[meteorite-list] Comet May Have Missed Earth By A Few Hundred Kilometres in 1883?

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Mon, 17 Oct 2011 16:23:42 -0700 (PDT)
Message-ID: <201110172323.p9HNNgok026234_at_zagami.jpl.nasa.gov>

http://www.technologyreview.com/blog/arxiv/27264/

Billion Tonne Comet May Have Missed Earth By A Few Hundred
Kilometres in 1883

Technology Review
October 17, 2011

A re-analysis of historical observations suggest Earth narrowly avoided
an extinction event just over a hundred years ago

On 12th and 13th August 1883, an astronomer at a small observatory in
Zacatecas in Mexico made an extraordinary observation. Jos?? 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. (Since then, others have adopted Bonilla's observations as
the first evidence of UFOs.)

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 kilometres 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 tonnes or more,
that's huge, approaching the size of Halley's comet.

That's an eye opening re-examination of the data. Astronomers have seen
a number of other comets fragment. The image above shows the
Schwassmann-Wachmann 3 comet which broke apart as it re-entered the
inner Solar System in 2006. There's no reason why such fragments
couldn't pass close by Earth.

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."

A sobering thought.

Ref: arxiv.org/abs/1110.2798 <http://arxiv.org/abs/1110.2798>:
Interpretation Of The Observations Made In 1883 In Zacatecas (Mexico): A
Fragmented Comet That Nearly Hits The Earth
Received on Mon 17 Oct 2011 07:23:42 PM PDT