Abstract We present high-angular-resolution observations of the third known interstellar interloper, 3I/ATLAS, from the Hubble Space Telescope. The object is clearly active at 3.8 au preperihelion, showing dust emitted from the hot, Sun-facing side of the nucleus and a weak, radiation-pressure-swept tail away from the Sun. We apply a simple model to estimate the mass loss rate in dust as dM / dt ∼ 12 a μ 1 / 2 kg s −1 , where a μ is the mean particle size in microns. With 1 ≤ a μ ≤ 100, we infer dM / dt ∼ 12–120 kg s −1 . A fit to the surface brightness distribution of the inner coma limits the effective radius of the nucleus to r n ≤ 2.8 km, assuming red geometric albedo 0.04. Conversely, the nucleus cannot be smaller than ∼0.22 km in radius if its coma is supplied by sublimation of carbon monoxide and must be larger if a less volatile molecule drives the mass loss.
Jewitt et al. (Thu,) studied this question.
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