Overview
- NASA and multiple missions classify 3I/ATLAS as the third confirmed interstellar comet, with Hubble constraining the nucleus to roughly 440 meters to 5.6 kilometers and imagery showing an active coma and tail features.
- The closest approach to Earth is forecast for 19 December 2025 at about 270 million kilometers (170 million miles), a safe distance that nonetheless offers a key opportunity for coordinated observations.
- James Webb infrared spectra indicate an unusually high CO2-to-water ratio in the coma, pointing to an atypical composition compared with many Solar System comets.
- Teams are modeling measured non‑gravitational accelerations, rapid brightness changes, jets and an anti‑tail, with directed outgassing under evaluation; blurry images reflect the expanding coma and the long ranges of observing spacecraft.
- Astrophysicist Avi Loeb continues to advance a speculative technological hypothesis tied to a March 2026 pass near Jupiter’s Hill sphere, but space agencies report no evidence of artificiality and maintain a natural comet explanation.