Concrete Example of this Threat
Back in 1883, the Mexican astronomer José Bonilla made a very strange observation: Over two days, some 450 objects, each surrounded by a kind of mist, passed across the face of the Sun. Strangely, other observatories around the world did not see them. At the time, there were various theories to try to explain the observation, but none very conclusive. It was only over a hundreds years later that scientists figured out what probably happened, and it’s a scary thought:
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. [...]
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.
600 to 800km is absolutely nothing when it comes to astronomical distance. It’s like a bullet grazing your head so close it shaves off some hair.
The scientists estimate that the objects ranged in size from 50 to 800 meters, and that the parent comet must have been in the billion-plus-ton range, close to the size of the famous Halley’s comet.
Read more: http://www.care2.com/causes/extinction- ... z2E6bT3mLY