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2w ago

Hubble and Infrared Observations Show 3I/ATLAS Is an Active Interstellar Comet

Thanks to Hubble’s sharp images and infrared spectroscopy, astronomers place the nucleus at 320 m–5.6 km and confirm active dust emission under a red spectral slope.

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This image provided by NASA/European Space Agency shows an image captured by Hubble of the interstellar comet 3I/ATLAS on July 21, 2025, when the comet was 277 million miles from Earth. (NASA/European Space Agency via AP)
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Overview

  • Hubble’s latest high-resolution images refine the nucleus diameter to between 320 meters and 5.6 kilometers, down from earlier multi-kilometer estimates.
  • Near-infrared spectra from NASA’s IRTF reveal a linear red spectral slope and no clear water-ice features in the comet’s coma.
  • Observations document dust outgassing from the sunward side, producing a dusty coma and faint tail typical of active comets.
  • Orbital calculations verify a hyperbolic interstellar trajectory with a record inbound speed of about 209,000 km/h and a perihelion inside Mars’ orbit around October 29–30, 2025.
  • NASA and international teams affirm 3I/ATLAS’s natural cometary nature while unsubstantiated artificial-probe theories persist and mission-retargeting proposals remain under review.