The interstellar comet 3I/ATLAS, the third object confirmed from outside the Solar System, passed through perihelion on October 29, 2025, 1.36 astronomical units from the Sun, between the orbits of Mars and Earth. Images obtained by the Nordic Optical Telescope on November 11, 2025 show the object as a single, active body, with no signs of fragmentation after solar approach. Astronomers monitor the trajectory to understand its composition and speed, which reaches 210 thousand km/h.
The discovery occurred in July 2025 by the ATLAS system in Chile, with confirmation of cometary activity in telescopes in Arizona and Hawaii. The object, with an estimated core between 0.6 and 5.6 km in diameter, follows a hyperbolic orbit and poses no risk to Earth, reaching 1.8 AU on December 19, 2025.
Data from NASA’s Minor Planet Center and Jet Propulsion Laboratory indicate non-gravitational acceleration, with radial parameters of 135 km/day² and transversal parameters of 60 km/day². Observations from the European Space Agency’s Juice mission, which began in November, capture details during the object’s departure.
Planetary trajectory and passages
The orbit of 3I/ATLAS aligns almost perfectly with the ecliptic plane, with an inclination of about 5 degrees. This configuration allows observations from both terrestrial hemispheresres.
The comet approached Mars at 0.19 AU on October 3, 2025 and Venus at 0.65 AU on November 3. In March 2026, it will pass by Jupiter at 0.36 AU, without significant gravitational interactions.
- Speed relative to the Sun: greater than 80 km/s post-perihelion.
- Minimum distance to Earth: 269 million km in December.
- Probable origin: direction of the constellation Sagittarius, possibly ejected from a young star system.
Post-perihelion observations
Images from the Lowell Observatory on October 31, 2025 reveal a compact structure, with a coma of 1.3 arcseconds. The Hubble Space Telescope and James Webb have recorded multiple jets in varying directions, with no traditional tail visible.
The spectrum shows bluish tones, with a hydroxyl signature detected by the Neil Gehrels Swift Telescope on November 2, indicating the presence of water and organic molecules. The core rotates every 5 to 16 hours, releasing heat efficiently.
Chemical composition revealed
Analysis by the Very Large Telescope in Chile identifies dust rich in carbon, silicates and complex organics, with little presence of water ice. This mixture suggests prolonged exposure to interstellar cosmic radiation.
Activity and accelerations detected
The object displays accontinuous eleration in the orbital plane, with coefficients A1 of 1.662 × 10⁻⁶ au/d² and A2 of 7.089 × 10⁻⁷ au/d², without an out-of-plane component. This dynamic differs from typical comets and requires mass loss greater than 13% for natural explanation.
Jets and gaseous emissions
On November 8, 2025, images from the Virtual Telescope Project captured a complex ion tail, with jets in multiple directions. The Discovery Telescope recorded CO2 emissions, reactive to solar heat.
The composition includes traces of carbon monoxide and nitrogen, similar to long-period comets. Observations from the Parker Solar Probe and Mars missions, such as ExoMars and Mars Express, complement the data.
- Observed jets: seven in hexagonal formation in some images.
- Propagation speed: up to 35 km/s.
- Spectral change: from reddish to bluish, in excess of 5,000 K.
Future visibility and monitoring
From mid-November 2025, 3I/ATLAS appears before dawn in the constellation Virgo, with an apparent magnitude of 11 to 12. Amateur telescopes with 200 to 300 mm aperture capture the object in dark skies.
In December, it moves into Leo, weakening to magnitude 12. The Comet Interceptor mission, scheduled for 2029, may intercept similar objects in the future.
