jumpjack said:
A nuke is totally useless in diverting an asteroid: it has the needed energy, but it acts in all directions. We'd need a "collimated nuclear blast"... which unfortunately has not yet been invented! But we could use nuclear power to power a huge ion engine.
Not correct, even allowing for hyperbole in "totally useless".
1: Even if the uncollimated blast is only 10% effective, if the nuke is ten times the notionally necessary size, that would suffice to justify the venture -- far from useless.
2: Suppose the nuke were triggered a millisecond before impact at high relative velocity, so that what hits the surface is a small, penetrating, but rapidly expanding fireball. The effect would amount to quite a highly collimated propulsion, with most of the reaction mass supplied by the target's own surface and shallow material. Even if the target fragmented (by no means certain for a largish object), most of its mass would be be accelerated in the desired general direction, though not all equally well or precisely.
3: Even if the fragmentation amounted to converting a "rubble-pile" object into a shotgun blast, not only would that blast be collimated in a favourable direction, but fine fragmentation would produce essentially a (probably harmless) meteor shower of particles. The effect would be drastically less disastrous than the rubble striking as a pile in a single spot at a single moment, rather than a shower over a few million square km over perhaps a period of some minutes.
4: Even if the blast neatly split a physically strong and dense missile into just a few large fragments, each too large to be acceptable "incoming", each being far from harmless on striking Earth, then if in fact in fact not all were effectively deflected, the chances are that not all would hit the planet, so the effect certainly would be mitigated by that at least. And even if every last fragment hit Earth with unaltered energy, divided impacts would be less catastrophic than a single large one. To achieve such a lesser disaster would be no means a "totally useless" investment, but a highly profitable or at least provident one.
5: Even if the cleanly split body left a few large missiles still on target, then if a flotilla of small nukes followed the large nuke by a few days, then in the interval before the trailing flotilla arrived, the trajectories of the major lumps could be measured and calculated with great precision. Surgically smaller nukes could nudge remaining really dangerous rocks the extra mile. Space billiards. No problemo!
Example: suppose something like our "rubber-duck" Comet 67P/C-G broke its neck when the nuke hit it, it is quite likely that the spinning halves would follow different courses, both missing Earth. But even if one did not miss Earth, the smaller impact would be a worthwhile mitigation. And a flotilla nuke now could tackle the dangerous remnant more surgically. And if there were no significant fragmentation, the smaller nukes could tackle the main body again for further longer-term improvements in prospects.
6: Gentler nudges could be negotiated in various ways. A series of blasts from space could vapourise part of the missile's surface each in turn till the reaction produced sufficient deflection.
7: Or the nuke could carry packing in the right orientation for the packing mass (wadding if you like) to be accelerated for impact, but not vapourised by the blast. That wadding then could hit the target at enormous velocity without vapourisation. Such a soggy solid wadding striking the target would amount to a high degree of collimation of blast. The mass of the carrier craft could act as a tamper from behind, or might act as the missile itself. The most probably efficient approach should be left to some intelligent engineer (if the project could afford one; if not, they could always get a lawyer or politician or priest instead... Details, details...)
8: I bet that a very high degree of collimation could be achieved, though expensively, by including an asymmetrical hollow shell around the nuke, so designed that it vapourises progressively from the far end, driving the target end with great acceleration. Whether this would be worth the extra mass required to deliver to the ignition site, is a different question. Whether such a mass as a missile would be more useful as a reflector or tamper, is also something to be calculated rather than argued.
It is dangerous to dismiss as absolutely useless any technology that offers wide ranges of modes of possible application. Someone might have worked out more ways of applying it than were at first obvious.
