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Welcome to The Nonlinear Library, where we use Text-to-Speech software to convert the best writing from the Rationalist and EA communities into audio. This is: Risks from Asteroids, published by finm on February 11, 2022 on The Effective Altruism Forum. When thinking about risks from space, you’ll likely think of comets and asteroids. The asteroid that caused a mass extinction event approximately 66 million years ago collided with an energy roughly ten billion times as great as the bomb dropped on Hiroshima. Far more significant than the damage caused directly by the impact, it caused a cloud of ash and dust to block out the Sun’s light across the globe — eventually rendering three quarters of the world’s species extinct. As recently as 1908, an asteroid exploded over a region of remote Siberia, flattening an estimated 80 million trees over more than 2,000 km2 of forest. There is no impact crater; instead the meteoroid likely disintegrated kilometers above the ground in an ‘air burst’ — powerful enough to throw people to the ground as far as 40 miles from the centre of the explosion. If a similar-sized asteroid instead exploded over a large metropolitan area, it might have killed more people in a single day than any event in history. Worse still, an asteroid greater than 10km in diameter colliding with Earth could massively disrupt Earth’s climate, likely causing a long ‘winter’ making it far harder to grow crops. The result could be the premature deaths of most living people, human progress thrown back by decades, or perhaps — given some mechanism by which survivors would fail to repopulate — permanent extinction. So wouldn’t the space programs of the world do well to urgently unite around building effective asteroid defence systems? The answer isn’t so clear. First, we know that catastrophic asteroid impacts must be very infrequent, and asteroids that pose significant existential risks like extinction are even rarer. Without even trying to count asteroids in the night sky, we can look at the Earth for indications of large craters or extinction events, and reason that asteroids over 1km in diameter can’t strike much more frequently than once every million years on average — any higher and the relatively small number of craters wouldn’t make sense. Of course, we can also try to spot asteroids in the sky. Astronomers have identified a large majority of near-Earth asteroids larger than 1km across, and many smaller examples. From these surveys, we know that the chance of an Earth-impact for asteroids 1-10km in diameter is about 1 in 6,000, and about 1 in 1.5 million for asteroids larger than 10km across — that is, roughly the size of the asteroid that caused the Cretaceous–Paleogene mass (dinosaur) extinction event. But we can be even more confident about the risk from asteroids in this century specifically, because astronomers can track the large asteroids they spot and check whether their trajectories are on course to intercept with Earth’s. With this more precise information, the risks look lower still. In particular, the chance of an impact from an asteroid greater than 10km in size following its natural orbit is effectively zero if NASA is correct in claiming it has identified all asteroids this size. In The Precipice, Toby Ord summarises the risks: Asteroid sizeTotalFoundAverage CenturyNext Century1-10 km~ 920~ 95%1 in 6,0001 in 120,00010 km~ 4> 99%1 in 1.5 million< 1 in 150 million Spotting and tracking asteroids is important: knowing whether they pose a risk this century means knowing more about how best to allocate our resources across different kinds of risk. Very likely, we needn’t worry about building asteroid defence until the deflection technology is cheaper, more effective, and easier to govern; but there’s a slim chance we may need an all-out effort on deflection now because we spot an asteroid heading our way. Clearly, it pays to know which world we’re in. Fortunately, the international community has already d...