3I/ATLAS: The third interstellar object ever found

Episode Summary

Executive Summary: Planetary Radio explores 3I/ATLAS, the third known interstellar object and a visibly active comet, with astronomer Bryce Bolin describing how his team rapidly studied its orbit, brightness, tail, and composition. The episode highlights why interstellar objects matter for understanding other planetary systems, how upcoming facilities like Rubin, JWST, and Comet Interceptor will expand discovery and characterization, and why this fleeting visitor is a rare scientific opportunity.

Main Topics: Discovery and significance of 3I/ATLAS (Priority: 5/5): Sarah Al-Ahmed frames 3I/ATLAS as only the third known interstellar object, discovered by ATLAS on July 1, 2025. Bryce Bolin explains why a macroscopic body from another star system is scientifically and emotionally extraordinary. Bolin’s path into planetary science and interstellar-object studies (Priority: 4/5): Bolin recounts a career shift from particle physics toward astronomy through planetary science, Pan-STARRS, and collaborations that placed him on the first-observation teams for all three known interstellar objects. What 3I/ATLAS reveals physically (Priority: 5/5): The discussion covers color, coma, near-infrared data, dust behavior, and the possibility of cometary volatiles such as water, carbon dioxide, carbon monoxide, cyanogen, and carbon-chain species. Comparing 1I/ʻOumuamua, 2I/Borisov, and 3I/ATLAS (Priority: 5/5): Bolin contrasts ʻOumuamua’s non-gravitational acceleration with the clearly cometary nature of Borisov and ATLAS, emphasizing that all three known interstellar objects appear comet-like so far. Observational constraints and future instrumentation (Priority: 4/5): The interview explains why rapid follow-up is essential, how Hubble, Keck, and ground-based telescopes complement one another, and why JWST, Rubin, and ELTs will change the field. Comet Interceptor and future rendezvous missions (Priority: 4/5): Bruce Betts describes ESA/JAXA’s Comet Interceptor as a mission waiting at Earth-Sun L2 to chase a pristine long-period comet or potentially an interstellar object, showing how future missions could enable in situ study. Community response and advocacy (Priority: 3/5): The episode also includes a call for listener stories about grant cuts affecting space science and a Planetary Society day of action to protect NASA science funding.

Key Arguments: Interstellar objects are rare natural samples from other planetary systems and offer a direct way to study how material is distributed beyond the Sun. Bolin’s work shows that fast-response observing campaigns are essential because these objects are visible only briefly before solar conjunction or departure. All three known interstellar objects have appeared comet-like, supporting expectations that ejected extrasolar material should often be volatile-rich. ʻOumuamua’s unusual motion is best interpreted as comet-like non-gravitational acceleration rather than a simple asteroid-like orbit. 3I/ATLAS’s dust morphology suggests large, slowly moving grains and weak activity rather than a conventional broad anti-solar dust tail. JWST will be especially valuable because it can detect key volatiles directly in the 3–5 micron range, beyond what ground-based optical observations can measure well. Rubin will discover many more interstellar objects, but most will be much fainter and therefore harder to characterize, increasing reliance on large telescopes and space-based spectroscopy. Comet Interceptor is strategically important because it could intercept a newly discovered target without needing a preselected object, making an interstellar rendezvous plausible in the future.

Data Points: Known interstellar objects: 3 - ʻOumuamua, 2I/Borisov, and 3I/ATLAS are described as the only three known interstellar objects. Discovery date of 3I/ATLAS: July 1, 2025 - ATLAS discovered the newest interstellar comet on this date. Publication date of the Bolin paper: July 18, 2025 - The paper 'Interstellar Comet 3I Atlas, Discovery and Physical Description' appeared in MNRAS Letters. Rotation period of ʻOumuamua: about 8 hours - Bolin cites the first object’s full light-curve coverage to infer its rotation period. Light-curve amplitude of ʻOumuamua: about 2 magnitudes - The amplitude supported the conclusion that it has a very elongated shape. Axial ratio of ʻOumuamua: about 6:1 - Bolin says geometric corrections led to this estimate; an uncorrected value could appear closer to 10:1. Brightness of 3I/ATLAS: 17th–18th magnitude - Bolin uses this to discuss size estimates and observability from smaller telescopes. Distance of 3I/ATLAS from the Sun at the time discussed: about 3–4 AU - Used as part of the rough size estimate and observing context. Estimated size of 2I/Borisov: a few hundred meters to ~500 meters - Bolin compares Borisov’s faintness to ATLAS to infer ATLAS may be around a kilometer across. Estimated size of 3I/ATLAS nucleus: about 1 kilometer - Derived by comparing brightness with Borisov and by Hubble-based size constraints. Hubble angular resolution: 0.04 arcseconds - Used to explain why Hubble can isolate the nucleus better than ground-based observations. Typical ground-based seeing: about 1 arcsecond - Bolin uses this to illustrate the advantage of Hubble for comet nucleus measurements. Resolution improvement: factor of ~600 - Cross-sectional contamination reduction when using Hubble instead of ground-based imaging. Dust speed from 3I/ATLAS: about 1 meter per second - Bolin infers slow dust escape from the comet’s unusual morphology. Expected post-conjunction brightness: about 12th magnitude - Bolin says the comet may brighten enough for smaller telescopes when it returns to view. Comet Interceptor launch year: 2029 - Bruce Betts says the ESA/JAXA mission is slated to launch then. Distance to Earth-Sun L2: 1.5 million kilometers - Comet Interceptor will wait at L2 for a suitable target.

Pivotal Quotes: "“For only the third time in history, astronomers have identified a macroscopic object from another star system traveling through our cosmic neighborhood.”" — Sarah Al-Ahmed: Introductory framing of why 3I/ATLAS is scientifically extraordinary. "“It was really the connection with particle physics because I didn't want to do asteroids and comets because I thought they were boring.”" — Bryce Bolin: Bolin explains the unexpected career path that led him into asteroid and comet research. "“We think it's most likely a comet because it has large non-gravitational perturbations like a comet.”" — Bryce Bolin: Discussion of ʻOumuamua’s behavior and why comets remain the leading interpretation.

Implications: The episode shows that interstellar objects are becoming a repeatable field of study, not a one-off curiosity. As Rubin, JWST, ELTs, and Comet Interceptor come online, listeners can expect many more discoveries, but also a sharper need for rapid follow-up and coordinated observing.