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JoshuaZ comments on Where I've Changed My Mind on My Approach to Speculative Causes - Less Wrong Discussion
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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?