Meterror Impacts Once Again

Hoisted from Comments: Frank:

Grasping Reality with Both Hands: The Semi-Daily Journal Economist Brad DeLong: impact rates based on the cratering record are pretty much in line with estimates based on the population of Earth-crossing asteroids. Size distributions roughly fit an inverse square law distribution (i.e., 2 x diameter = 1/4 probability; 1/2 diameter = 4 x probability). Using this one can calculate impact rates based on a probability of a 500 m impactor every 100,000 years. So on average, 50 m projectiles should impact every 10,000 years or so (10 x 10 times more frequent). Once you get down to about 50 m the probability of the projectile exploding in the atmosphere (like Tungusaka) is quite high. Much smaller events might be like nuclear explosions, but they are rare and very unlikely to hit populated areas.

More to the point of the Easterbrook article, I've searched the Web of Science publication database for papers by the chief protagonist in his article. DH Abbot has not published a significant paper in 5 years and has never published anything other than unreviewed abstracts on this subject that I can find. Looks like a squeeky wheel getting Easterbrook's attention, but no follow-up to credible experts in this field.

And then there is the cross-section problem: we have what? 6000 cities each of roughly 100 square miles = 600,000 square miles of devastating impact cross section in a world of 200M square miles. That means only 1 out of 400 50M impacts will be "devastating" if we say that a 50 meter meteorite--2 megatons, Barringer crater-sized--hitting a city is our threshold for "devastating."

So we are down to one devastating every 4,000,000 years--not the one every thousand years of the Atlantic Monthly's lead to Gregg Easterbrook's article.

Comments

[Frank]

lol. That last calculation is a bit off as it only considers the 50 m objects. While it's reasonable to say something like "there's about a 1 in 400 chance that a 50 meter asteroid will score a direct hit on what is now a major population center some time in the next 10,000 years" it's incorrect to say that the chance of a devastating impact is limited to one every 4 million years. In 4 million years, the odds favor about 40 impacts of projectiles larger than 500 m (about 12 gigatons TNT [5E+19 Joules]), and likely one as large as 3 km, any of which could be quite devastating.

[If they hit cities...]

I'd also suggest that projectiles smaller than the Barringer Crater event could also be very dangerous, but certainly not with the frequency suggested by Easterbrook.

Statistics can be a bit wierd in these kinds of discussions. For example, you have a higher probability of getting killed by an asteroid than in a plane crash. While a very large impact has an extremely low probability it could also be capable of killing most of the Earth's population. In a million years there won't be enough plane crashes to kill a billion people.

Finally, one should note that these are stochastic events. They have an equal probability of occurring at any time, with the caveat that populations of astronomically observed objects can be accounted for.

[Cranky Observer]

Keep in mind too that any asteroid strike with an equivalent energy in the 20 kt range and up anywhere near a populated or sensitive area of the US, Russia, PRC, UK, France, India, Pakistan, or Israel could very easily trigger a nuclear launch against that nation's Enemy of the Week. So both the target area and damage estimates have to take that into account.

Cranky

[wood turtle]

Just because an asteroid doesn't land on inhabited areas directly doesn't mean that it couldn't affect them very severely indirectly.

[Tom Fuller]

I believe that the frequency of meteor/asteroid strikes should be decreasing dramatically over time. First, objects within the solar system should be both fewer and more stable--fewer, as each meteor/asteroid strike removes it from play, second because orbital capture should anchor more objects in a stable pattern. The expanding nature of the universe should make it much less likely that we will be threatened by extra-solar intrusions. I wonder if Easterbrook takes any of this into consideration--it seems more likely that he read a couple of popular magazines and said to himself 'there's a paid piece in there bringing up this 'threat' again. I seem to get a lot of criticism when I write about politics--maybe this will be safer...'

[Monte Davis]

This is, after all, the same Easterbrook who wrote cheerily a year or two back that we'll solve CO2 just as we solved other airborne pollutants like lead from gasoline or airborne sulfur from coal. He seems unaware that (1) CO2 is not a side product, but the inescapable consequence of getting energy from carbon, and (2) the quantities are incomparably greater.

Don't try to confuse HIM with your whining about orders of magnitude.

[frank]

"[If they hit cities...]"

not really. When they get this big, the 70% of them that hit the ocean can make enormous tsunamis, perhaps more destructive than the one in the Indian Ocean a few years ago.

But then on million year time scales, if the human race is still around it will surely have methods to deal with these dangers.