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

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 rh=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−3.8h, 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 Md˙∼5kgs−1 in early May of 2025 (rh∼6 au), increasing to Md˙∼30kgs−1 towards mid-July 2025 (rh∼4 au) assuming 100 micron dust grains, in line with the more recent Hubble Space Telescope measurement made at rh=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 rh∼9 au, coinciding with the turn-on distance of CO₂ ice.

We also conduct a deep search of 3I/ATLAS with multiple nights of data taken in 2024 when the comet was at rh=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.

Quanzhi Ye, Michael S. P. Kelley, Henry H. Hsieh, Eric C. Bellm, Tracy X. Chen, Richard Dekany, Andrew Drake, Steven L. Groom, George Helou, Shrinivas R. Kulkarni, Thomas A. Prince, Reed Riddle

Comments: Submitted to ApJL; photometry data at this https URL
Subjects: Earth and Planetary Astrophysics (astro-ph.EP); Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:2509.08792 [astro-ph.EP] (or arXiv:2509.08792v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2509.08792
Focus to learn more
Submission history
From: Quanzhi Ye
[v1] Wed, 10 Sep 2025 17:21:09 UTC (880 KB)
https://arxiv.org/abs/2509.08792
Astrobiology,