Comet 3I/Atlas, detected in July 2025 by the ATLAS telescope in Chile, crossed the Solar System at a speed of 57 km/s, exceeding the Sun’s escape velocity. This object, the third confirmed as interstellar, followed a hyperbolic trajectory that prevented its capture by solar gravity. The passage took place at the end of October 2025, close to the orbit of Mars, where agencies such as NASA and ESA monitored the event.
Astronomers classified 3I/Atlas as a visitor from another star system due to its high initial speed, which exceeds the limits of local gravitational binding. The phenomenon occurred in a region between 1.4 and 4.5 astronomical units from the Sun, allowing detailed observations without risk to Earth.
- Hyperbolic speed: 57 km/s, greater than the 33 km/s of 2I/Borisov and 26 km/s of 1I/Oumuamua.
- Trajectory: Slightly altered by the Sun, but without a closed orbit.
- Discovery: July 1, 2025, with cometary activity confirmed in following days.
Hyperbolic trajectory defines passing visit
The orbit of 3I/Atlas has an eccentricity greater than 6, which guarantees its exit from the Solar System after perihelion inOctober 29, 2025. Calculations from international observatories indicate that solar gravity acted like a slingshot, deflecting the path at a precise angle without slowing enough for retention. This dynamic reflects the kinetic energy inherited from its galactic origin.
Data from telescopes in Chile, Arizona and Hawaii recorded the marginal coma and a 3-arcsecond tail shortly after initial detection. Preliminary spectroscopic analysis confirms that the object maintained stable activity, with no outbreaks detected until August 2025.
Chemical composition reveals distant origins
Initial studies point to a coma rich in carbon dioxide in relation to water, distinguishing 3I/Atlas from local comets. Observations by the Hubble Space Telescope, carried out in July 2025, identified dust grains measuring several micrometers in a reddish coma, caused by irradiated organic compounds. This reddish color aligns with type D asteroids and comet 2I/Borisov, suggesting formation in similar environments.
The core, estimated to be between 320 meters and 5.6 km in diameter, displays a surface crust formed by billions of years of cosmic radiation. Analysis indicates a potential age of up to 7 billion years, greater than that of the Solar System, with evidence of prolonged exposure in the thick disk of the Milky Way.
Early detection, at 4.5 astronomical units, allowed continued tracking, including pre-discovery data from June 2025. Instruments like NASA’s TESS captured possible cometary activity as early as May, extending the study period to months.
Comparison with previous visitors
3I/Atlas stands out for its speed of 57 km/s, twice that recorded in 1I/Oumuamua, discovered in 2017. While Oumuamua exhibited non-gravitational acceleration attributed to the release of hydrogen gas, with no visible coma, the 2019 Borisov had an active plume and rich organic composition, but fragmented as it approached the Sun.
- 1I/Oumuamua: Speed of 26 km/s; elongated shape; observed anomalous acceleration.
- 2I/Borisov: 33 km/s; red tail; high carbon fractions detected.
- 3I/Atlas: 57 km/s; increasing brightness; inverted blue spectral slope.
These differences point to varied ejection mechanisms in exoplanetary systems, such as gravitational interactions or stellar explosions. The estimated size of 1 to 10 km in 3I/Atlas contrasts with Oumuamua’s 400 meters, expanding the spectrum of known interstellar objects.
Current observations and upcoming passages
Ground-based telescopes have been tracking the comet since November 2025, with magnitude 14.7 visible on 20 cm aperture instruments before dawn, in the constellation Virgo. ESA’s Juice mission set course for additional observations, capturing data on water vapor and dust.
Perihelion to Earth occurs on December 19, 2025, at 270 million km, without threat. In March 2026, the object will cross Jupiter’s orbit, leaving definitively in mid-2027. Dynamical models test shape theoriesplanetary tion based on this trajectory.
Initial detection details
The discovery occurred via the NASA-funded ATLAS system on July 1, 2025. Archival observations extended the trail to June 14, confirming the hyperbolicity. The object arrived from the direction of Sagittarius, with a vertical speed of +18.5 km/s in relation to the galactic plane, indicating its origin in the thin or thick disk of the Milky Way. Cometary activity, including diffuse coma, was validated by telescopes in Hawaii and Australia, solidifying its classification.
