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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mo>∝</mml:mo> <mml:msubsup> <mml:mrow> <mml:mi>r</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">h</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>3.8</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> , 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover accent="true"> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">d</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> <mml:mrow> <mml:mo>̇</mml:mo> </mml:mrow> </mml:mover> <mml:mo>∼</mml:mo> <mml:mn>5</mml:mn> <mml:mspace width="0.25em"/> <mml:mi mathvariant="normal">kg</mml:mi> <mml:mspace width="0.25em"/> <mml:msup> <mml:mrow> <mml:mi mathvariant="normal">s</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> </mml:math> in 2025 early May ( r h ∼ 6 au), increasing to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover accent="true"> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi>M</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">d</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> <mml:mrow> <mml:mo>̇</mml:mo> </mml:mrow> </mml:mover> <mml:mo>∼</mml:mo> <mml:mn>30</mml:mn> <mml:mspace width="0.25em"/> <mml:mi mathvariant="normal">kg</mml:mi> <mml:mspace width="0.25em"/> <mml:msup> <mml:mrow> <mml:mi mathvariant="normal">s</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> </mml:math> 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.