Nucleus and Postperihelion Activity of Interstellar Object 3I/ATLAS Observed by the Hubble Space Telescope

Abstract We report the detection of the nucleus of interstellar object 3I/ATLAS, using a nucleus extraction technique on Hubble Space Telescope (HST) observations taken between 2025 December and 2026 January. The product of the V -band geometric albedo, p V , with the physical cross section of the nucleus is 0.22 ± 0.07 km 2 , which corresponds to an effective radius of 1.3 ± 0.2 km if assuming a comet-like albedo p V = 0.04. This size is in agreement with an independent estimate based on the reported nongravitational acceleration and activity of the interstellar object. If the measured photometric variations are solely due to the rotation of an aspherical nucleus, the axis ratio must be 2:1 or greater, and…