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What are the reported sizes of 3I/Atlas objects in 2023–2025 observations?
Executive summary
Reported size estimates for interstellar comet 3I/ATLAS during 2023–2025 vary widely: Hubble-based papers and major press summaries place the nucleus diameter between roughly 320 meters and 5.6 kilometers (1,050 ft–3.5 mi) [1] [2], while some observatory and commentator estimates have been much larger — up to ~12–20 km (7–12 miles) or even "about 7 miles" in press accounts — and a few commentators infer masses in the billions of tons from those larger size claims [3] [4] [5]. Coverage is heterogeneous because measurements mix nucleus limits, coma extents and model-dependent mass inferences; available sources do not mention any single, definitive size agreed by all teams.
1. Hubble and telescope teams: a constrained but wide range
Professional telescope analyses cited in multiple outlets report Hubble observations that constrain the nucleus to a broad interval — Hubble-derived limits often quoted are between about 320 meters and 5.6 km in diameter (1,050 ft to 3.5 miles) [1] [2]. NASA and other space‑science summaries emphasize that the nucleus size remains uncertain because the bright coma surrounds the solid nucleus, complicating separation of nucleus and dust in images [6] [2]. These Hubble-based constraints are the most commonly cited numerical bounds in mainstream science coverage [1].
2. Larger estimates from ground observatories and early alerts
Some observatory press releases and early ground-based reports gave much larger upper limits: Gemini North’s release noted estimates "at most 20 kilometers (12 miles)" in diameter as an upper bound from initial observations [3]. ESA and other agencies repeatedly cautioned the coma makes it hard to tell whether apparent large size reflects a huge bare nucleus or an extended, active coma [7] [8]. That caveat explains why larger kilometer-scale headlines appeared even while Hubble teams reported smaller nucleus bounds [3] [2].
3. Popular-press headlines and commentator claims — wide dispersion
Popular outlets and individual commentators amplified a wide range of claims. Live Science published a headline stating "7 miles wide" based on Rubin Observatory imagery coverage [4], and other outlets summarized Hubble as saying "no wider than 3.5 miles and possibly as small as 1,400 feet" [9] [10]. Opinion pieces and some commentators (notably Avi Loeb and associates) publicly argued for diameters "larger than 5 km" or compared the object to Manhattan in scale, producing mass estimates on the order of tens of billions of tons [11] [12]. These larger figures often rest on different interpretive choices about coma removal and grain/dust models [11] [7].
4. Coma extent vs. nucleus diameter: a persistent source of confusion
Several sources stress that reported "sizes" sometimes refer to different things: nucleus diameter, coma full width at half maximum, or the full visible coma/tail extent. For example, coma angular sizes in July 2025 were reported as ~2 arcseconds (core) to ~10 arcseconds (full extent), and some imaging teams reported coma diameters of many kilometers to tens of kilometers — numbers that are not directly equivalent to nucleus diameter [13] [14]. Press summaries also highlighted coma extents of up to ~15 miles (~24 km) in some imagery discussions, conflating nucleus and coma in headlines [4].
5. Why estimates disagree — methodology and observational geometry
The disagreement reflects three clear factors noted in the reporting: (a) the coma masks the nucleus so photometric nucleus estimates depend on uncertain coma subtraction models [2]; (b) different instruments and viewing geometries (ground telescopes, Hubble, Webb, Mars orbiters) probe different spatial scales and wavelengths, producing model-dependent size inferences [15] [16]; and (c) some larger claims rely on early, conservative upper limits or interpretations of the entire dusty envelope as a solid body, which other teams explicitly caution against [3] [7].
6. What authoritative sources say and where uncertainty remains
Authoritative teams (Hubble analysis and journal/preprint work) provide the most-cited numeric bounds (320 m to 5.6 km) while warning that coma contamination limits precision [2] [1]. ESA and NASA materials stress active follow-up (Hubble, Webb, Mars orbiters) to refine physical properties and that currently reported ranges are provisional [6] [8]. Available sources do not mention a single consensus size for the nucleus; instead they document a range and emphasize measurement limitations [2] [6].
7. Bottom line for readers
If you want a cautious, science‑team‑backed range: cite the Hubble‑constrained interval (~320 m to ~5.6 km) and note that larger, often publicized figures (several to tens of km) exist as upper-bound or model-dependent claims from some observatories and commentators [1] [3] [4]. All reporting agrees the coma complicates nucleus measurement and that further analysis of Hubble, Webb and spacecraft data will refine these numbers [2] [15].