Astronomers at the National Aeronautics and Space Administration (NASA) and the European Space Agency (ESA) identified 3I/ATLAS as the third confirmed object originating from outside the Solar System after the Asteroid Terrestrial-impact Last Alert System (ATLAS) telescope in Chile detected the body on July 1, 2025. Moving along a hyperbolic trajectory and emitting a bluish glow, the comet reached perihelion — its closest point to the Sun — on October 30, 2025, at a distance of 1.4 astronomical units. The pass posed no threat to Earth, which remained 1.8 astronomical units away during the approach, while coordinated international observations focused on mapping its trajectory and chemical makeup to investigate the formation of ancient galaxies.
The interstellar body displays verified cometary activity, characterized by a diffuse coma and a dust tail. Estimates place its age between 7 billion and 14 billion years, making it older than the Solar System itself. In July 2025, space observatories including the Hubble Space Telescope captured direct images of the nucleus and the material being ejected from its surface.
Retroactively processed archival data from June 2025 confirmed the object’s hyperbolic orbit and a travelling speed of 137,000 km/h. Those initial tracking campaigns also established three baseline physical characteristics of the visitor:
- Coma diameter: expanded from 13,000 km to 18,000 km over the course of July 2025.
- Brightness profile: absence of sudden brightness spikes, indicating structural stability.
- Chemical signatures: confirmed presence of carbonyl sulfide and water vapor.
Telescopes in Chile, Hawaii, and Australia track the reddish dust coma
The ATLAS survey instrument in Chile initially spotted 3I/ATLAS as a faint, fast-moving point of light against the background stars. The velocity and orbital path immediately confirmed its interstellar origin to planetary defense and astronomy teams.
Ground-based observatories in Hawaii, Chile, and Australia activated rapid-response observation protocols to follow the target. On July 21, 2025, the Hubble Space Telescope recorded a reddish dust coma surrounding the core.
Flybys of Mars and Venus allow spacecraft to collect spectral data
The comet passed within 30 million kilometers of Mars on October 3, 2025. That proximity allowed European orbiters operating around the red planet, specifically Mars Express and the ExoMars Trace Gas Orbiter, to gather direct spectral measurements of the object.
Maintaining its hyperbolic speed of 137,000 km/h, the body later passed by Venus on November 3, 2025, at a distance of 97 million kilometers without experiencing significant gravitational interactions. Earlier in the campaign, on August 6, 2025, the James Webb Space Telescope observed gas emissions from the nucleus, while the European Space Agency scheduled its Jupiter Icy Moons Explorer (Juice) mission to use its onboard sensors for observations in November 2025.
When 3I/ATLAS emerged from behind the Sun on October 31, 2025, ground-based visibility from Earth resumed. During the period of solar conjunction, solar observatories including the Solar and Heliospheric Observatory (SOHO) and the Polarimeter to Unify the Corona and Heliosphere (PUNCH) monitored the object’s path.
James Webb and Swift instruments measure carbon dioxide and nickel vapor
Spectroscopic readings collected by the James Webb Space Telescope and the Spectro-Photometer for the History of the Universe, Epoch of Reionization and Ices Explorer (SPHEREx) showed that carbon dioxide is the dominant component of the outgassing material. Water ice and carbon monoxide appear in smaller proportions within the coma.
Cyanide gas and nickel vapor were detected at concentrations comparable to those found in native Solar System comets, while the reddish hue observed earlier originated from micrometer-sized dust grains. As the comet moved closer to the Sun, its color shifted toward blue, indicating surface changes driven by sublimation without visible mass loss.
The Neil Gehrels Swift Observatory recorded water vapor emissions throughout July and August 2025. Those same ultraviolet and X-ray monitoring sessions showed nickel levels rising as solar heating intensified.
NASA confirms natural sublimation after Harvard researcher notes eight anomalies
Tracking teams documented non-gravitational acceleration without visible mass loss alongside an irregular rotation pattern. Avi Loeb, a researcher at Harvard University, grouped these physical behaviors into eight specific anomalies.
Despite the unusual rotational and acceleration metrics, NASA classifies 3I/ATLAS as a natural comet with no evidence of technological origin. Agency scientists base this conclusion on spectral data that match standard gas sublimation processes.
Computer models trace the comet to the Milky Way thick disk
Computer simulations indicate an age between 7.6 billion and 14 billion years for 3I/ATLAS, pointing to a birthplace in the thick disk of the Milky Way. That region of the galaxy is populated by older stars and contains lower concentrations of heavy elements.
Separate dynamical analyses place the age in a range from 3 billion to 11 billion years, noting that the object’s high vertical velocity reflects an inclined galactic orbit altered by stellar encounters over billions of years. Because of the duration of its journey across space, the comet cannot be traced back to a specific parent star and serves instead as a preserved sample of early interstellar chemistry.
ESA prepares 2029 interceptor mission as 3I/ATLAS exits the Solar System
To study similar objects in the future, the European Space Agency is preparing its Comet Interceptor mission for launch in 2029. The spacecraft will wait in a parking orbit to intercept pristine long-period or interstellar comets like 3I/ATLAS as they enter the inner Solar System.
Astronomers continue to compile global datasets from the 2025 pass to refine models of planetary formation before the object leaves the Solar System over the next decade at a velocity of 50 km/s. Multi-instrument observations that bridged the gaps during solar conjunction confirmed that the comet’s predominant blue glow stems from diatomic carbon (C2) and amidogen radical (NH2) emissions.
