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This was discussed in another question related to reaching earth's core using nukes. I re-ask it here to (1) stop unintentionally hijacking other topics, and (2) be able to respond (as someone cut my newbie score from 50 to 49).

My position, to be clear, is that it might be possible (depending on the scale of things) but is highly improbable today, despite the landing of a small unmanned craft on such a celestial body. I stress that point since either long-term contact or penetration of such a body would seem to be a must, given the fact that a nuke does little in the vacuum of space other than produce enormous light, heat, EMP and other forms of EM radiation for the briefest instant. Landing and anchoring a robot capable of drilling into an object possibly made up largely of iron would be a far cry from anything that's been done.

Surely one giant step has been taken. Further, a small nuclear reactor (Rover) has been successfully deposited on Mars. But the details of a mission that cannot fail have to be ironed out. I have a good deal of experience with defense projects, and while most are remarkably successful, they are also remarkably time and effort consuming, including very long test phases. Asteroid test environments aren't easy to come by.

So (a) could it be done at all, (b) if so, what stages of reasonable development effort would have to be met, and (c) how long would a concentrated effort to produce something with likelihood of success take?

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    $\begingroup$ I think the best solution might actually be to explode a bomb designed to direct its radiation mostly in one direction at some distance. The gamma radiation will vaporise parts of the surface which will produce thrust spread over the entire surface, which avoids the creation of Earth bound meteoroid shrapnel. At that point diverting the asteroid comes a simple question of hitting it early enough. I think there was discussion about using large mirror or laser in a similar way. The bomb would be more compact and faster to deploy, though. $\endgroup$ – Ville Niemi Jul 7 '16 at 4:25
  • $\begingroup$ Using multiple bombs in sequence would further reduce the chances of breakup. $\endgroup$ – Ville Niemi Jul 7 '16 at 4:26
  • $\begingroup$ I've heard the alter-trajectory-via-proximity-blast theory. It sounds plausible if enough thrust due to vaporization could be imparted in that short time to alter the trajectory. As you suggest, the farther away, the better the odds. $\endgroup$ – leoman Jul 7 '16 at 4:31
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It's actually trivial to change the course of an asteroid, even if you don't have a nuke. Any impact will do. When you do, you apply a change in velocity (delta-V) to the asteroid. The further the asteroid is away from the Earth, the less delta-V you need to protect the Earth because you have a longer time for that change in velocity to take effect.

I don't remember the study, but a few years back I read that we could easily redirect an asteroid with the present day technology. However, our ability to detect said asteroid early enough to avoid needing a stupendous amount of delta-V was lacking. By the time our current technology can detect an asteroid and determine that its trajectory intersects the Earth, its too late.

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Placing a nuclear device on the surface or even buried inside an asteroid (as suggested in some SF movies) could be positively dangerous, especially if the asteroid is a loose pile of rubble. Instead of a diverted asteroid you now have a cloud of fragments mostly rushing towards you on nearly the same trajectory as before.

A nuclear device at a stand off from the asteroid can ablate some of the surface material with the X-ray pulse, essentially creating a rocket motor out of the asteroid. Much more efficient would be to create a sort of nuclear shaped charge and direct a high velocity plasma stream at the asteroid, much like the ORION nuclear pulse drive, transferring a great deal of momentum. This would,be a high thrust/high ISP drive, one of the few known to science.

enter image description here

This image (via Scott Lowthar's Unwanted Blog) gives you a good idea of what is being described, and the scale image of the asteroid in question is the Chelyabinsk meteor. For a larger one, a series of small devices detonated in sequence is probably more efficient and provides more control than trying to apply one massive impulse.

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  • $\begingroup$ Good info. This relatively small object exploded with about the energy of a small thermonuclear warhead. What about much worse (and hopefully rarer) cases? E.g. an irregularly shaped iron object with the volume of a sphere one mile in diameter would have mass of around 1.7x10^13 kg. Has anaylsis been done on what energy might be required to deflect something like that, and at what distance? Or is this scaled out of the realm of current possibility? $\endgroup$ – leoman Jul 7 '16 at 21:10
  • $\begingroup$ The main variables we need to know is the distance from Earth and the relative velocity. The farther away you "kick" the object, the less overall energy you will need to deflect it from its current orbit. If you somehow detect it very close to Earth, you will most certainly need far more energy to kick it out of its intersecting orbit. Jerry Pournelle's "A step Farther Out" first brought up the idea, and I think Atomic Rockets might have a link to how to calculate this. $\endgroup$ – Thucydides Jul 7 '16 at 23:12
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http://www.nasa.gov/pdf/171331main_NEO_report_march07.pdf

To quote this 2007 NASA study

  • Nuclear standoff explosions are assessed to be 10-100 times more effective than the non-nuclear alternatives analyzed in this study. Other techniques involving the surface or subsurface use of nuclear explosives may be more efficient, but they run an increased risk of fracturing the target NEO. They also carry higher development and operations risks.
  • Non-nuclear kinetic impactors are the most mature approach and could be used in some deflection/mitigation scenarios, especially for NEOs that consist of a single small, solid body.
  • “Slow push” mitigation techniques are the most expensive, have the lowest level of technical readiness, and their ability to both travel to and divert a threatening NEO would be limited unless mission durations of many years to decades are possible.
  • 30-80 percent of potentially hazardous NEOs are in orbits that are beyond the capability of current or planned launch systems. Therefore, planetary gravity assist swingby trajectories or on-orbit assembly of modular propulsion systems may be needed to augment launch vehicle performance, if these objects need to be deflected.

So in summary, nuclear is actually our most effective option at the moment, but it is crude and very "brute force-ish". And as others have mentioned, the composition of asteroids is often unknown, and if they are more or less loose piles of rubble, an explosion will only fragment it into pieces and turn it into buckshot headed towards Earth. Also worth noting that there are treaties forbidding nuclear weapons in space, although in a scenario like this, international law probably isn't the top priority.

In the long term, slow push technologies (gravity tractors, pulse drives) could work provided that we have enough warning.

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Can you? Sure.

Should you? Probably not.

Blowing stuff up might look cool and dramatic in movies, but blowing up an asteroid with a nuke isn't a very good idea - instead of one big rock, you'll wind up peppering the Earth with a whole bunch of smaller ones - which means that the chances of major population centers being hit will be much higher. Also, the rocks will now be radioactive.

Instead of blowing it up, deflecting the asteroid is a better idea, and yes, you could use nukes for this. The problem is that detonating a nuke on the outside of an asteroid means that most of the nuke's energy is being blown into space rather than deflecting the asteroid itself. Kind of a waste.

The most sensible way of dealing with an incoming asteroid threat, unfortunately for Hollywood, is also the most boring - gently push it out of the way over the course of several months or years. You can do this with rockets, reflected sunlight, or laser ablation. Slow and steady saves the species.

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