Effectiveness of nuclear weapon in vacuum

  1. I saw a science fiction movie in which some guys planted an atomic bomb in a cave in an asteroid which was headed toward Earth. The bomb blew the asceroid into a million meteroites which harmlessly burned up in the atmosphere.

    Is this possible ?

    An atomic bomb, detonated on Earth, creates an enormous amount of heat. That heat can melt things, and also creates an enormous outwardly radiating pressure pulse in the air. That pulse causes damage.

    However, in a vacuum, there is no air. Hence, no pressure pulse. The matter of which the bomb is constructed may be ejected at high velocity, but that is a very small amount of ballistic stuff.

    I guess the radiation could melt the asteroid. But it seems that, given the heat capacity of rocks, and computing the total heat capacity of an asteroid which is, say, 10 miles in diameter, even the energy in a large atomic bomb is insufficient to melt that astreroid.

    And what good would melting do anyway ?

    So can an atomic bomb destroy an asteroid in space ?

    Thank you.
     
  2. jcsd
  3. One thing you must consider is that the majority of the energy will be deposited locally (relative to the size of the asteroid) reaching temperatures hot enough to turn the rock into a plasma. Theoretically this could cause a pressure pulse through the rock/ice splitting the asteroid into many bits.

    The composition of asteroids is not well known. Some appear to be mostly ice, some solid rock. Some asteroids are thought to be very solid and others very porous and loose. The net result would really depend on these types of things. For a loose asteroid I could see it working as described in the movie, for a solid one I don't think it would work that way unless it was very small or used a very large nuke. See Tsar Bomba http://en.wikipedia.org/wiki/Tsar_bomba.

    More realistically (IMHO), nuclear weapons could be used to move asteroids out of the way instead of blowing them up. Detonating a nuke right next to an asteroid (or even slightly under the surface) could deflect an asteroid. The idea is that some material will be ejected away from the surface changing the momentum of the asteroid to avoid a disastrous conclusion. This has never been tried however and there is a lot of debate if shooting nukes at asteroids is the best idea. There are many other suggestions, such as using a laser to continuously eject bits of mater off the surface of the asteroid to change it's direction.
     
  4. There are quite a few people with considerable expertise in (a) astrophysics and (b) building really big bombs who seriously study the disruption and deflection of potentially dangerous asteroid-type objects using nuclear explosives.

    One of those people is LLNL's David Dearborn.



    This is very interesting stuff :)
     
    Last edited by a moderator: Sep 25, 2014
  5. The thing about using a bomb is, I think it would be impossible to calculated the new trajectory you will sent the asteroid on. You might be gaurenteed it wont hit you on it's first pass, but who's to say you won't just introduce it on a new trajectory that will hit you next decade or on the return trip?

    I think a system that can supply a much more controlled thrust so we could steer the object far away from the earth would be a better approach personally, however such a system would be more expensive and require a lot more warning time.
     
  6. FlexGunship

    FlexGunship 739
    Gold Member

    I'd like to rephrase the OP's question and try to answer it: "how does an atomic weapon's damage scale with no atmosphere to provide a medium for compression."

    The most interesting aspect of this discussion is the component of vacuum. The most damage coming from the weapon would likely be in the temporary formation of an atmosphere due to the rapid boiling of the surround asteroid and then the immediate dissipation of that atmosphere into vacuum.

    The result would be similar to a vacuum bomb (or fuel bomb) which attempts to burn all of the oxygen in the atmosphere as quickly as possible to create a reverse pressure front. Interestingly, the magnitude of damage is similar.

    It's hard to really answer your question since the composition of asteroids is still in question, however, it seems likely that conservation of energy in the system would do the work regardless of the manner in which it is applied.
     
  7. Astronuc

    Staff: Mentor

    In a vacuum and at some distance from a nuclear detonation, the effect would be purely radiative, and would decrease proportionally to 1/r2, and the radiative effect would have to trigger some ablation to be effective.

    Nearby, there might be a mass effect if the nuclear system has substantial mass.

    In contact, the blast would have an effect, but it would be mostly dominated by whatever mass was blasted from the asteroid.
     
  8. Morbius

    Morbius 1,160
    Science Advisor

    minerva,

    Until he retired recently, Dr. Dearborn was one of my colleagues. His work on deflecting
    asteroids was a frequent topic of discussion by several of us around the lunch table at
    the Lab cafeteria.

    If a comet like Shoemaker-Levy 9 which hit Jupiter a few years ago were headed toward
    Earth instead, a nuclear weapon would be the ONLY means we would have to save the
    Earth. The "gravity tractors" and other proposed solutions take YEARS to work - you have
    to discover that the asteroid is going to hit the Earth years ahead of time.

    If a comet comes screaming out of the Oort cloud at us - we are only going to have a few
    months notice of the impending disaster; and we won't have years and many orbits to
    deflect the asteroid - we have to deflect it NOW on this orbit.

    The desired degree of deflection can usually be done by either slowing or speeding up the
    asteroid or comet in order to change its orbital parameters. Detonating the weapon out in
    front of, or directly behind the object can either slow / accelerate the object.

    I also attended a seminar Dr. Dearborn gave at the Bankhead Theater in Livermore as part
    of the Lab-sponsored "Science on Saturday" lectures for high school students.

    Dr. Gregory Greenman
    Physicist
     
    Last edited by a moderator: Sep 25, 2014
  9. Morbius

    Morbius 1,160
    Science Advisor

    Melting of the entire asteroid isn't what would happen - and it's not what you are
    attempting anyway.

    The energy of the weapon would be deposited in a small part of the asteroid. That energy
    would go way beyond just melting this portion - it would VAPORIZE it. The vaporized material
    would be ejected off the asteroid.

    By conservation of momentum - the asteroid has to recoil away - in the opposite direction.
    That will be enough to deflect the asteroid into a new trajectory that will miss the Earth.

    The real worry would be a comet like Shoemaker-Levy 9 that struck Jupiter a few years ago.
    In that case, we don't get years of warning, like we can with asteroids. We would only have
    a few months notice to avoid getting hit like Jupiter. In that case, a nuclear weapon is the
    ONLY thing we have that could save the Earth.

    Dr. Gregory Greenman
    Physicist
     
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