How much time will we get to evacuate the right city before an asteroid impact?

Factual Questions
1

Say next week a smallish asteroid, about 100m across, is discovered to be on a possible collision course with Earth. That’s big enough to ruin some town or city’s day, but not big enough for regional devastation. Say it’s going to arrive in only 30 days, which is way too soon to get an intercept mission launched with enough time to significantly alter its trajectory.

I understand that the longer astronomers can observe the object, the smaller the probable area of impact becomes. How fast will that shrink? How much observing time would astronomers need to determine which city it will hit? At an early point, it could be anywhere between NYC and Centralia, PA, so it’s useless to try to issue an evacuation order yet. Would we be able to tell with enough time if they should evacuate Manhattan?

2

Won’t matter, we can’t get people to freaking evacuate from a hurricane, and they hit our coasts all the freaking time. You will have at least a quarter of the people going ‘mah freeeedumb’ and refusing to move, another ten percent ‘way freaking cool, I got to stay and see this shit!’ and probably fifteen to twenty percent won’t go because their animals can’t be evacuated and they have no vehicles or place to go that will take a dozen chickens, 3 goats and a pig …

3

Moderator Note

Since this is in FQ, let’s focus on the factual aspects of the question, please.

The questions are:

  1. How fast will that shrink?
  2. How much observing time would astronomers need to determine which city it will hit?
  3. Would we be able to tell with enough time if they should evacuate Manhattan?

For (3) let’s at least try to find some sort of cite for how quickly Manhattan could be evacuated. Somewhere around a million people were evacuated from Manhattan during 9/11, so that could be used as a reasonable reference.

4

Looking at the table of currently observed close to Earth approach objects here, it looks as if most objects have an accuracy of between ±1 and ±15 minutes, but many are in days, not minutes of accuracy of time of closest approach. Not quite the same as estimated time of impact, but it gives one a start on estimating the accuracy possible. At the Equator the Earth spins at about 28 km per minute, so at say Manhattan, the Earth rotates at about 20km per minute. So even a ±1 minute error makes disaster planning almost impossible, accuracies in hours to days are useless.

Based on that source, there is little to no hope of getting significant advance notice down to the right city. Not even the right US state of the US, or European country.

Obviously if there is clear danger lots of effort could be expended quickly to try to get much more accurate orbital elements. But accuracy depends on the closing angle, and since it is pointed at us, these are, by definition, tight, and not good for accuracy. Throwing major resources at the problem could be expected to narrow the error down fast. But getting it down to the nearest minute may be impossible, and that puts a clear limit on what can be done. Then one needs high accuracy modelling of the Moon/Earth system as it affects the orbit. If the moon happened to be close to the approach it may become impossible to model.

Whether one could close the gap in useful time is hard to know. One problem is that moving too early and you risk evacuating one area into the actual impact zone. Given the enormity of the problem determining when to pull the trigger on evacuation orders is just as important as determining where to evacuate.

5

There’s a very good chance that it won’t even be detected until it begins to burn its way through the earth’s atmosphere. Periodically, I read about how we are shocked to learn that an undetected asteroid whizzed by us. So, the answer to your question is, we would have no time at all.

6

I don’t know whether the OP’s choice of Manhattan was deliberate or not, but you’ve picked just about the worst-case place on Earth to try to stage an evacaution of. Singapore might be a close second.

The combination of population density and natural and man-made barriers to mass movement are formidable. Evacuating, e.g., Omaha would be trivial by comparison.

Though as @Francis_Vaughan says, getting actionable accuracy more than a couple minutes before impact is very unlikely.

7

Although Manhattan has the advantage of multiple commuter lines that run out of the city; the New Haven, Harlem and Hudson lines of Metro-North, the PATH trains to New Jersey and the LIRR trains to Long Island. I imagine if every train was packed full of passengers in some organized fashion, I think quite a few people could be evacuated.

Edited to add, there’s also the Amtrak train routes on the Northeast Corridor and the ones that head north towards Albany and Montreal.

8

Would that rely on someone driving the empty trains back ?

9

Yes. But I know there are multiple tracks at Grand Central, so if there was a train at each track, quite a few people could leave. Anyone know the max capacity of an LIRR or Metro-North train?

10

And as we found out during 9-11, there is even evacuation by sea. But that raises a good point that the number of routes out is a huge variable. Take Denver, effectively two freeways, one N/S and one E/W and both are backed up during the morning commute meaning we’re all dead here.

11

Plan C involves evacuating all the wrong cities as well, since you won’t know which is the right one.

If you have a city-killing asteroid coming, Plan A [shoot it from the sky] and Plan B [guess and empty the right city] have too high a probability of failing, with many deaths, plus you’ll have wasted valuable running-away-screaming time.

If you try for high levels of evacuation of all at-risk cities into their hinterland [say 200 km radius] over the week before then wherever it lands will kill a lowish density of people. Also take and disperse as much machinery needed for disaster recovery and emergency response so it it ready wherever the thing lands.

You can thank me later.

12

Also depends on possible damage. The recent meteor(?) that hit Russia was estimated to be about 20m but most does not appear to have hit the ground, speculation being it was ice not rock.
Chelyabinsk meteor - Wikipedia.

Other interesting details were that it entered at an angle - not every hit on earth is straight on. The angle, and the unknown factor as to how it will slow as it goes through the atmosphere could seriously widen the potential impact site area. But by the time the object was a week away, i would hope the trajectory would be well established, to within the minute. There are devices like Hubble that can get a very accurate location and so velocity estimate, and certainly it will be a priority. (And of course, 70% of earth is water, and a significant area - Siberia, Alaska, northern Canada, Greenland - is so sparsely settled that may be no worse than an ocean impact.)

the real question is how organized? I would imagine that like a hurricane evacuation, there would be arenas and schools in the safe areas turned into evacuation centers. A week or two to organize bedding, food supply etc. Rush hour is maybe what, 10% of the population trying to go somewhere in an hour, so loading the whole household into a car and driving out sometime over a week or so should be manageable, until the foolish ones who waited until the last minute all take to the road at once. I imagine a state of FEMA group marshalling all school busses, public transit, etc. to assist in evacuation for those without cars. When do authorities start limiting things like deliveries and other vehicles going in? Ensure sufficient gas supplies? Who runs the gas stations? Communications, internet? Should we just switch all traffic lights to flashing red, in case they malfunction during the evacuation time? These are the details that a FEMA organization is supposed to think about before it’s urgent.

To be fair, the problem zone in a hurricane is fairly small, and keeps changing. Ian, for example, was originally going to impact Tampa, but ended up hitting down the coast which was not determined definitely until close to impact. So people who though it prudent to evacuate possibly ended up not needing to, and many who should have - did not. The same uncertainty will apply to impact zone from a meteorite, and I imagine there will be cantankerous types that will refuse to budge. BUT… There’s a huge difference between “I think my house can take 100mph winds” and “I think my house can take a billion and a half kilograms of burning rock landing in it.”

i would think the logical thing would be to note the impact “zone” and evacuate it all, ordering out the less mobile earlier and the essential people being the last to go. Plus, companies with valuables will have a week to arrange moving them - I’m imagining compounds full of every rentable truck and trailer holding quasi-valuable goods from local stores. One fun question is looting - who will volunteer to patrol the zone, knowing maybe 80% of it is a wild guess and won’t be impacted - but which 80%? It’s not like a hurricane, where a few well-sheltered law enforcement can patrol to the last minute and then go indoors. OTOH, someone could patrol up until the last hour or two and then floor it out of there if the roads are open. It’s not like there’s floods or high winds to impede rapid departures.

13

This was in the news…

It was not a tiny space rock either, estimated at somewhere between 15 and 40 feet in diameter, said Liliya Posiolova, the orbital science operations lead at Malin Space Science Systems in San Diego, which built and operates two of the Mars Reconnaissance Orbiter cameras.
The impact released the energy equivalent to somewhere between 2.5 and 10 kilotons of TNT, Dr. Posiolova said. (The atomic bomb dropped on Hiroshima at the end of World War II was the equivalent of 15 kilotons of TNT.) It left a crater wider than a football field.
During a NASA news conference on Thursday, Ingrid Daubar, a planetary scientist at Brown University who leads InSight’s impact science working group, said a meteor this big enters Earth’s atmosphere about once a year.
“We see those pretty regularly,” Dr. Daubar said. “But because Earth has a thicker atmosphere, asteroids of this size burn up and are generally pretty harmless.”

The interesting part to me is “We see those pretty regularly…” for a rock between 15 and 40 feet diameter. Probably we don’t see them until they hit the atmosphere.

14

“It has happened before, but there is nothing to compare it to now.”

15

Huh? In other words I don’t know what you are saying.

16

Second sentence of Gravity’s Rainbow, which starts with almost the same scenario you’re describing, except it’s a rocket attack on London.

17

Did you assume that others would recognize the quote?

18

In Chapter 13 of his book A Short History of Nearly Everything, Bill Bryson asks two experts how much warning we would have if an asteroid were on a collision course with Earth.

The answer he gets is the answer @Jasmine provides here:

And it would burn through the atmosphere for approximately one second before impact.

19

BOOM

20

Said another way, if you hear the noise you know you’ve survived the impact. You may still get whacked by the splatter, but you made it past Step 1.