OK please clarify this… meteorites are constantly impacting the atmosphere and creating spectacular light shows, without any apparent problem. Are you saying that transforming a large impact mass into multiple smaller impact masses that will burn up in the atmosphere will cause a problem? If so what problem? The only one I can think of is heat related. What mass-velocity (momentum?) combo burned up in the atmosphere is going to significantly heat up the global average temperature? (Hitting the actual Earth is very very different than burning up in the atmosphere).
http://neo.jpl.nasa.gov/ NASA keeps track and this site is how they inform us. They are called near earth objects and the odds of them hitting is given in case you don’t want to sleep well.
I haven’t read Plait’s book, but again, the problem is converting the gross energy output of a nuclear device (which is mostly x-rays, gamma rays, and fast neutrons) into a useable impulse, which means converting the radiative energy into kinetic energy carried by a large mass that can transfer momentum to the object in question.
Let’s try a simple back-of-envelope calculation to see the scale of this. Let’s take a spherical pure nickel-iron asteroid of 1 km in diameter, equivalent in mass to a solid water ice approximately 2 km in diameter, which envelopes all but a tiny fraction of catalogued potentially hazardous objects (PHO). If we want to change its speed by 100 m/s (which would be sufficient to affect a deflection of 100,000 km at a distance of ~30,000,000 km, resulting in a near-certain miss), we’ll need to apply impulse so as to cause a change in kinetic energy of 20.6∙10[sup]6[/sup] GJ. A 1 MT nuclear weapon releases 4.184∙10[sup]6[/sup] GJ. Assuming we can harness 10% of the total output into useful impulse (which I’m basing on some rough extrapolations from Project Orion) that requires approximately 50 bomblets; expensive, but not technically prohibitive. If we change our deflection delta-v to 10 m/s (still giving us a deflection of approximately 10,000 km at the same distance, which is probably enough to push it a couple standard deviations out from a nominal impact trajectory) then you only need a single 500 kT device. Of course, this usefulness of this whole scheme is dependent upon identifying and plotting PHAs early enough to effect a deflection mission; if you only detect a threat at the last minute, or cannot put together a mission in time to deflect the object at distance, no credible amount of effort will deflect it enough at the last minute to make a change.
The calculation scales linearly for mass, so a much smaller threat can be readily deflected by such a method; conversely, a significantly larger one–the size of a small moon or larger–can’t feasibly be deflected sufficient to avoid collision. Fortunately, such larger free objects (the size of Manhattan Island, to use your example) are extremely rare, for reasons that aren’t fully understood but thought to be due to coupling effects with Jupiter and the Sun that cause large masses to be more easily captured into stable orbits. Only one of the catalogued list of Near Earth Object approaches from NASA/JPL’s Near Earth Object Program has an absolute magnitude of less than 17 (roughly corresponding to a size of greater than 1 km in diameter).
So I would agree with Plait’s conclusion that it is expensive but technically feasible, provided we can collectively act to identify threats and be prepared to deflect them. Unfortunately, the current state of space programs in general doesn’t provide a lot of optimism about this. While we spend tens of billions of dollars trying to develop ineffectual strategic defense systems from terrestrial threats, and noodle about in Low Earth Orbit in a manned program with no explicit direction or goal, we ignore a threat that is both capable of reducing humanity to Stone Age civilization and yet readily reducible with only a slight extension of conventional technology.
Stranger
Probably a very ignorant question, but how can they so precisely make the census of “hazardous” asteroids, as those are far, rather-small objects that don’t emit light ?
The bulk of PHOs are objects with orbits that lay close to or periodically intersect Earth’s orbital trajectory, coming within 7.5 million miles of Earth, and are larger than 150 m in diameter. Although they don’t self-radiate, they do reflect sunlight. The catalogue of of PHOs certainly underrepresents all threats, as it won’t capture most long period objects (Kuiper objects, long period comets) and may not capture many low albedo objects. We can infer, by the number of near passes that were not discovered until just before or after the pass, that the current number of PHOs that have been catalogued are only a small fraction of potential threats, although they are the objects that pass by most frequently. Our survey of PHOs is currently almost exclusively limited to objects that can be seen from terrestrial observatories, which also limits how accurately their orbits can be plotted. A space-based survey with an array of extended orbit or sun synchronous orbit observatories would expand our catalogue immensely for a cost of a single year’s budget of the manned space program.
Stranger
Yes, but it does make a difference whether you get shot with a large iron pellet, or a bunch of radioactive bird shot.
What’s important is the total energy (which, yes, ends up as heat), and for that, there’s no difference at all between hitting the surface and burning up in the atmosphere. Those spectacular light shows you mention range from grains of sand to small pebbles, with an absolutely amazing meteor being perhaps as large as your fist. We’re not talking here about rocks the size of a fist; we’re talking things the size of mountains. Even if you break up a mountain into pieces the size of gravel or sand, that’s still a hell of a lot of rock.
You seem to be saying two different things:
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The pieces will still be too big to burn up in the atmosphere
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Even if they all burn up, there will still be some sort of problem
I can understand #1 - actually being hit by the same total mass even if it’s not in one piece, will still cause major damage. But I’m asking about #2 - assuming a mass large enough to cause a major catastrophe, but broken up into pieces that are small enough to burn up in the atmosphere, what is the problem? I understand conceptually that, hypothetically, you are still introducing the same amount of destructive ‘energy’ if in a different form - but what is the actual physical thing which will occur? Will the atmosphere heat up to dangerous levels? Or will there be some kind of physical impact damage on the land from a wave of comet mist?
Tunguska is widely believed to have resulted from an asteroid or comet that came apart in the upper atmosphere without hitting the ground. It still did enough damage on the ground to take out a modern city, and produced enough light to be visible around much of the world.
The object that caused Tunguska is generally estimated as 20-50 meters in diameter. While that’s big in terms of average objects, it’s nothing compared to some of the monsters that are out there. Take Tunguska and multiply it by a million and it won’t matter whether anything hits the ground or not. At a certain scale, you’d have enough energy to turn the whole atmosphere into a fireball and smash everything on the ground with the shock waves.
More, there’s speculation that the Chicxulub impact was part of a multiple strike - there are possibly similarly dated craters, and the impact is suspiciously close in date to the formation of the Deccan Traps.
Looked at another way, all a nuke does is heat up the atmosphere, too. But a good-sized asteroid impact would be many orders of magnitude more heat than a nuke.
seems to me that one of the best things that could happen to Humanity is for us to detect a planet killer asteroid with approx 5 years warning.
The resulting combining of forces of all nations to develop a manhattan project style race to avert it would change the course of history more than any other event short of confirmed ET contact.
Assuming we figure it out of course.
Chronos, I was thinking the same thing but a quick BOE calculation makes me wonder – it also makes me think I forgot something like accounting for gravitational potential energy. 
100 m wide sphere of rock (2500kg/m[sup]3[/sup]) moving at 10km/s relative (asteroid plays catch up with us) to the earth would have a kinetic energy of 6.5x10[sup]16[/sup] J. If we flip that so the relative velocity is 44 km/s (now we’re playing chicken) it jump’s to 1.3x10[sup]18[/sup]J.
With an atmospheric heat capacity of 1000 J/kg/K and a mass of 5x10[sup]18[/sup]kg we would only see fractional changes in the atmosphere’s temperature on the order of 10-260 uK. Not an awful lot of heat assuming the thing is broken down to small enough pieces to burn up in the atmosphere.
Now a dino killer with a diameter of 10000 m is completely different - at 10 km/s it could heat the atmosphere by 13 degrees.
Even worse I suppose since really you’ll only shed energy into a section of the atmosphere and not really over the entire globe. Would make for a very cool thesis - Effects of localized thermal shedding on the global atmosphere during boloid events.
I think most agree that only chance would be to detect the asteroid early and far enough away to actually have the time to react and possibly divert it’s course from a direct hit.
Although, there’s always the chance a near miss will strip the earth of it’s atmosphere as it passes by anyway.
Not likely. We’re talking about asteroids with a diameter of a few tens of kilometers at most; the gravitational pull of these things is negligible compared to that of the earth. The atmosphere will stay put.
If we’re dealing with asteroids bigger than that, we won’t be able to divert them anyway, so we’ll just have to wait and see what happens. 
So from Grey’s BOE calcs, one can see that when dealing with a small asteroid (on the order of 100 meters diameter), breaking it up prior to impact may be beneficial. The total atmospheric temperature rise due to burnup is negligible, and we would save ourselves from an impact that could kick up climate-changing amounts of debris into the atmosphere. When dealing with a much larger one (on the order of 10 km diameter), we’re just about screwed either way: this embodies energies on the order of 15 million megatons (the largest nuclear device ever detonated was around 50 megatons). A surface impact would cause ridiculous seismic events and kick up life-extinguishing amounts of debris; if instead we could somehow shatter a meteor that large prior to imact, the atmospheric temperature rise would be on par with the worst clime-change forcasts, except it would happen literally overnight.
Move from a 10km-diameter bolide to a 20km one, and the energy (and therefore the temperature rise from atmospheric burnup) increases by a factor of 8, to almost 100 Kelvins. Anything not living in the ocean would be dead in fairly short order, and I imagine the eventual increase in temps of the oceans’ surface layers would upset the marine ecology so much that life throughout the oceans might eventually die off completely.
Thing is we don’t know if a blast would fragment the object. Some are piles of rubble which might scatter while others are massive chunks of nickle and iron. We simply don’t know enough to say that a nuclear blast would work - consider the oil spill in the gulf and all the various failed methods of containment used.
In all cases the most valuable thing is time. With time you can shift an asteroid’s velocity by cm/s and let time move it out of the way. Problem is we don’t know what’s out there and we’re bound to be surprised when we do find out.
Interesting JPL Paper on asteroid deflection.
The Chicxulub impactor is believed to have thrown enough material into space that the fragments’ re-entry worldwide set the atmosphere on fire and led to worldwide firestorms. That’s just the rubble bouncing up and falling back in. Presumably an equivalent mass of fragments on its way into the atmosphere would have at least as much energy.
A side effect of what was mentioned above about MIRVs is that, because the energy of impacts or explosions falls off over distances, several medium-sized impacts are probably going to cause wider destruction (because they will have more total infrastructure and people near the various ground zeroes) than one large impact. Obviously this has its limits; if something splits the planet like an axe blow, all bets are off.
Also, in theory anyway, if one large impact destroys and entire country, other countries less affected could send aid. If numerous medium-sized impacts inflict crippling damage on most countries, everyone may have their hands full locally, and there will be no relatively undamaged area to coordinate aid.
It’s even worse than that in some respects. After the intitial blast and firestorms die down the dust and debris in the atmosphere would cause a nuclear winter, so anything on land that didn’t burn would freeze or starve to death as any remaining plant life dies without sunlight.
shrug Maybe, although comparing to the Manhattan project (or the Super, or even Apollo) is probably overstating the complexity. For a reasonable sized PHO, this would be just an extension of existing space launch and space vehicle technology. The only real challenges are meeting a space booster production and launch schedule to support this (current production and launch rates of Atlas V and Delta IV boosters are 4-5 per year each for the single common core vehicles, so you’d either have to increase this substantially or enlist the aid of other nations that produce heavy-lift vehicles and integrate the payloads to this spectrum of vehicles) and develop the explosive focusing and ablative pusher system described above. Neither is trivial in terms of the effort and cost involved, but I don’t think it represents a dramatic advance in technology. The real advantage wouldn’t be so much international cooperation–which would likely fall apart after a successful effort anyway–as justifying the development of a robust and less costly space launch infrastructure rather than the haphazard and primarily military/surveillance customer base that currently exists.
One possibly deleterious side effect is that once this capability is developed, it does potentially offer the involved parties to use this as a threat, i.e. using the same technology to deliberately target another nation with a PHA. I personally don’t think this is very likely–it is substantially more difficult to target a precise intercept than it is to just push a PHA out of a collision zone, and would represent almost as much of a hazard to the aggressor nation as to that being targeted–but it would be virtually unstoppable and this has been used to argue against developing such a capability until we actually need it.
Stranger