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Monday, February 18, 2013

Asteroids! The Not-So-Game.

Most of us noticed that a significant meteorite exploded and impacted near Chelyabinsk a few days ago. And also in the news is that asteroid DA14 flew by earth at a surface-to-surface distance of approximately two earth diameters. It does seem like a pretty big coincidence, doesn't it?

One article by a renowned physics professor cites this coincidence of having a 1:100,000,000 chance of happening. It's like we just won the lottery! But I don't think it's really cause to celebrate.

Firstly, I would say that this remarkable coincidence reveals that our way of computing the odds may not be as accurate as we hoped. Does this imply that such dangerous natural space objects are more common than we once believed? I would say no. It could, however, imply that they are more clustered than we previously thought.

Secondly, I would say that we have a ways to go to even detect the dangerous objects from space. Take the Chelyabinsk meteorite, for instance. We didn't even know it was coming until it hit the atmosphere at more than mach 50 (18 kilometers per second). Yes, it exploded with a half-megaton yield about 32.5 seconds later in a fireball easily visible from Chelyabinsk, a city of 1.1 million people. And then pieces of it hit areas near Lake Chebarkul another 5 seconds later.

Clearly we are going to need a bit of warning.

So, how lucky were we to even have seen the Chelyabinsk meteorite? First, it was a shallow entry, less than 20 degrees. This was lucky because it gave people a chance to photograph and video it. Put simply, this lengthened the time-span of the event. Second it was very near a highly-populated city. This meant that many people were able to capture the event. Third, it happened in Russia, where apparently most of the cars have dashboard cameras, due to prevalent issues with collisions and liability. This allowed us to have pictures and videos of the meteorite's reentry and explosion from many different angles. Fourth, it was large enough, apparently only about 15 meters in diameter to create an event that burned up in the atmosphere, creating a light show (and other not so good effects). But it wasn't large enough so most people who saw the event wouldn't have survived.

So we are very lucky that we saw it at all. It could have landed in Antarctica or the Pacific Ocean, and nobody would have seen it.

Unfortunately all these lucky reasons are also unlucky reasons for the people of Chelyabinsk. They noticed the flash outside and rushed to the windows to see a burning contrail, the remnants of the place where the meteorite, still traveling in excess of mach 20, had compressed the air so much that it turned atmosphere to plasma, igniting the bolide. A few seconds later the blast wave, caused by atmospheric overpressure of maybe 200 psi shattered the windows that they all flocked to. Causing a lot of injury from broken glass!

But it could have been worse. If the meteorite had had a steeper angle of entry then it could have hit the ground and exploded on contact, releasing all its kinetic energy at ground level instead of at an altitude of 30-50 km. Such an explosion could easily destroy even a large city. The heat of the blast and the overpressure wave would essentially flatten the city. It would be similar to a half-megaton nuclear weapon, but without the lingering radiation.

Actually the illustration at the top of this post gives an indication of what a large (0.2 km) meteorite would do when striking land from a very steep angle. It is shown at maybe one or two seconds after the impact. The amazing thing is that it would push out the air when it strikes, sending out a compression ring igniting the air: plasma. Instead of an explosion cloud, it would look more like many nuclear fireballs at the periphery as it found more oxygen as fuel. At this size, I presume the main strike wouldn't punch through the crust, but it would send ejecta (really pieces of itself) out, each one as large as a building. The plasma ring would propagate out at around mach 10, slowing to mach 5 at maybe a 4 km distance. This size meteorite on land would be a region killer with about a half-gigaton yield.

Yes, it can be much worse.

You know, I don't believe in coincidences, but when it comes to nature, I have an open mind. Still, even if the asteroid DA14 and the Chelyabinsk meteorite strike hadn't happened on the same day, we should still seriously consider the consequences of complacent belief that our luck will continue.

Yes, I'm advocating that we do something about the situation.

The main failure was the inability to predict that the asteroid would hit the Earth ahead of time. I believe it's a good idea to be able to track smaller near-Earth objects (NEOs) than our current plans would suggest. So, how many objects are capable of approaching Earth and have about the same size as the Chelyabinsk meteorite? About 250,000 (see this link for a table of population estimates).

And our current plans? The Spaceguard Survey's goal is to spot 90 percent of the NEOs greater than 1 kilometer in diameter. The George E. Brown, Jr. Near-Earth Object Survey's goal is to spot 90 percent of the NEOs greater than 140 meters in diameter.

Of course, neither of these two would have spotted the Chelyabinsk meteorite, since it is 1/10th the size of the smallest NEO found by the most accurate survey.

So much for the bit of warning.


2 comments:

  1. An interesting aside that is not generally known, is the possible existence of a large cluster of "dust" like meteors within our solar system that could come our way and, according to astronomers, potentially disable and destroy the thousands of communication satellites orbiting earth. The impact on global communication, commerce and safety would be hard to calculate. Ironically it could be a large "storm" of these smaller meteors that could cause more damage than one big one.

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    1. It is true. The literature almost always talks about the fact that, the smaller the asteroid, the more numerous they are. This means that Chelyabinsk-sized meteors can cause problems. And with a half-megaton yield, we should certainly be concerned that they can hit us without any warning at all.

      Clusters are actually quite possible because a lot of asteroids are actually loose rubble, according to other articles. And we all remember that Shoemaker-Levy, when it hit jupiter, had already broken into several pieces. OK it was a comet, but collisions don't cause obliteration. They cause a mass to split into multiple pieces, all traveling mostly the same direction.

      Clusters consist of many small pieces, which would mostly burn upon reentry, but your point is clear: our satellites are vulnerable. Fortunately they are amazingly small compared to the Earth and the chances of them being struck by an asteroid is fairly small. The Leonids and the Geminids are clusters of cometary and asteroidal matter respectively. Each piece is generally smaller then a pea, and so we do actually have some experience with clusters.

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