Astronomers Discover New Interstellar Comet 3I/ATLAS

On July 1, 2025, astronomers at the Asteroid Terrestrial-impact Last Alert System (ATLAS) in Río Hurtado, Chile, identified a new celestial object, later designated as 3I/ATLAS. This discovery marks the third confirmed interstellar object to traverse our solar system, following 'Oumuamua in 2017 and 2I/Borisov in 2019. Unlike 'Oumuamua, which lacked a visible coma, 3I/ATLAS exhibits a pronounced coma and a developing tail, akin to 2I/Borisov.

3I/ATLAS possesses an exceptionally high orbital eccentricity of approximately 6.3, indicating a hyperbolic trajectory that confirms its origin from outside our solar system. The comet reached its perihelion—the closest point to the Sun—on October 29, 2025, at a distance of 1.37 astronomical units (AU) from the Sun and 2.31 AU from Earth.

Initial observations in July and August 2025 recorded the comet at magnitudes between 17 and 18. However, by September, 3I/ATLAS exhibited a rapid increase in brightness, with its luminosity scaling with heliocentric distance ( r ) as ( r^{-7.5±1.0} ). This brightening trend was documented by space-based instruments, including STEREO-A's SECCHI HI1 and COR2, SOHO's LASCO C3, and GOES-19's CCOR-1. Notably, LASCO color photometry indicated that the comet appears distinctly bluer than the Sun, suggesting a significant contribution of gas emission to its visible brightness near perihelion.

Precovery data from NASA's Transiting Exoplanet Survey Satellite (TESS) revealed that 3I/ATLAS was active as early as May 2025, when it was approximately 6.4 AU from the Sun. The comet maintained an average TESS magnitude of 19.6 ± 0.1 during this period, indicating that its activity commenced at a considerable distance from the Sun.

The detection and study of 3I/ATLAS provide a rare opportunity to analyze the composition and behavior of interstellar objects. Infrared spectroscopic observations from the James Webb Space Telescope (JWST) have focused on the rovibrational fluorescence emission from H₂O, CO₂, and CO—key drivers of coma activity in typical solar system comets. These observations aim to derive molecular production rates and mixing ratios, enabling a chemical characterization of the comet's volatile gas inventory.

3I/ATLAS shares similarities with 2I/Borisov, such as a pronounced coma and tail, but differs from 'Oumuamua, which lacked these features. The high eccentricity and rapid brightening of 3I/ATLAS distinguish it from its predecessors, offering new insights into the diversity of interstellar objects.

The discovery of 3I/ATLAS has captivated both the scientific community and the public, highlighting the dynamic nature of our solar system and the potential for interstellar objects to traverse it. Such discoveries underscore the importance of continuous sky surveys and international collaboration in monitoring and studying celestial phenomena.

As 3I/ATLAS continues its journey through our solar system, ongoing observations and analyses are expected to yield further insights into the nature of interstellar comets and the processes governing their behavior.