Comment on "Discovery and Preliminary Characterization of a Third Interstellar Object: 3I/ATLAS" [arXiv:2507.02757]

Abstract

The interstellar object 3I/ATLAS shows a weak cometary activity. Its brightness suggests a maximum radius of ~10km (A/0.05)^{-1/2} for an asteroid with an albedo A. I show that interstellar objects with that radius would amount to an interstellar mass density that is well above the expected mass budget of interstellar comets or asteroids. Given this budget, the detection rate of objects like 3I/ATLAS implies that it is a comet with a small core radius <0.6km, or a member of a rare population with a number density <5x10^{-8}au^{-3} for R>10km. The second possibility would suggest that the rare population of 3I/ATLAS objects favors plunging orbits towards the inner solar system to accommodate their inferred detection rate.