Why is nuking an asteroid a bad thing

I’m watching the history channel about asteroids and how we can keep them from wiping us out. There many theories as to how we could avert disaster by defecting the path of an incoming asteroid. Everyone seems to agree that nuking one is a bad idea. I’ve heard this many times before. The explanation goes something like this - if the astroid hits earth as a whole it would be getting hit with a single bullet, it we nuke it into pieces it will be like getting hit with a shotgun blast.

This doesn’t make sense to me. This explanation seems to assume it’s the difference between a .22 long rifle (for example) and a shotgun shell full of pellets. But if instead of a shot shell from a shotgun wouldn’t an equal comparison be a shotgun slug? Or in the case of a .22 the difference between a standard 40-grain bullet and a .22 LR shot shell? I’ve shot quite a few of both and I can say with certainty I would much rather be hit with the shot shell.

Also, wouldn’t the mass be about the same? In fact, if the asteroid was in many pieces wouldn’t there be a greater chance that it would burn up before it hit the earth as there is now a much higher surface to mass ratio. It seems the smaller pieces that did hit the earth wouldn’t dig in as far as one large unit and thus not kick up as much dust to block out the sunlight.

What’s the flaw in my thinking?

Nuking it breaks into many pieces, each of which has the potential of landing in an inhabited area. A swarm of pellets will create a larger “danger zone” than the one intact asteroid would.

A nuke will not deflect (much) the course of an asteroid.

I tried to type up an explanation, but the Bad Astronomer does a better job:

I think the comparison between shotgun pellets and slug is apt, given that the Earth has a layer of “armor” in the form of its atmosphere. While as mlees notes, a shattered asteroid might have a bigger destructive footprint, the smaller pieces are also more likely to break up or be deflected by the atmosphere. Moreover, a lot of the footprint might fall over oceans where its effects would be minimal (not that an actual impact on the ocean would necessarily be harmless).

Earth is apparently routinely hit by objects that cause multi-kiloton blasts in the upper atmosphere without anyone noticing. If we could reliably shatter an incoming object into pieces small enough to have that effect, I think it would obviously be preferable to having a single object airburst over a populated area or impact to create climate changing dust or devastating tsunamis.

I guess I should have clarified that we are talking about one like that hit the coast of Mexico 65 million years ago. The fact that it increased the ‘danger zone’ should be irrelevant as life would be wiped out anyway.

The atmosphere isn’t a protective layer around the “real Earth”; it’s a part of the Earth itself. It’s like saying that you don’t mind getting shot, since the energy will be dissipated in your organs, not in you.

Right. I agree. Choosing between a “planet killer” and “well, we only lost Europe” is an exercise in choosing the lesser of two evils.

Edit: The reason I didn’t mention the dinosaur killer type asteroids was because we don’t get hit by them all that often, in Human terms. That’s why I envisioned smaller “city killer” type objects. I should have been more clear.

I’m curious. The consensus seems to be to let one big asteroid hit us than many little ones. In 50 or 100 years, let’s say many smaller countries get nukes. An asteroid is coming. Every country except one wants to nuke it. Would we stop them, destroy them if we have to in order to prevent them from launching?

Just look to nuclear weapons for the answers - and these are tested and highly thought-out answers. Here’s what Wikipedia says about MIRV warheads:

The Tsar Bomba (at 52 MT with a theoretical 100 MT yield) was always considered a propaganda weapons - impressive for the cameras, but impractical for actually killing the enemy. If you look at the warheads in actual nuclear arsenals, they are a lot smaller than most people realize.

All of the same thinking remains intact for an asteroid or comet impact.

While merely fragmenting a threatening object may not be effective, you could use one or more nuclear devices to deflect it by converting the radiative output to thrust by converting a sacrificial “puck” of material into a large, fast-moving cloud of gas that transfers momentum to the object and retards or otherwise deflects its motion so that it passes ahead or behind the Earth as it crosses Earth’s orbit. Even if such an impulse does cause the object to fragment (quite possible, as many cometary and asteroid bodies are actually collections of small objects cemented together by water ice or silicon oxides) the distributed thrust should push the collection of objects more or less equally as a group. The further away you do this, i.e. well out beyond Earth’s sphere of influence, the better, so this would require the capability for a launch system including Earth escape stage and interplanetary spacecraft, but it isn’t really a far reach for conventional heavy lit booster systems (like Atlas V-Centaur or Angara V-KVRB).


Stranger, it’s my impression that a lot of asteroids are detected fairly close to passing (or already passed) by the Earth.

How much “lead time” is the average in detecting dangerously large asteroids in fact?

Hard to say, since it depends on what you mean by “dangerous” (an asteroid that even just barely misses could be said to be not dangerous at all, and of course statistics are sparse for ones that actually hit). But there have in fact been some near-misses (by Solar System standards) that were indeed detected only after closest approach.

I’ve seen it argued that on an average or expected-value basis, better asteroid monitoring would save more lives than any other government endeavor. There’s a small chance of a significant impact in any given span of time, but if that small chance comes through, it could kill billions, and such consequences should be enough for us to prepare against it even if it is unlikely.

I asked because Stranger said that lead time was important. I can understand why that’s important, so I was wondering if we could count on having enough time with things the way they are.

My impression was that we get surprised by asteroids looming out of the dark on near miss courses more often than we detect stuff far out, assuming that we had to actually launch nukes at one.

After looking here: http://spaceweather.com/ , I see “On August 5, 2010 there were 1140 potentially hazardous asteroids.” Looks like our solar system is a busy place.

I’m not understanding the analogy. My understanding is that the asteroid that killed the dinosaurs did so by kicking up a layer dust that blanked the earth by striking the earth itself. Is there some damage to the atmosphere it caused that I’m not aware of?

First of all, many scientiests do not believe that it was a single event that wiped out the dinosaurs. There’s speculation that the hit triggered much of the other activity that may have led to the mass extinction, but we don’t know for sure the extent that can be directly attributed to the lone strike.

The explanation of the dangers of fragmenting a comet before it hits Earth by the Bad Astronomer were with regards to the strategy used in the movie Armagedden. BA was pointing out that breaking up the comet so close to Earth was likely ineffective in preventing the damage. The scenario suggested to him that it wouldn’t lead to any net drop in energy from the impact, merely that it would dump a great deal of it into the atmosphere instead of impacting the ground. As chunks of rock fall through the atmosphere, they build up a tremendous amount of ram pressure because they travel faster than the air can get out of the way (i.e., supersonic). This is what causes most fragments to explode before they reach the ground. If that was happening over a significant percentage of the face of the Earth exposed to the asteroid strike, it would be like having thousands of massive conventional explosions going off. An un-fragmented rock might well come down intact, and while it would ring the Earth like an unholy gong, a significant percentage of the energy would be delivered into the ground. Of course, it’s going to make the locale of the landing spot hellish, and kick up a vast amount of debris that will cause global calamity, but it might still be better than a large portion of the atmosphere being turned into an inferno.

It’s important to realize the scales of things we’re talking about. While we think of the atmosphere as huge, it’s really a very thin layer surrounding the Earth (think: like the skin of an apple). It’s ability to absorb energy is miniscule compared to the immensity of the planet.

Couldn’t we have nuclear device that is not large enough to pulverize the asteroid embed into the asteroid, and have the asteroid material ejected from the explosion act to deflect it? At some point on the asteroid 90 degrees from its trajectory, both the ejected material and asteroid should deflect to bypass the earth. There might be some matter left on course, but wouldn’t most smaller pieces be deflected much further than the larger part of the asteroid?

The determination that deflection doesn’t work is quite old. It seems like we are picking up large objects much further out now.

I’m not sure where, but I saw some reasonable analysis that the smithereens approach could create an enormous moving plasma in the atmosphere that would impact the earth. The followup included a number of ways that was much worse than a solid impact, the greater area directly affected, converting much of the atmosphere into nitrogen compounds, ejecting large portions of the atmosphere into space, etc.

That’s pretty much what Stranger suggested.

And PlainJane, I understood the question, and the reason for asking it. It’s a perfectly reasonable question; it’s just really hard to answer. About all we can say is that, with the current status quo, we stand an uncomfortably large chance to not have anywhere remotely near enough lead time.

Basically we need to

  1. Map the near earth asteroids a hell of a lot better
  2. Locally examine a decent sampling to determine their composition (rubble, stone, metallic)
  3. Identify leading “killer” candidates
  4. Test long term deflection methods on them

Personally I like the gravity tug idea but like a lot of the effective, subtle approaches it needs time.

It makes a huge difference whether you get shot with a large pellet or bird shot.

The problem is the material of an asteroid. If it’s solid iron it would be tough to break it into small enough pieces. If it’s fractured rock then it should be easier to break it into small enough pieces to mitigate the damage. the more pieces there are the more surface area exposed to the atmosphere which means more of it is consumed before it hits.

Stranger, see the book Death from the Skies! by Phil Plait; it goes into this in detail, with calculations.

Basically, from my understanding of the book, you are overestimating the effect of a nuclear explosion on a chunk of rock the size of Manhattan Island, already moving at high speed.

The author does say that it IS possible to deflect an asteroid, just very expensive and requiring a great amount of technology.