What are the technical differences between the R2 Swan and 3I/ATLAS aircraft models?
Executive summary
C/2025 R2 (SWAN) is a Solar System long‑period comet discovered in SWAN/SOHO imagery on 11 September 2025 and reached a reported peak brightness near magnitude ~5.9 during an outburst; it made closest approach to Earth at ~0.26 AU on 20 October 2025 [1]. 3I/ATLAS is an interstellar, hyperbolic visitor widely reported as the third confirmed interstellar object; observers emphasize its hyperbolic orbit and unusual spectral/tail features that sparked intense follow‑up including Hubble and JWST [2] [3] [4].
1. Origins and orbital class: local Oort Cloud comet vs. interstellar intruder
The clearest technical distinction is origin and orbit: C/2025 R2 (SWAN) is presented in reporting as a long‑period Solar System comet likely from the Oort Cloud and was cataloged with standard cometary orbital elements (per JPL ephemerides cited) [5] [1]. By contrast 3I/ATLAS is characterized as interstellar because its orbit is hyperbolic (eccentricity >1), a defining dynamical signature that indicates it did not originate bound to the Sun [3]. Several outlets explicitly contrast SWAN as Solar System and 3I/ATLAS as interstellar [6] [3].
2. Discovery circumstances and observing platforms
SWAN was discovered in imagery from the SWAN instrument aboard SOHO and initially noted very close to perihelion, which limited early ground visibility but allowed immediate SOHO‑based detection [5] [1]. 3I/ATLAS drew rapid, multi‑facility attention — Hubble, Gemini and JWST were tasked to observe it — because of its interstellar status and because its hyperbolic trajectory made it scientifically exceptional [3] [2]. Reporting notes different discovery vectors on the sky (Aquarius for SWAN vs. Sagittarius for 3I/ATLAS) which emphasize they are unrelated arrivals [7] [8].
3. Brightness, activity and morphology — how they looked to telescopes
SWAN underwent a notable outburst in September 2025 reported to reach magnitude ~5.9 and developed a diffuse coma and tail visible to amateurs and binocular users during parts of its apparition [1] [6]. 3I/ATLAS often appears fainter in some reports but earned attention for complex tail structure (multiple tails/jets) and spectral hints from JWST/HST that may differ from typical Solar System comets — material composition and jet morphology are part of why observers call it “unusual” [4] [2] [3].
4. Size and nucleus constraints
Available reporting gives an upper limit for 3I/ATLAS’s size from Hubble‑based constraints (a maximum diameter around ~5.6 km, with caveats it could be much smaller) [3]. For SWAN, reporting states there was no precise nucleus measurement published in the cited articles; discussions focus on integrated brightness and coma rather than a firm nucleus size [3] [1]. Therefore the better nucleus constraint in these sources applies to 3I/ATLAS [3]; precise nucleus data for SWAN is “not found in current reporting.”
5. Scientific significance and follow‑up priorities
Journalistic and expert commentary frames 3I/ATLAS as a high‑priority target because an interstellar object offers potential insight into extrasolar small bodies and formation environments; that motivated intensive campaigns and speculation about atypical composition in JWST spectra [2] [3]. SWAN’s value is different: it supplied a bright, easily observed Solar System comet with an outburst that engaged amateur and professional observers and illustrated SWAN/SOHO’s discovery role [1] [5].
6. Public narratives, misinformation risks, and context
Popular pieces and social media amplified dramatic comparisons — size ratios (“100× bigger”), “alien spaceship” narratives, or a supposed “battle” behind the Sun — but multiple outlets explicitly flag those as unsupported by data; Star Walk and explanatory sites call out misinformation and emphasize that orbital timing and directions contradict conspiracy framings [6] [2]. Avi Loeb and some commentators explored speculative scenarios (e.g., different arrival directions arguing SWAN is not a fragment of 3I/ATLAS), but the underlying observational point reported is simple: the arrival directions and orbital solutions differ [7] [8].
7. What the sources don’t settle
None of the provided sources give a definitive SWAN nucleus diameter, and they do not present a complete compositional comparison grounded in peer‑reviewed spectral fits; JWST and Hubble data are referenced for 3I/ATLAS but full compositional conclusions remain framed as preliminary or evocative in the cited reporting [2] [3]. If you’re seeking peer‑reviewed spectroscopic abundance tables or a SWAN nucleus size, “not found in current reporting” in these sources.
Bottom line: technically, the primary differences reported are dynamical (Solar System long‑period vs. interstellar hyperbolic), discovery and follow‑up intensity (SOHO/SWAN discovery vs. multi‑facility campaigns), and the degree to which nucleus/size/composition have been constrained (3I/ATLAS has tighter upper limits from HST; SWAN’s nucleus is not precisely reported) [5] [1] [3].