Prediscovery Activity of New Interstellar Object 3I/ATLAS: Rapid Brightening from 6 to 4 au

Abstract

We report on the prediscovery observations and constraints of the new interstellar comet 3I/2025 N1 (ATLAS), made by the Zwicky Transient Facility, for the inbound leg of the comet out to a heliocentric distance of r 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 h  = 6.5 au. The comet followed a brightening rate of ∝ r h − 3.8 , which is significantly steeper than that of 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 M d ̇ ∼ 5 kg s − 1 in 2025 early May ( r h  ∼ 6 au), increasing to M d ̇ ∼ 30 kg s − 1 toward 2025 mid-July ( r h  ∼ 4 au) assuming 100 μ m dust grains, in line with the more recent Hubble Space Telescope measurement made at r h  = 3.8 au. Comparison with the prediscovery photometry by the Transiting Exoplanet Survey Satellite suggested that 3I started producing constant dust outflow probably around r h  ∼ 9 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 h  = 13–17 au, and conclude that the comet was no brighter than 2–5 mag 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.