3I/ATLAS

December 2025: a band is present but the model fits poorly, so Q is not trusted at its quoted precision.

Real SPHEREx frames, centred on the comet

SPHEREx D5 · 3.90 µm · 2025-12-09 · obs 2025W50_1A_0348_3 Outside the CO₂ band SPHEREx: NASA/JPL-Caltech/IRSA (DR1 + QR3, L2 l2b-v27); ephemeris: NASA/JPL Horizons

Wavelength of the frame (µm) Frame 1 of 10

Each frame is one exposure at one wavelength, tracked on the comet with the JPL Horizons ephemeris and shown with the stack's common stretch. No continuum is subtracted and no gas map is made: where the CO₂ band is bright you see it as a brighter, larger coma only in the frames marked “in band”.

Spectrum around the CO₂ band

D5, sky subtracted, 3 px aperture, window 3.85–4.42 µm. One point per exposure.

H I PaβH I PaαH I BrαH₂OCH₄CO₂ ν₃CO (1–0)

Wavelength (µm) / Flux (mJy)

µm	mJy	σ
3.8514	12.03	0.267
3.8867	11.28	0.262
3.9209	12.58	0.275
3.9541	8.094	1.42
3.9897	8.060	1.44
4.0239	9.571	1.13
4.0598	9.170	0.274
4.0946	9.096	0.252
4.1305	9.779	0.250
4.1663	11.83	0.283
4.2031	77.05	0.533
4.2387	623.8	1.43
4.2756	448.0	1.21
4.3124	54.21	0.471
4.3493	18.44	0.338
4.3873	18.19	0.346

Spectrum around the CO window

D6 spectrum used for the CO census; the CO (1–0) band sits at 4.55–4.80 µm.

H I PaβH I PaαH I BrαH₂OCH₄CO₂ ν₃CO (1–0)

Wavelength (µm) / Flux (mJy)

µm	mJy	σ
4.4670	14.52	0.388
4.5002	15.94	0.387
4.5331	33.25	0.470
4.6014	53.20	0.546
4.6358	71.13	0.663
4.6696	82.56	0.673
4.7028	78.67	0.664
4.7411	34.70	0.504
4.8080	29.44	0.504
4.8428	31.44	0.528
4.8825	29.95	0.510
4.9513	34.38	0.577

Numbers behind the result

Status: With systematics

Q(CO₂) 3.42 × 10²⁷ [2.78 × 10²⁷–4.18 × 10²⁷] s⁻¹

S/N 41.1 · σ_F/F 0.025 · 2 core channels · χ²ᵣ 185.2 · T_rot 80 K · r_h 2.15 au · Δ 1.81 au

Alternative recipes: 3.82 × 10²⁷ with other τ and g; 2.05 × 10²⁷ with a lower outflow speed.

Q against aperture: 3.32 × 10²⁷ (2 px), 3.39 × 10²⁷ (3 px), 3.48 × 10²⁷ (5 px), 3.33 × 10²⁷ (8 px) s⁻¹. A strong trend means the gas is not a simple parent-molecule coma.

CO: candidate; CO/CO₂ 1.5 [1.2–1.8]

H₂O: no coverage

Compared with published values

CO₂ · 2025-08: 1.17 × 10²⁷ s⁻¹ ↔ 9.40 × 10²⁶–1.60 × 10²⁷ s⁻¹ (inside the published range)

Lisse et al. 2025, RNAAS, arXiv:2508.15469 (0.94e27, preliminary); Lisse et al. 2026, ApJL 1000, L52, arXiv:2512.07318 (1.6e27 +/- 0.16e27)

Provenance

Tracked with JPL Horizons target 2025 N1, observer SPHEREx.

Stack 2025-12 · D5 · 10 frames · 10 shown (≥ 95% valid pixels)

Frames used

2025W50_1A_0348_3 · 2025W50_2A_0685_1 · 2025W50_2A_0685_3 · 2025W50_2A_0685_4 · 2025W50_2A_0698_2 · 2025W50_2A_0698_3 · 2025W50_2A_0712_1 · 2025W50_2A_0712_2 · 2025W50_2A_0712_4 · 2025W50_2A_0827_1

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