Comet 3I/ATLAS: A Visitor from Beyond the Solar System

On July 02, 2025, astronomers confirmed the discovery of Comet 3I/ATLAS, marking only the third time in recorded history that an interstellar object (ISO) has been identified passing through our Solar System. This follows the landmark discoveries of 1I/‘Oumuamua in 2017 and 2I/Borisov in 2019.

The confirmation was discussed during a SETI Live broadcast hosted by Dr. Simon Steel, Deputy Director of the Carl Sagan Center at the SETI Institute, and featured expert commentary from University of Washington astronomer Dr. James Davenport, SETI Institute postdoctoral fellow and comet researcher Dr. Ariel Graykowski, and SETI Chair and Allen Telescope Array Project Scientist Dr. Wael Farah.

Discovery by the ATLAS Survey

Comet 3I/ATLAS was first identified by the Asteroid Terrestrial-impact Last Alert System (ATLAS), a global network of four robotic telescopes located in Hawaii, Chile, and South Africa. ATLAS systematically surveys nearly the entire night sky every 24 hours, primarily searching for near-Earth objects that could pose a risk of impact.

Initially, 3I/ATLAS appeared as a faint, fast-moving point of light. However, follow-up observations revealed the object was traveling at a hyperbolic velocity, meaning its speed and trajectory exceeded the Sun’s gravitational binding energy. This is characteristic of interstellar objects on open-ended orbits, distinguishing them from the typical elliptical orbits of bodies in the Solar System.

Cometary Activity Detected: A Distinctive Feature

Unlike 1I/‘Oumuamua, which lacked any visible comet-like features, 3I/ATLAS has displayed clear cometary activity, even while still four astronomical units (AU) from the Sun (one AU is the average distance between Earth and the Sun), making 3I/ATLAS more similar to 2I/Borisov in that regard.

According to Dr. Graykowski, this activity manifests as a coma: a diffuse envelope of gas and dust created by the sublimation (transition from solid to gas) of volatile compounds. The early onset of sublimation indicates the presence of highly volatile ices, potentially providing a pristine record of the object’s origin outside the Solar System.

Dr. Davenport emphasized that 3I/ATLAS is moving faster than either of the previous interstellar visitors at roughly 60 km/s, suggesting it was ejected from its parent stellar system with substantial kinetic energy, possibly due to gravitational interactions with massive planetary bodies or close stellar encounters.

Multi-Wavelength Observations in Progress

Optical Monitoring Through Citizen Science

Dr. Graykowski leads optical monitoring efforts using the UNISTELLAR Network, a decentralized array of small, smart telescopes operated by citizen scientists worldwide. Despite the object's faintness, UNISTELLAR telescopes (with apertures of just 3-4.5 inches) have successfully detected it using image stacking techniques to compensate for the comet’s rapid motion.

Early brightness measurements reveal a steep increase in luminosity, which is consistent with dynamic new comets making their first close approach to a star. This rapid brightening reflects the sublimation of volatile compounds that have remained unaltered since their formation, offering a unique opportunity to probe primordial interstellar materials.

Radio Observations at the Allen Telescope Array

Radio observations began promptly at the SETI Institute’s Allen Telescope Array (ATA), led by Dr. Wael Farah. The ATA team initiated monitoring on the night of July 2, targeting both narrowband emissions (a potential indicator of artificial (technological) signals) and natural emissions such as continuum radiation from dust and specific spectral lines associated with common molecules like hydroxyl (OH).

Within just a week, the ATA had amassed over 21 terabytes of data, equivalent to thousands of hours of high-resolution video. This dataset is under active analysis, with researchers scouring the data for both natural radio signatures and hypothetical technosignatures.

Prospects for Spectroscopic Analysis

Future observations from large facilities, including the Vera C. Rubin Observatory, will enable detailed spectroscopic studies, which are a technique used to decipher the chemical composition of astronomical objects. Spectroscopy will allow scientists to determine whether 3I/ATLAS harbors exotic compounds not commonly found in Solar System comets.

Scientific Significance: A New Era of ISO Studies

One particularly compelling aspect of 3I/ATLAS is the extended opportunity for observation. Unlike 1I/‘Oumuamua, which was discovered after perihelion (closest approach to the Sun) and while outbound from our Solar System, 3I/ATLAS was identified well before its perihelion passage. This provides a full inbound observational window.

Dr. Davenport noted that its galactic trajectory aligns with the thick disk of the Milky Way, a region populated by older stars. This suggests the object may have wandered through interstellar space for millions, if not hundreds of millions, of years before encountering our Solar System.

When and How to Observe 3I/ATLAS

Comet 3I/ATLAS will reach perihelion in October 2025, approaching closest to Mars. Unfortunately, Earth will be on the opposite side of the Sun during this period, meaning ground-based observations will be temporarily impossible due to solar conjunction.

Even at peak brightness, the comet is not expected to become visible to the unaided eye, remaining accessible only to telescopes. However, citizen scientists with small telescopes can still make significant contributions, especially before and after perihelion, by submitting their observations to the UNISTELLAR network or databases such as the Minor Planet Center (MPC).

The Future of Interstellar Object Detection

The discovery of 3I/ATLAS underscores the transformative impact of modern astronomical surveys. Dr. Davenport highlighted that upcoming facilities, such as the Rubin Observatory, are expected to revolutionize the detection of interstellar visitors. Current projections estimate that Rubin may identify dozens to hundreds of such objects during its operational decade, enabling statistical studies on the frequency, composition, and origin of interstellar objects.

As Dr. Steel concluded, interstellar objects are no longer rare curiosities – they are becoming a critical tool for understanding the processes shaping planetary systems beyond our own.

Watch the whole conversation here to learn more about this exciting new discovery.