I did. I am not impressed by the statistics quoted in it. In particular, there is a neat trick in transitioning from “NASA reports that all near-earth asteroids larger than 10 kilometers in diameter … have already been identified” to “This eliminates much of the estimated risk due to {note the glaring empty space here where words “near-earth” used to be} asteroids”
Asteroid impact is mostly a black-swan type of problem: you can identify the risk you see but you have very little idea of the remaining risk from things you do not see.
Near earth asteroids are the primary threat set here, with only a tiny fraction not in that set that have any chance of hitting Earth. That’s precisely why they say it eliminates much of the estimated risk due to asteroids.
Near earth asteroids are the primary threat set here
I think it used to be the primary threat set. The claim is that near earth asteroids are not a threat because we looked at them and established that large ones are not going to hit Earth in the near future. Thus near earth asteroids are not the primary threat set any more.
This seems like an odd use of language which misses the fundamental point: the observation of the near Earth asteroids reduces the estimated risk level by orders of magnitude. Whether the remaining risk is still concentrated in the near-earth case is a secondary consideration and not relevant to what was being discussed.
I don’t know why you think this use of language is odd. Saying that “we thought X was dangerous, we looked at it closely and it turns out X isn’t dangerous at all” has the same meaning as “we mis-estimated the danger from X and then corrected the estimate”.
If your updated belief is that there is little danger from near-earth asteroids, then the original belief that near-earth asteroids were the primary threat set was incorrect.
Because it misses the point that the total risk was from asteroids isn’t that high. Yes, of the remaining asteroid threat, more of it is from non near Earth asteroids, but that’s not relevant to the discussion at hand. Hence the phrase in the report that you objected to “”This eliminates much of the estimated risk due to asteroids” makes complete sense.
Let me try to reformulate my point. We’re talking about existential risk of an asteroid impact (where “asteroid” is defined as anything large enough moving fast enough). Large asteroids have hit Earth before, we have a fairly good idea how often such things happen. The historical record gives us the basis for a guesstimate of the risk.
That risk estimate is, of course, quite low. Still, we went out looking for things which might hit us in the near future. Note the asymmetry here: were we to find something our risk estimate would skyrocket, but were we to find nothing, it wouldn’t perceptibly change.
So we looked at near-earth asteroids because, well, they are near-earth. Turned out none of them is on a collision course with Earth in the foreseeable future. This is good, of course, but it does not mean that the estimated risk went down—what happened was that it did not go up and that’s a different thing.
My original objection was to the characterization of asteroid risk as a “solved problem”. It is not. Saying this is like looking up, noticing that the ceiling isn’t about to collapse, and then on this basis confidently pronouncing that things falling on your head is a solved problem.
So we looked at near-earth asteroids because, well, they are near-earth. Turned out none of them is on a collision course with Earth in the foreseeable future. This is good, of course, but it does not mean that the estimated risk went down—what happened was that it did not go up and that’s a different thing.
I had the impression that the near-earth ones were the ones that, averaged over earth’s history, are the bulk of the problem. So if the current crop of near-earth asteroids aren’t likely to hit us in the historically-near future, doesn’t that mean that our near-future risk of impact is below the long-term average risk?
(I am not an astronomer and do not vouch for “NEAs are the main part of the risk” from personal knowledge.)
Well, IANAAE (I Am Not An Astronomer Either) but I think that with respect to historical record, there are these considerations:
We’re pretty sure that large asteroids (defined as above) have struck Earth before. We are not sure where they came from.
With the passage of time the frequency of collisions should decline as Earth sweeps a path free of other space objects. So the future risk of impact is below the historical risk of impact.
The extinction-scale impact risk seems to be very small. In geologically recent times Earth was not bombarded by asteroids.
So we looked at near-earth asteroids because, well, they are near-earth. Turned out none of them is on a collision course with Earth in the foreseeable future. This is good, of course, but it does not mean that the estimated risk went down—what happened was that it did not go up and that’s a different thing.
Yes, it does mean the estimated risk has gone down. It means that the largest set of obvious candidates aren’t doing that. If seeing them would make it go up, not seeing asteroids on collision paths must push it down. This is the conservation of expected evidence.
If seeing them would make it go up, not seeing asteroids on collision paths must push it down.
Yes, technically. But I’ve already been though that in a thread here—that was the whole thing about how checking your garbage can and not finding a tiger in it happens to be evidence for non-existence of tigers.
I’m willing to grant that not finding any near-earth asteroids on a collision course reduces the probability of an impact during, say, next 50 years, but that reduction is miniscule. In fact I’d call it “undetectable”.
To throw in another metaphor, if I’m driving on a highway, look around, and see that no cars are headed straight at me—technically speaking, that reduces the probability that I’ll get into a car accident this year. But it reduces this probability by an infinitesimal amount only. On the other hand, if I see a car that’s about to ram me, the probability of getting into an accident this year HUGELY increases.
Ok. So the thing is, is most asteroids don’t change their orbits that drastically. NEOs aren’t just things near Earth’s position right now, but are all asteroids that orbit the sun on roughly the plane of the elliptic. They are about .9 to about 1.4 AU from the sun. So the vast majority of objects which have any substantial chance of hitting Earth fall into this category. And we can plot their trajectories out far into the future.
...and we’re back to me pointing out that once you have determined that these are not a threat, these are not a threat.
But let’s try another tack. Do you know of any data-supported estimates of the asteroid impact risk? I’m not interested in the number per se, but more in the data on which it is based and the procedure of estimation.
...and we’re back to me pointing out that once you have determined that these are not a threat, these are not a threat.
Which we’ve already addressed, since what is relevant is trying to estimate the total risk. I thought I had explained that already. Is there something that is wrong with that logic?
But let’s try another tack. Do you know of any data-supported estimates of the asteroid impact risk? I’m not interested in the number per se, but more in the data on which it is based and the procedure of estimation.
So, one thing to look at is the Near Earth Object Program, which has a lot of links and discussions. Most of the specific asteroids are targeted by ground based telescopes, although a lot of the initial data comes from the WISE mission which was able to spot objects in a fairly broad range (for most purposes, out to a bit beyond where the main asteroid belt is). In addition to this, we have models of the solar systems which try to estimate how many large objects are likely to be missed, as well as estimates from prior background impact rates. Since the Earth is highly active, only some of the largest of asteroid impacts end up leaving a direct trace here, so we have to use the Moon and other objects to make those sorts of estimates. The links Carl gave earlier are also worth reading and discuss some of this in further detail.
what is relevant is trying to estimate the total risk
Right, and I’m asserting that finding that risk from near-earth asteroids in the next 100 years or so is negligible should not affect the estimate of the total risk in any meaningful way (compared to the pre-NEO-survey estimate).
Now, for the actual estimates we’re interested in, that’s what is called the background frequency, aka unconditional expectations of impacts. The source for that goes to Chapman, C. R., and D. Morrison 1994. Impacts on the Earth by asteroids and comets: Assessing the hazard. Nature 367 ,33–40 which, unfortunately, is behind a paywall and I’m too lazy to scour the ’net for an open copy. The basic expectation of frequency of impacts, though, is visible through other sources (see e.g. http://neo.jpl.nasa.gov/risk/doc/palermo.pdf)
To re-express my point in these terms, the survey of near-earth asteroids does not change the background frequency.
(One of the nice things about my Xmonad setup is that I have a shortcut which yanks the current copy-paste buffer and searches Google Scholar for it; so the net effort looks like ‘highlight “Impacts on the Earth by asteroids and comets: Assessing the hazard”, hit mod-Y, right-click on PDF link, copy, paste’.)
Background frequency over a few hundred years or more, yes. Is anyone asserting that we should be planning out now exactly how much resources we put into this past anytime beyond the next fifty years or so? And if not, how is that relevant?
Are you saying that your current impact risk estimates for the next, say, 50 years are significantly lower than the background?
Yes. And we can conclude that because we have detailed understanding of the orbits of the big near earths and can predict their orbits out reliably for a few decades.
You need the assumption that (geologically) recent impacts that the background frequency reflects came from near-earth asteroids.
Yes, but we’re pretty confident about this also. It is very difficult for an asteroid out past 1.4 au to end up changing orbit enough to run into Earth. It requires a large force. It can happen if it gets just the right luck with collisions or with the right gravitational pull (such as if it happens to past just right near Mars at the right time in its orbit). But these events are rare, and moreover, we can model their likelyhood pretty well. Stochastic aspects of orbital dynamics are decently approximable by a variety of methods (such as Monte Carlo).
I don’t know about that. First, there are comets. Second, large forces are not uncommon with collisions in space. More to the point, any collision (or maybe even a close pass) could change the trajectory of an already-catalogued asteroid to something different and possibly dangerous.
The Chapman & Morrison paper points out that “Because of stochastic variability in the process of asteroid and comet break-up, there is a chance for significant temporal variation in the impact flux”.
I really don’t think we understand the movement of various objects in the Solar System well enough to declare “problem solved”.
Comets != asteroids. Comets have much harder to predict orbits (outgassing and change in mass both make them much trickier to predict). There’s some positive side here in that in order for them to be remotely close to us they generally need to be pretty visible (there’s some worry about comet remnants who are in weird orbits but are no longer highly visible when they are in the inner solar system area).
More to the point, any collision (or maybe even a close pass) could change the trajectory of an already-catalogued asteroid to something different and possibly dangerous.
Yes, and this is a problem, and I mentioned this explicitly in my last comment. The issue is how frequent such events are.
The Chapman & Morrison paper points out that “Because of stochastic variability in the process of asteroid and comet break-up, there is a chance for significant temporal variation in the impact flux”.
Yes. And what’s your point? No one is saying anything otherwise.
I really don’t think we understand the movement of various objects in the Solar System well enough to declare “problem solved”.
The argument isn’t that the problem is solved. The issue is that the chance of an issue in the short-term is much lower than we would have thought 10 or 20 years ago. That’s not the same as problem solved: the problem won’t be completely solved until we’ve got much better tracking (I’d prefer radio beacons on every object greater than 1 km) and have a system that can deal with sudden threats. But that’s not the issue at hand.
In the grand...grandparent I explicitly defined asteroids as anything large enough and fast enough to make a noticeable impact on Earth precisely to avoid terminology issues like this.
The argument isn’t that the problem is solved.
That’s how the whole thing started. If you go to the origin of this long sub-thread you’ll see CarlShulman saying “That’s mostly solved, all the dinosaur-killers have been tracked” and me replying “I don’t think so”.
The issue is that the chance of an issue in the short-term is much lower than we would have thought 10 or 20 years ago.
Yep—that’s what I mean by having an wrong estimate and then correcting it.
In the grand...grandparent I explicitly defined asteroids as anything large enough and fast enough to make a noticeable impact on Earth precisely to avoid terminology issues like this.
Ah, I see. Yes, in that case, using that broad class of objects, we have a much poorer understanding of comets then. The same basic argument goes through (because comets are not nearly as common an object as what is normally called asteroids), but not as by much.
. If you go to the origin of this long sub-thread you’ll see CarlShulman saying “That’s mostly solved, all the dinosaur-killers have been tracked” and me replying “I don’t think so”.
Yeah. I think Carl’s wording here is important. “Mostly solved” is different from “solved”. In this sort of context problems are very rarely solved completely, but more solved in the sense of “we’ve put a lot of effort into this, the most efficient thing to do is to put our next bit of resources into many other existential risks”.
The issue is that the chance of an issue in the short-term is much lower than we would have thought 10 or 20 years ago.
Yep—that’s what I mean by having an wrong estimate and then correcting it.
With respect to the wrong estimate—there is the “background frequency”, right? Tracking a bunch of near-earth asteroids does not lower it significantly (I am not sure, we may disagree on that). So if 10-20 years ago we thought that the threat of an asteroid impact was much higher, I think I’d call it a wrong estimate.
Saying that “we thought X was dangerous, we looked at it closely and it turns out X isn’t dangerous at all” has the same meaning as “we mis-estimated the danger from X and then corrected the estimate”.
Risks and dangers here are percieved risks and dangers. In that context, such talk makes sense—obviously percieved risks depend on your current state of knowledge. Maybe god knows whether the bad thing will happen or not—but without a hotline to Him, percieved risks and dangers will remain the best we have.
I did. I am not impressed by the statistics quoted in it. In particular, there is a neat trick in transitioning from “NASA reports that all near-earth asteroids larger than 10 kilometers in diameter … have already been identified” to “This eliminates much of the estimated risk due to {note the glaring empty space here where words “near-earth” used to be} asteroids”
Asteroid impact is mostly a black-swan type of problem: you can identify the risk you see but you have very little idea of the remaining risk from things you do not see.
Near earth asteroids are the primary threat set here, with only a tiny fraction not in that set that have any chance of hitting Earth. That’s precisely why they say it eliminates much of the estimated risk due to asteroids.
I think it used to be the primary threat set. The claim is that near earth asteroids are not a threat because we looked at them and established that large ones are not going to hit Earth in the near future. Thus near earth asteroids are not the primary threat set any more.
This seems like an odd use of language which misses the fundamental point: the observation of the near Earth asteroids reduces the estimated risk level by orders of magnitude. Whether the remaining risk is still concentrated in the near-earth case is a secondary consideration and not relevant to what was being discussed.
I don’t know why you think this use of language is odd. Saying that “we thought X was dangerous, we looked at it closely and it turns out X isn’t dangerous at all” has the same meaning as “we mis-estimated the danger from X and then corrected the estimate”.
If your updated belief is that there is little danger from near-earth asteroids, then the original belief that near-earth asteroids were the primary threat set was incorrect.
Because it misses the point that the total risk was from asteroids isn’t that high. Yes, of the remaining asteroid threat, more of it is from non near Earth asteroids, but that’s not relevant to the discussion at hand. Hence the phrase in the report that you objected to “”This eliminates much of the estimated risk due to asteroids” makes complete sense.
We are talking past each other.
Let me try to reformulate my point. We’re talking about existential risk of an asteroid impact (where “asteroid” is defined as anything large enough moving fast enough). Large asteroids have hit Earth before, we have a fairly good idea how often such things happen. The historical record gives us the basis for a guesstimate of the risk.
That risk estimate is, of course, quite low. Still, we went out looking for things which might hit us in the near future. Note the asymmetry here: were we to find something our risk estimate would skyrocket, but were we to find nothing, it wouldn’t perceptibly change.
So we looked at near-earth asteroids because, well, they are near-earth. Turned out none of them is on a collision course with Earth in the foreseeable future. This is good, of course, but it does not mean that the estimated risk went down—what happened was that it did not go up and that’s a different thing.
My original objection was to the characterization of asteroid risk as a “solved problem”. It is not. Saying this is like looking up, noticing that the ceiling isn’t about to collapse, and then on this basis confidently pronouncing that things falling on your head is a solved problem.
I had the impression that the near-earth ones were the ones that, averaged over earth’s history, are the bulk of the problem. So if the current crop of near-earth asteroids aren’t likely to hit us in the historically-near future, doesn’t that mean that our near-future risk of impact is below the long-term average risk?
(I am not an astronomer and do not vouch for “NEAs are the main part of the risk” from personal knowledge.)
Well, IANAAE (I Am Not An Astronomer Either) but I think that with respect to historical record, there are these considerations:
We’re pretty sure that large asteroids (defined as above) have struck Earth before. We are not sure where they came from.
With the passage of time the frequency of collisions should decline as Earth sweeps a path free of other space objects. So the future risk of impact is below the historical risk of impact.
The extinction-scale impact risk seems to be very small. In geologically recent times Earth was not bombarded by asteroids.
Yes, it does mean the estimated risk has gone down. It means that the largest set of obvious candidates aren’t doing that. If seeing them would make it go up, not seeing asteroids on collision paths must push it down. This is the conservation of expected evidence.
Yes, technically. But I’ve already been though that in a thread here—that was the whole thing about how checking your garbage can and not finding a tiger in it happens to be evidence for non-existence of tigers.
I’m willing to grant that not finding any near-earth asteroids on a collision course reduces the probability of an impact during, say, next 50 years, but that reduction is miniscule. In fact I’d call it “undetectable”.
To throw in another metaphor, if I’m driving on a highway, look around, and see that no cars are headed straight at me—technically speaking, that reduces the probability that I’ll get into a car accident this year. But it reduces this probability by an infinitesimal amount only. On the other hand, if I see a car that’s about to ram me, the probability of getting into an accident this year HUGELY increases.
Ok. So the thing is, is most asteroids don’t change their orbits that drastically. NEOs aren’t just things near Earth’s position right now, but are all asteroids that orbit the sun on roughly the plane of the elliptic. They are about .9 to about 1.4 AU from the sun. So the vast majority of objects which have any substantial chance of hitting Earth fall into this category. And we can plot their trajectories out far into the future.
...and we’re back to me pointing out that once you have determined that these are not a threat, these are not a threat.
But let’s try another tack. Do you know of any data-supported estimates of the asteroid impact risk? I’m not interested in the number per se, but more in the data on which it is based and the procedure of estimation.
Which we’ve already addressed, since what is relevant is trying to estimate the total risk. I thought I had explained that already. Is there something that is wrong with that logic?
So, one thing to look at is the Near Earth Object Program, which has a lot of links and discussions. Most of the specific asteroids are targeted by ground based telescopes, although a lot of the initial data comes from the WISE mission which was able to spot objects in a fairly broad range (for most purposes, out to a bit beyond where the main asteroid belt is). In addition to this, we have models of the solar systems which try to estimate how many large objects are likely to be missed, as well as estimates from prior background impact rates. Since the Earth is highly active, only some of the largest of asteroid impacts end up leaving a direct trace here, so we have to use the Moon and other objects to make those sorts of estimates. The links Carl gave earlier are also worth reading and discuss some of this in further detail.
Right, and I’m asserting that finding that risk from near-earth asteroids in the next 100 years or so is negligible should not affect the estimate of the total risk in any meaningful way (compared to the pre-NEO-survey estimate).
Now, for the actual estimates we’re interested in, that’s what is called the background frequency, aka unconditional expectations of impacts. The source for that goes to Chapman, C. R., and D. Morrison 1994. Impacts on the Earth by asteroids and comets: Assessing the hazard. Nature 367 ,33–40 which, unfortunately, is behind a paywall and I’m too lazy to scour the ’net for an open copy. The basic expectation of frequency of impacts, though, is visible through other sources (see e.g. http://neo.jpl.nasa.gov/risk/doc/palermo.pdf)
To re-express my point in these terms, the survey of near-earth asteroids does not change the background frequency.
http://schillerlab.bio-toolkit.com/media/pdfs/2010/03/16/367033a0.pdf
(One of the nice things about my Xmonad setup is that I have a shortcut which yanks the current copy-paste buffer and searches Google Scholar for it; so the net effort looks like ‘highlight “Impacts on the Earth by asteroids and comets: Assessing the hazard”, hit mod-Y, right-click on PDF link, copy, paste’.)
Thanks.
Interesting shortcuts you have :-)
Background frequency over a few hundred years or more, yes. Is anyone asserting that we should be planning out now exactly how much resources we put into this past anytime beyond the next fifty years or so? And if not, how is that relevant?
Huh? Are you saying that your current impact risk estimates for the next, say, 50 years are significantly lower than the background?
Yes. And we can conclude that because we have detailed understanding of the orbits of the big near earths and can predict their orbits out reliably for a few decades.
That’s not enough to conclude that.
You need the assumption that (geologically) recent impacts that the background frequency reflects came from near-earth asteroids.
Yes, but we’re pretty confident about this also. It is very difficult for an asteroid out past 1.4 au to end up changing orbit enough to run into Earth. It requires a large force. It can happen if it gets just the right luck with collisions or with the right gravitational pull (such as if it happens to past just right near Mars at the right time in its orbit). But these events are rare, and moreover, we can model their likelyhood pretty well. Stochastic aspects of orbital dynamics are decently approximable by a variety of methods (such as Monte Carlo).
I don’t know about that. First, there are comets. Second, large forces are not uncommon with collisions in space. More to the point, any collision (or maybe even a close pass) could change the trajectory of an already-catalogued asteroid to something different and possibly dangerous.
The Chapman & Morrison paper points out that “Because of stochastic variability in the process of asteroid and comet break-up, there is a chance for significant temporal variation in the impact flux”.
I really don’t think we understand the movement of various objects in the Solar System well enough to declare “problem solved”.
Comets != asteroids. Comets have much harder to predict orbits (outgassing and change in mass both make them much trickier to predict). There’s some positive side here in that in order for them to be remotely close to us they generally need to be pretty visible (there’s some worry about comet remnants who are in weird orbits but are no longer highly visible when they are in the inner solar system area).
Yes, and this is a problem, and I mentioned this explicitly in my last comment. The issue is how frequent such events are.
Yes. And what’s your point? No one is saying anything otherwise.
The argument isn’t that the problem is solved. The issue is that the chance of an issue in the short-term is much lower than we would have thought 10 or 20 years ago. That’s not the same as problem solved: the problem won’t be completely solved until we’ve got much better tracking (I’d prefer radio beacons on every object greater than 1 km) and have a system that can deal with sudden threats. But that’s not the issue at hand.
In the grand...grandparent I explicitly defined asteroids as anything large enough and fast enough to make a noticeable impact on Earth precisely to avoid terminology issues like this.
That’s how the whole thing started. If you go to the origin of this long sub-thread you’ll see CarlShulman saying “That’s mostly solved, all the dinosaur-killers have been tracked” and me replying “I don’t think so”.
Yep—that’s what I mean by having an wrong estimate and then correcting it.
Ah, I see. Yes, in that case, using that broad class of objects, we have a much poorer understanding of comets then. The same basic argument goes through (because comets are not nearly as common an object as what is normally called asteroids), but not as by much.
Yeah. I think Carl’s wording here is important. “Mostly solved” is different from “solved”. In this sort of context problems are very rarely solved completely, but more solved in the sense of “we’ve put a lot of effort into this, the most efficient thing to do is to put our next bit of resources into many other existential risks”.
I’m confused here. What exactly are you saying?
With respect to the wrong estimate—there is the “background frequency”, right? Tracking a bunch of near-earth asteroids does not lower it significantly (I am not sure, we may disagree on that). So if 10-20 years ago we thought that the threat of an asteroid impact was much higher, I think I’d call it a wrong estimate.
Risks and dangers here are percieved risks and dangers. In that context, such talk makes sense—obviously percieved risks depend on your current state of knowledge. Maybe god knows whether the bad thing will happen or not—but without a hotline to Him, percieved risks and dangers will remain the best we have.