Prediscovery Activity of New Interstellar Object 3I/ATLAS: Rapid Brightening from 6 to 4 au
Creators
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1.
University of Maryland, College Park
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2.
Boston University
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3.
Planetary Science Institute
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4.
University of Washington
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Infrared Processing and Analysis Center
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6.
California Institute of Technology
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 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 rh = 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 (rh ∼ 6 au), increasing to M_d ̇ ∼ 5 kg s⁻¹ toward 2025 mid-July (rh ∼ 4 au) assuming 100 μm 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 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 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.
Copyright and License
© 2025. The Author(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
Acknowledgement
Q.Y. is supported by NASA grant 80NSSC21K0659. Based on observations obtained with the 48 inch Samuel Oschin Telescope at the Palomar Observatory as part of the Zwicky Transient Facility project. ZTF is supported by the National Science Foundation under grant Nos. AST-1440341, AST-2034437, and currently Award #2407588. ZTF receives additional funding from the ZTF partnership. Current members include Caltech, USA; Caltech/IPAC, USA; University of Maryland, USA; University of California, Berkeley, USA; University of Wisconsin at Milwaukee, USA; Cornell University, USA; Drexel University, USA; University of North Carolina at Chapel Hill, USA; Institute of Science and Technology, Austria; National Central University, Taiwan, and OKC, University of Stockholm, Sweden. Operations are conducted by Caltech’s Optical Observatory (COO), Caltech/IPAC, and the University of Washington at Seattle, USA.
Facilities
PO:1.2m - Palomar Observatory's 1.2 meter Samuel Oschin Telescope.
Software References
astropy (Astropy Collaboration et al. 2013,2018, 2022), matplotlib (J. D. Hunter 2007), numpy (S. Van Der Walt et al. 2011; C. R. Harris et al. 2020), pysynphot (STScI Development Team 2013), sbpy (M. Mommert et al. 2019).
Files
Ye_2025_ApJL_993_L31.pdf
Files
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Additional details
Additional titles
- Alternative title
- Prediscovery Activity of New Interstellar Object 3I/ATLAS: A Dynamically-Old Comet?
Related works
- Is new version of
- Discussion Paper: arXiv:2509.08792 (arXiv)
- Is supplemented by
- Dataset: https://content.cld.iop.org/journals/2041-8205/993/1/L31/revision1/apjlae147bf3.tar.gz (URL)
Funding
- National Aeronautics and Space Administration
- 80NSSC21K0659
- National Science Foundation
- AST-1440341
- National Science Foundation
- AST-2034437
- National Science Foundation
- 2407588
Dates
- Accepted
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2025-10-15
- Available
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2025-10-29Published