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Do we know how many big asteroids could impact Earth?

by Milan_Griffes 1 min read7th Jul 20197 comments


Will MacAskill discussing asteroids with Joe Rogan, starting around 1:24:00 in this recording

Rogan: When you think about how big that thing was, that killed the dinosaurs 65 million years ago, and that there are hundreds of thousands of those things floating around in space.
MacAskill: I was asking some people at NASA, just two days ago actually, how many of them we've managed to identify – cause they're serious about scanning the skies to find them all...
I thought we had it covered. I thought this was something where NASA was like "Yeah yeah, we know where all the earth-killers are."
And their response was like "No, we have no idea. We don't know how many of them are out there, and so we don't know [what proportion] we've managed to track."

That was in 2017. It cuts against this 2013 Open Phil report (a) which states:

Unlike other GCRs (e.g., nuclear war), asteroid risk is extremely quantifiable: scientists have estimated the number and size of near-earth asteroids and are able to track how many have been discovered.

I'm inclined to follow MacAskill here, as the Open Phil investigation is based on public sources and NASA may have incentive to overstate their handle on the problem in public-facing communications.

But I haven't looked into this closely. Has anyone in EA thought about asteroid risk, recently?

(It looks like this isn't totally neglected – the B612 Foundation was set up to "to protect Earth from asteroid impacts." I haven't poked them enough to know how good their stuff is.)

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Global Catastrophic Risks (now slightly outdated with a 2008 publication date) has a chapter on comets and asteroids.

It estimates that an impactor with a diameter of 1 or 2 kilometers would be "civilization-disrupting" and 10 kilometers would "have a good chance of causing the extinction of the human species". So that's what the "big" means in this context.

We can estimate the population of possible impactors via impact craters, telescopic searches and dynamical analysis. Using these techniques, "[i]t is generally thought that the total population of near-Earth asteroids over a kilometre across is about 1100." But there are other classes of impactors with greater uncertainty-comets and Damocloids. "Whether small, dark Damocloids, of, for example, 1 km diameter exist in abundance is unknown - they are in essence undiscoverable with current search programmes."

This sounds like a plausible reconciliation of the apparently conflicting claims. OpenPhil is specifically talking about near-earth asteroids where we do indeed have fairly accurate estimates. The NASA employee referenced by MacAskill may be referring to the larger class of all possible impactors where uncertainly is much greater.

After reviewing the literature pretty extensively over the last several months for a related project (the risks of human-directed asteroids), it seems to me that there is a strong academic consensus that we've found most of the big ones (though definitely not all - and many people are working hard to create ways for us to find the rest). See this graphic for a good summary of our current status circa 2018: https://www.esa.int/spaceinimages/Images/2018/06/Asteroid_danger_explained

Don’t know much about this but I thought you could estimate a ballpark for the total frequency by looking at craters on moons and mars.