3I/ATLAS (C/2025 N1)

Orbital Parameters

Observation Campaign

Visualizations

Sky-Plane Trajectory (RA / Dec)

Apparent motion of 3I/ATLAS across the celestial sphere. The nearly linear track reflects the object's extreme hyperbolic velocity as it crosses from Sagittarius through Virgo. Data: MPC ephemerides, 30 epochs.

Why it's nearly a straight line: Most solar system comets trace curved arcs across the sky as they swing around the Sun. Atlas barely curves. Its path is so close to a straight line because it's moving so fast relative to us that the Sun's gravity hardly bends it — like a bullet passing through a room. That visual straightness is the eccentricity of 6.139, made visible.

Brightness Light Curve (V-band)

Predicted V-band magnitude evolution of 3I/ATLAS from July 2025 through January 2026. The ~4-magnitude brightening around perihelion (October 2025) indicates significant cometary outgassing activity as solar heating increases.

What the brightening tells us: A 4-magnitude increase means Atlas became roughly 40 times brighter as it approached the Sun. That's not just reflecting more light — it's outgassing. Volatile ices formed in another star system, preserved for billions of years in interstellar cold, are now sublimating in our Sun's warmth. We're watching another star's building materials evaporate in real time. That's what makes Atlas fundamentally different from 'Oumuamua, which showed no outgassing at all.

Key Scientific Findings

Observations Since Publication (2025–2026)

The sections above describe what was known at the time of the original notebook analysis. In the months since, 3I/ATLAS reached perihelion and was characterised across the UV–to–IR by JWST, HST, VLT, Subaru, SPHEREx, and ground-based campaigns. What follows is a synthesis with provenance: a side-by-side comparison against the two prior interstellar objects, a placement of the new water D/H measurement against Solar-System cometary families, and a sourced audit of five claims that have circulated in popular coverage. Every value here is vendored from the primary literature with an arXiv ID or DOI; nothing is summarised from secondary reporting.

1I / 2I / 3I — what each one taught us

Full long-format table (38 rows, source per cell) and the wide pivot are in the notebook outputs: interstellar_comparison_long.csv and interstellar_comparison.csv. Primary sources include Meech et al. 2017 (Nature), Micheli et al. 2018 (Nature), Guzik et al. 2020 and Bodewits et al. 2020 (Nature Astronomy), and for 3I/ATLAS: Roth et al. 2026 JWST (arXiv:2603.20460, 2603.20445), Salazar Manzano et al. 2026 (arXiv:2603.07026), Opitom et al. 2026 (arXiv:2603.07187), Spada, Królikowska & Dones 2026 (arXiv:2603.00782), Hutsémekers et al. 2026 (arXiv:2605.07652), Ahuja & Ganesh 2025 (arXiv:2511.16247).

Water D/H — placing 3I/ATLAS against the local family tree

The deuterium-to-hydrogen ratio in cometary water is the closest thing we have to a birth certificate for ices. Protosolar gas sits at ~2×10⁻⁵; Earth ocean water is ~1.56×10⁻⁴; Jupiter-family and Oort-cloud comets span roughly 10⁻⁴–5×10⁻⁴; cold prestellar / interstellar water sits above 10⁻³. The new 3I/ATLAS lower limit lands above even the ISM water band — consistent with an origin in a cold, lightly-processed reservoir.

What the news got wrong — sourced audit

Five claims that have circulated in popular coverage, each graded against the primary literature. Verdicts use plain language: confirmed, partly confirmed, wrong, unsupported.

• Confirmed CO₂-dominated coma JWST NIRSpec, Subaru, and SPHEREx independently confirm an elevated CO₂/H₂O signature. Primary: arXiv:2603.20460 (Roth et al. 2026, JWST) and companion observations.
• Confirmed (with context) Nickel emission persisting to ~2 AU UVES + VLT high-resolution spectroscopy detects Ni I (and Fe I) at heliocentric distances where most comets show no metals. Real, but in line with a small but growing population of Solar-System comets with the same behaviour — not unique to 3I. Primary: arXiv:2605.07652 (Hutsémekers et al. 2026).
• Unsupported “Green glow without carbon chains” C₂, C₃, and CN are all clearly detected. The premise of the headline is contradicted by the spectra. Primary: arXiv:2603.07187 (Opitom et al. 2026, submitted to Nature).
• Wrong “Minimal or absent non-gravitational acceleration” Non-gravitational acceleration is not only present but characterised — consistent with normal outgassing-driven thrust, not exotic propulsion. Primary: arXiv:2603.00782 (Spada, Królikowska & Dones 2026) and follow-ups.
• Unsupported — actively contradicted Association with the “WOW” signal or any technosignature Three independent targeted radio searches return clean nulls. No narrowband emission, no anomalous broadband structure, no spectral artefact attributable to 3I/ATLAS. Primary: arXiv:2512.19763, arXiv:2512.18142, arXiv:2603.19023 (three independent 2025–2026 null results).