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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[1]. 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[2] 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[3] 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 in an average century 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[4], Toby Ord summarises the risks:

Asteroid sizeTotalFoundAverage CenturyNext Century
1-10 km~ 920~ 95%1 in 6,0001 in 120,000
10 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 done a surprisingly good job at tracking asteroids. In fact, the story of how asteroids have been handled is mostly a demonstration of how (space) governance which takes big risks seriously can succeed. It took just over a decade from the emergence of a scientific consensus around the risks from asteroids to the point where governments began to discuss what to do about them. Since then, a UN-sponsored network of ‘spaceguard’ programmes has spotted and tracked almost every asteroid which could pose a major threat, and these programmes’ funding has increased more than ten times between 2010 and 2016[5]. There is room for expanded efforts to spot asteroids — on one estimate, we have spotted around 60% of the near-Earth asteroids and short-period comets 1.5 km or larger. Nonetheless, as Toby Ord writes in The Precipice: “no other existential risk is as well handled as that of asteroids or comets”.

Waiting a very long time without building a way to proactively defend against asteroid impacts would indeed be reckless — as time passes, the cumulative chance that a large asteroid impact will have occurred increases unsustainably.

Even in this century, I’m not saying that spending on defensive capabilities against asteroid impact would necessarily be a waste of money, assuming the technology couldn’t be misused in dangerous ways. Some back-of-the-envelope calculations from Matheny (2007)[6] show that the cost-effectiveness of an asteroid deflection program this century could work out at about $2.50 per life-year saved in expectation,[7] easily competitive with other programs governments and individuals are willing to spend on.[8]

But spending on such an enormous risk mitigation program would probably mean using up people and resources which could be deployed on similar but even better programs, so what matters most is how asteroid deflection should be prioritised among other interventions. On that front, it just looks like there are far more urgent (and comparably tractable) risks from Earth and space this century — especially those posed by technology rather than nature. And as we address those more urgent risks, we might also expect the cost of an effective asteroid detection and deflection program to fall dramatically as spaceflight technology improves. As such, diverting a great deal of attention and money[9] to asteroid deflection in the next few decades could look like getting lost on a jungle expedition, and worrying about being killed by falling coconuts.

Plus, work on defection technology doesn’t look especially neglected compared to work on mitigating other risks, either. In particular, NASA’s Planetary Defence Coordination Office plans to test its ‘kinetic impact’ technology by crashing a spacecraft into a near-Earth asteroid at a speed of 6.6 km/s and measuring the effect on its orbit. The program has a budget of over $300 million.

Dual-use concerns

But there’s also a reason we might want to positively avoid rushing into building asteroid defence systems: any technology capable of deflecting an asteroid away from a collision course with Earth will probably make it easier to divert it toward Earth.

Suppose diverting an asteroid into Earth is 10 times harder than deflecting it, and that deflection efforts are always successful. In that case, to think that developing deflection technology makes us safer, we would need to be confident that the risks from malicious use of the diversion technology are less than ten times as great as the natural risks from asteroid impacts. But this would mean being confident that the chance of malign use of the technology[10] was less than 1 in 10,000 (0.01%) this century. This seems overly confident.

As Carl Sagan and Steven J. Ostro conclude in their short article in Nature; “premature deployment of any asteroid orbit-modification capability, in the real world and in light of well-established human frailty and fallibility, may introduce a new category of danger that dwarfs that posed by the objects themselves.” In fact, I suspect the possibility of deliberate deflection of near-Earth objects could pose the greatest threat to Earth from space this century.[11]

This ‘dual-use’ concern mirrors other kinds of projects aimed at making us safer, but which pose their own risks, like ‘gain of function’ research on diseases. In such cases, effective governance may be required to regulate the dual-use technology, especially through monitoring its uses, in order to avoid the outcomes where a malign actor gets their hands on it. With international buy-in, a monitoring network can be set up, and strict regulations around technology with the potential to divert planetary bodies can (and probably should) be implemented.

When people think about existential risks for the first time, they often think of asteroids. But the real lessons we should learn from asteroids are unexpected. First, the international effort to track near-Earth asteroids is potentially humanity’s most successful effort to date to directly address an existential risk — that should be a source of inspiration as much as the risks themselves should be a source of concern. Second, expanding beyond mere detection to building deflection systems probably shouldn’t be a priority right now — not just because other comparably tractable risks look far more urgent, but because deflection technology could pose risks of its own from malign use.

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  1. ^

    Eyewitness reports can be found in Jenniskens et al. (2019)

  2. ^

    Roughly the size of the Chicxulub impactor which wiped out the dinosaurs.

  3. ^

    Meaning they trace an orbit around the sun relatively close to Earth's orbit.

  4. ^

    Toby Ord, The Precipice (2020) p. 71, Table 3.1

  5. ^

    Comets also pose a threat of collision with Earth roughly on par with asteroids, and they can be much harder to identify in advance, because their highly elliptical orbits mean they can arrive from beyond the outer planets of the solar system — becoming visible with little time to spare. There might therefore be some scope for expanding existing frameworks for spotting and tracking near-Earth objects to focus more on comets, but this will be a far more expensive undertaking, since comets are extremely difficult to detect for most of their orbital periods.

  6. ^
  7. ^

    Assume saving future lives is as valuable as saving lives today, and suppose that (conditional on surviving this century) humanity can be expected to survive as long as our closest relative, homo erectus (1.6 million years) at an average population size of 10 billion people. Suppose it could cost $20 billion to buy an asteroid detection-and-deflection program which would reduce the chance of an asteroid causing extinction by 50% this century and impose no other risks or benefits. Finally, assume the probability of an extinction-level asteroid impact this century is around one in a million. Then the expected value of such a program would work out as 0.5 × 1.6 million × 10 billion × one in a million = 8 billion life-years. So the cost-effectiveness would work out at 20 billion ÷ 8 billion life-years = $2.50 per life-year. Matheny notes, “​​it is common for U.S. health programs to spend, and for U.S. policies and citizens to value, more than $100,000 per life-year”.

  8. ^

    Spending on asteroid deflection (ignoring dual use concerns) is probably not crazy even when you only consider presently existing people. On one reasonable estimate, the chance of being killed by an asteroid impact (or its effects on Earth’s climate) in an average century is about 1 in 40,000 — larger than the chance of dying from a venomous bite or sting, and comparable to your chance of dying in a flood. On cost estimates given in the previous sidenote, the U.S. government alone could tax its median taxpayer by an extra $2 per year over the next century to halve this risk for its citizens (and everyone else in the world as a nice bonus). Similar spending to halve the deaths from flooding sounds not unreasonable to me.

  9. ^

    Note that an asteroid deflection program is currently an ‘all-or-nothing’ affair, in the sense that at present small-scale programs are probably just impossible. As such, advocacy for asteroid deflection should also be ‘all or nothing’ (it doesn’t make any sense to push for a bit more asteroid deflection). Since I’m arguing ‘all’ might not be a good option, ‘nothing’ might currently be the better one.

  10. ^

    Or just a really bad accident.

  11. ^

    Risks from supernovae and gamma ray bursts are laid out in Toby Ord, The Precipice (2020) p. 79, Table 3.3. Credit is due to Michael Dello-Iacovo for more recently elaborating on and restating this argument from Sagan & Ostro.

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This paper on the concern of nuclear explosives for asteroid deflection increasing the risk of nuclear war is relevant.

I find this post very interesting. However, I don't think the dual-use should worry us much. I cannot estimate how much harder it is in general to divert an asteroid toward Earth than away from it, but I can confidently say that it is several orders of magnitude higher than 10x (the precision needed would be staggering). In addition, to divert an asteroid toward Earth, one needs an asteroid. The closer the better. The fact that the risk of a big-enough asteroid hitting the Earth is so low indicates that there are not too many candidates. This factor has to be taken into account as well.

But, even if diverting an asteroid towards the Earth would be only 10 times harder than diverting it from the Earth, dual-use does not need to be a big concern. To actually manage to divert an asteroid towards the Earth one does not only need to divert it, one also needs to prevent the rest of humanity from diverting it away on time, which is much easier. So, as long as a small bunch of independent institutions are able and ready to divert asteroids, dual-use does not seem a concern to me.

Thanks, these are great points.

Of course, we can also try to spot asteroids in the sky. Astronomers have identified a large majority of near-Earth asteroids[3] 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 in an average century 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.

Interestingly enough, the importance of asteroid size might be overestimated, compared to impact angle and impact site. The asteroid that killed the dinosaurs wouldn't have been nearly as deadly, hadn't it struck at one of the worst possible places at one of the worst possible angles.

This 2017 paper used computer models to see if the rock composition of the impact site could've made a difference. The computer calculated the amount of soot and sulfates that would be ejected into the atmosphere as well as what that would mean for our planet, since both soot and sulfates can block the sun’s light. The blocked out sun started a global winter that lasted years and this is what killed the dinosaurs, not the impact of the asteroid directly. The researchers found that the composition of the impact site was especially unlucky. And since the Earth is constantly spinning and moving in space, this means that if the asteroid had just arrived a couple minutes later it wouldn't have hit such a problematic piece of land or might have even hit the ocean where a lot of its impact would have been lessened (in terms of the amount of rock that got ejected into the atmosphere).

This 2020 paper concluded that the asteroid hit from a pretty steep angle, about 45-60 degrees. This vaporized more rock than a shallow strike and released more climate-changing gases than other angles, with 2-3 times as much carbon dioxide released as a vertical impact and 10 times as much as a shallow impact.

Seeing how unlucky the impact timing was means that asteroids probably aren't as big of a risk as they are imagined to be. And even if we don't develop the technology to completely deflect asteroids, changing the angle or delaying it so it hits a different impact site might be enough to change a mass extinction into a mere disaster.


If I could give more than a Strong Upvote for your bringing up the dual-use issue as a crucial consideration for working on asteroid deflection capabilities, I would. I was considering doing a write-up on this as well. It is a wonderful example of second-order considerations making the effort to reduce risk actually increase it. 

I think this is strong enough as a factor that I now update to the position that derisking our exposure to natural extinction risks via increasing the sophistication of our knowledge and capability to control those risks is actually bad and we should not do it. Maybe this generalizes to working on all existential risks...

Thank you for the kind words!

I think this is strong enough as a factor that I now update to the position that derisking our exposure to natural extinction risks via increasing the sophistication of our knowledge and capability to control those risks is actually bad and we should not do it.

I would feel a bit wary about making a sweeping statement like this. I agree that there might be a more general dyanmic where (i) natural risks are typically small per century, and (ii) the technologies capable of controlling those risks might often be powerful enough to pose a non-negligible risk of their own, such that (iii) carelessly developing those technologies could sometimes increase risk on net, and (iv) we might want to delay building those capabilities while other competences catch up, such as our understanding of their effects and some meaure of international trust that we'll use them responsibly. Very ambitious geoengineering comes to mind as close to an example.

Maybe this generalizes to working on all existential risks...

Perhaps I'm misunderstanding you, but I'm very hopeful that it doesn't. One reason is that (it seems to me) very little existential risk work is best described as "let's do build dual-use capabilities whose primary aim is to reduce some risk, and hope they don't get misused"; but a lot of existential risk work can be described as either (i) "some people are building dual-use technologies ostensibly to reduce some risk or produce some benefits, but we think that could be really bad, let's do something about that" and (ii) "this technology already looks set to become radically more powerful, let's see if we can help shape its development so it doesn't turn to do catastrophic harm".


I think the meme of x-risk and related will spread and degrade beyond careful thinkers such as readers of this forum, and a likely subset of  responses to a perception of impending doom are to take drastic actions to gain perceived control, exacerbating risk. The concept of x-risk is itself dual-use.

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

I don't know how I'm supposed to interpret this statistic without a time frame. Is this supposed to be per century?

Thanks for the pointer, fixed now. I meant for an average century.

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