Something did fly past Earth last week, and it left a trail of uneasy questions behind it: could interstellar comet 3I/ATLAS have shed toxic material that made it all the way to us? Harvard astrophysicist Avi Loeb has publicly raised that possibility, framing the object as a potential 'intergalactic cyanide tablet'.
However, in the same analysis, Loeb also outlined why physics makes an actual 'poison fallout' scenario highly unlikely. His provocative question comes after telescopes detected a plume containing cyanide and hydrogen cyanide around the object as it approached the Sun.
Why 3I/ATLAS' Cyanide Talk Grabbed Attention
When 3I/ATLAS made its Earth fly-by on 19 December, Loeb used the moment to ask a stark question in a Medium post: 'Will any of the material shed by 3I/ATLAS arrive on Earth?'
His concern centres on the chemistry reported around the object, including a plume described as containing cyanide and hydrogen cyanide, which was observed as the comet approached the Sun.
Loeb has also stressed the basic risk factor: 'Hydrogen cyanide at large concentrations is a poison,' and he has previously pointed to its historical use as a chemical weapon during the First World War.
What Telescopes Detected in the Plume
Loeb cited observations from the Atacama Large Millimetre/Submillimeter Array (ALMA) in Chile, which reported methanol and hydrogen cyanide associated with 3I/ATLAS during the autumn. Separate research reporting ALMA detections also describes methanol detections on multiple 2025 dates and hydrogen cyanide detections on specific September 2025 dates, reinforcing that these molecules were indeed observed.
Those detections fed Loeb's darker analogy that the interstellar comet could resemble 'a serial killer spreading poison'—'like an intergalactic cyanide tablet'. Still, the presence of molecules in a coma is not the same as an exposure event on Earth, and Loeb's own follow-on assessment leans heavily on that distinction.
Why a 'Poison Fallout' is Unlikely
Distance is a key factor, with reporting on the flyby placing 3I/ATLAS at roughly 270 million kilometres (168–170 million miles) from Earth at closest approach on 19 December.
Loeb also referenced this scale when discussing why an Earth impact scenario is not something the public needs to 'brace' for. The key point is not simply that it missed Earth, but that the gap is so large that any material would need to survive harsh conditions and remain on a trajectory that intersects with Earth's position.
Loeb's central argument is that solar wind and sunlight make the delivery of hazardous material to Earth improbable. He wrote that, 'Given the mass loss rate measured by the Webb Space Telescope, the gas around 3I/ATLAS would be swept up by the solar wind at a distance of just a few million kilometres (several million miles) from 3I/ATLAS.'
He added that tiny dust grains smaller than a micrometre would be pushed away by solar radiation pressure, while larger fragments would burn up in Earth's atmosphere if they were under three feet across. For objects larger than that, he argued the odds of an impact remain negligible because of the comet's distance and the speed at which material is expelled.
The Alien-Tech Caveat
He then flagged a caveat that keeps the controversy alive: the outcome could differ if those projectiles can 'manoeuvre by technological propulsion'.
🚨 3I/ATLAS X- rey ALERT — DEC 23, 2025 ☄️
— 3I/ATLAS updates (@Defence12543) December 23, 2025
⚠️The interstellar visitor 3I/ATLAS is exiting the solar system, but new data is revealing unexpected behavior.😱
🛰️Recent spacecraft observations show: • X-ray emission surrounding the comet
• A complex, multi-wavelength signature… pic.twitter.com/qpKag2esrT
Loeb has continued to argue that 3I/ATLAS may not be purely natural, suggesting its trajectory could be consistent with a technological origin and even theorising it may be dispatching 'satellites' towards Jupiter.
He told the New York Post that the comet could use its time near the gas giant, expected on 16 March 2026, to 'seed' it with additional probes.
