Well you’d have to build a stable spacebased laser capable of delivering significant power to a rotating object millions of kilometers away while somehow keeping the beam divergence small enough to actually hit the thing and not half of asteroid belt.
So, tricky.
Sorry, but we’re in GQ. The phrase ‘planet-killer’ should be taken to mean exactly that. If you want to talk about ‘merely’ an extinction-level event, then you need to say so.
A true planet killer would need to deliver enough energy to melt the earth’s crust, in order to kill all the bacteria. There is nothing we could do to affect an impactor of that size.
High power lasers are fundamentally hugely inefficient in terms of energy throughput. You’d need to take the energy requirements to achieve a satisfactory deflection and multiply them by a hundred or more, not even counting for the massive inefficiency in this type of propulsion, i.e. without a nozzle to catch energy from propellant plume expansion the propulsive efficiency is going to be very low.
The problem with mounting a rocket or radiating atomic pile or whatever propulsive method to the surface of the object is that you’d need to be able to distribute the thrust across the entire structure without fragmenting it, and control the orientation. By providing impulse via a fast-moving cloud of vaporized material, you push at the entire facing aspect of the object, regardless of whether it is rotating or in fragments. You’ll get a lot of losses due to energy that isn’t effectively transferred, but it is the most even distribution of load across the entire object.
While this technique obviously hasn’t been used to divert objects in space, it is drawn from the Teller-Ulam method in thermonuclear fusion weapons by which a fission Primary is used to create the conditions to initiate fusion in the Secondary (and higher stages). The x-ray flux is focused by the case (hohlraum) to generate heat and pressure into a sacrificial polystyrene layer that compresses the tamper case of the Secondary to cause fusion. In our case, we’d replace the Secondary with more foam or other material to be vaporized and focused on the target in a controlled impulse, not dissimilar to the Orion nuclear pulse propulsion rocket concept. This was the method used for testing the yield and reliability of small fission Primaries to thermonuclear fusion weapons after the 1974 Threshold Test Ban Treaty, which limited total yield of nuclear testing to 150 kT by assessing the conditions on an inert, instrumented tamper and comparing to hydrocode simulations, weapon designers could assess the yield of the Secondary. So this really is an extension of already existing technology. The only think novel about it is the application.
Stranger
Quartz said:
Okay, we’re using vague terms and then arguing over what they mean. My point was that a million city killers would essentially be a global extinction event. The effects are not radically different in scope of damage. One large impact that sends a global fire storm around the planet, then coats the atmosphere with smoke and ash for months, killing all plant life; or a million somewhat smaller impacts that create localized firestorms that effectively cover the surface of Earth, then throw up dirt and ash to fill the sky for several months and kill off all plant life.
Musicat said:
You really think there is a difference? By the way, it wasn’t weakening internal forces, the body broke up on a previous close pass to Jupiter where the tidal stresses exceeded the body’s internal structural forces. Yes, the close pass was July '92, first seen as fragments in March '93, impact July '94. Wiki says there were 23 identifiable parts (labeled A through W) that ranged in size from a few hundred meters to a couple kilometers each.
Bottom line: most of the factors that make blowing it up to potentially reduce the amount that impacts Earth at one time (if at all) useful are offset by factors that make moving the whole object more sensible.
If it is far away and blowing it up would give time for scatter and other factors to make the impact possibly more survivable, then you have the time and same amount of energy to direct the whole body and make tracking easier and reduce the long term risk of getting hit (esp repeatedly hit). If you have very little time to affect a change so redirecting the whole won’t seem effective, then blowing up will not substantially reduce the amount of energy that the Earth absorbs, and so will not reduce the overall effects significantly.
Stranger, you seem well up to speed on this topic, and your proposed method certainly sounds feasible to me (much more so than any alternatives I’ve heard of, including gravity tractor). My reservations are mostly of the form that we haven’t yet demonstrated that this works (actually affects an asteroid), so the folks who are actively studying the topic are not set on any one method yet. (And for all I know, you are one of those experts working with my buddy. Say hi to Paul for me. ;))
Any of those methods could in principle work, if we had enough lead time. And while, as Stranger points out, a laser is pretty inefficient, that might be made up for by the fact that the laser would presumably be Earth-mounted, and not have to be launched to the asteroid.
What’s the best method to use for deflection would also depend significantly on the nature of the object. Any method based on vaporizing material for propulsion, for instance, would work a lot better on a comet than an asteroid, since they’re mostly made up of easily-vaporized water, not hard-to-vaporize rock or metal.
And if the object were rotating significantly, we could probably just put the propulsion system (whatever it is) at or near the pole. We don’t much care which way we deflect it, just so long as it’s away from the Earth.
So, tricky. 
But if I had to make a choice I would go with shaped nuclear charge deflecting the object. Though I would prefer to validate the gravity tug and nuclear deflection methods before we have to roll the dice.
You probably wouldn’t want to do that. Thermal blooming effects from a high energy laser would distort and sap the beam. Using a low frequency that is transparent to the atmosphere (microwave) would give to large of a div angle to be useful. And, of course, you’d be subject to Earth’s rotation with respect to the object. A laser in an Earth polar orbit or libration point would be better and would possibly allow you to use solar energy to power your system, but regardless, high energy lasers are still massively inefficient, and also fall into the category of not-yet-ready-for-prime-time technology.
Well, yes and no. We wouldn’t just want to push it straight up out of the ecliptic, because it’ll then just orbit an an angle that still intercepts Earth orbit. Assuming that it is orbiting in the same direction as Earth and the intercept is a converging angle, the best bet would probably be to retard its trajectory so that it falls inward (toward a smaller orbit). It depends on the particular, though if you’re expending enough energy to significantly change the orbit, the the amount of energy to change its orientation is probably not of issue. This assumes, of course, that it isn’t already spinning rapidly or erratically enough to make landing upon it unfeasible. Some kind of standoff method of altering it that doesn’t rely upon the particulars of the objects rotational dynamics or surface integrity is probably the safest bet.
Stranger
I should have said “internal forces rendered too weak to hold the object together when acted upon by gradually increasing external forces.” I didn’t mean that it spontaneously lost all its gravity.
Is there a difference? I would think so. A satellite separating due to forces like the Roche Limit would be gradually breaking apart and slowly separating, but one blown up by a very powerful bomb would have particles rapidly start moving in many directions. I don’t think a nuked object would look much like the Shoemaker-Levy comet after a few years or decades.
First remember the velocities we are looking at here. Typical impact velocities are 17 km/s for asteroids and 51 km/s for comets. The maximum Earth impact velocity for objects orbiting the sun is 72 km/s. The Atmosphere is ~50Km. So most objects would hit the ground almost instantly(If they are large enough). By breaking the object up you spread the area effect . Like grilling with charcoal. You don’t cook with one large charcoal brick. You cook with several dozen smaller ones to spread the heat around. Thats what breaking one up would do to the Earth.
But if you spread the charcoal around enough, in very small pieces, you can’t cook anything at all.
And I think you are ignoring what air resistance does to small objects entering the atmosphere. It prevents them from hitting the ground “almost instantly.”
Sure, if you give it several seconds to work. But you’re not.
Besides, as mentioned many times before, it’s not hitting the ground that’s the problem, it’s hitting any part of the Earth, including the atmosphere.
This is an example of why argument by metaphor is such a misleading way to reason. As has already been detailed, not only in explanation but in calculation, that the effects of a nuclear explosion will not pulverize or fragment a large object into small fragments of harmless size; that even if it could fragment a large object into smaller fragments (of ~1m[sup]3[/sup]) there would be thousands or millions of them in orbits that would periodically intercept that of the Earth, and they would still contain as much kinetic energy each as a strategic nuclear weapon; that it would take as much or more energy to divert or disperse a collection of fragments from an orbital intercept course as it would a single larger object; and that the gross energy input of of such an object, either composite or in fragments, would be destructive to the atmosphere and climate even if it did not reach the ground intact.
The point you persist in making --that by fragmenting or pulverizing it–has been repeatedly addressed and demonstrated to be inaccurate. Continuing to reiterate this argument without basis is bordering on intentional obtuseness. “Nuking” a threatening object is neither a desirable or workable solution.
Stranger
Strange, almost any clear night I can look up at the sky and see objects plummeting harmlessly to Earth after hitting the atmosphere. Sometimes during meteor showers there are many.
So many objects, if sufficiently small and spread out in time or space, do not present a hazard to us. The problem remains to insure such fragmentation. If a nuclear burst won’t do it – I don’t care if you nuke it or pour applesauce on it – isn’t there some kind of action that will?
How about hitting it with intentional obtuseness?
Even assuming that you do somehow pulverize it into pieces that are small enough to be individually safe, you still need to scatter those pieces so they don’t all hit at once, and that’s significantly harder than deflecting it in one piece. As Stranger especially has said many times.
And moreover, you’ll need apply at least as much energy to divert or distribute the resultant debris field as to deflect the original object. By distributing the field, you also increase the uncertainty of intercept, and increase the sensitivity of the smaller objects to gravitational perturbations, further increasing uncertainty.
Even if you could somehow pulverize the object into a cloud of fine dust and discount the effects of the kinetic energy impulse into the atmosphere, you’d still end up with a major hazard to spacecraft and satellites, costing tens or hundreds of billions of dollars in replacement cost. That cost alone could justify the effort to divert the object.
Stranger
Musicat said:
Sure - nanobots. They can disassemble it at the molecular level, and then turn it into building materials or fuel or whatever.
Oh, you want existing technology, not science fantasy.