Sunday, November 25, 2012

August 13, 1883 -- The Day the Earth Almost Died?

Instapundit linked to an an article from MIT Technology Review from October of last year that suggests that the Earth nearly collided with a large comet in 1883. The article states:
On 12th and 13th August 1883, an astronomer at a small observatory in Zacatecas in Mexico made an extraordinary observation. José Bonilla counted some 450 objects, each surrounded by a kind of mist, passing across the face of the Sun.

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. ...

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.

... 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."
However, not everyone is buying into this explanation. The "Bad Astronomy" blog at Discover Magazine had this to say:
Doing some simple math, the authors calculate the comet fragments were no closer than about 500 km (300 miles) from the Earth’s surface, and no farther than about 65,000 km (40,000 miles).
This right there is enough for me to be extremely skeptical of this idea. When a comet breaks up, it spreads out. Even when intact, the material surrounding a comet can be tens or even hundreds of thousands of kilometers across! Claiming that a comet broke apart, yet managed to constrain its pieces to volume of space less than a few thousand kilometers across strains credulity.

Mind you, Bonilla claimed to have seen these objects over the course of two days. That means they would’ve been stretched out along a path that was a million km long at least, yet so narrow that only one observatory on Earth saw them transit the Sun. That is highly unlikely.
Worse, the very fact that no one else saw anything makes this claim even less tenable. A comet, even one that’s broken up, can be very bright, and the closer it is to us the brighter it can be. I have personally seen a comet in broad daylight! Even if this purported comet couldn’t be seen during the day for some reason, at some point in the days or weeks before or after these observations, the comet should have been visible in the evening sky (or at least at dusk or dawn). Yet no one saw anything.
Worse than that, there were no meteor showers reported that day or night. Comets are basically giant snowballs peppered with dust and gravel. Every time they get near the Sun, some of that ice sublimates (turns from a solid into a gas), forming the fuzzy ball we associate with comets, but also releasing some of the embedded rocky material. When the Earth passes through this stuff, we get meteor showers as it burns up in our atmosphere.
A comet passing a few thousand klicks from the Earth would have generated a lot of meteors. It’s practically impossible for me to believe that one could get that close to us and not even be noticed, and not create a meteor shower that would have been practically biblical in size.
It’s not like there wouldn’t have been ammo for such a meteor shower. The authors of the new study calculate the sizes of the fragments given their distance and the size Bonilla reported. They find the fragments would have been a few dozen meters across to as large as a kilometer. If there were hundreds of objects this size, there would’ve been millions as small 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.
Also, the Earth is moving, and covers a lot of ground (OK, space) in a day. Having the Earth move at least 2.5 million km during that time, and never getting closer or farther than 500 – 65,000 km is too much to ask.

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