[killthebuddha]

I'm having a weirdly difficult time finding some basic ELI5 answers.

  - What is the probability that this asteroid will hit us?
  - What is the time interval where that probability applies?
  - Do we have a probability distribution for where it might hit? I don't know anything about anything, but I assume we know what *general direction* it's coming from?
  - Do we have a probability distribution for the potential blast radius?

In general, I am very confused by this news because "400m diameter asteroid 3% chance of impact" is something I would expect literally everybody to be talking about all the time. It's also something where if I learned that everything north of Kansas has a 5% chance of getting hit but everything south of Houston has a 1% chance, I'd seriously consider taking an impromptu vacation.

[Analemma_]

When a new potentially-hazardous asteroid is discovered, it's normal for the probability of impact to go up a couple times before abruptly plummeting to zero, as the radius of uncertainty shrinks until it shrinks past the Earth. See https://en.wikipedia.org/wiki/Torino_scale#/media/File:Apoph....

A 3% chance of impact right after discovery with the initial error ellipse isn't all that unusual; it will almost certainly be revised to 0 with more observation (and if it's not, you'll hear about it).

[apatil]

This makes sense, thanks. However, it doesn't mean that the 3% estimate of the chance of impact was wrong at the time of initial observation given data available, and that still makes it a huge deal at that time. At minimum, it would seem to justify using the best available instruments to characterize the asteroid more precisely as soon as possible.

If these large numbers happen so often that asteroids with initial impact probabilities of 3% are known to actually impact much less frequently than that, then the model is poorly calibrated, no? In other words, the reported probabilities aren't really probabilities and that is what has caused the confusion and anxiety in these comments.

[cfraenkel]

It's not a model that is poorly calibrated - you seem to be taking a software-centric concept far away from where it's useful. The uncertainty at initial observation is because when you first observe an object, you only have observations covering a tiny bit of the orbit, resulting in very wide error bars. The "model" (Newtonian orbital dynamics) is one of the most precise models we have. Doesn't help when the observations are noisy.

[MauranKilom]

Unless one in every 33 asteroids that have 3% impact probability at some point in time actually impacts earth, there is clearly some unwarranted assumption in the error bar/distribution calculation.

"The measurement data has noise" does not explain why the noise has a bias towards "the asteroid will hit earth" whereas reality so far has been biased towards "the asteroid will not hit earth".

(This assumes that significantly more than 33 asteroids have had >= 3% impact probability predicted at some point. The opposite would not be less concerning.)

[gyrovagueGeist]

To simplfy, let's assume you have perfect knowledge of everything else & that the only variable that matters is asteroids current position. By triangulating observations you have a point estimate. Due to calibrating your instruments in the past you know that they tend to have uniform additive noise that is the same in each dimension. Let's say it shifts measurements by up to 1km randomly.

So the best guess you have is that the true asteroid is 99% likely to be somewhere within a 2km box centered at the observation point.

For each possible location in this box you use it as a hypothetical starting point and run a simulation forward creating a trajectory. In 3% of these trajectories the asteroid hits the earth.

The 3% is only a probability over the measurement uncertainty. It represents our knowledge about the system in a bayesian sense. The true asteroid was always ever going to hit the earth or not. There is no uncertainty inherent in the system.

That many asteroids have non negligible probability only means the physics is sensitive to initial conditions or that the measurements are loose. (Both are true)

[MauranKilom]

Given everything you said is true, under those assumptions 3% of those asteroids that we identify as being in said 2km box will hit earth, unless the forward simulation is wrong (implausible) or the measurement error distribution is substantially wrong (also seems unlikely).

What your analysis is not touching on is the prior probability that an asteroid will hit earth (you collapse this to "any asteroid will either hit or not", but that is not helpful for "model calibration" or whatever you want to call this) - or, equivalently, the prior probability of making (a series of) observations with a certain uncertainty/error distribution. If that prior were actually as uniform as each measurement error suggests, I don't see any Bayesian wiggle room left for why we don't have those 3% of impact actually happen.

(I'm no expert, but presumably you need multiple measurements to predict a trajectory, and while their measurement error distributions may be independent, it seems plausible to me that the prior probability of making two specific noise-affected observations, i.e. of the asteroid being on a certain trajectory, is most likely not so uniform. That's the part that I'd like to learn more about though.)

[jryb]

> Unless one in every 33 asteroids that have 3% impact probability at some point in time actually impacts earth

There would be a ~63.4% chance that at least one would hit us if there were 33 such asteroids. To compute this, take 1-(0.97^33). I agree with your broader point though.

[jacquesm]

That's because Earth has gravity, and an asteroid that comes close enough can get deflected onto the planet even if right now it seems to be on a trajectory to miss it entirely. The closer they get and the lower the relative speeds the larger the chance that they will collide and that's not a linear relationship. Beyond a certain boundary impact is certain, then the question is what the time of the impact is and how precise the observations up to that point are in order to figure out where and when exactly it will come down. That won't happen very long before the impact itself happens even if you could say some time in advance roughly in which hemisphere and roughly when. But not precise enough to be very useful.

[fouc]

I wouldn't expect earth gravity to affect it sufficiently enough to cause it to crash unless it was moving very slowly, but I'm not sure asteroids ever move that slowly?

[msandford]

Imagine you see a car 1 mile away as you're preparing to cross the street. 1 sec later, it's a bit closer. You wonder "will this car hit me?". It's hard to say since the car is so far away and your measurements of its speed are so poor.

You wait 5 sec and it's still only imperceptibly closer. You realize there is no way it could possibly hit you. You cross the street unconcerned.

[s1artibartfast]

That makes perfect sense. Where it breaks down is if you put percentages on it. If you say the car is a 3% chance of hitting you, it doesn't and you repeat the process a thousand times, and it never hits you something is wrong with your math

[muzani]

3% seems much higher though. If I crossed the street at 3%, I probably would be dead by now. Cars may not be a great analogy, because they swerve, but it is quite high. Space is pretty damn big too so the odds are really low of being hit by space things. But unlike cars, space stuff tend to swerve towards the larger bodies.

[noduerme]

One explanation would be the Anthropic Principle. In 3% of universes you were killed today, you're just not living in one of those.

[jacquesm]

In 97% of the universes dinosaur descendants rule planet earth. But on this one they got unlucky.

[MauranKilom]

This only works if there is nothing between "no impact" and "you die the same day as the impact". But we know that's not the case.

[credit_guy]

Most likely the estimation is conservative.

In many situations, erring on one side results in worse outcomes than erring on the other side. In our case, a false positive has pretty much zero consequences, while a false negative could wipe out the dinosaurs.

[snewman]

If you have very wide error bars, shouldn't your estimate of impact probability be much lower than 3%? Most trajectories within your error bars will not intersect the Earth.

[jacquesm]

The 3% is likely the median of the probability range. You need more observations (and more accurate ones to narrow it further down, but for a first estimate it will do).

[abathur]

I don't know, but I suspect this is more about the limits of the observations (which I imagine are mostly from terrestrial observatories) needed to obtain much certainty about the object's size, course, speed, density, etc.

[apatil]

In hindsight this comment was too short. Clarifying some points:

By "This makes sense", I meant that this kind of thing can happen; as more data are gathered, the Bayesian probability of a candidate value can increase and then suddenly decrease. Here's a Colab notebook demonstrating the general phenomenon: https://colab.research.google.com/drive/1Eb1_humiGPdKb0c3qr_...

"Calibration" in this context means "statistical consistency between distributional forecasts and observations" in the words of https://sites.stat.washington.edu/raftery/Research/PDF/Gneit... . If the model's early forecasts predict impact with probability >3% for a class of objects that end up impacting with frequency much less than 3%, then the model is not well calibrated with respect to its early forecasts for those objects.

Based on the GP, it sounds like these early impact "probabilities" are no one's subjective (Bayesian) probability of impact because people who are closely familiar with this model know it is not well calibrated. The reported probabilities may still be useful to them as indicators or flags. However, those of us who are _not_ closely familiar with the model have found it confusing to see things that are not really probabilities reported as probabilities.

[aaron695]

> This makes sense, thanks.

It makes no fucking sense.

There is 3% chance it'll be revised to 100% chance and 97% chance it'll be revised to 0% chance.

Are you Yogi Berra?

It'll be reported like possible hurricanes hitting landfall.

If this has happed ~33 times then one will hit us.

If it's 400m it will kill 200,000 people assuming Vox is reporting correctly - https://www.vox.com/future-perfect/2019/7/26/8931776/near-ea...

That will be 9 in 10 it hit's boring ocean and looks cool on satellites and one in 10 kills 2 million people and there will be some cool live streams.

Or the 3% in the title is a lie.

[jryb]

> If this has happed ~33 times then one will hit us.

There would be a ~63.4% chance that at least one would hit us if this happened 33 times. To compute this, take 1-(0.97^33). But I agree with your broader intuition that these predictions must be getting inflated.

[mortehu]

What percent of the times when the initial probability is 3% will it later be revised to zero? Is it more than 97%?

[jacquesm]

To date the percentage that got revised to zero is 100%. And the only time we had a really close call we didn't see it coming.

[Mrkvak]

That's not correct at all: https://newton.spacedys.com/neodys2/NEOScan/index_past_imp.h...

[jacquesm]

Fair enough, but those are all relatively small, and detected only hours before impact, their effect would not be such that anything major would happen on the surface of the planet beyond some broken windows and maybe a sunburn.

Anything that size aimed straight down would most likely not reach the ground but burn up in the atmosphere and any remaining bits would just fall at regular terminal velocity.

But from 10 meters and up things change and the Chelyabinsk meteor is remarkable in that it (1) was large enough to have been detected but wasn't and (2) struck while we were apparently focused on one that was more visible but that ultimately missed us. We were very lucky that it impacted where and at the angle that it did, otherwise the airburst might have happened far closer to the ground or to might have impacted directly over much more populated territory. That would have been very bad news.

It doesn't matter how many 1 through 5 meter objects we can track because we have the atmosphere to protected us from the worst of these if we miss the 20 meter ones (or apparently even much larger) that travel at speeds high enough to give their relatively modest mass tremendous energy and for which the atmosphere does not give sufficient (or even any) protection.

[mabbo]

I think, given the expected impact time windows have passed now, we can update it to 0% (this time).

[Leary]

Found this tweet with an impact probability map that is around northern US/Asia/Europe : https://twitter.com/JoelSercel/status/1691549821629526219

"If it is real this IS the worst asteroid threat ever discovered and the impact location and times are ugly (will post pics shortly). Note that the impact is in the next couple days! However, an Italian colleague of one of our astronomers suspects there is an error in the reported observations and there has been no chatter about this object and no followup. This probably means that it is not real. " - Joel C. Sercel, PhD

I'm guessing the impact death area will be around 3600 km2 (I have no idea, please correct), so ultimately the chance of it falling on me is 1 in 10,000 to 1 in 100,000 provided the asteroid hits. Which means 1 in 1,000,000 to 1 in 10,000,000 in total? (assuming 1% hit probability)

[pdabbadabba]

What should we make of the fact that this tweet has since been deleted?

[DouweM]

With this retraction at https://x.com/joelsercel/status/1691588629079138391:

> I would like to make an apology to the small body community. Some information was shared with me on an internal company message board that I did not have full perspective on and I posted a tweet, which I now understand was not appropriate. It was preliminary data and I did not have full perspective on it.

“Preliminary data” is not very reassuring.

[radicaldreamer]

Pretty concerning update imo ^^

[tophi]

There is a disclaimer from the data source: “Scout data are about unconfirmed objects and all information should therefore be treated as potentially unreliable”

I’m hoping this ztm0038 is an error or hoax.

[ortusdux]

Do NEOscan and CNEOS share data? They have slightly different details:

https://cneos.jpl.nasa.gov/scout/#/object/ZTm0038

[noduerme]

If I'm looking at the time chart correctly, that basically corresponds to "daylight hours", when you're facing the sun, tomorrow within the northern hemisphere...

[radicaldreamer]

We should look for secondary evidence, did the gov start continuity of gov operations etc. If it is real and if nothing can be done about it, we cannot expect official confirmation.

[noduerme]

Well, that map tweet has now been deleted, and the guy posted this:

>> I would like to make an apology to the small body community. Some information was shared with me on an internal company message board that I did not have full perspective on and I posted a tweet, which I now understand was not appropriate. It was preliminary data and I did not have full perspective on it.

Does that sound like everything's gonna be okay?

[onlyrealcuzzo]

If it hits the ocean, how big will the tidal wave be?

[radicaldreamer]

Not that large, I think ocean impact is best case scenario (<1% of kinetic energy transferred).

Land impact would be catastrophic (1x-100x 50 megaton explosion)

[Mrkvak]

Is there any chance of you having a screenshot?

[judging_you]

Someone on twitter managed to grab them: https://twitter.com/s53001/status/1691615280726704235

[1970-01-01]

We need an expert to explain these facts and numbers. Until then, please buy necessary amounts of toilet paper.

[mrb]

I can answer 2 of your questions:

  - What is the probability that this asteroid will hit us?

It's listed in the article: Impact probability 0.034, meaning 3.4% chance of impact.

  - What is the time interval where that probability applies?

It's also listed: impact was estimated to potentially occur between 2023/08/14 04:48 TDB and 2023/08/15 12:22 TDB. (TDB seems to be UTC time without leap seconds? not sure). In other words the asteroid already passed Earth, and is currently no longer a risk.

[Karellen]

TDB: https://en.wikipedia.org/wiki/Barycentric_Dynamical_Time

> [TDB] is a relativistic coordinate time scale, intended for astronomical use as a time standard to take account of time dilation when calculating orbits and astronomical ephemerides of planets, asteroids, comets and interplanetary spacecraft in the Solar System. TDB is now (since 2006) defined as a linear scaling of Barycentric Coordinate Time (TCB). A feature that distinguishes TDB from TCB is that TDB, when observed from the Earth's surface, has a difference from Terrestrial Time (TT) that is about as small as can be practically arranged with consistent definition: the differences are mainly periodic, and overall will remain at less than 2 milliseconds for several millennia.

https://en.wikipedia.org/wiki/Terrestrial_Time

> TT is distinct from the time scale often used as a basis for civil purposes, Coordinated Universal Time (UTC). TT is indirectly the basis of UTC, via International Atomic Time (TAI). Because of the historical difference between TAI and [Ephemeris Time] ET when TT was introduced, TT is approximately 32.184s ahead of TAI.

Phew!

[mabbo]

It seems pedantic, but I'll bet the differences matter when you're talking about objects that have relative speeds in the tens of km per second.

2ms at 10km/s is off by 20m.

[onlyrealcuzzo]

Probability without confidence interval is not very useful.

If there's <1% confidence - this is not exactly news.

[Enginerrrd]

The probability takes into account the error on the measurements so I'm not sure what you're getting at

[hollerith]

I don't know what you mean.

[lijok]

Spotted 3 days ago. There's an incentive not to publicize this. Lots of close passes only get coverage after the fact.

[apatil]

Also, is it possible to quantify the risk of unseen companion asteroids, which someone else mentioned in the comments.

[ortusdux]

I found this breakdown of JPL's CNEOS data to be every helpful. The close approach time chart shows % chance of impact vs day. From this I take it that we just didn't have enough info at the time of publishing.

[vkou]

Uncertainty in measurements make it almost impossible to tell where an object like this is going to impact, until we are hours, if not minutes away from impact.