1 impact vs multiple impacts

[Aghanim]

Which is more devastating, a single big asteroid, multiple medium asteroid that won't burn in atmosphere, or a lot of small asteroid that burn in atmosphere?

[Algiark]

A single big asteroid, definitely, since it can affect the entire biosphere of a planet. Multiple medium asteroids could still cause heavy damages, but the effects will most likely be localized. Lots of small asteroids that burn in atmosphere already happens every day.

[Ralathon]

Depends on what counts as "devastating".

If we turn 1 big asteroid into a bunch of small ones that burn up before they hit the ground they both deliver the exact same energy. It's just that the big asteroid dumps most of it into the ground while the small ones dump it into the atmosphere.

This means that either way you'll have a bad time. In one case you'll be torn apart by shockwaves and earthquakes while in the other case everything in a large area catches fire thanks to the thermal radiation. Both probably have the same net effect on the biosphere: Lots of ash gets tossed up into the atmosphere causing a long winter. The big asteroid might be slightly worse in this regard since the impact will cause it to rain molten rock all over the planet, causing much more forest fires than the other scenario.

[Godot]

I agree with Algiark ...

I would guess that the kinetic energy released by a single large impact has the potential to hurl the dust/ashes further into the atrmosphere, than the multiple medium impacts would have

(thereby probably reaching altitudes, which the dust/ashes wouldn´t have reached by means of normal stmospheric convection).

Therefore I would think that the nuclear winter of a single large impact lasts longer (as the ashes rom the higher altitudes might take much longer to get down to earth again)...

and there are much more dangers of particles from the impact reacting with gases in the upper atmosphere (and causing negative effects for life on earth)

[Starwhip]

Well, think about it: An asteroid, similar to the size of Everest (Like the one 65 million years ago), would hit the Earth, cause tsunamis, incinerate foliage, rain molten rock, and generally cause a bad day. But a hundred million pebbles raining down from space would cause casualties and destruction, but wouldn't have the same "massive impact" aftereffects. And if the asteroid was big enough, it could affect the Earth's orbit and spin, leading to large climate changes. If I were to choose, I'd say we nuke it. Perhaps several times.

[jwenting]

Multiple impacts are more devastating, have a larger total effect than a single one of the same total mass.

That's one of the reasons current nuclear strategy calls for multiple smaller bombs rather than a single very large one for most targets (reducing the potential lost strikes from failing weapons is another, obviously).

[Albert VDS]

It depends on how big the big asteroid is and how big the medium asteroid are.

If the big one is big enough to kill us all and the medium ones are continent killers then it's both as bad.

Edit: Let's put it in an other context. Let's say we have a melon and it's smashed to pieces with this big rock.

Now let's take a new melon and hit it with multiple smaller rocks, with the same mass as the big rock, which hit at the same time.

Now what do you think would happen?

Edited November 25, 2013 by Albert VDS

[Starwhip]

That's what I'm saying: if you literally pulverize it, and nothing is left but dust and pebbles, perhaps some boulders, then most if not all of it would be either knocked off course or burn up in the atmosphere. A huge asteroid (especially a stony-iron or iron) would just slice through the atmosphere and impact.

[DonLorenzo]

That's what I'm saying: if you literally pulverize it, and nothing is left but dust and pebbles, perhaps some boulders, then most if not all of it would be either knocked off course or burn up in the atmosphere. A huge asteroid (especially a stony-iron or iron) would just slice through the atmosphere and impact.

I'm not so sure they would. If you pulverize the big rock, but keep all the pebbles together somewhat (I'm assuming they still hit at the same time, otherwise it's a bit moot) they might still behave a bit similarly to the big rock? Ie. the first pebbles to enter the atmosphere shielding the rest a bit. Some of them would vaporize, but the rear ones wouldn't and still impact, perhaps much the same as the big one? It's still the same mass that's entering, just a bit more spread out. I'm assuming you're not significantly altering the trajectories, obviously if you let the pebbles enter one by one or from completely different angles it wouldn't do a lot if anything.

If the scenario is something like '1. One impactor with X mass or 2. N impactors with X/N mass, on different trajectories but in the same day' I feel that scenario 2 cause the least amount of damage, if for nothing else that there's more surface area per mass of entering rock and thus probably more material shed/vaporized/exploded before impact

[Kryten]

I'm not so sure they would. If you pulverize the big rock, but keep all the pebbles together somewhat (I'm assuming they still hit at the same time, otherwise it's a bit moot) they might still behave a bit similarly to the big rock? Ie. the first pebbles to enter the atmosphere shielding the rest a bit. Some of them would vaporize, but the rear ones wouldn't and still impact, perhaps much the same as the big one? It's still the same mass that's entering, just a bit more spread out. I'm assuming you're not significantly altering the trajectories, obviously if you let the pebbles enter one by one or from completely different angles it wouldn't do a lot if anything.

How exactly could an asteroid reduced to smaller pieces without 'significantly altering the trajectories'? Astronauts with hammers and chisels?

[Awaras]

From the review of the movie 'Deep Impact' by Phil 'Bad Astronomer' Plait:

Bad: Minutes before final impact, the astronauts blow up the second comet, and we are treated to a spectacular light show.

Good: Aaaaarrgg! This was the Biggest Baddest Astronomy in the movie. Blowing up a comet does no good at all, and might even make matters worse. Just because the pieces are smaller doesn't mean you have changed anything. If every piece still impacts the Earth (by that I mean actually is stopped by the Earth or its atmosphere) you are still dumping all the kinetic energy of The Comet into the Earth's atmosphere! That's a HUGE amount of energy, dumped in practically all at once. It would still create a massive explosion, dwarfing all of our nuclear bombs combined. Even if you could somehow soften the blow, all that heat would wreak havoc with our weather. Some people actually think it might be better to simply let a big one hit rather than blow it up, because the Earth itself can absorb the energy of impact better than the atmosphere can. This is still argued, though. I'd prefer not to try any experiments!

[Brotoro]

Multiple medium asteroids would be more devastating than a single large asteroid of equivalent mass (for the same reason that multiple lower-yield nuclear warheads are more devastating than a single warhead of the same total yield).

[mdatspace]

That's what I'm saying: if you literally pulverize it, and nothing is left but dust and pebbles, perhaps some boulders, then most if not all of it would be either knocked off course or burn up in the atmosphere. A huge asteroid (especially a stony-iron or iron) would just slice through the atmosphere and impact.

Stony-iron and especially iron asteroids tend to survive reentry the best.

But pulverizing it is no easy task, assuming it is a rigid body with very little or no porosity.

Edited November 25, 2013 by mdatspace

[brdavis]

How exactly could an asteroid reduced to smaller pieces without 'significantly altering the trajectories'? Astronauts with hammers and chisels?

No; blowing it up should work just fine. A 1 [km] asteroid would have a mass of 8.4e+12 [kg] and a surface escape velocity of 1.1 [m/s]. If you "blow it up", plenty of stuff will be moving faster than that, and a lot of stuff slower than that. Assume you over-estimate, and accelerate every bit of gravel to 10x faster than needed to disassemble the asteroid - at 10 [m/s], with an asteroid approaching Earth at 15 [km/s], the pieces will travel 1,500 [m] forward for each 1 [m] sideways perpendicularly-blown bits will spread - the expanding cloud will have an opening angle of about 0.04°. I think a shotgun blast may have a wider spread than that. If the asteroid had been aimed "dead center" for the disk of the Earth, it would take more than two months for those debris to just start clearing the edge of the Earth.

[brdavis]

On the whole, you probably want an impactor hitting intact, not as a "shotgun blast". As others have noted, the amount of energy dumped into the biosphere is identical in either case - in the case of a single impact, you have an amazingly hot impact zone. That means (if it is on land), it can radiate at least some of that energy away efficiently… and a lot more energy is partitioned into throwing around lots of rock. Not a lot of fun in the immediate vicinity (and, admittedly, for a big enough rock the immediate vicinity starts being measured in hemispheres), but it does allow the thermal radiation a chance to radiate away from a high-intensity source.

Now picture all that as gravel. Pea gravel. Falling at hypervelocity into the atmosphere. Same energy… but now spread over the entire visible sky (if the debris cloud had spread that far). The result is a lower temperature, so heat is not radiated away as efficiently as in the single impactor case, but the total energy load is the same. Above a certain size, the result is a global* firestorm, with the entire sky horizon to horizon going very very rapidly to the radiative equivalent of an oven. Poof, there goes the biosphere on that side of the planet**. For similar reasons you probably would prefer the impact hits land, not water - a water impact is going to be extremely good at capturing the bulk of the energy (either thermally… look up "hyper canes"… or in the form of really largish tsunamis), while a land impact just really drastically overheats a very small area that can radiate the heat back to space more effectively than most alternatives.

Yeah… don't send Bruce Willis. If it's big enough, deflect, don't break, and if it's on the borderline you'd almost certainly want it to hit intact anyway (and evacuate that continent… or that entire oceanic coastline).

*OK, it wouldn't immediately be a global firestorm, just the facing hemisphere. Still, a pretty poor day to suntan.

**And given the amount of dust and soot generated by this, the biosphere on the far side of the planet is unlikely to be happy either.

[Sternface]

http://www.purdue.edu/impactearth

Enjoy.

[DonLorenzo]

How exactly could an asteroid reduced to smaller pieces without 'significantly altering the trajectories'? Astronauts with hammers and chisels?

Hehe, nice mental image. I wasn't really discussing how to get from scenario 1 to 2, just treating the two separate ones 'as is'. Ie. would you be more worried if a big rock approached or a cloud of pebbles. Not 'would it be smart to turn 1 into 2'.

[shand]

Interesting question.

First, to aknowledge the good points made above: yes the kinetic energy is the same (assuming same mass and average speed and all that), and yes its fair to say that blowing it up shouldnt reduce the probabilty of bits hitting you and would just spread the bits out.

shotgunning will result in more of the material being burnt up in atmosphere - so more affect there compared to the more terrestrial affects of a larger material impacting the earth. Both will have a similar effect on the oribt/spin/inclination of the planet (due to the same kinetic energy). so really it comes down to, what is more devastating A) massive thermal energy increase in atmosphere or relatively slight thermal increase in atmosphere but massive physical impact.

A) will result in local extreme climate changes, high winds (very high, 200mph+) and general weather being terrible for a time, depending on how big the little bits are will effect the affect at sea level (smaller bits = more energy in high atmosphere so less affect at sea level for example). things like jetstreams may be disrupted, and we can expect global weather patterns to change drastically. perhaps best to remain inside/in a bunker for a month or so.

a single impact will result in... simply put, a massive explosion. so initial shockwaves, earthquakes globally (if its big enough), expect semi active volcanoes to join the party. weather systems will be disrupted, but less than in A. now factor in the massive amount of ash/debris thrown up, a lot more debris than in A and a lot more ashy. this will result in a massive ash cloud, again dependant on how big it was, this could well result in temperature drops that last years, also a noticeable decrease in sun light penetration. so a noticeable reduction in crop yields. Also air travel would be a no-no for a long time. those in europe will know how disruptive that one volcano was.

So to summarise: in my opinion a single impact would be a lot worse globally. A is more local, B is more global. Feel free to say if i missed anything from A or if i've got anything completely wrong.

[brdavis]

shotgunning will result in more of the material being burnt up in atmosphere - so more affect there compared to the more terrestrial affects of a larger material impacting the earth. Both will have a similar effect on the oribt/spin/inclination of the planet (due to the same kinetic energy).

Agreed; ‘shotgunning’ results in a higher percentage of the energy deposited in the atmosphere. There is, actually, a difference in terms of things like changing the planets rotation, because that depends on where (not just with what momentum) the objects hits. A ‘shotgun’ blast would result in less torque (center of impact being equal, a ‘shotgun’ blast may have a portion of the debris miss the disk of the planet)... but honestly it makes no difference even for a ‘dinosaur killer’ - they are just way way too small to deliver much angular momentum on these scales.

what is more devastating A) massive thermal energy increase in atmosphere or relatively slight thermal increase in atmosphere but massive physical impact.

There’s another thing to consider. That ‘massive thermal energy increase’ in the atmosphere is actually pretty dang small, but the intensity matters (and it is intense). Take that hypothetical 1 [km] radius asteroid, massing 8.4e+12 [kg] (and let’s say hitting at 15 [km/s]). In terms of energy, that’s popping in with 9.45e+20 [J]... but just one day worth of sunlight on one hemisphere of the Earth is around 7.6e+21 [J]. So as drastic as this is... it’s the equivalent of having two Sun’s in the sky for a total of an hour and a half. It’s not going to effect the weather, or the global circulation, or the jet stream... at least not for more than a few days to a week at best.

The intensity might be the issue however. If the incoming objects are dropping enough energy to briefly heat the air to incandescence, then you have the potential for this glowing oven-temperature sky to ignite fires... over an entire hemisphere. That can put out far far FAR more particulates than the original impact would. That’s why in certain size ranges... you’d really like the thing to hit intact, on land. Because the “dump the energy into the atmosphere†solution is far worse.

Nuclear weapons do far more damage in air bursts than in ground detonations. There’s a lesson there, and it has to do with shock physics (dumping that energy into the ground really doesn’t result in much destruction... there’s a LOT of ground) and thermal physics (the heat pulse in the air is deadly and a distance, and couples energy into an air shock... not so much for ground impacts, er, I mean detonations).

Perhaps the biggest problem in this discussion is what scale of impact we're talking about. For really large ones (10 [km] scale), the shotgun effect results in a global firestorm that is far more damaging than the "localized" impact, but those pack so much energy that re-entering ejecta from the main impact are enough to start their own global firestorms. At the very small end, it doesn't matter. At the very large end, it also doesn't matter. In between… it probably depends on the size in a rather complex way.

I don't think global earthquakes are a significant problem however… at least not until you start hitting Chicxulub-style impacts or Caloris-basin-forming events (that one did leave geologic traces on the far side of the planet).

[Starwhip]

Alright, fair points. And if we think, more surface area (a dust cloud) would heat more air than a single asteroid. So it might be worse.

The best strategy would be to find the asteroid several years in advance, send up a "pusher" (A probe which anchors to the asteroid) and alter the trajectory enough to avoid impact. But if we had to choose, what would you pick? Leave it alone or attempt to destroy it?

[Motokid600]

Both scenarios destroy civilization. But multiple impacts would save the planet and allow life to recover faster.

[brdavis]

…and if we think, more surface area (a dust cloud) would heat more air than a single asteroid. So it might be worse.

Although to be fair to your point, for a small (enough) object, spread enough, it might be better (if a significant portion of the mass misses the disk of the Earth entirely… or if the air is heated by the impact but only to the point of raising it a couple hundred degrees or so - uncomfortable, but no global firestorm).

The best strategy would be to find the asteroid several years in advance, send up a "pusher" (A probe which anchors to the asteroid) and alter the trajectory enough to avoid impact.

Or hit the side of it with a really big bag of chalk dust wrapped around a firecracker, and let Yarkovsky do the driving. You don't even have to worry much about direction - if it's on a collision course, " any orbital change is for the better (speeding it up or slowing it down likely being the best… don't even think about kicking it sideways).

...if we had to choose, what would you pick? Leave it alone or attempt to destroy it?

Going to try to pin me down to an answer? sigh, let me guess…

- below 100 [m] scale, evacuate (and film… YT will be saturated)

- 100 [m] to 1000 [m], maybe… maybe… try to fragment it, if it is really weak (gravel pile style)

- 1 [km] to 5 [km], probable let it hit intact and consider having Lloyds of London write a policy for the continent

- 5 [km] and up, ignore it - get a possibly self-sustaining colony to Mars if you have a decade or two, and use the rest of the world economy on the biggest end-of-the-world party you've ever seen. Or ever will see.

[PakledHostage]

- 100 [m] to 1000 [m], maybe… maybe… try to fragment it, if it is really weak (gravel pile style)

- 1 [km] to 5 [km], probable let it hit intact and consider having Lloyds of London write a policy for the continent

Currently, the asteroid with the highest known impact probability is 1950 DA: a 1.1 - 1.4 km diameter rock with a mass greater than 2 x 10¹² kg. Fortunately, the risk isn't for another 800 years. It is currently unknown whether the Yarkovsky effect will increase or decrease the probability of an impact. If impact ever does become a certainty, lets hope it isn't just (as K^2 once said on these forums) "a rude surprise for some future feudal kingdom" and that we've got the technology to do something about it.

[minimuffin]

[DISCLAIMER] The following is my opinion based on what I know and common sense. I might be beyond wrong, or I might be perfectly right. [END DISCLAIMER]

I am no scientist, but based on common sense it seems to me that a bunch of smaller pieces would be less damaging. Assuming the pieces are quite closely packed, and they all equal the mass of one big impact, the multiple ones would be better. Reasons are:

Small ones would burn up faster then big ones; It is not concentrated as much, meaning that the force would be spread out a small amount;

Also, if the pieces all scattered, then that would probably be safest, as the force is spread out. People might find asteroid pieces in their cars, but overall it is safest. (In my opinion)

Edited November 27, 2013 by minimuffin

[jwenting]

Both scenarios destroy civilization. But multiple impacts would save the planet and allow life to recover faster.

nonsense. A single big impact won't "destroy the planet", neither will multiple smaller ones. And if you'd read the facts before posting that nonsense you'd have found that multiple impacts are in fact worse for the ecosystem than a single large one.