Empirical Predictions for 3I/ATLAS (October–December 2025)
Posted for independent verification and discussion
These predictions concern the expected compositional and photometric behaviour of the interstellar object 3I/ATLAS as it approaches and passes perihelion (~29 October 2025, r ≈ 1.36 AU). They are model-agnostic and intended purely as empirical expectations for comparison with forthcoming JWST and VLT datasets.
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CO₂/H₂O Ratio Inversion
• At heliocentric distances > ~2.8 AU, the coma will remain CO₂-rich with weak H₂O signatures.
• Near perihelion (~1.4 AU), the CO₂/H₂O ratio should decline, marking activation of deeper H₂O ices.
• A measurable inversion of the ratio (relative to early observations) is expected between ~1.8 – 1.5 AU. -
Early Nickel Emission
• Spectral lines of neutral or ionised Ni are predicted to appear before any Fe features or strong H₂O lines, likely between 3.0 – 2.5 AU.
• Nickel activity should plateau or decline as the H₂O phase begins. -
Rotational Compositional Modulation
• CO₂ and Ni emissions are expected to vary strongly with rotational phase, reflecting surface heterogeneity.
• H₂O emission should be more uniform once deeper layers activate. -
Polarimetric and Grain-Size Evolution
• Pre-perihelion dust will show higher polarisation (finer, porous grains).
• As perihelion approaches, larger and less-porous grains from deeper layers will reduce polarisation by ~10–20 %. -
Post-Perihelion Hysteresis
• Outgassing ratios will not retrace inbound values: CO₂ emission will drop faster than H₂O, producing a distinct hysteresis loop in compositional trends. -
Possible Isotopic Offset
• Isotopic ratios in CO₂ may differ slightly from those in later-released H₂O (Δ(¹³C/¹²C) or Δ(¹⁸O/¹⁶O) ≈ few × 10⁻³), indicating stratified material layers.
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Purpose:
To provide an observational reference for colleagues tracking 3I/ATLAS through perihelion. These outcomes are testable using existing or planned JWST MIRI/NIRSpec and VLT VISIR/CRIRES+ datasets.
Should any of these behaviours be confirmed or contradicted, please share updates here so that independent analyses can be refined accordingly.
— Christopher Portelli, Independent Researcher (UK)