A New Visitor From Beyond: Discovery and First Look at 3I/ATLAS

In their recent paper, Darryl Seligman and collaborators announce the discovery and initial characterization of a third interstellar object, named 3I/ATLAS (also designated C/2025 N1). Like its predecessors, 1I/‘Oumuamua and 2I/Borisov, 3I/ATLAS is a hyperbolic interloper -- an object not bound to our Solar System -- but with properties that distinguish it from both earlier discoveries. The authors present their early observations, describe its orbit and activity, and offer suggestions for future observations.

Introduction

Interstellar objects -- bodies that originate outside of our Solar System -- are extremely rare discoveries. The first two, 1I/‘Oumuamua and 2I/Borisov, were found in 2017 and 2019, respectively, and displayed remarkably different properties. While ‘Oumuamua showed no cometary tail but experienced unexplained acceleration, Borisov appeared as a more typical comet with strong activity and dust. The discovery of these objects raised many questions about how and where such bodies form, and what they can tell us about other planetary systems. This paper adds a third example, 3I/ATLAS, to this small but growing family, and begins to explore its unique characteristics.

Discovery and Hyperbolic Orbit Characterization

3I/ATLAS was discovered on July 1, 2025, by the ATLAS survey in Chile, which scans the sky for near-Earth objects. The object was first detected as a faint, fast-moving point of light in the crowded star field of the Galactic plane. Follow-up observations confirmed it was on a hyperbolic orbit -- meaning it is on a trajectory that will carry it out of the Solar System, never to return. The orbit has an extreme eccentricity (about 6.2) and an incoming velocity of about 60 km/s, making it clear that it originated from beyond our solar neighborhood. Its path appears to come from the direction of the constellation Sagittarius, near the Galactic Center.

Light Curve and Activity

To study whether 3I/ATLAS behaves like a comet or an asteroid, the team observed its brightness over time -- called its light curve -- and looked for signs of activity like a coma or tail. Using telescopes in Chile, Hawaii, and Canada, they found a surprisingly flat light curve, with brightness variations less than 0.2 magnitudes over about 29 hours. This contrasts with ‘Oumuamua’s highly variable light curve, suggesting that 3I/ATLAS is not as elongated or tumbling dramatically. Stacked images from the Canada-France-Hawaii Telescope revealed faint activity, hinting that some dust or gas is being emitted from its surface, though much weaker than that of Borisov.

Color

The team also measured the colors of 3I/ATLAS in several visible and near-infrared filters to study its surface or coma composition. These observations revealed a moderately red spectral slope -- meaning it reflects more red light than blue -- similar to both ‘Oumuamua and Borisov. This color is typical of objects from the outer Solar System and may indicate organic-rich material. Some of the light may come from a faint coma, as 3I/ATLAS does appear to be slightly active, but the redness is consistent with both an irradiated surface or dusty coma.

Discussion

Based on its brightness, the authors estimate that 3I/ATLAS has a radius of about 10–12 kilometers if it reflects light like a dark asteroid, though this is an upper limit because some light comes from the coma. Interestingly, this makes it likely much larger than Borisov. From its discovery, the team estimates a rough density of similar-sized interstellar objects in space: about one object per 1,000 cubic astronomical units, which is less than previous estimates. Unlike its predecessors, 3I/ATLAS combines properties of both: weak activity like a comet but with a stable brightness and red surface like an irradiated asteroid. The authors emphasize that more observations are urgently needed, especially since the object will be hard to observe at its closest approach to the Sun.

Call to Action

The authors urge the astronomical community to monitor 3I/ATLAS with as many techniques as possible -- including photometry, spectroscopy, and polarimetry -- to understand its rotation, activity, and any unusual forces acting on it. Since it will pass close to Mars at perihelion, spacecraft near Mars or Jupiter could potentially observe it when Earth-based telescopes cannot. These observations will help determine whether 3I/ATLAS represents yet another distinct type of interstellar object, expanding our understanding of the diversity of visitors from other star systems.

Source: Seligman