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If an extinction-level asteroid were to be detected on an impact course with Earth, could we avert it effectively with existing technology and systems, such as course adjustment via nuclear missiles? (How much advance warning would we need in order to do so?)

It seems like the ideal answer would take the form of an equation relating the mass and velocity of the asteroid to the warning time and detonative tonnage needed to avert impact.

Organic Marble
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DJG
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    https://en.wikipedia.org/wiki/Asteroid_impact_avoidance – DJG Sep 29 '21 at 15:10
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    "In May 2021, NASA astronomers reported that 5 to 10 years of preparation may be needed to avoid a virtual impactor based on a simulated exercise conducted by the 2021 Planetary Defense Conference." (from above-linked article) – DJG Sep 29 '21 at 15:11
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    Planetary Defense Conference and FEMA tabletop simulated exercises (link to previous exercises at CNEOS) have focused on 100-300 meter sized impactors not extinction-level ones to make a more likely and feasible scenario. The scenarios also tend to have somewhat contrived detection times and visibility to make a more challenging scenario where you have to make tough decisions based on incomplete info. For the OP: how much warning time between detection and impact are you assuming ? This makes a major difference in the possibility of any mitigation plan. – astrosnapper Sep 29 '21 at 16:39
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    Related: https://space.stackexchange.com/q/7773/58 – called2voyage Sep 29 '21 at 17:33
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    As I mention here: https://astronomy.stackexchange.com/a/39875/16685 The Earth's orbital speed is around 29.8 km/s, thus it covers a distance equal to its own diameter in around 7 minutes. If you want to determine if some rogue body is going to hit the Earth, your trajectory calculations need to have that level of precision. (That 7 minute window is for a body in the ecliptic plane heading towards the Sun, so it crosses the Earth's orbit radially). – PM 2Ring Sep 30 '21 at 10:37
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    ...could we avert it...? By the time the committee was approved, organizational rules decided upon, officers elected, roles assigned, a plan created, and funding received to possibly implement a prototype, the Earth would have a very sizable dent... and, of course, if anybody survived, some minority group would get blamed as the scapegoat... – End Anti-Semitic Hate Sep 30 '21 at 10:54
  • @RockPaperLz-MaskitorCasket you assume western countries would be the ones managing such projects. – Famous Jameis Sep 30 '21 at 23:58
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    Do people really think bureaucracy would still move at a snail's pace in the face of a species-wide imminent threat? I don't. – DJG Oct 01 '21 at 14:06
  • @DJG Well, once all the forms are signed. :) Hoping they'd move quickly, but I think it's possible they wouldn't. Just not sure. – Don Branson Oct 01 '21 at 21:10
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    @DJG It's not the bureaucracy I'm worried about, it's Mitch McConnell. – J.G. Oct 02 '21 at 19:07
  • For serious example, see here & here. – J.G. Oct 02 '21 at 19:09
  • @DJG COVID-19 was discovered in, well, 2019 (no Kellyanne, this isn't the 19th COVID - https://www.complex.com/life/2020/04/kellyanne-conway-covid-1). Here we are nearing the end of 2021, and the majority of the world's population still has no access to the vaccines (almost all the vaccines have gone to a handful of rich nations). So to answer your question, *yes*. – End Anti-Semitic Hate Oct 02 '21 at 21:51
  • @FamousJameis Doesn't matter who is going after it, there are always those who will try to personally benefit. Personally I expect an ELE that requires major effort to deflect we would fail due to such infighting and not allowing for the inevitable failures. (SpaceX: To deflect that rock we need 80 hits with nuclear-tipped Starships, launches starting in two weeks. Given the inevitable failures of such a pace please fund and arm 100 rockets. Congress: Here's 90. SpaceX: 78 hits, it's too late now.) – Loren Pechtel Oct 04 '21 at 15:31

4 Answers4

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Yes.

(But it requires a bit of dishonesty.)

Simply because an "extinction level" asteroid does sufficiently limit the possible nature of the object.

To be extinction-level, it would need to be quite large. More than 3km in diameter.
To be asteroid, it would need to be an inner-solar-system object. This pretty much limits it to being a large asteroid, perturbed from its previously stable orbit by the gravity of Jupiter or one of the solar system bodies inward from Jupiter. Even at the absolute latest, we would detect it directly after such an encounter and have half an orbit to prepare for it. This means six years of warning, if the encounter event is such that it is directed directly to an Earth encounter, which is ludicrously unlikely.

The thing is, we have already mapped all, or very nearly all, asteroids of that size range. And we certainly will notice if one passes close enough to Jupiter to be deviated into an Earth collision in one gravitational event.

We have the ability to build a defense against a very large asteroid, although with only six years warning the result will be very messy indeed. We should be able to fragment it, and divert much of the asteroid mass from collision, but the resultant buckshot effect from the fragments will be extremely nasty. Still, we could.

What is much more dangerous is those classes of objects not eliminated by our clever wordplay analysis at the beginning.

But actually, NO

A not-quite-extinction-level but still civilization-ending asteroid of 500m to 1km in the exact same scenario might not be noticed in time.

And worst a visitor from the outer solar system, or even from interstellar space (thus technically a comet not an asteroid), does not need to be perturbed by Jupiter to enter an Earth-targeting orbit. It might already be on such a path. It would potentially require a lot less than six years to cross the same distance. And it would come from somewhere out of the sky, not from the very highly observed space near Jupiter.

  • We would have much less warning of an object from the outer system, and even less from an interstellar visitor. (assuming the object is rocky and not gassy)
  • Its mass could be very much higher. We have mapped pretty much all the inner system asteroids >1km in size. We most certainly have not mapped such objects for the outer system or interstellar space.
  • It would be moving faster, thus more impact energy for the same mass.
  • and it would potentially come from any direction.
J.G.
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CuteKItty_pleaseStopBArking
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    The odds of an interstellar object impacting Earth are quite small, to say the least... – PearsonArtPhoto Sep 29 '21 at 18:58
  • The question presupposes that we can detect it in sufficiently advance time hence most of your answer is irrelevant. – user2617804 Sep 30 '21 at 06:54
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    @user2617804 That is an assumption, minus the umption. I have no idea where you get it from. There is absolutely no context within the question that states when the detection occurs, yet the detection time is everything in this situation. If we detect a 10km asteroid 100 years ahead of time, we can divert it easy. If we detect even a 1km asteroid just a week before impact, we need to go to church not the science labs and launch facilities. – CuteKItty_pleaseStopBArking Sep 30 '21 at 08:55
  • @user2617804: It wouldn't help as much as you like. The energy difference by being hyperbolic with the sun makes it much harder to deflect. – Joshua Sep 30 '21 at 16:22
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    @PearsonArtPhoto one might even call those odds astronomical. – Mark Ransom Sep 30 '21 at 19:16
  • @user2617804: No, the question doesn't presuppose it will be detected sufficiently far in advance. It asks how long in advance is sufficient, something PcMan hasn't addressed other than saying 6 years should be enough. – Ben Voigt Sep 30 '21 at 21:21
  • "To be asteroid, it would need to be an inner-solar-system object." Why not an asteroid from the Oort Cloud? This article says "Given the ratio found above and assuming a population of 1012 comets in the outer Oort cloud, then there are currently ∼8 * 10^9 asteroids in the outer cloud." – Don Branson Oct 01 '21 at 20:09
  • I'm wondering about this, too: "even from interstellar space (thus technically a comet not an asteroid)." Not everything from interstellar space is a comet - they could be comets, but they could be asteroids, too. Even a rogue planet, I suppose. – Don Branson Oct 01 '21 at 20:16
  • Horribly wrong--turning it into buckshot is absolutely the wrong thing to do. Against rocks of that size breaking them up is the worst possible outcome. Deflect it or spend the effort on hunkering down. – Loren Pechtel Oct 04 '21 at 15:34
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Perhaps. Perhaps not.

  • There are programs to look for these asteroids. Sometimes they work. Sometimes not. You assumed in your question that it would be detected. Just how much warning time is there? Hours? Weeks? Decades?
  • If a body is detected very far out, it will take only a tiny nudge to make it miss Earth.
  • Current space programs are very conscious of safety. Nobody wants the death of a crew as breaking news, and nobody wants to splatter fragments of a nuclear power source on Earth. People were more risk-accepting during the the Cold War. Once the reality of an "extinction level event" sinks in, risk acceptance will be adjusted even beyond Cold War levels.
  • Giving that nudge e.g. with nuclear weapons could shatter the body, creating many slightly smaller impacts rather than a single big one.
o.m.
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  • "creating many slightly smaller impacts rather than a single big one." I don't have the reference in front of me, but have read this doesn't really make a difference, except to the extent that some of the pieces miss Earth. Otherwise, the amount of energy delivered is about the same. Still, good answer, +1. – Don Branson Oct 01 '21 at 20:40
  • @DonBranson, if there is an explosion next to a stable rock, that entire rock may be diverted just a bit. If an unstable asteroid shatters, there could be significant chunks still on collision course and the follow-up missions to divert those would be messy. If it can be done at all in time/in sufficient numbers. I have faith in the ability of NASA and the USAF, or Russia, or China to put one megaton bomb next to one asteroid within a year. Less so for a swarm within a debris cloud. – o.m. Oct 02 '21 at 05:49
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It would depend massively on the size of the asteroid, it's composition and how far out it was detected.

For the true planet killers it is extremely likely that the asteroid would be detected years in advance. Asteroids "planet killer" like Apophis are simply too big (Apophis is over 300m) to be missed. This gives us a significant advantage as it both gives us time to put together a mission and vastly reduces the amount by which the asteroid has to be re-directed. In this circumstances there are three main solutions:

Gravitational tractor - the good

Fly a spacecraft next to the asteroid, stop it stationary to the asteroid (using it's thrusters to maintain distance) and allow the gravitational attraction between the asteroid and the spacecraft to act as a tow rope, towing the asteroid along. This has the advantages of having very little which can go wrong and creating no orbital debris. However, the forces involved are simply too small to redirect a "planet killer" size asteroid in a reasonable timeframe

Nuke thruster - the cool

Fly a spacecraft packed with a (for a this kind of asteroid, really really big) nuke next to one side of the asteroid and blow it up. The energy released by the nuke will vaporise one side of the asteroid creating thrust. If a planet killer was discovered today this would probably be our best bet, as (thanks to the cold war) we have the technology to make some extremely powerful nukes as well as the rockets to get them there. This does, however, have some major downsides. While there is no threat of nukes going off in the event of a launch failure (setting off a nuclear bomb is an extremely precise process - the US has accidentally set fire to them and dropped them out of aeroplanes on multiple occasions and they have never gone off) it would scatter a lot of highly radioactive material. Additionally, it is likely that this would create a vast amount of orbital debris.

Smaller asteroids

Our current system - ATLAS https://atlas.fallingstar.com/home.php - gives one day's warning for a 30 kiloton asteroid, a week for a 5 megaton and three weeks for a 100-megaton. For context the Chelyabinsk meteor had a mass of about 12000 tons (note at 11km/s - escape velocity and therefore the lowest velocity at which an asteroid can hit Earth - each ton of matter has about the same energy as 15 tons of TNT). Depending on the accuracy with which the asteroid's trajectory can be calculated this warning it may allow the impact area to be evacuated (though considering that even a 5 megaton asteroid "only" has a TNT equivalent of about 3 Tsar Bomba's there are still a lot of places on earth where this could hit and do almost no damage). Therefore while this class certainly isn't good news their yields are low enough and the warning time long enough that they don't pose an existential threat.

However, the 100kt+ range is the real worry, with our current detection capabilities we'd need something ready to go - which we don't have. And their yield (equivalent to hundreds of Tsar Bomba's) is sufficient to obliterate the area for hundreds of kilometres and cause something similar to a nuclear winter.

ICBM's

Before someone inevitably suggest ICBM's, no they wouldn't work - while they look good on paper. They lack the delta V to intercept an asteroid at sufficient distance to redirect it, they also lack the yield to vaporise all but the smallest asteroids (it is also doubtful they they would have the yield for either). To destroy a large asteroid after a late detection would require an extremely large rocket carrying an extremely large nuke and guided very precisely to it's target.

Pioneer_11
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  • You don't want one bomb, you want many bombs delivered over a period of time. Hit it too hard and it breaks up and you're in a worse position than if you didn't shoot at all. – Loren Pechtel Oct 04 '21 at 15:36
  • True, although, I was presuming a late redirect in which case you need a large impulse and don't have time to analyse the change in orbit/composition from a large number of small blasts. Making one (or a small number of) large blasts the only practical option – Pioneer_11 Oct 05 '21 at 17:02
  • In that case you don't shoot in the first place. Breaking it up makes it worse, it's not merely ineffective. Breakup is only of value if you can blast it into small enough pieces that it burns up in the atmosphere. – Loren Pechtel Oct 05 '21 at 21:07
  • At the kind of distances we are talking for a 3 week warning. Breaking it up is a huge advantage. Not only would it make a large number of pieces burn up in the atmosphere (and vastly reduce the impact of those which made it through). But it would (depinning on the yield of the nuke and resulting impulse) push a some, or maybe even all, of the pieces out of an intercepting orbit – Pioneer_11 Oct 06 '21 at 20:33
  • No. It's a matter of overkill. The blast radius goes at the cube root of the energy, the area of destruction goes at the square of the radius. Thus damage goes at the 2/3 root of energy. Break a rock into 8 equal parts and it now does twice the damage. 64 parts = 4x the damage. – Loren Pechtel Oct 06 '21 at 21:10
  • Firstly, I was mostly talking about how breaking it up would many of the pieces and that (depending on the level of fragmentation) many of the others would burn up in the atmosphere (doing minimal damage). Secondly, while the destruction radius per unit mass (again the greater ablation in the atmosphere will greatly reduce this mass) is higher. A single massive asteroid hitting the earth would create a massive earthquake and/or tsunamis (which would probably be the main source of destruction) whereas the same mass of small asteroids wouldn't. – Pioneer_11 Oct 12 '21 at 00:48
  • It depends on how small the pieces are. An extinction rock is going to be a mile or more across. To burn it in the atmosphere would require breaking it into literally millions of pieces. That's not what's going to happen. – Loren Pechtel Oct 12 '21 at 23:36
  • As I said in my original answer, planet killers are big enough that they are/would be detected years in advance. The danger zone of high tens to low hundreds of meters (what I was referring to in the "three week" part of my answer) is small enough that we have, or could easily build, nukes (and the rockets required to launch them) which could either vaporise, or sufficiently fragment them – Pioneer_11 Oct 29 '21 at 13:39
  • We know where most extinction level rocks are, but something in a very long orbit could be undetected. – Loren Pechtel Oct 30 '21 at 21:01
  • True. However, the number of those rocks is vastly lower and once again the bigger you go the further out it will be detected giving more time and making it easier to redirect. – Pioneer_11 Jan 14 '22 at 22:33
  • These kinds of discussions tend to assume the earth is a huge target hoovering up asteroids - it isn't. Astronomical distances dwarf the earth - the earth is 12,700km in radius. The asteroid belt is hundreds of millions of kilometres away and even an asteroid heading for earth at 10km/s detected 3 weeks in advance is still 20,000,000km away. You only need to redirect an asteroid a comparatively miniscule amount to make it miss earth. In this case a change in velocity is only 50m/s would half way though that path would be more than sufficient – Pioneer_11 Jan 14 '22 at 22:51
  • How far away it is only relevant for shooting, what counts is time to impact. – Loren Pechtel Jan 15 '22 at 03:22
  • @LorenPechtel I assume you mean how far away it is is only relevant for detection, which is of course absolutely true. However, it is vastly more likely that an asteroid would have several close approaches before hitting earth than randomly come out of the blue and smack into earth. Additionally the more eccentric the orbit of a rock the smaller a target the earth presents. This is because the rock will be going faster when it passes earth's orbit, meaning the radius at which it can be sucked up by the earth's gravitational field is smaller. – Pioneer_11 Oct 07 '23 at 12:58
  • Additionally for asteroids at high eccentricities the angle between the velocity vector of the asteroid and that of the earth is large. This again decreases the chance of the asteroid being captured by earth's gravity as it will spend a far smaller proportion of it's orbit close to earth and therefore the number of small perturbations which can put it on a collision course is far reduced. Finally because the period of the orbit is longer the asteroid has fewer chances to hit the earth. Therefore, while these asteroids are the hardest to detect they are also the least likely to hit earth. – Pioneer_11 Oct 07 '23 at 13:02
  • No, not only does distance matter for detection, but it also matters for outbound flight time. Rock #1 is seen crossing Jupiter's orbit and closing at 5 km/sec. Rock #2 is 1/16th the mass but closing at 20 km/sec. Approximately equal damage. You have much less time to get stuff in flight against #2 and even if you had birds ready to fly they're going to engage much closer in. – Loren Pechtel Oct 07 '23 at 15:22
  • @LorenPechtel first those two rocks will not do approximately equal damage. Both will pick up 11km/s worth of kinetic energy when falling into earth's gravity well (note that is 11km/s worth of kinetic energy not of velocity) so the first asteroid will do far more damage than the second. Secondly I wasn't arguing that highly eccentric asteroids aren't a risk, only that they are far less likely to hit us than closer asteroids, and thus less of a threat overall. Yes they are a threat and no we couldn't stop them in time but that is also true of asteroids which are far more likely to hit us. – Pioneer_11 Oct 07 '23 at 23:40
  • True, I was neglecting the energy it will pick up as it approaches--reasonable for the fast one, not reasonable for the slow one. I was simply making a point about speed reducing the intercept window. – Loren Pechtel Oct 08 '23 at 03:56
  • And against the smaller, faster rock you are more limited in how hard you can hit it--you can't throw anything at it that might break it up. – Loren Pechtel Oct 08 '23 at 14:25
  • @LorenPechtel it depends on how many pieces, how many of those pieces will hit earth and how large those pieces are. As long as all those pieces miss earth or break up in the upper atmosphere you're good. Again I don't think you are appreciating just how small earth is on the scale of the solar system or just how much energy we are talking about in these interactions. Unless the asteroid is incredibly close you only need to redirect the debris by a matter of meters per second and for an object travelling at multiple km/s it's kinetic energy alone is comparable to a small nuke. – Pioneer_11 Oct 09 '23 at 12:49
  • Agreed--but generating even meters per second takes a lot of energy if it's a dinosaur killer. And that's assuming you have a lot of time--if your intercept is one year from impact 3m/s will address almost every situation. But if you shove an asteroid more than it's escape velocity you might have a bunch of pieces to deal with instead. – Loren Pechtel Oct 11 '23 at 04:11
  • @LorenPechtel You need to get REALLY close before the velocity gets large. Even if the asteroid was intercepted at a distance equal to the orbit of the moon going 5km/s you would only need to change it's velocity by about 100m/s to make it miss the earth. If you run the calculations for a 1MT warhead (which the US has many of), assuming a nuclear to kinetic energy conversion efficiency of 10% (this is a guess I don't know of any studies which have worked what the number would be and it would likely differ between asteroids) you can still divert an asteroid with a mass of up to 80MT. – Pioneer_11 Oct 11 '23 at 12:29
  • Considering the scenario I laid out above is the kind of thing that could conceivably be made to work with a reasonably chance of success with only a few weeks warning. (the concept I am basing this off of is taking the US's most powerful nuke, the B83 with a yield of 1.2 mega tonnes, and strapping it and a bunch of RCS to the top of a falcon 9) You need either a very big rock or very little warning present a target that couldn't be dealt with in time. – Pioneer_11 Oct 11 '23 at 12:37
  • There are asteroids which are either too massive to stop at short notice or would be detected at too short notice to stop at all. But you need a really big rock for an existential threat and it needs to be detected fairly late in order for us to not be able to stop it. The proportion of asteroids which fill those criteria is very small and earth is a very small target. We as a planet also have a huge number of nukes and the capacity to build a huge number of rockets, so given sufficient warning almost any asteroid can be stopped. Therefore, the risk of a planet killer is extremely minimal. – Pioneer_11 Oct 11 '23 at 12:48
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So we should first be clear about how big an object we are talking about. The dinosaur killer was thought to be about 10 km across, so that is the size range we should be considering, if we are serious about "extinction level." (The lower end may be below that, but it's a good round number and we know mammals and birds did survive.)

To my understanding, surveys have already concluded that there are no threats of that size in the inner solar system, at least as far into the future as orbits can be projected.

The Wikipedia article Asteroid Impact Avoidance gives a good overview of the many approaches. Apparently it has been proposed to develop a 1 gigaton nuclear explosive, which could deflect a 10 km asteroid with only perhaps six months warning. That would perhaps be sufficient for one detected coming inward from the outer solar system, but only if a bomb was kept prepared at all times.

With decades of notice, a 10 km asteroid could apparently be deflected with just a long series of small impacts, with no nuclear explosives involved, according to this article in Space.com. In general, if you read a lot about research on asteroid deflection, you will find that people feel more comfortable proposing methods that don't involve nuclear explosives. I think a lot of the reason for this is that nuclear devices are so tied up in military secrecy, international treaty rules, and security (falling-into-the-wrong-hands) concerns that other approaches just seem safer. For instance, building a 1 gigaton bomb would be politically problematic. Better, rather, to improve surveillance to catch outer-solar-system threats sooner, and then deal with them with greater advance notice if necessary.

Mark Foskey
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