Interstellar Object Research
3I/ATLAS carries dark, porous comet dust
A multi-observatory spectral and polarization analysis finds unusual scattering behavior but broadly familiar albedo and grain sizes.
Briefed October 6, 2026 · Aliens in the Clouds
Preprint · Natural Dust Model Favored
Researchers combined new Pirka Telescope polarimetry of interstellar comet 3I/ATLAS with Hubble, VLT, SPHEREx, and Webb observations spanning visible through mid-infrared wavelengths. Their model favors a coma dominated by dark, absorbing dust with an effective visible albedo near 0.04, a grain-size power-law index of about 3.0 to 3.3, and a small amorphous-silicate component.
The full polarization dataset points toward a highly porous scattering structure. Yet the central comparison is deliberately conservative: despite 3I/ATLAS's unusual polarimetric curve, its inferred albedo and grain-size distribution fall within the range measured for Solar System comets. Composition and structure models are not a returned sample, and the paper makes no claim of artificial manufacture, biology, or technology.
Why it matters
An interstellar comet lets astronomers compare another planetary system's debris with local material. Finding a mix of unusual optical behavior and familiar dust properties is more informative than forcing the object into either ‘ordinary’ or ‘extraordinary.’
What the source establishes
Fresh preprint submitted October 4. The polarimetry and assembled spectral energy distribution are observations; porosity and component fractions are model-dependent interpretations.
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