3I/ATLAS Warning: Harvard's Avi Loeb Claims Comet Carries WWI Chemical Weapon

An interstellar comet passing through the solar system has triggered fresh scientific scrutiny after a Harvard astrophysicist warned it carries chemicals once used in warfare. The object, known as 3I/ATLAS, made its closest approach to Earth in December 2025, prompting questions about whether any of its material could pose a danger.

The warning came from Avi Loeb, a senior Harvard scientist, who highlighted the comet's toxic gas content and its historical links to WWI chemical weapons. Concerns focus on chemistry and physics, not intent, but the stakes are clear.

Could anything from 3I/ATLAS reach Earth, and if so, would it matter?

Why 3I/ATLAS Is Being ComparedTo a WWI Chemical Weapon

The comparison to a World War I chemical weapon stems from the substances detected in the comet's gas plume. According to Avi Loeb, the surrounding cloud of the 3I/ATLAS comet contains cyanide and hydrogen cyanide. Both chemicals are highly toxic and were used in early chemical warfare during World War I.

Loeb, the Baird Professor of Science at Harvard University, emphasised that the reference is historical rather than sensational. In his analysis, he described hydrogen cyanide as 'a poisonous gas which was used as a chemical weapon during World War I.' The point, he explained, is not that the comet represents a weapon, but that its composition warrants careful examination.

The scientific concern lies in whether such gases, once released into space, could survive long enough to interact with Earth. That question has driven much of the public attention surrounding 3I/ATLAS.

How Close Did 3I/ATLAS Come to Earth — and Why That Matters

The comet's proximity played a major role in raising those questions. On 19 December 2025, 3I/ATLAS reached its closest point to Earth at a distance of 269 million kilometres. While that is vast by human standards, astronomers measure risk on a cosmic scale.

Loeb noted that the comet travelled through the inner solar system but remained well beyond Earth's orbital path. Still, the timing of the fly-by sparked public curiosity. He observed that, as the holiday season approached, some wondered whether the comet might deliver an 'interstellar gift of sorts.'

From a scientific perspective, the key issue was not closeness alone. It was whether any gas, dust or debris released by the comet could bridge the immense distance between its path and Earth's orbit.

Could Toxic Gas or Dust From 3I/ATLAS Reach Earth?

Loeb's assessment points strongly towards no. He explained that the solar wind plays a decisive role in neutralising the risk. Data from the James Webb Space Telescope allowed scientists to estimate the comet's mass loss rate.

Based on those measurements, Loeb said the gases released by 3I/ATLAS would be swept away within a few million kilometres of the comet itself. That distance is far smaller than the 55 million kilometres separating the comet from Earth's orbital path.

'This risk is mitigated by the solar wind,' Loeb wrote, adding that no harmful gas could persist long enough to reach Earth.

Even finer dust offers little cause for concern. Particles smaller than a micrometre are dispersed even faster by solar radiation pressure.

What About Solid Debris — And Loeb's Key Caveat

Larger material was also considered. Loeb explained that small solid particles would burn up harmlessly in Earth's atmosphere if they ever arrived. Objects larger than a metre are far rarer.

Based on the comet's mass loss, fewer than one million such objects may have been released in recent months. Their trajectories mean the closest would still remain at least ten times Earth's radius away. As a result, the chance of impact is negligible.

Loeb did add one caveat. All conclusions assume the released objects cannot manoeuvre. As he noted, this holds 'under the assumption that the released objects cannot manoeuvre by technological propulsion.'