Interstellar Comet 3I/ATLAS Shocks Astronomers: Trajectory, Origins, and How to See It Tonight

Updated 9 November 2025 — Astronomers worldwide are tracking a rare visitor, interstellar comet 3I/ATLAS, as it races through the inner solar system on a hyperbolic, one-way trip toward deep space. What is 3I/ATLAS? 3I/ATLAS is only the third confirmed object known to have originated beyond our Sun’s gravitational reach, after ʻOumuamua (2017) and 2I/Borisov (2019). It was first logged on 1 July 2025 by the ATLAS survey telescope in Chile and quickly flagged as interstellar because its velocity (≈61 km/s) is too high to remain bound to the Sun. Key discovery facts • Earliest archived images now trace the comet back to 14 June 2025. • The nucleus is estimated to be between 440 m and 5.6 km wide, shrouded in a teardrop-shaped dust coma seen by Hubble. • Spectra from NASA’s Webb and SPHEREx missions show abundant water-ice and volatile carbon compounds, hinting at pristine chemistry from another star system. Trajectory and important dates • 30 Oct 2025: Perihelion at 1.4 AU (just inside Mars’ orbit). • 3 Nov – 5 Dec 2025: Behind the Sun; unobservable from Earth. • Early Dec 2025: Re-emerges in the predawn sky; magnitude ~11–12, visible in 8-inch or larger amateur telescopes. • Mar 2026: Crosses Jupiter’s orbit and exits the planetary region forever. Why scientists are excited 1. Chemical fingerprints of water and CO₂ offer a direct sample of another stellar nursery, free from billions of years of Sun-light alteration. 2. Comparing 3I/ATLAS with 2I/Borisov helps refine models of planet-formation ejecta and the expected rate of interstellar objects. 3. NASA assets from the Perseverance rover to Parker Solar Probe are opportunistically gathering data, providing a multi-mission case study in rapid-response planetary defense workflows. How to observe 3I/ATLAS • From mid-December, look southeast 90 minutes before sunrise; the comet drifts across Virgo toward Libra. • Use star charts or apps to locate the faint, fuzzy “star” moving a few arcminutes nightly. • Long-exposure astrophotography (ISO 1600+, 30–60 s) can reveal its subtle green coma caused by diatomic carbon fluorescence. Looking ahead The comet will fade beyond reach of even large observatories by summer 2026, but its brief passage is expected to yield a decade’s worth of research papers. Each spectrum, light-curve and dust-tail measurement adds a new clue to the puzzle of how planetary systems eject debris—and how often those icy vagabonds cross our cosmic doorstep. Stay tuned as NASA releases fresh imagery and peer-reviewed results in the coming months, turning 3I/ATLAS from a fleeting streak of light into a cornerstone of interstellar science.