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yesterday

3I/ATLAS Enters Solar Conjunction as Hera and Europa Clipper Eye Tail Encounters

New modeling ties the sunward anti-tail to CO2-driven ejection of large ice grains.

NASA's Hubble Space Telescope captured this image of the interstellar comet 3I/ATLAS on July 21, 2025, when the comet was 277 million miles from Earth. Hubble shows that the comet has a teardrop-shaped cocoon of dust coming off its solid, icy nucleus.
Keck II telescope observations of interstellar object 3I/ATLAS found "evidence for a puzzling anti-tail extension" pointing at the Sun.
Image

Overview

  • Earth-based views are paused at solar conjunction this week, with the comet reaching perihelion on October 29 out of direct terrestrial sight.
  • Fresh arXiv studies released October 22 model an ice-dominated coma, explaining the sunward anti-tail as scattering by large H2O-ice and refractory grains produced by CO2 sublimation.
  • Pre-perihelion photometry from the Nordic Optical Telescope finds a heliocentric activity index of n ≈ 3.8, consistent with CO2-driven activity and dust production comparable to active comets.
  • Mission analyses indicate brief opportunities for in‑situ sampling downwind of the ion tail: Hera between October 25–November 1 and Europa Clipper between October 30–November 6, with Europa Clipper’s plasma and magnetometer instruments suited to detect ions and magnetic draping.
  • Most planetary scientists judge 3I/ATLAS to be a natural interstellar comet, while minority claims of technological origin remain speculative and would require unanticipated trajectory changes to gain support.