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Monday, October 31, 2011

Did we miss a cometary impact extinction event on 1883?

  "On 12th and 13th August 1883, an astronomer at a small observatory in Zacatecas in Mexico made an extraordinary observation....
Bonilla published his account of this event in a French journal called L'Astronomie in 1886. Unable to account for the phenomenon, the editor of the journal suggested, rather incredulously, that it must have been caused by birds, insects or dust passing front of the Bonilla's telescope. ....
Today, Hector Manterola at the National Autonomous University of Mexico in Mexico City, and a couple of pals, give a different interpretation. They think that Bonilla must have been seeing fragments of a comet that had recently broken up. This explains the 'misty' appearance of the pieces and why they were so close together.
But there's much more that Manterola and co have deduced. They point out that nobody else on the planet seems to have seen this comet passing in front of the Sun, even though the nearest observatories in those days were just a few hundred kilometers away.
That can be explained using parallax. If the fragments were close to Earth, parallax would have ensured that they would not have been in line with the Sun even for observers nearby. And since Mexico is at the same latitude as the Sahara, northern India and south-east Asia, it's not hard to imagine that nobody else was looking.
Manterola and pals have used this to place limits on how close the fragments must have been: between 600 km and 8000 km of Earth. That's just a hair's breadth.
What's more, Manterola and co estimate that these objects must have ranged in size from 50 to 800 metres across and that the parent comet must originally have tipped the scales at a billion tons or more, that's huge, approaching the size of Halley's comet....
One puzzle is why nobody else saw this comet. It must have been particularly dull to have escaped observation before and after its close approach. However, Manterola and co suggest that it may have been a comet called Pons-Brooks seen that same year by American astronomers.
Manterola and co end their paper by spelling out just how close Earth may have come to catastrophe that day. They point out that Bonilla observed these objects for about three and a half hours over two days. This implies an average of 131 objects per hour and a total of 3275 objects in the time between observations.
Each fragment was at least as big as the one thought to have hit Tunguska. Manterola and co end with this: "So if they had collided with Earth we would have had 3275 Tunguska events in two days, probably an extinction event."

  My guess is probably not but only probably. Either he really did have birds flying past the telescope or more likely it was further away but no other astronomers happened to be looking in the right direction. Even so cometary debris passing at a mere 10s of thousands of km would be pretty close. The Earth has a diameter of 8,000 km so in astronomic terms anything closer than the Moon is to close to the bull's eye. Although the dinosaur killer extinctio9n event was 65 million years ago it is quite possible that we have had many more which "only" killed 99% of land animals. IN most cases that would mean populations back to normal with a thousand years of breeding - not enough to even register on a geological record 1os of millions or even millions of years ago but no fun for the 99%v of us.

   We need a spacegoing civilisation, to be able to spot these quickly and a large instant launch capability to reach and divert them. Anything less is the human race playing Russian Roulette - perhaps only with 1 chamber in 10,000 loaded but that is still far to much.

HT Pournelle.

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Earth's diameter is a little over 12000Km (not the 8000 you quote..
regards sandy
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