Prediscovery Activity of New Interstellar Object 3I/ATLAS: A Dynamically-Old Comet?

Abstract

We report on the prediscovery observations and constraints of the new interstellar comet 3I/2025 N1 (ATLAS), made by the Zwicky Transient Facility (ZTF), for the inbound leg of the comet out to a heliocentric distance of $r_\mathrm{h}=17$ au, or approximately a year before its discovery. We find that 3I/ATLAS has been active inward of a heliocentric distance of at least $r_\mathrm{h}=6.5$ au. The comet followed a brightening rate of $\propto r_\mathrm{h}^{-3.8}$, which is significantly steeper than the only other known interstellar comet 2I/Borisov, and is more consistent with dynamically old long-period comets and short-period comets in the Solar System. By measuring the brightening of the dust coma, we estimate that 3I had a dust production rate of $\dot{M_\mathrm{d}}\sim5 \mathrm{kg s^{-1}}$ in early May of 2025 ($r_\mathrm{h}\sim6$ au), increasing to $\dot{M_\mathrm{d}}\sim30 \mathrm{kg s^{-1}}$ towards mid-July 2025 ($r_\mathrm{h}\sim4$ au) assuming 100 micron dust grains, in line with the more recent Hubble Space Telescope measurement made at $r_\mathrm{h}=3.8$ au. Comparison with the prediscovery photometry by the Transiting Exoplanet Survey Satellite (TESS) suggested that 3I started producing constant dust outflow probably around $r_\mathrm{h}\sim9$ au, coinciding with the turn-on distance of CO$_2$ ice. We also conduct a deep search of 3I/ATLAS with multiple nights of data taken in 2024 when the comet was at $r_\mathrm{h}=13$-$17$ au and conclude that the comet was no brighter than 2-5 magnitudes above the coma or bare-nucleus lightcurves. This suggests that the comet did not exhibit strong outbursts during these periods, consistent with 2I/Borisov as well as most long-period Solar System comets.