Astronomers Confirm Discovery of 3I/ATLAS, Third Interstellar Object

On July 1, 2025, astronomers announced the discovery of 3I/ATLAS, marking it as the third confirmed interstellar object, following the earlier discoveries of the asteroid 1I/ʻOumuamua in 2017 and the comet 2I/Borisov in 2019. The interstellar comet 3I/ATLAS was detected by the NASA-funded Asteroid Terrestrial-impact Last Alert System (ATLAS) survey telescope located in Rio Hurtado, Chile, approximately 670 million kilometers away from Earth at the time of discovery. This comet is of particular significance due to its size and trajectory, which has prompted astronomers to conduct further investigations regarding its physical properties and origins.

Professor Avi Loeb, head of the Galileo Project at Harvard University and director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, provided insights into the implications of this discovery. "If 3I/ATLAS’s brightness is a result of reflecting sunlight at an albedo of about 10%, its estimated diameter of 20 kilometers could be significantly larger than that of 'Oumuamua, and is also anticipated to be much more massive," Loeb stated. His analysis indicates that 3I/ATLAS may possess a mass over ten million times greater than that of 'Oumuamua, raising questions about the rarity of such high-mass interstellar objects.

The comet will reach its closest approach to the Sun around October 30, 2025, at a distance of 1.4 AU, which is just inside the orbit of Mars. Observations suggest that 3I/ATLAS is on a retrograde orbit inclined at 175 degrees relative to Earth’s orbital plane, an anomaly that challenges existing models of celestial object trajectories. Following its closest approach, the comet will become too close to the Sun for observation, although it is expected to re-emerge for study by early December 2025.

The significance of 3I/ATLAS extends beyond its discovery; it opens avenues for further research into the composition and behavior of interstellar objects. The NSF/DoE Vera C. Rubin Observatory in Chile and the James Webb Space Telescope are anticipated to play crucial roles in this ongoing research. According to NASA astronomers, pre-discovery observations of 3I/ATLAS have been compiled from multiple telescopes, including the Zwicky Transient Facility at the Palomar Observatory in San Diego County, California, dating back to June 14, 2025. These collaborative efforts highlight the importance of international cooperation in astronomical research.

As the scientific community continues to analyze the data collected, the implications of 3I/ATLAS's discovery will likely reshape our understanding of interstellar objects and their formation mechanisms. This comet serves as a reminder of the vastness of our universe and the mysteries it holds, prompting further inquiry into the origins of celestial bodies that traverse beyond our Solar System. Future investigations are expected to yield insights that could challenge existing paradigms in astrophysics and cosmology, as researchers strive to comprehend the nature of these enigmatic visitors from the depths of space.