Fragmented comet impact


Updating Comets

A fragmented comet impact

Cosmic traffic accidents were the main news story when The Comet is Coming! first appeared in 1980. The collision with a comet or asteroid that wiped out the dinosaurs – then newly indicated – was certainly not the only event of its kind, as my book stressed at the time. Now Prof. Bill Napier at Cardiff University’s Astrobiology Centre suggests that 12,900 years ago the Earth strayed into a trail of fragments from a very large but disintegrating comet. And that encounter, he proposes, caused a drastic change of climate.

Hubble image (2006) of a broken chunk of Comet Schwassmann-Wachmann 3 dividing into dozens of “house-sized” fragments. NASA, ESA, H. Weaver (JHU/APL), M. Mutchler and Z. Levay (STScI)

Comets certainly can break up, and Napier interprets meteor streams of the Taurid Complex as traces of a large comet that entered the inner Solar System system 20,000-30,000 years ago. His idea is that the Earth later ran into a dense trail of its material over the course of an hour, with North America bearing the brunt and experiencing thousands of fireball explosions each comparable with a nuclear bomb.

That would explain a layer of soot found at many sites in the USA, telling of continental-scale wildfires, together with microscopic diamonds of the type produced by meteoritic shocks. And 35 genera of North American mammals were wiped out. All this is persuasive.

Napier also suggests that the impacts provoked a major cooling that lasted 1300 years. The event is well-known to palaeoclimatologists as the Younger Dryas, which takes its name from a flower, Dryas octopetala or mountain avens, which likes cool conditions. The climatic downturn occurred just as the world was emerging from the most recent Ice Age. It was a miniature ice age of its own, with temperatures falling by perhaps 8 deg. C and glaciers re-advancing. Drastic regional changes of climate drove people in the Middle East to invent agriculture, just to survive.

Napier’s multi-impact event coincided with the start of the Younger Dryas. Although you can imagine that impact debris in the stratosphere and smoke from the wildfires would have caused a dimming of the Sun for months or perhaps years, it’s not at all clear why the effects should have persisted for 13 centuries. Napier doesn’t discuss the climatology in his report for the Monthly Notices of the Royal Astronomical Society.

Some climate theorists will offer to explain the Younger Dryas by melt-water from the receding ice sheets stifling the North Atlantic circulation. For me, it’s no coincidence that the Younger Dryas was a time of intense cosmic radiation associated with a long period of weak solar activity. But having faced a lot of flak myself, back in 1980, for “buying” the dinosaur impact story, I’ll not scorn Napier’s latest idea, which is astronomically convincing. His impacts might well have helped to trigger, accelerate and/or intensify the Younger Dryas, even if the depth and persistence of the cooling may need more explaining.

By the way, it’s amusing to note that not until March 2010, 30 years after The Comet is Coming!, did an international panel of 41 experts solemnly pronounce in Science (the Schulte reference below) that a cosmic impact really did do for the dinosaurs. They disposed of the only surviving geophysical hypothesis, namely that massive flood-basalt volcanism in India (the Deccan traps) caused the mass extinction 65.5 million years ago. The main Deccan eruption phase was half a million years too soon. That finding also rules out a proposed compromise between the hypotheses, namely that shock waves from the impact might have triggered the flood-basalt event.

Schulte et al. said without discussion that the impactor was an asteroid. The celebrated astrogeologist Eugene Shoemaker would have begged to differ. He estimated that whatever made the Chicxulub crater in Mexico was 15 kilometres wide and he noted that no asteroids as large as that are known in the Earth’s vicinity. As reported in Magic Universe:

Not impossible that it was an asteroid,” Shoemaker said. “But if you’re a betting man like I am, you put your chips on a comet. My own hunch is that we will find that the impact of large objects, probably chiefly comets, has profoundly influenced the evolution of life.” That was Shoemaker’s last word on the subject. Soon after recording it for television, in 1997, he lost his life in a car smash in Australia while prospecting other impact craters.


W.M. Napier, “Palaeolithic extinctions and the Taurid Complex”, Mon. Not. R. Astron. Soc., in press, released online 1 April 2010.

Peter Schulte, et al., “The Chicxulub Asteroid Impact and Mass Extinction at the Cretaceous-Paleogene Boundary”, Science, Vol. 327, pp. 1214 ff. 2010

Nigel Calder, “Impacts” in Magic Universe, OUP 2003

About Comets


Occasional postings will comment on news of comets and asteroids. This introduction explains my long-standing interest.

About Comets: Speculation and Discovery Dover Classic re-issue of The Comet is Coming!

Comet Ikeya-Seki at dawn, 1965. NOAO/ R. Lynds.

In 1968 the BBC producer Philip Daly and I visited a teacher of classical guitar called Tsutomu Seki in Kochi City, Japan. From his small rooftop observatory, he had co-discovered the spectacular Comet Ikeya-Seki 1965 when it was just a faint smudge in the sky. For the first BBC-TV science blockbuster “The Violent Universe”(1969) about quasars, pulsars and cosmic microwaves, Seki’s patient searching provided some light relief. As professional telescopes grew bigger, comet-hunting became a job mainly for the amateur astronomers.

Ahead of the competition, the BBC anticipated the public interest in the impending return of Halley’s Comet in 1985 and in 1980 published my book The Comet is Coming!: The Feverish Legacy of Mr Halley. In the following year BBC-TV broadcast a 60-minute TV documentary also called “The Comet is Coming!” produced by Martin Freeth. We labelled the programme “a prank” because the comic actor Tim Brooke-Taylor played the ghost of Edmond Halley fated to ride in limbo on his eponymous comet, voiced by Leo McKern.

Both the book and the programme climaxed with important new science, the first strong evidence that the impact of a comet or an asteroid 65 million years ago wiped out the dinosaurs. My inquiries took me to a gorge near Gubbio in Italy, where the thin red clay layer marking the end of the Mesozoic Era had proved to be doped with extraterrestrial atoms of iridium. As so often with my science reporting, the story was highly controversial at the time (“Unbelievable arrogance” one dinosaur expert called the impact theory) but now it’s standard stuff.

At mission control in Darmstadt in 1986 I was present for the tumultuous interception of Halley’s Comet by the European Space Agency’s Giotto spacecraft. Although badly damaged in the high-speed encounter with the comet’s dusty head, Giotto not only returned excellent data but survived to intercept Comet Grigg-Skjellerup in July 1992. Then my book Giotto to the Comets was published very quickly by Presswork, because most of it was drafted in advance of the second encounter.

When The Comet is Coming! re-appeared from Dover in 1994 under its time-adjusted title Comets: Speculation and Discovery, a preface briefly summarized the results from March 1986, when five spacecraft, the Soviet Vega 1 and Vega 2, the Japanese Sakigake and Suisei, and Europe’s Giotto, all intercepted Halley’s Comet.

In Magic Universe (2003) my story about “Comets and asteroids: snowy dirtballs and their rocky cousins” stressed that the distinction between the two kinds of micro-planets had become fuzzy. It also looked forward to NASA’s Deep Impact encounter with Comet Tempel 1 in 2005. By then political interest had been engaged in the search for wayward objects that threaten to collide with the Earth, under the tag Spaceguard. Cross-links to “Impacts” and “Extinctions” led readers to the re-interpretation of key events in evolution on the Earth as consequences of traffic accidents like the one that killed the dinosaurs.

Rosetta and its lander. ESA.

Writing for the European Space Agency kept me up to speed with the discovery of vast numbers of sungrazing comets, spotted by the ESA-NASA SOHO spacecraft, and with ESA’s ambitious Rosetta mission, launched in 2004 to rendezvous with Comet Churyumov-Gerasimenko in 2014. Rosetta will put a small lander onto the icy nucleus, and spend the next two years orbiting the comet as it heads towards the Sun and swings around it, spewing out its tail. Rosetta is the de luxe mission that comet investigators have waited for ever since the Space Age began.

Comment on The Comet is Coming!

As we would expect from Calder, Britain’s most entertaining science writer, The Comet is Coming! is factual, easy to understand, and great fun. Timothy Ferris, New York Magazine

Comment on Giotto to the Comets

Nigel Calder has risen to the occasion. A most interesting book about a most interesting and important mission. Patrick Moore

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