Dust activity of interstellar comet 3I/ATLAS (C/2025 N1) at 4.3–4.5 au from VLT/FORS2 imaging

Abstract We present R-band coma characterisation of interstellar comet 3I/ATLAS (C/2025 N1) from archival VLT/FORS2 imaging spanning 2025 July 4–21 at rh = 4.35–4.53 au—the earliest VLT/FORS2 morphological dataset for this object. After Moffat PSF fitting to remove field stars, aperture photometry yields Afρ = 110–458 cm (ρ = 2.″5, ~7000 km) across four reliable epochs, consistent with independent contemporaneous measurements. Radial surface-brightness slopes γ = −2.6 to −5.0 are far steeper than the canonical γ = −1, implying radiation-pressure sweeping, grain fragmentation, or time-variable emission at these distances. The coma is nearly circular (e ≲ 0.07) within ρ 5 arcsec (~14 000 km), with no position-angle alignment to the antisolar direction. A systematic jet search using Larson–Sekanina filtering and azimuthal-median subtraction yields no coherent features; injection-recovery tests confirm 90 per cent completeness for jets with ≳ 8 per cent peak contrast at ρ = 5.″0. Since a wobbling jet was detected from August 3 onward, our non-detection through July 18 constrains the emergence of this feature to a 16-d window (July 18 – August 3, rh ≈ 4.5 → 4.0 au), consistent with thermally activated onset. Dust activity at rh 4.3 au requires a driver more volatile than H2O, consistent with the CO2-dominated coma confirmed by JWST spectroscopy.