalactic origin, being 3 billion years older than the solar system. It was, in fact, very difficult to follow 3I/ATLAS on its trajectory post-perihelion due to its proximity to the Sun with a low elevation angle in the sky, but thanks to the hard work of astronomer Qicheng Zhang at the Lowell Discovery Telescope.
During the time before the perihelion of the comet, it was observed to be bright with a peculiar blue coloration. The blue colors represent the high gas emission rates, which are dominated by carbon-based materials as opposed to dust reflection. During the time of the comet’s perihelion passage, the V magnitude was approximated to be 9, which is much brighter than the original estimates. The spectral observations showed a high concentration of carbon dioxide with low water vapor content at a ratio of 7.6:1, the highest ever recorded in the composition of the comet. The high concentration of carbon dioxide is characteristic of the irradiation of galactic cosmic rays in the outer parts of the comet, which have converted carbon monoxide to carbon dioxide over billions of years.
Analysis of data from the JWST and SPHEREx missions indicates that the cosmic rays have reached a depth of 15-20 meters and resulted in the formation of a layer of ice rich in CO2 on top of the pristine ice. This points towards the fact that most of the outgassing now comes from altered ice as opposed to pristine ice. The data from the European Southern Observatory’s Very Large Telescope added a layer of complexity and revealed an unusually large ratio of nickel to iron and an unusually large ratio of nickel to cyanide. These values may point towards differences in the release or formation processes in the original star system.
In addition to its composition, 3I/ATLAS has demonstrated the existence of measurable acceleration, which is non-gravitational, and this represents tiny changes in its orbit caused by gas and dust jets that act like mini propulsion systems. This has been carried out close to the time of its perihelion, and there has been an actual positional change of a number of arc seconds. A recent study has shown that this acceleration could be caused by the standard release of volatile materials, thus not requiring any special models. Its activity also includes a multi-jet phenomena with at least seven jets, including anti-tails, and it remains a very fascinating body for future imaging.
Orbital assets have offered unique perspectives. NASA’s Parker Solar Probe took photos on a daily basis while the comet was beyond the solar visibility range for Earth. The Europa Clipper spacecraft, destined for Jupiter, utilized the Ultraviolet Spectrograph to detect the presence of oxygen, hydrogen, and dust. In this way, it has been confirmed that the comet has a high outgassing rate following perihelion. These data will allow astronomers to reconstruct the comet’s path during its passage in the solar system.
For a community of observers, comet 3I/ATLAS has offered a brief opportunity in late 2025. The comet reached its peak during its closest point to Earth on December 19th at a distance of 1.8 AU. While it was much too faint for the naked eye, it became detectable by larger amateur telescopes. In this way, it became a prime target for professional astronomers. The comet’s departure marks the end of observational activities. However, it will aid in planning future missions. These could well include ‘wait in space’ interceptors for the next messenger from another star system.
Comets like 3I/ATLAS could well be considered natural space probes for the exploration of diversity in planetary systems. The study of these natural space probes could aid in the development of a clear understanding of the formation of worlds in the galaxy. As 3I/ATLAS departs into space, it will go down in history for the scientific information it has yielded. In addition, it serves as a warning that the next messenger from another star system could arrive at any time.
