The comet exploded, or it didn’t

6 min read

Scientists get things wrong all the time. Look up “cold fusion” for a famous example. When they do make mistakes, how does the scientific establishment react? Mistakes must be called out, certainly. But how does that happen? Is there public scorn? Gloating? Name-calling?

Well, here’s an example. Judge for yourself.

In February 2022, a team led by Kenneth Tankersley, a geologist at the University of Cincinnati, published a paper in Nature magazine (The Hopewell airburst event, 1699–1567 years ago (252–383 CE), Kenneth Barnett Tankersley et al., Nature, 1 February 2022,

They had been studying the so-called Hopewell people, who lived in the Ohio River valley, near Cincinnati, over 1,500 years ago. Tankersley and his team made a remarkable claim: that a “cosmic airburst event”—a comet explosion—happened over the valley sometime in the 3rd or 4th Century CE. The Hopewell people survived this calamity, they said, but “it likely contributed to their cultural decline”.

Their paper cited plenty of evidence of this explosion. Near the epicentre, the Hopewell people constructed a “comet-shaped earthwork”. Archaeological sites in the area “contain an anomalously high concentration and diversity of meteorites when compared to all other cultural periods”. The sites all had surfaces that had been “exposed to extreme heat”—the scientists even were able to estimate that temperatures there had once risen above 765°C. All these suggested that the airburst had caused “widespread synchronous fires”.

Besides these findings, the Hopewell sites also yielded “microspherules”—tiny spheres, rich in iron and silicon. These are known to be produced when a meteor enters the Earth’s atmosphere, melting and vaporizing as it races towards the ground. So, their presence at the Hopewell sites would support the hypothesis of a comet exploding.

Finally, radiocarbon dating of material from the sites suggested that the airburst happened between 252 and 383 CE.

A fascinating theory, really. Who wouldn’t find a certain tragic romance in the story of a huge cosmic explosion sending a large settled community into terminal “cultural decline”?

Except that there were questions about Tankersley’s findings almost as soon as they were published.

Just months later, a husband-wife team of German astronomers, Ralph and Dagmar Neuhäuser, published a paper exploring some of the questions (Arguments for a comet as cause of the Hopewell airburst are unsubstantiated, Ralph and Dagmar Neuhäuser, Nature, 15 July 2022, According to them, the microspherules, the burned surfaces, and other evidence Tankersley offered—well, none of it spoke of a comet or its explosion.

And there’s something else. One thing that Tankersley cited to support the airburst thesis was that the Earth was at “increased risk” of such an event at the time. Why? Because historical records show that as many as 69 comets approached the Earth in those 131 years, an unusually high number. But the Neuhäusers disputed that. The figure of 69 actually is in reference to the years 220-589 CE, not 252-383. 369 years, not 131, so not unusual. In any case, there’s no evidence that these were “near-Earth” comets; in fact, some of them may have been other astronomical or atmospheric phenomena.

What about the earthwork? It is just “one feature in a larger structure”, and thus, an assumption that it is comet-related must also explain the rest of the structure. Also, if the explosion sent the Hopewell people into cultural decline, why would they build such a large structure afterwards?

Tankersley and his colleagues did respond to the Neuhäusers—strangely enough, in Nature on the same date as the Neuhäuser paper. Expectedly, they refute much of the refutation. To my untrained eye, some of it looks like a matter of differing interpretations of the evidence. Tankersley underlines this: “There will always be some degree of ambiguity in explaining the cause of ancient airburst events and tracing the origin of ancient impactors on the Earth.”

Ambiguity or not, there was more to come, for Tankersley. Another year later, another set of researchers published an even more critical rebuttal of the airburst hypothesis and evidence. Their paper begins by calling the claim of a cosmic airburst “extraordinary in the face of hundreds of archaeological investigations in the [area] that have heretofore provided no evidence of a widespread cataclysm or ‘social decline’ in need of explanation”. (Refuting the sensational claim of a Hopewell-ending cosmic airburst, Kevin C. Nolan et al., Nature, 9 August 2023,


* A chronological analysis of what was found at the Hopewell sites “does not support the notion of a single event spanning 15,000 sq. km”.

* It was not catastrophic synchronous fires that left behind burned surfaces, this team asserted. Instead, the surfaces were used by the Hopewell people for ceremonial fires.

* Microspherules were found, certainly, but a kilometre away from other “supposed evidence” of the airburst. Not just that—there’s no evidence at all that the microspherules have extraterrestrial origins. Their observed chemical characteristics are “likely indicative of a local soil composition”.

Nolan’s team goes into considerable detail in its refutation. They sum things up pretty damningly with this: “Tankersley et al. misrepresent primary sources, conflate discrete archaeological contexts, improperly use chronological analyses, insufficiently describe methods, and inaccurately characterize the source of supposed extraterrestrial materials to support an incorrect conclusion…[T]heir observations fail to demonstrate any aspect of this cosmic catastrophe.”

Elsewhere, Nolan even referred to “possibly intentional data manipulations” to support the comet explosion theory.

Taken together, the Neuhäuser and Nolan papers spelled doom for Tankersley’s theory. His team’s paper is still available on the Nature website, at the link above, but if you visit that page today, you’ll find the title preceded by two words in capitals: “RETRACTED ARTICLE”.

In a “Retraction Note”, the editors say they “no longer have confidence that the conclusions presented are adequately supported.” Tankersley et al., they report, “did not respond to correspondence from the Editors about this retraction”.

Here’s a question to ask, at the end of this episode. Because it played out as it did, is science better for it? I believe the answer is, “Yes”.

Once a computer scientist, Dilip D’Souza now lives in Mumbai and writes for his dinners. His Twitter handle is @DeathEndsFun.

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Updated: 09 Nov 2023, 11:36 PM IST

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