The total mass loss rate and outflow speed from its 3I/Atlas surface was used to infer its mass. The mass of 3I/ATLAS must be bigger than 33 billion tons. The diameter of its solid-density nucleus must be larger than 5 kilometers. This suggests that 3I/ATLAS is more massive than the other two interstellar objects, 1I/`Oumuamua and 2I/Borisov by 3–5 orders of magnitude, constituting a major anomaly.
A new paper using data on the motion of 3I/ATLAS — as compiled by the Minor Planet Center between May 15 and September 23, 2025, set an upper limit on the deviation of 3I/ATLAS from a trajectory sculpted by gravity alone. Momentum conservation implies that 3I/ATLAS must be massive in order not to be pushed back by the flow of material from its warmed Sun-facing side.
Normally there is distribution of objects in space by size where there are many more smaller objects. This suggests we should have discovered on the order of a hundred thousand interstellar objects on the 0.1-kilometer scale of 1I/`Oumuamua before finding 3I/ATLAS, yet we only detected two interstellar objects previously.
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Hi Brian
Remember that’s a minimum mass estimate. Could be bigger/heavier. Odds are it’s somewhere around the 5.6 km size estimated by other means. I’m disappointed because I had hoped it was a very low density Graphene Wiffle-ball sent our way by ETI’s – as I explored in the paper with Adam Hibberd & Avi Loeb – but a pretty theory is slain by hard fact.
thanks for the feedback Adam and thanks for the work you and Avi and Hibberd have done.