Comet 3I/ATLAS, also known as C/2025 N1 (ATLAS), marks the third confirmed interstellar object passing through our solar system. Discovered in July 2025, this rare comet offers unprecedented insights into extrasolar chemistry and origins.
Discovery Story
Astronomers detected 3I/ATLAS on July 1, 2025, using the NASA-funded ATLAS telescope in Río Hurtado, Chile, at an apparent magnitude of 18. Precovery images from Zwicky Transient Facility and ATLAS extended observations back to June 14, 2025, confirming its hyperbolic trajectory within hours. The Minor Planet Center designated it 3I/ATLAS on July 2, highlighting its interstellar nature after 1I/ʻOumuamua and 2I/Borisov.
Interstellar Trajectory
This comet follows a hyperbolic path with eccentricity of 6.139, entering at 58 km/s hyperbolic excess velocity from Sagittarius near the Milky Way’s center. It reached perihelion on October 29, 2025, at 1.36 AU from the Sun, between Earth and Mars orbits. Closest to Earth at 1.8 AU on December 19, 2025, it poses no threat and passed Mars at 0.194 AU on October 3.
| Closest Approach | Date (UT) | Distance (AU) |
|---|---|---|
| Mars | 2025-Oct-03 | 0.194 |
| Sun (Perihelion) | 2025-Oct-29 | 1.36 |
| Venus | 2025-Nov-03 | 0.65 |
| Earth | 2025-Dec-19 | 1.80 |
| Jupiter | 2026-Mar-16 | 0.36 |
Physical Characteristics
Hubble Space Telescope images constrain the nucleus diameter to under 1 km, surrounded by a coma up to 26,400 km wide with a reddish hue from organic dust. JWST detected high CO₂ (129 kg/s), low water (6.6 kg/s), CO, and OCS, suggesting formation beyond its star’s CO₂ frost line. VLT observations revealed cyanide and nickel emissions similar to solar system comets, with a Sun-facing dust plume and anti-solar tail.
Scientific Significance
Likely from the Milky Way’s thick disk, 3I/ATLAS may exceed 7 billion years old, predating our solar system. Its CO₂-rich, carbon-chain-depleted composition hints at unique formation or processing by galactic cosmic rays over eons. Missions like Mars Express, Juice, and JWST provided multi-wavelength data, advancing interstellar object studies.
Observation and Future Path
Peaking at magnitude 11.5, visible via mid-sized telescopes through 2026 in constellations like Libra and Leo. Post-perihelion, it fades toward the Oort cloud by 2189, offering trailing observations of its ion tail by probes like Europa Clipper. Ongoing JWST and Hubble monitoring track its outbound journey.
