I’ve never been a big fan of nuking asteroids, but this test should cause some concern:
Don Korycansky of the University of California, Santa Cruz, and Catherine Plesko of the Los Alamos National Laboratory in New Mexico simulated blowing up asteroids 1 kilometre across. When the speed of dispersal was relatively low, it took only hours for the fragments to coalesce into a new rock.
“The high-speed stuff goes away but the low-speed stuff reassembles [in] 2 to 18 hours,” Korycansky says. The simulations were presented (pdf) last week at the Lunar and Planetary Science Conference in Houston, Texas.
So you have to have a big enough bomb to really do the job. I think there are better, more controllable ways.
At the distances involved, I think diverting the rock would take a lot less effort. A nuclear nudge or two would probably do the trick.
I tend to favor nudging asteroids with technologies where you can push steadily for months on end, and use a portion of the mass of the asteroid itself as reaction mass:
http://writings.mike-combs.com/averting_armageddon.htm
“Too small a bomb” is useless, yes. But “big enough” turns out to be relatively easy. If you read on in the cited piece, you’ll find a reference to another study in 2009 that showed a 900 kiloton nuke would be enough to disperse a 1 km rock beyond danger of reassembly. A conservative rule-of-thumb for modern fission devices is about a half-ton of bomb mass per megaton of yield. Real warheads in the US inventory actually do somewhat better. And you can scale up fission device yield pretty much at will – the biggest ever tested was the 50+ MT Soviet “Tsar Bomba”, and that was deliberately scaled down from a design yield near 100 MT.
All this talk of subtler methods is fine – but if we needed to divert a big incoming rock this year, the smallest package that would do the job (with low velocity-matching requirements too) would be a fission bomb (or more likely a series of fission bombs.)
If you used a bigger bomb wouldn’t there be a danger of turning the rock from a rifle to a shotgun?
Given the raging controversy over what should or shouldn’t be done regarding the possibility of Global Warming, I can just envision the type of debate that would be ignited by scientists discovering a 2km rock heading our way. If the rock is allowed to proceed on path it could be considered and “act of God” whereas any attempt to alter its course immediately shifts culpability to those who changed its course. In that case, fragmenting the rock could be worse than doing nothing if one considers the wars that may follow.
There’s a reason why snipers don’t use shotguns. Beyond a certain range the shot spread is too great, and the impact energy of the individual pellet that might hit the target is small enough to be survivable.
Of course, making sure all of the “pellets” are too small to penetrate the target at all, either as a ground impact or an air burst, is key.
Assuming the objective is to divert the asteroid rather than scattering it, the asteroid reassembling in a few hours under self gravity is good news for a diversion with a nuclear pulse.
Well, if you shatter it, wouldn’t it tend to break up in the early stages of re-entry rather than survive intact into the lower atmosphere where it could explode or crash in a large piece? Wouldn’t the effect of the same total mass hitting the atmosphere at the same velocity be far less if said mass was dispersed, or at least loosened?
Two words: mobile laser.
As far as I know the purpose of using a nuke was to deliver enough energy in order to move the asteroid- as in. Orion type propulsion.
One problem with using nukes was it was feared that some percentage of asteroids were “rubble piles” and this was seen a problem because a nuke might blow up an asteroid instead of moving it.
So therefore if asteroid do re-form quickly there isn’t any reason not to use nukes in all cases.
You don’t blow up asteroids with nukes, that far too risky. You use stand-off nukes to ablate the surface of asteroids, generating thrust which pushes them off course.
My understanding of the issue behind the “shotgun versus rifle” argument is that if you disperse a large asteroid into a cloud of smaller rocks that all still hit the Earth, it is possible to actually increase the total damage. As I understand it, one mechanism involved is the obvious one of widely-scattered smaller surface strikes rather than one big localized strike – rather like the case of being able to do more damage with ten one-megaton nukes than with one ten-megatonner.
The other issue is less obvious – it’s what you might call the “toaster-oven effect”, with enough small rocks reentering across a wide swath of sky producing enough radiant heat to incinerate the land below. (This has also been proposed as a worldwide damage mechanism after really big single asteroid strikes, as the debris splashed from the original impact reenters across much of the rest of the world.)
In any case, if you disperse an incoming rock early enough and widely enough so the debris cloud is significantly wider than the Earth, you’ve reduced the fraction that will impact Earth – at some point you’ll have reduced it from globally disastrous levels to really-cool-meteor-shower.
Or as others have suggested, if you use a series of bombs small enough so the asteroid reassembles, you may well then be able to usefully steer aside the bulk of the asteroid with bombs after all.
Bottom line, if we were to discover a big one inbound this year, we don’t yet have practical mass-drivers or gravity tractors or propulsion lasers. Any or all of those may well eventually provide more elegant asteroid steering. But if we spot the big one this year, what we have is bombs and rockets.
Which is a whole lot better than where we would have been if we’d spotted a big one inbound in, say, 1940.
Our first efforts at asteroid mining should be focused on the near-Earth asteroids. Steer them into orbit around the Earth, then mine them till there’s nothing left. Rinse and repeat.
I’m probably getting a little ahead of our current technology, but that’s at least something to aim for.
I think you mean fusion not fission bombs, Henry.
Yup, I brain-farted there – it’s fusion bombs that scale up at something under a half-ton mass per megaton yield, not fission bombs.
And before someone jumps in and gets technical about the differences between fission, and boosted fission, and fission-fusion, and fission-fusion-fission devices (the actual cycle in what most people call H-bombs or fusion weapons) yes, those of us who care know, and those of us who don’t care, well, don’t care…
I’ve never been a big fan of nuking asteroids, but this test should cause some concern:
The problem is, it wasn’t a test. It was a simulation — based on a model of what they think some asteroids might be like.
There are a lot of uncertainties there. We will need to visit and study quite a few asteroids before we can have any reasonable certainty that we know how to deal with the problem.
That’s one reason why General Bolden’s plan is a step in the right direction.
Something does not add up here.
First, we need to remember that there is no blast effect in space. (there’s no atmosphere to carry it). So, the only way you’d end up shattering an asteroid is with a surface or subsurface detonation, and what would be the point?
The second point is that the only way to attain a re-assembly with zero net delta-v is to detonate in the exact center of mass. Otherwise, you’d impart a net delta-v regardless of whether the thing reassembled. (and imparting a new delta-v is what you want to do.)
As another poster mentioned, the ideal use of a nuke would be a close detonation, so that the heat and radiation pulse vaporizes a thin layer of the asteroid, imparting thrust.
A few small three-stage devices (colloquially called an H-bomb) in the 500KT range would be ideal (And we have them). Send them, in sequence (at least a few hours but preferably days or weeks apart at encounter point) for detonation, a few hundred yards from an asteroid. As long as they triggered at the same relative distance and direction, it wouldn’t really matter which. It would probably be easier to “shoot for the side (as seen from Earth)” and detonate at closest approach. One way this would help is that you can do an intercept with no need to match velocity or course with the target. (though, that could result in a closing speed of around 30,000 mph or higher, so timing and terminal course adjustments would be tricky, to say the least).
The key is that even for a big rock, you only need a very tiny nudge in any direction if you’re years from impact. Early detection is the key IMHO.
A stand-off nuclear nudge, IMHO is the most viable option we have based on our current tech, hardware, and delta-v abilities. (With chemical propulsion, it would be massively difficult to match course with an asteroid without a gravitational-assist and redirection course (such as a swing by Venus) that would take months or years after launch, assuming there’s even a gravitational window for it when you need it. The best combination of time and initial delta-v requirements is likely a direct-intercept trajectory with very high relative velocities at encounter.
It’s all well and good to talk about mass drivers, gravity tugs, etc, but those involve technologies we don’t presently have for that application, and have propulsive requirements far beyond our means.
My choice for a warhead would be the W88 from the Trident D5: nominal yield 475Kt, weight on the order of 800 pounds. The carrier vehicle (for minor terminal course correction and detection of closest approach) would, as a guess, not need to weigh much more than a ton. That would theoretically put it within the launch capability of several existing launchers.
None of this would be easy (understatement alert!) and it would be easier if the oncoming space rock obliged us by coming relatively close (within a few million miles) on a pass years before impact.
Exit question: If, tomorrow, we discovered a 2-mile wide asteroid seven years from impact, can anyone think of a more viable option than launching a series (months apart if need be) of W88’s with carrier vehicles (one per launcher) at it for stand-off ablative deflection? Bear in mind that every day closer to impact increases the required delta-v for a deflection, so it would definitely be a case of the sooner, the better.
I played Asteroids a lot in college. Clearly, you nuke the rock–then nuke the pieces until they are no more. Unless the simulation showed a little space ship coming at us with uncannily accurate aiming capabilities, we’d be all right.
The fact that (depending upon the asteroids structure) the little rocks will recombine is immaterial. The question is, what will the resulting vector be? If we can predict this, and manage it, that’s all we need.
One thing to note is that a surface blast of a nuke on an asteroid will impart amazingly little energy to the asteroid. Most of the damage nukes do on Earths surface comes from the tamping effect of having so much atmospheric mass ABOVE the blast.
The method the Armageddon movie used is the correct one, but its not going to blast it to smithereens unless you drilled reasonably closer to the core. A few hundred meters is only going to give it enough tamping mass to impose some worthwhile force on nudging it.
There is no blast in space, since there is no atmosphere, but this also means the energy from the bomb is delivered straight to the asteroid in the form of a very short pulse of radiation. This radiation (mostly in the form of thermal x-rays and neutrons) does cause an explosion — in the surface material of the asteroid. A layer on the surface is rapidly heated to extremely high temperature and pressure. This launches a shock wave into the asteroid as well as ejecting the expanding surface layer outward at high velocity.