The Day All Life on Earth Almost Ended

On August 12th, 1883, a pack of life-extinguishing comets came within a few hundred miles of slamming into the Earth, nearly killing everything on the planet.

That's what scientists at the National Autonomous University of Mexico are saying after re-analyzing the findings of José Bonilla, a Mexican astronomer who may have unknowingly come close to witnessing the destruction of the world as we know it.

Back in 1883, Bonilla observed 447 objects passing in front of the sun over two days from Zacatecas, Mexico; he published his findings in a French astronomy journal. But these objects weren't observed at larger observatories in Puebla or Mexico City, where much bigger-time scientists were standing their nighttime vigils. Why? At the time, nobody knew for sure. The editor of L'Astronomie, where Bonilla's findings were published, suggested that birds, insects, or dust may have passed in front of the telescope.

The team at UNAM has a different explanation. They propose that what Bonilla observed were actually fragments of a massive, billion-ton comet, and the reason that no one else was able to observe the objects is that they passed so close to the Earth that they were only visible to observers perched on a very narrow slice of the planet. It's a parallax kinda deal. But here's where it gets crazier: Since Mexico City and Puebla are both only about 400 miles away from Zacatecas, the team at UNAM calculate that the comet passed between 300 and 5,000 miles from Earth. 300 miles is about the distance from Philadelphia to Boston, and possibly all that stood between us and total extinction.

So Bonilla may have observed 447 objects over 2 days, but he was only counting for a total of 3.5 hours. To calculate the actual total number of fragments, the researchers at UNAM took the average number of fragments Bonilla saw per hour and multiplied it out over the entire two-day event. The total: a mind-slapping 3,275 individual pieces, all of which they believe were as big or bigger than the Tunguska event meteor in 1908. The impact at Tunguska was by moderate estimations about 15 megatons, or 1,000 times more powerful than Hiroshima. 3,275 of those bombarding the planet over two days would likely be, as the authors of the paper put it, "an extinction event."

Comet fragmentation was a known phenomenon in 1883, but only two occurrences had been observed. That's probably why Bonilla and his bird-loving editor didn't put two and 447 together. Some of Bonilla's trajectories coincide with one of the comets that had been observed that same year, but it's just as possible that another, equally massive comet slipped by unnoticed and only missed destroying our planet by what is, in astronomical terms, a galactic hair's breadth. [Cornell University Library via Technology Review, SlashDot; Science; Bad Astronomy; Image credit: NASA, ESA, H. Weaver (APL/JHU), M. Mutchler and Z. Levay (STScI) via APOD]

Skeptic's Update: Phil Plait, who writes the exceptional Bad Astronomy blog, has some serious concerns about the paper out of UNAM. He argues that a fragmented comet with thousands of large pieces would have millions of smaller pieces, and the spread of those would certainly carry many of those into Earth's atmosphere at the speculated range:

If there were hundreds of objects this size, there would've been millions as small [as] a few centimeters across. Objects that size make brilliant fireballs as they burn up in our atmosphere, and would've been visible during the day, even with the Sun shining. Again, no reports of any meteor storms, despite a comet being a few thousand kilometers away and a million kilometers long.