The biggest event which never happened and no one knows about? Offering a novel reinterpretation of some forgotten historical data, several Mexican researchers say a billion-ton comet may have passed a few hundred miles from Earth in 1883. Via Technology Review:
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. Today, Hector Manterola at the National Autonomous University of Mexico in Mexico City, and a couple of pals, 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.”