Asteroid ZTm0038 with a >3% impact probability | Hacker News Reader

Asteroid ZTm0038 with a >3% impact probability

newton.spacedys.com

211 pointsisbn230 comments

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.

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).

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.

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.

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.)

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)

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.)

I think some confusion here seems to come from the following interpretations:

-Then what does 3% mean? Surely it means "given the data we have, one in every 33 will hit" -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.

Both of these statements are false. The probability density is over our knowledge of the state variables/state space for this asteroid, not over asteroids. The hypothetical sample of asteroids is not drawn from the distribution I'm talking about.

Going back to the simplified example: With the uniform prior on the box, our probability means that 3% of the volume of this box would lead to an impact if an asteroid was centered at a point in that volume at this time of measurement.

It doesn't say anything about hypothetical realizations of this asteroid (it is not clear what this would be sampled from or what it means in a precise sense to repeat a 1 time event) and says even less about the sample of (nearly) independent asteroids observed in the past. The probability measure only describes the measurement uncertainty on properties of this particular asteroid. It is not conditioned on or related to statistics on impacts of "general asteroids".

But "presumably you need multiple measurements to predict a trajectory" and your notes about independence and uniformness being bad assumptions are absolutely correct tho. I agree 100%

My comment above is mostly an attempt to make a simple example to clarify what the probability measure being measured here is. It's not a physically realistic example :) and definitely doesn't make good assumptions about what information is needed and what error distributions that information would have! I don't do space and didn't want to make guesses

Calibration here would have to be over multiple measurements of the same asteroid (which my example doesn't touch on). Likely by predicting trajectories at different intervals and matching the likelihood of later observations.

Verifying multiple observations leading up to a 1 time event is a very different than, say, verification of simulations of an internal combustion engine design where measurements of a real world prototype can be conducted repeatedly and independently to learn/calibrate some fundamental properties or initial conditions like chemical kinetic coefficients and such.

For general interest/lectures/fun, the general field that studies how to push uncertainties forwards/backwards/calibrate a mathematical model and simulation is called "Uncertainty Quantification". Also not an expert lol, I was just surrounded by a bunch in my cohort

> 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.

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.

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?

We're not talking about the asteroid stopping with a screech of tires and then taking a hard left turn to crash into earth.

It's just that anything traveling through the earth/moon gravitational sphere of influence will have it's trajectory tweaked just a bit. How close to the center of gravity the pass is determines exactly how much of a tweak. There is a small section of space, we'll call it the keyhole, where if the asteroid happens to pass exactly through that area the tweak will result in a collision next time the asteroid comes around. That could be decades hence.

There could even be a case where an unlucky keyhole pass this time lines up another unlucky keyhole pass the next time to an eventual collision in the distance future.

The technology to nudge the asteroid just far enough to miss a critical keyhole pass is within the realm of possibility with today's knowhow. We just need to have these missions ready to go on short (order of a few months to a year) notice.

We see big ones with a few days to hours of notice, sometimes we see them when they hit.

Most likely: this will never come up.

Less likely: if it does we're fucked.

Even less likely: if it does and surprisingly we see it in time we will act for the good of all and not bicker about who pays and we'll make things better rather than worse. If not, see above.

Like the moon is moving slowly? About 1 km/second for an object that weighs ~10^18 tons at a distance of 300,000 km?

Now think of what that kind of force would do on a much lighter object that moves faster.

I'm not sure I understand your point... The object mass does not impact its trajectory (unless it either touches our atmosphere or is so massive as to measurably change earth's orbit). The gravitational force earth exerts on the moon and some asteroid is also very different, because the force is proportional to both object masses.

Think 'gravitational slingshot' but without missing the planet. The object will change direction and accelerate into us. It could end up grazing the atmosphere or it could go from grazing the atmosphere or even non-impact to impact.

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.

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

I wonder if it's the difference between "this asteroid" and "all asteroids". As we learn more about it, we can start to treat it like a process that has repeated, but initially we can't be sure if it's like other asteroids.

Consider a 6-sided dice roll. What is the chance it will roll a 1?

A person might think, "1 in 6". But what if this is a loaded die? In that case, we need more information before we can classify it as "a die like other dice". We can observe two rolls, and try to ascertain whether or not it is like other fair 6-sided dice; however, two rolls is not enough to be sure.

So as we're gathering data, we start to classify this instance of a thing (a die, an asteroid) as part of a series of things we already know about. The more rolls we observe, the more sure we can be that this is a fair die or a loaded die, for instance.

If I'm understanding how asteroids' trajectories are calculated, we can simulate THIS asteroid's trajectory (3% chance of hitting you, based on a little data), or we can just decide to classify it (perhaps prematurely?) in the series "an asteroid like every other asteroid that we've observed" and arrive at a 0.000001% chance of hitting you (I'm making up a number here).

I think you're right. The 3% number must be ignoring repeat sampling bias. This is basically the same issue as P hacking or false positives and medical testing.

You have one confidence margin for a single single measurement and a different confidence margin if you make 1 million measurements.

Let's say you can measure marble diameters and your tool has a calibrated standard deviation of 1 mm.

If you pull one marble and measure it to be 10 mm larger than expected, you can calculate the chance you are wrong using only the standard deviation of your measurement tool.

However, if you pull 1 million marbles and measure one to be 10 mm larger than expected, you need to take into account the number of marbles you have measured.

The uncertainty is epistemic not aleatoric. The percentage represents our knowledge about the system at the time of measurement propagated through the forward model and is not an inherit random process in the system/model itself.

If your model is consistently wrong in a statistically predictable way, either your measurement or model is inaccurate.

A 3% chance that never occurs is an inaccurate prediction.

Right! Yes absolutely!

It's wrong because the measurements are suggestive of possibility, rather than certain of it.

If we observe an asteroid that with two poor measurements is determined to be headed away from Earth, that's the end. Look no further.

If we observe an asteroid with two poor measurements that has some significant chance of hitting, more and better measurements are made. Then very often those better measurements show it was never actually going to hit anyhow.

But we never would have known without the better measurements, and we never would have devoted more time to making better measurements without a reason to do so.

A 3% chance that never occurs is because that 3% is based on data that's at the limit of what the telescopes can provide, not based upon bad math.

Then what does 3% mean? Surely it means "given the data we have, one in every 33 will hit". Since that empirically doesn't happen, it must be that "the data we have" has a very low prior probability of being real. In other words, the measurement noise seems distributed in a way that over-represents unlikely trajectories.

Hence it seems that it would lead to more accurate predictions if the measurements and their uncertainties were fitted to a model that corrects for the prior probability of observing an asteroid on a given trajectory/making a certain observation.

This discrepancy between distribution of measurement error vs distribution of actual trajectories is what people are wondering about, because it seems interesting to know more about (e.g. "why are certain trajectories less likely?").

Despite all the people coming out of the Woodworks with weird theories, my best one is that the 3% number doesn't take into account their entire measurement process and sampling.

It's is similar to P hacking.

I don't think you understand how this works at all. You might read up on this here if you want to learn more. https://astronomy.stackexchange.com/questions/8450/how-is-th...

If you just want to argue with people, feel free. But based on how this conversation has been going it doesn't seem like you want to learn.

Setting your condescension aside, I browsed the thread.

I understand that calculating trajectories is difficult.

If someone claims something like a 3% impact probability, and they are wrong 99.999% of the time, that speaks to a methodological error in how the numbers are conveyed and or defined.

I work in medical devices and testing. I perform tests like X percentage of patients will die based on the statistical calculations. You may undergo treatment with a medical device that I have worked on.

Calculating trajectories is easy. Getting good data points is hard. Two pictures using a telescope on back to back nights is probably the smallest reasonable sample one could get. Take another picture the 3rd night and you've just doubled the size of the arc.

Wait a week and get another sample and your arc is now approx 5x as long. Wait a month and get another and now your arc is 30x as long as the original. More observations shrink your error bars.

There are systemic errors here for sure. Two kinds, really:

1. Limits of resolution of telescopes 2. Short sample lengths

You absolutely can't do anything about error type 1. You can fix 2 by getting more data. But there's no point in getting data on asteroids that have absolutely no possibility of hitting. So only asteroids that have some probability with limited measurements get enough better measurements that are high quality in order to find out where they're really headed.

All of these measurements of trajectories are completely uncorrelated, so you can't use the priors to adjust probabilities. I mean you can do whatever you want, but we haven't been hit by a big asteroid yet since we've had telescopes and tracking databases.

If we made adjustments based on priors we'd have to discount all collisions down to 0 irrespective of the trajectories. Seems absurd, so there must be something else going on here.

This is a statistics problem, not a measurement problem. The problem is that there are different well understood formulas that must be applied depending on if a measurement is taken of a single sample in isolation, or if it is one measurement of many.

Illustrate the point, imagine a pass/fail AIDs test with 99% accuracy and 1% false positive. If you test one patient only and they are positive, You can conclude that is 99% likely to be correct. However, if you test a hundred different people and one of them comes up positive, you can no longer claim the 99% certainty for that patient. You know that you administered a hundred different tests to different people and would have to reduce your confidence accordingly because you expect one false positive. This second statistical approach is what is not happening with the asteroids, and why asteroids with a 3% chance of hitting Earth suspiciously get revised down to zero more than 97% of the time.

>If we made adjustments based on priors we'd have to discount all collisions down to 0 irrespective of the trajectories. Seems absurd, so there must be something else going on here

Not quite true. If you measure a million asteroids in the data from one says it has a trajectory towards Earth, you need to Discount that observation by the fact that you made 1 million different measurements. The outlier might still be close to zero statistically, but it did have a outlier data. This would be a reason to remeasure the asteroid multiple times. It is only through that process that the number will climb from zero, or stay at zero.

It's not that you're applying the prior that we have never observed Earth colliding asteroid. You're simply accounting for the fact that with the error bars on your measurement system, you expect one false positive in 1 million measurements.

My inference is that the 3% number we are talking about for this specific asteroid what's not calculated using the proper statistical treatment, and that's why it wasn't published in the first place.

This is also why it is similar to P hacking. If you run 20 experiments and analyze them as if they were the only experiment you did, you will get one of them that says a wrong result with 95% confidence, which is the common threshold for publishing outside of physics.

That conclusion may be too early to reach with confidence, based on the limited data!

You're standing in a four-lane road and see a car approaching. You're looking at an angle and the lanes are poorly marked, so you can't tell which one it's in. Your observation lets you estimate the chance you need to move at 25%.

When it gets a little closer, you can tell at least which half it's on, the left or the right. Now your estimate is either 0% or 50%.

Closer still and you tell which lane it's in, so now you're sure.

again, that makes perfect sense.

What wouldn't make sense is if you repeat this 1000 times and a car is never in your lane.

That means that something is wrong about how you are modeling the road and cars.

The claim that people are confused by is (asteroids with a 3% chance of hitting get the change revised to 0% more than 97% of the time).

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.

> But unlike cars, space stuff tend to swerve towards the larger bodies.

That's exactly it. And at the speeds these objects are going and the uncertainty of the observations you would have to be observing an object for a really long time to get the kind of accuracy required to pick a mitigation method that would work. And even then, assuming you could nail the point of impact of something going 2000 km / second of unknown mass in a strong gravity field: given the COVID response I have a hard time believing that the response to 'Houston, Texas is going to be obliterated on Jun 1st 2024' would be met with anything but skepticism and laughter. Right up to the moment of impact.

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

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

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.

Why "same day" and not same week or month? If it's not the same instant, then you're hypothesizing some kind of back-propagation (where alternate futures in which you die influence the likelihood of current events)[0]. Under that hypothesis, it would only matter whether some event would cause you to die sooner than you otherwise would.

[0] https://arxiv.org/abs/0707.1919

[edit] FWIW, I actually corresponded with one of the authors of this paper back in 2007, and from what I could tell, this wasn't an attempt at parody, although now it might be dismissed as one. Personally I'm not willing to declare my (non)commitment to the theory either way.

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.

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.

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).

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.

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.

> 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.

> 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.

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

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.

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

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.

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

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)

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

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.

Pretty concerning update imo ^^

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.

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

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

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...

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.

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?

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

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)

Is there any chance of you having a screenshot?

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

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

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.

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!

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.

Probability without confidence interval is not very useful.

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

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

I don't know what you mean.

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

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

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.

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.

Here's a visualization of its orbit, Earth in green, object in teal: http://orbitsimulator.com/gravitySimulatorCloud/yr/gsim2023....

It passed about 9 hours ago.

VSauce with packages summary for passive consumption. https://www.youtube.com/watch?v=4Wrc4fHSCpw

And jacquesm's replies to my other comments, https://news.ycombinator.com/item?id=37139144

FYI VSauce is a different content creator than Veratasium (unaffiliated).

Oops, thoughtless basic error in posting something quickly. Good note.

> Since 1988 over 1,200 asteroids bigger than a meter have collided with the Earth. And of those we detected only 5 before they hit, never with more than a day of warning.

Yikes

It was first seen on August 12, midday UT1. About a dozen observations.

   ZTm0038\* C2023 08 12.49542 06 32 33.23 +15 57 35.5          18.86rUNEOCPI41

Is the first entry, and asterisk-marked, on https://cneos.jpl.nasa.gov/scout/#/object/ZTm0038 at the bottom, in the Observations section. Unusual (new to me) format. Appears to me, first guess to read:

   [object] [yr] [m] [day.time-decimal] [position] [elevation-angle]   [observatory]

Likely oriented relative to the plane of the ecliptic, and absolute direction relative to Earth at time of observation.

I don't know what I'm talking about. Just giving best-guess interpretations.

> Closest approach time [...] t_max = 2023/08/15 12:22 TDB [...] Run started at 2023-08-14 23:51 UTC and ended at 2023-08-15 00:06 UTC

So this computation result became known after the closest approach would happen at the latest.

Looking at the asteroid I saw hitting us, 2023CX1 (known as Sar2667 before being designated), it says:

> Closest approach [...] t_max = 2023/02/13 03:22 TDB [...] Run started at 2023-02-13 11:47 UTC and ended at 2023-02-13 11:53 UTC

Again, hours after the impact actually occurred. And that's from the 7-observations page, there are 3 more pages with 28, 76, and 125 observations, all listed here: https://newton.spacedys.com/neodys2/NEOScan/index_past_imp.h...

The next one on the past impactors page has a 3 day delay between when it hit us and when its orbit was computed

What's the point of this?

Can you elaborate on the question you're asking?

Put differently: am I missing something if I say this website is similar in function to https://hasthelargehadroncolliderdestroyedtheworldyet.com ?

At least in terms of a public service announcement; I understand it's interesting for science but not so much for linking "hey look there's an X% chance of impact....yesterday!" on news websites

When I was taking physics at the University 20 years ago, one of my professors said that the closest we have ever come to predicting an asteroid impact was "whoa that was close" as it zipped by.

Sounds like that's still state of the art.

I find it very interesting even if the approach was yesterday, as the chances were seemingly quite high and asteroid was quite big. Also, Hacker News isn't exactly only news, just any interesting things, current or past.

I'm aware that it already passed its closest approach, but what does >3% even mean? 100% is >3%, but 3.000001% is >3% too. Was an impact certain at some point and the probability degraded to a bit above 3% over the course of the asteroid's trajectory? If so, I think I'd like to have a heads up when the probability is still closer to 100%, before it drops?

It was 3.4% https://newton.spacedys.com/neodys2/NEOScan/risk_page/ZTm003... If you're not logged in to Twitter, I don't think you can scroll from the linked tweet up to see the one this page was linked from, so I put it here.

I think the link works just fine and I can see it's at 3.4% right now, but was mostly wondering why it's written as >3% in the title. Most likely it's meant as ~3% like other people suggested and I shouldn't be reading to much in it :) Thank you for taking the effort to write your comment.

From the page: Impact probability 0.034

I guess OP decided to abbreviate to >3% instead of writing 3.4%

Personally I would’ve written ~3%

The linked page indicates:

Impact probability 0.034

Edit: Not familiar with the site, but I get the sense that this probability reflects the latest run. The probability hopefully gets more accurate as observations rise?

I couldn't readily figure out how to see the probability at each of the 8 observations for this one (perhaps this is the first run it's included in--all 8 observations predate this run?), but the page for actual impactors (https://newton.spacedys.com/neodys2/NEOScan/index_past_imp.h...) at least implies that accuracy may improves with each (and then maybe flip to 0/100?)

Think of it as the most sensational number you can write without lying. So it's between 3% and 4%.

I think it specifies that it’s a 3.4% probability in the page.

So it was just an odd choice of title by the poster.

Is there an english / eli5 if you like, summary of what this means?

I gather that today or yesterday an asteroid ~400m along it's biggest dimension ultimately came within x km of earth, that was close enough there was a nontrivial chance it would hit us? And this is big enough to make a 30 mile crater? And we only found out it was coming a couple days ago? Sounds like a pretty big deal if that's accurate.

It's accurate and it could have been a pretty big deal but for now it isn't, unless there are very low magnitude companions to it that hit unexpectedly because they have not been observed at all. There is some precedent for that sort of thing.

> ~400m along it's biggest dimension

Where did you find this information? I don't understand all the fields on the page, but don't see any number being 400 within an order of magnitude

Everything I wrote was from skimming the comments here. This is the 400m: https://news.ycombinator.com/item?id=37138510

I was hoping someone more familiar with this stuff could write a definitive summary.

400m in one dimension leaves a lot of room for variation. 400m x 1m x 1m is a lot less concerning than a 400m diameter sphere.

400x1x1 would be a pretty nasty spear that must have been made by someone.

It’s the Spear of Longinus

That would sound like a kinetic bombardment weapon and that we were about to get rodded. My level of concern would be sky high!

Depending on how it is aimed... straight in that would be messy.

That would be an interesting analysis to see, my guess is it wouldn't be that different than if it was a sphere of equivalent mass, and vs a 400m sphere would be 400/(4*pi*200^3/3) = .0012% the volume (and so kinetic energy), and it would not penetrate in a way that does specifically worse damage. Just guessing though.

Edit: I confused radius and diameter in my calculation and revised it. I may have made other mistakes

That would highly depend on what the composition of the asteroid was and where it impacted. On a fault line? In the middle of an ocean, or a city?

For sure the depth of penetration would be different for a rod shaped object of equivalent mass compared to a spheroid, the latter would penetrate much less deep. Angle of incidence would be a factor as well as (obviously) the speed of the impacting body. But if it impacted at a higher speed than that the ejecta could get out of the way it would cause an absolutely massive crater. As though you'd exploded an absolutely enormous nuclear bomb deep underground, but not so deep that the explosion would still reach the surface. That would probably be the worst case scenario for an impact like this.

Without fins, it's unlikely to go straight in. It would start tumbling, and probably break up and burn up. A sphere would more likely get to the surface intact.

If it has the same density than earth, it wont pierce deeper than its length. If it is made of uranium a bit over twice that (assuming earth is made of iron).

I wonder if there have been any simulations of such an impact and what the worst case would look like for equivalent mass but different shapes and asteroid compositions.

I wonder what the distribution is like; the most extreme length to height ratios we've seen for example. Not sure I've ever heard of something weirder than on the order of 2:1

'Oumuamua was closer to 8:1 https://en.wikipedia.org/wiki/%CA%BBOumuamua

That's just measure of uncertainty in out measurements, unless we're saying there was a ~3% chance someone would do an emergency asteroid redirection to make it hit.

I don't really know how to read this page, but it looks like the time of closest approach has already passed -

> Closest approach time of impactors: t_min = 2023/08/14 04:48 TDB and t_max = 2023/08/15 12:22 TDB

If so then we're all good. Can the poster or anyone else chime in with background on this?

I saw this tweet: https://twitter.com/ByronDrury/status/1691487518590402560?s=...

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

It has some cool graphs that seem to show a lot of uncertainty in closest approach time

[edited to fix second link]

Well, that NASA page has the closest approach going into next week and a ~10% of impact, so... hopefully the big disclaimer at the top is relevant.

Are you sure? I chose another asteroid by random in the selector and it looked even worse.

I am not an expert, but my interpretation was the same as yours.

TBD seems to be roughly UTC time. In which case, this passed by 8 hours ago.

I plugged this into neal.fun/asteroid-launcher.

I'm not sure if I did it right (I used the 15.84 km/s as impact speed, picked 45 deg impact, and an iron asteroid). It's certainly serious - millions dead - especially if it hits Manhattan as neal.fun seems to invite you doing, but not world-ending.

I used the Earth Impact Effects Program calc w/ 90 deg impact, rock asteroid, and 15km/s.

https://impact.ese.ic.ac.uk/cgi-bin/crater.cgi?dist=100&diam...

4 mile crater, 2300 MT of TNT equiv. so about 45 Tsar Bombas.

Where did you find the mass/size?

I had to guess without albedo data, but an H of 19.2 puts it in the 0.4 to 0.5 km range, so 400m could be on the conservative side. If it randomly has an albedo of 0.05, it could be as large as 0.96km

https://cneos.jpl.nasa.gov/tools/ast_size_est.html

Impact time is in the past. Mission accomplished, everyone!

Am I reading this data correctly?:

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

Because it looks like today is the most probable day of impact at 10%, but there is a good 25%+ chance it will impact sometime in the next two weeks.

I would also very much like to know what the consequences of an impact might be

This paper is an interesting read:

https://www.sciencedirect.com/science/article/pii/S009457651...

Not my area of expertise, but it sounds like a deep ocean impact would not be completely catastrophic.

With only ∼1% of the asteroid kinetic energy being converted into tsunami waves and with the stronger decay with distance implies that moderate size asteroids (100–500 m in diameter) striking the deep ocean basins off the continental shelves are not a significant overall hazard...

So like a 50% chance it wouldn't be that bad. Larger asteroids would vaporize enough ocean water to cause long lasting atmospheric affects.

Otherwise, a land impact would be somewhere in the 1-100 Tsar Bomba range energy wise.

If it hit land anywhere other than Siberia or the Sahara, it would be pretty bad!

Even that could be pretty bad if it turns out the object has lower Albedo than estimated. The difference between 500 meters and 1000 meters isn't all that big in terms of astronomical observations of a fast moving object and the composition is also quite uncertain and could add another factor of three or so to the potential impact. It could be very bad everywhere.

How bad is the question

Can’t find any information on this anywhere else.

0.10% and 0.25%?

If you're looking at the "closest approach" distribution, that's not just for impacts, and the numbers are probability masses for the closest approach occurring on that day as opposed to some other day (not the probability of impact on that day).

I did my part - go team

I must have put a decimal point in the wrong place or something. I always do that. I always mess up some mundane detail.

Here are some more numbers:

https://www.projectpluto.com/neocp2/mpecs/ZTm0038.htm

In particular the MOID (Minimum Orbit Intersection Distance, the minimum distance between the orbit of the object and the Earth, in AU) is 0.0199 AU which is still 3 million km

From other comments in this thread, it seems this risk has passed, but was known three days ago?

Was any action taken at all? A 3% risk isn't small.

Is there any action that could have been taken? Genuinely curious, I read about a test of some technology for deflecting asteroids a few weeks ago on HN, but I don't think we really have those capabilities yet?

There is likely a capability in the classified world to launch a vehicle to hit this object, but it won't be used because disclosing this ability would spark an arms race.

“Hit” and “do something useful” would seem to be distinctly different questions. A rock that you discover just before it hits us can’t be easily deflected, and even if you destroy it all of the mass is still headed our way.

I don't know what action could be taken. The risk of mass anxiety and the behavior that comes with that is probably greater than any additional safety a vague 3 day warning would provide.

I would want to know, especially if it’s a large object and higher percentage probability, because then it’s a way to tell loved ones what we need to tell them and go forward together in love. We should always strive to do this, of course, but it would still be better to have the opportunity to do it.

Oddly, I cannot find where on this page it says anything about the size of the asteroid in question. It's probably there somewhere, but I'm not seeing it somehow.

H=19.9

~400m diameter

https://cneos.jpl.nasa.gov/tools/ast_size_est.html

Where did you get its albedo (reflectivity)?

That's in the submission, there's a column called H in the first table that says 19.90

The chart is H vs. Albedo, and is awkwardly formatted. H is the brightness observed on earth. Albedo is the inherent reflectivity of the object. What the chart is saying is that if we observe an object that is 19.5 H bright, it could either be highly reflective (0.30 albedo) and 310m across, very dim (0.05 albedo) and 760m across, or somewhere in between.

As I understand it, we can only calculate the albedo if we know the size and apparent brightness.

I'll note that the linked NEOscan page has an H of 19.9, while JPL's CNEOS has an H of 19.2.

True. At least it gives us a range this way unless it's a block of ice or whatever else. I still remember some of this from rendering days.

H is basically the same as M, an absolute magnitude, but for solar system objects. Difference being that for solar system objects H, as you've noted, is an absolute magnitude for an object 1 AU from Earth and 1 AU from Sun in a triangle where phase angle is zero meaning it's a straight line instead of a triangle and M is an absolute magnitude for an object (usually self illuminating) outside of solar system set at 10 parsecs from Earth. A star with M = 1 and a solar system object with H = 1 would roughly be the same brightness. It's a log scale, lower the number the brighter it is, so if you put that star at the same spot as the object it'd be a difference of about 26 where each unit is roughly 2.5 times brighter. What's interesting is that at zero it used to (roughly) be Vega (star). Both M and H are band-dependent, I guess it's V-band (visual or green-ish) if it's not mentioned. If you have B (blue) and V band, the difference between the two tells you a (visual) color which can be also interpreted as temperature, the lower the number the bluer it is and higher means red.

I'm all out of trivia for the night. I'm sure there are (amateur and not) astronomers and astrophotographers here that know this stuff way more about. All I knew is that without albedo you couldn't get to the size with absolute magnitude alone, we could if we had both absolute and apparent. What I didn't know if that apparently there's a rough table of correlation between H and albedo values probably based on most rocks we've seen in space around us.

That's what I get for reading HN before going to sleep...

If you wake up: no problem. If you don't wake up: no problem.

> If you wake up: no problem

Depends on what you wake up from: sirens and broken windows everywhere but not a direct hit, for instance, I wouldn't call 'no problem' :(

Fair enough, that would definitely be a possibility. The dashcam and other videos from Siberia are still very fresh in my mind. That was a very close call. Slightly different angle in a more populous part of the world...

Looks pretty bad in this asteroid impact simulator: https://neal.fun/asteroid-launcher/

How accurate is this? I just feel implicitly that a 1mi asteroid would leave a creater bigger than 30mi wide upon direct max impact ;)

I also wouldn't expect an impact in rural Kansas to create a 2,500 foot tall tsunami.

If you scroll you'll see that the fireball is much larger than the crater.

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

Which following the contained link to Terrestrial Time (TT) states that TBD is relativity corrected TT, which itself is about 30 +/- seconds from UTC -- something much more familiar to we groundhogs...

How long ago was it spotted. That's what I'm curious about, did we have any lead time?

3 days ago, it was first seen. About a dozen observations. F***, it's not fun that these pass with so little forewarning.

   ZTm0038* C2023 08 12.49542 06 32 33.23 +15 57 35.5          18.86rUNEOCPI41

Is the first entry, and asterisk-marked, on https://cneos.jpl.nasa.gov/scout/#/object/ZTm0038 at the bottom, in the Observations section.

Unusual (new to me) format. Appears to me, first guess to read:

   [object] [yr] [m] [day.time-decimal] [position] [elevation-angle]   [observatory]

Likely oriented relative to the plane of the ecliptic, and absolute direction relative to Earth at time of observation.

Not to spoil your lunch or anything:

https://www.youtube.com/watch?v=bU1QPtOZQZU

Of course 500 Km is a big difference with 400 meters but to those near the point of impact it wouldn't matter and the global effects would still be beyond anything in our history.

This is one interpretation of what this could look like:

https://youtu.be/ZyyrfB8s5cY?t=152

The difference between the 500km asteroid impact in the video, and a 400m asteroid impact is the difference between a man jumping, and a man jumping to the moon.

Yes, it is the difference between a sterilization level event and one that is probably survivable. But it would still be the most significant impact in all of recorded history and to get to a larger one you'd have to go back a long, long time.

Even a 400m land hit sounds pretty survivable for whole humanity, significant climate impact is expected beyond the 1km range?

Yes, it would be survivable for humanity. But depending on where it would happen, how dense the impactor is and at what speed it is traveling the effects would nevertheless be quite serious. Imagine NYC or SF taken out like this (of course the chances of that happening are really small given their area relative to the total of the planetary surface):

https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/201...

Here's a visualization of its orbit, Earth in green, object in teal: http://orbitsimulator.com/gravitySimulatorCloud/yr/gsim2023....

According to https://www.projectpluto.com/neocp2/mpecs/ZTm0038.htm#elemen... the observations were taken a few days ago, August 12-13. I don't know when someone calculated the future trajectory and impact probability though.

Let's hope there are no 'dark companions' like with the Chelyabinsk meteor.

Looks like it is about the same size as 25143 Itokawa that the Hayabusa probe visited. That has a mass of 35 billion kg. The Burj Khalifa is 0.45 billion kg so that is about 80 Burj Khalifa's.

Yes, but how many ant-sized swimming pools deep is it?

did this pass ~4000km from earth?

[1] MOID given as 0.0000296

https://newton.spacedys.com/neodys2/NEOScan/risk_page/ZTm003...

[2] AU to km https://www.google.com/search?q=au+to+kilomore&rlz=1C1GCEA_e...

Something weird is going on. This site [1] suggests the MOID was about 0.02 AU.

[1] https://www.projectpluto.com/neocp2/mpecs/ZTm0038.htm

Other source[1] says 0.0002 AU, still very close (about 40,000 km).

[1]: https://cneos.jpl.nasa.gov/scout/object/ZTm0038

This, as I understand it, being the closest approach to our orbit, not our atmosphere or so. To put this in perspective, the Earth is some 6.4 Mm in radius, so that's 40-6.4 = 33.6 Mm above the surface.

MOID being minimum orbit intersection distance, for anyone else wondering. Sounded like someID (identifier) to me, was wondering why that value was given in AU

MOID is an id that is just that big. You wanted more id, you got MOID.

Is it possible to estimate where the astroid is most likely to hit? I'm not sure how to interpret most of the info on that page.

Pretty graphs of possible impact sites:

https://twitter.com/JoelSercel/status/1691549821629526219

That tweet which had an image showing bands of latitude that it may hit and times (london/newyork ish tomorrow and day after) has been deleted by the author and now says "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."

I don't feel very reassured.

there seems to be a second entity on the risk list (but less than 3 observations) https://newton.spacedys.com/neodys2/NEOScan/index_risk.html

Also both these entities are "lost" but not "high priority" https://newton.spacedys.com/neodys2/NEOScan/index_nspl.html

Is this the highest probability assigned to such an object?

That depends on the timescale. Over a long enough time it is 100%, for a human lifetime maybe one in a million or so.

Well, no, because some have impacted Earth before. But if we limit ourselves to large bodies, yes. The current record holder is 99942 Apophis, which briefly had an estimated probability of 2.7% of an impact in 2029 (which has since been eliminated as a possibility). Apophis and ZTm0038 are around the same size.

If ZTm0038 does indeed have a 3% chance of impact, it would, like Apophis, land as a 4 on the Torino scale, tying the record for the most threatening asteroid in history. Unlike Apophis, however, the impact has a lead time of hours, not decades. Efforts might have been made to deflect Apophis; no such effort could realistically be made for ZTm0038.

It's worth noting that NASA's Scout page (https://cneos.jpl.nasa.gov/scout/#/object/ZTm0038) for ZTm0038 has a much higher probability of impact: of their 1000 sampled orbital solutions, 160 of them impact Earth. A 16% chance of impact for a 400m asteroid would put ZTm0038 on the border of a Torino 5, and would make it by far the most threatening asteroid ever discovered.

That said, note that the prior here has to be that an impact of such a size is very unlikely. Such impacts are exceedingly rare. The Tunguska event, for example, was an asteroid 1/8 the diameter (so 1/8^3 = 512 times smaller in volume) as ZTm0038, and it was by far the largest impact in recorded history. Since impacts roughly follow a power law, the much larger putative impact of ZTm0038 would be proportionately rarer, the sort of thing you'd see every hundred thousand years or so.

This is why numbers should always be bounded. Is this 3.3-3.5%?

Or 0-3.4%?

Or 0-6.8%?

Etc

Do we know how close it was?

According to a site [1] pointed to by another post in this page, the estimate for "Minimum Orbit Insertion Distance" was about 0.02 AU or about 3 million km (in round numbers).

Luna's distance is roughly 384,000 km -- so call it 8 lunar orbital distances...

(edited to add the link) [1] https://www.projectpluto.com/neocp2/mpecs/ZTm0038.htm

> Closest approach time of impactors: t_min = 2023/08/14 04:48 TDB and t_max = 2023/08/15 12:22 TDB

If you can read this that means it hasn't hit you.

Not threatening, mostly harmless.

At 400m diameter we should expect an impact energy of ~3Gt, which would be one of the most energetic events in human history, if not the most.

For a comparison, the Lake Toba eruption (which is suspected of causing homo sapiens to almost go extinct 76kya) is estimated to have been in the 1-2Gt range.

400m diameter is certainly pretty bad if hit.

And why is there no articles about it on Wikipedia yet?

How do you tell?

Only 400m in diameter.

... The Chelyabinsk meteor had ~20m of diameter.

The impact energy of a 400m diameter asteroid is somewhere in the same ballpark as the deployed nuclear arsenal of the United States.

Not extinction-level impact, but wherever it hit would definitely feel it.

Yes some place will feel it somehow, 70% chance it strikes an ocean and creates a tsunami.

But the world will go on, people will shrug their shoulders and go on with life.

If it was a 60 mile wide asteroid, now that would truly be the end.

This would wipe out an area equivalent to a mid sized country and would possibly have global effects. The threshold for that is somewhere between 250 meters and a kilometer but we fortunately do not have any data to make that much more precise. You definitely wouldn't shrug your shoulders and get on with life, it would be the most important event in your life assuming it didn't end. Depending on where it happened the economic impact of such an incident would destabilize the world economy for decades.

How much of that would burn up in the atmosphere if it did hit?

https://www.quora.com/Would-an-asteroid-measuring-400-metres...

I googled your question :)

Quora is pretty much unreadable now. Is the main answer just chat gpt? That's how it seems from mobile view. The page is too noisy to navigate for me.

Not sure why this happened but it went viral in India as a place to practice english for internet points or an online place to for Indians to do endless chit-chat, which is a lot more fun in person there than it is online!

Quora has been trash for awhile now.

Yep, they've added a ChatGPT answer on top of all the human answers. Although in this case the human answer isn't really better, but it's the fault of the person who asked the question for being vague about the kind of speed they meant.

How could Quora suck worse? Rub a little AI on it.

The more I think about it, this might be a good thing for the world, and amazingly stupid on the part of Quora. Since the whole Quora shtick is to heavily gamify your experience on the site, perhaps knowing there's always going to be an ML 'summary' above your incredibly erudite reply (/s in case it wasn't obvious) will make most people realize "hey...this place actually does suck" and maybe move on to something better...a life or something.

It's very dumb for Quora. The original selling point was that they had a lot of experts/celebrities answering. Now that the userbase has degraded, you would think they would want to avoid saying "go to ChatGPT, it's better than asking the randoms here".

This video has comparisons of devastation that would be caused by different sizes (around 7:50 mark) of asteroids: https://www.youtube.com/watch?v=4Wrc4fHSCpw

So a 500m is roughly a couple thousand or ten thousand times more than a Hiroshima nuke?

More like 50,000 or so (back of the envelope).

As a fraction of the overall asteroid, very little, thanks to the square-cube law.

The better question to ask is, how much energy would that transfer to the atmosphere on its way to dumping the rest into the planet proper?

Not enough.

A lot.

Going at 55~ km/s

Or 8. The page isn't quite sure.

Bad news for any life forms on its path either way, to be fair

I'm surprised Elon Musk didn't pick up on this a couple days ago and launch something at it.

The public is unanimously in favor of funding asteroid protection mechanisms and research. It's even higher support than the military.

Naturally what do we do with that money and goodwill instead? Yes! Let's build a massive space telescope which takes pictures that are only marginally better than the other multi-billion dollar space telescope!

Protect the Earth? Fuck that, don't ya know kid? She's toast anyway because the Sun will explode soon! Also get in loser, we're going to Mars!

It seems any serious asteroid prevention program is going to require the learnings of "pointless" (as you might put it) scientific expeditions like putting an object into L2.

There have been many studies about what could be done to deal with asteroids that are on track to impact earth and the simple answer is 'nothing'. Frustrating, but unless you can think of something that none of the very clever people on those committees could think of it's going to stay that way.

There have been many studies about what could be done to deal with asteroids that are on track to impact earth and the simple answer is 'nothing'.

The National Academies report that you link to down-thread did not state that nothing can be done. They discuss multiple options, and they discuss which options would be most effective given different amounts of warning lead time. Figure 5.5 (https://nap.nationalacademies.org/read/12842/chapter/7#85) is a great high-level view of the option space.

A real message to take away from the "Mitigation" section of that report is that it's important to identify dangerous objects as early as possible.

No, the lead author is on the record as saying exactly that. The report is a fantastic read and chapter 5 (the mitigation section) leaves little doubt about the futility of any attempts to mitigate such a threat. Your chances of making things worse are just as large as making things better and given the masses involved your 'window of opportunity' is very likely going to be preclude doing something about an impact that you are 100% sure is going to happen. Of course if you want to feel good about this that's fine with me but this is just blunt physics, an impactor that weighs 8.4 billion metric tons (500 meter asteroid) isn't going to respond to much of anything that we do to it in the time between detection and the time that it will intersect Earth's orbit. And smaller ones are likely to go undetected right up to the moment that they hit.

The Siberian one was a very nice illustration of how completely blindsided we were and now that Arecibo is gone we have lost one very powerful tool in our arsenal that could have helped with this.

But don't let it ruin your day, the chances of this happening are very low, one in a million or larger.

Here is some more recent stuff on this subject:

https://earthsky.org/space/dart-mission-deflected-asteroid-u...

(Doesn't make a huge difference but gives experimental validation of their previous theories.)

Some choice quotes from the report:

" As addressed in Chapter 5, the time required to mitigate optimally (other than only via civil defense) is in the range of years to decades, but this long period may require acting before we know with certainty that an NEO will impact"

"The amount of destruction from an event scales with the energy being brought by the impacting object. Because the range of possible destruction is so huge, no single approach is adequate for dealing with all events. For events of sufficiently low energy, the methods of civil defense in the broadest sense are the most cost effective approach for saving human lives and minimizing property damage.[+] For larger events, changing the path of the hazardous object is the appropriate solution, although the method for changing the path varies depending on the amount of advance notice available and the mass of the hazardous object. For the largest events, from beyond global catastrophe to events that cause mass extinctions, there is no current technology capable of sufficiently changing the orbital path to avoid disaster."

[+] So, in the case of say the Siberian meteor if we had seen it coming (which we did not) you could have called on all the people in a 100 km (1/20th of a second of travel!) radius or so to go to the nearest shelter. This likely would have caused more injuries and casualties than the event itself did, but if the impact had been a bit more steep and closer to a city (or even in a city) then it may well have saved (some) lives. Note that that was only 18m across, was going close to 70 K km/hour, weighed 9000 tons and that it exploded nearly 30 Km up in the air.

"Finding: No single approach to mitigation is appropriate and adequate to fully prevent the effects of the full range of potential impactors, although civil defense is an appropriate component of mitigation in all cases. With adequate warning, a suite of four types of mitigation is adequate to mitigate the threat from nearly all NEOs except the most energetic ones."

Note the careful qualifications, 'adequate warning' does a lot of heavy lifting there.

Pages 70 and onwards are pretty realistic and I think that the table really tells it all, none of the methods outlined are going to be practical given realistic times of warning and the kind of effect that you would have to create to make a meaningful difference in the outcome. Unless you happened to be able to pinpoint the trajectory with extreme precision and you had plenty of time and the impactor would be small enough. But that's playing the lottery. 'Civil defense' is code for 'shelter and evacuation', but assuming we're talking about an impact the size of the one that we are talking about here (400 meters, a couple of hours notice) utterly futile, especially if you don't know exactly where it is going to come down, you might end up moving people in the wrong direction, besides the mass panic. I don't want to be overly pessimistic but I'm with Jewitt in the sense that I do not believe we are geared up to deal with a challenge at that level.

Here it is in his own words in case you don't believe me:

https://youtu.be/4Wrc4fHSCpw?t=871

I don't buy that. It shouldn't be that hard to launch a cloud of space drones that produce a lot of dusty debris when they hit. Have them adjust their courses when the incoming asteroid gets close.

That way, we could paint a huge smiley face on the asteroid.

I would much rather be exterminated by an asteroid with a smiley face than a big dumb pile of rocks. (Even if the smiley face keeps spinning out of view while it's coming in.)

The masses you're talking about are such that you'd need to detect the asteroid well before impact (ideally: years) and that your efforts would not accidentally make matters worse rather than better. Even then it likely won't do anything at all. Calling Bruce Willis on line 3... But agreed on the smiley.

I'm not sure if we're talking about the same thing. The smiley drones would work fine hitting pretty close to Earth, even an hour or two out. And the mass shouldn't matter much, just the surface albedo.

Oh. You're probably talking about deflection. Yeah, deflection requires massive lead time, way more ballistic forecasting ability than I suspect we're capable of (isn't a clump of rocks going to heat up and throw things off as it comes in closer to the sun?), massively efficient engines to match velocity, and a whole lot of wishful thinking.

The smiley face might still be useful in that implausible scenario, I guess, if it changes the reflectivity enough to let the sun slowly nudge it out of the way? It at least avoids the velocity matching problem; you're intentionally crash "landing" anyway. And spinning is probably ok, as long as Galileo was right and the sun isn't orbiting around the asteroid. It's not likely to head straight at the sun.

But as you say, intervention seems just as likely to make things worse as better.

The obvious answer to that is being able to reliably detect asteroids on a collision course, and compute where is it gonna fall in order to proceed the emergency evacuation.

But first you have to be able to detect it, second you have to do it with reasonable time to save as many people as possible

Again, these people aren't stupid. You will have a very large amount of uncertainty based on a series of observations which essentially project a circle of probability on the planet that shrinks as more information from newer observations comes in and then just before impact that probability will either drop to zero because it is deemed to be a near miss (sorry, George) or it will then become a certainty. By that time it will be too late to evacuate. I can dig up the report from one of these committees if you want (I should be able to find it), they make for very interesting reading, it is a really nice example of science at work, even if the result is a negative. Anything more advanced makes for great special effects in movies and science fiction but isn't going to work. You'd have a very short time to move the entire population of a circle with a radius of a few hundred kilometers to outside of that circle starting from the center. The longer you waited the fewer people you'd have to move but the bigger the chance you'd be too late.

Any links to these reports?

Found it:

https://www.boulder.swri.edu/~bottke/Reprints/National_Acade...

Note that Arecibo is now gone.

I wonder how many people already used this https://neal.fun/asteroid-launcher/ to see what happens if it falls in moscow downtown

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