Extinction Event Near Miss: Fragments From Billion-Ton Comet Missed Earth by a Hair in 1883

We Need to be More Careful About Low-Probability/High-Risk Events

There are some very serious risks to life on Earth that most environmentalists don't talk about nearly enough, in my opinion, and I think it's because they seem very unlikely to happen at any point in time. Nuclear war, super-volcanoes, near Earth objects hitting us, deadly plagues, etc. While these things are improbable on any single day, their negative consequences would be so huge that they have to be taken seriously. After all, what's the point of doing our best socially and environmentally and such if it all gets wiped out by some mega-huge natural disaster? Especially if it's something we could have avoided with enough forethought...

Which brings me to near Earth objects, like comets and asteroids. It might not be possible for us to do much about the Yellowstone super-volcano, but we can definitely monitor the orbits of asteroids and comets and, with enough lead-time, figure out how to make them change course.

CometNYT 1923/Public Domain

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 8000 km 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.

So how close did Earth come to catastrophe? Very close:

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.”

For those of you not familiar with the Tunguska event, it was a large explosion over northern Russia in 1908; it flattened an estimated 80 million trees down over 2,150 square kilometres (830 sq mi). The explosion was caused by what is believed to be a meteroid or comet fragment. It's power is estimated to have been in the 5 to 30 megaton range, which is about three orders of magnitude more than the nuclear bomb that was dropped on Hiroshima during World War II! Not imagines hundreds or thousands of these all hitting Earth within a couple of days, and that makes for a very, very bad weekend. It's not clear exactly how high that would be on the Torino Impact Scale, but the multiple impacts would certainly add up to a lot.

Asteroid ImpactNASA/Public Domain

What Can We Do?

That's a whole other article, but there are things we can do to greatly reduce the risks. The first thing to do is to find and monitor near Earth objects above a certain size. NASA is already doing a pretty good job with that, but the work is not over as they are still finding more and more every year. Calculating their orbits far in the future is the next step, and if any seem to be coming a bit too close to Earth, we need to monitor them extra-closely and refine orbits until we are sure one way or the other.

The second part is having the capabilities of rapidly deflecting an inbound object. Ideally we would have all this stuff tested and ready to go within a relatively short time frame, rather than having to actually do the R&D and build the stuff after we've discovered a dangerous orbit. You don't want your test-run to have millions, if not billions, of lives hanging in the balance...

NASA asteroidNASA/JPL/JHUAPL/Public Domain

Via Technology Review

See also: NEOShield: What is Earth's Best Defence Against Asteroids?

Extinction Event Near Miss: Fragments From Billion-Ton Comet Missed Earth by a Hair in 1883
Thousands of Tunguska events over a couple days? Ouch.

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