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Machinegun vs giant asteroid

  1. Apr 24, 2015 #1
    I've heard that "blowing up" asteroids as seen in films would be ineffective, as it would turn the individual object into a shotgun spray of smaller asteroids with a larger perimeter. Which would be more likely to damage our planet.

    But what about firing rubber bullets (or objects with little destructive power, but decent kinetic "knockback") to slow down the asteroid or knock it off trajectory? I've never heard of this idea so I assume it has obvious flaws. What are they?
  2. jcsd
  3. Apr 24, 2015 #2
    Theoretically they could of course deflect the asteroid, but I think the difficult part is placing and operating such a pellet gun in space. After all, the propellant to accelerate the pellets might as well be straight applied to the object (by attaching a rocket to the asteroid) instead of going through a middleman.
  4. Apr 24, 2015 #3


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    Meteor Crater near Flagstaff, Arizona is an example of what a "small" asteroid can do when it strikes earth:


    The crater itself measures about 1.2 kilometers in diameter and is approximately 170 meters deep. The object which created this crater is thought to be a nickel-iron asteroid which measured approximately 50 meters across and weighed some 300,000 tons. It is estimated that this asteroid struck the earth at a speed of at least 12.8 kilometers per second. The energy of the impact is calculated to be about 10 megatons of TNT, or the yield of a rather large hydrogen bomb.

    You are not going to shove aside such a massive and speedy object with a rubber bullet. Remember this is what a "small" asteroid can do, and most of its mass was vaporized after it entered the atmosphere:

  5. Apr 25, 2015 #4
    Well since the question originally referred to the "disaster movies" I'll just offer a quote I remember from Armageddon:

    "General, if you consider your target: size, composition, sheer velocity, you could fire every nuke you've got at her and she'd just smile at you and keep on coming".

    Theoretically of course it is possible under F=MA, but not practical on any terms that we have today. For one thing, you need your "launcher" to be in space already so the earth's gravity does not rob any of the rubber bullets of their acceleration. And even if somehow we were able to build such a device, as rumborak points out you could just apply the force directly to asteroid/comet/meteor/derelict starship/whatever.
  6. Apr 25, 2015 #5


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    quite difficult to attach and apply a directional force to change the asteroid orbit, when the asteroid can be rotating and tumbling all over the place.
    Shooting a series of projectiles might be a way to go to change the orbit.
  7. Apr 25, 2015 #6
    1. True, but if it's rotating and tumbling all over the place you're also going to need to factor that into your calculation for the vector of your rubber bullet stream. The asteroid's rotation would have to be accounted for in either circumstance.

    2. I was trying to think in context of the question. If, hypothetically speaking, we had the technology to build a giant structure in space with the capability to fire so many rubber bullets at such high velocity that they could significantly alter the course of a projectile the likes of which we see in disaster movies like Armageddon and Deep Impact, we'd probably have the technology to do so with something much more efficient than rubber bullets.
  8. Apr 25, 2015 #7


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    Shoot the rubber bullets in the opposite direction, let the gun hit the asteroid (at relevant impact speeds, every material acts similar to rubber anyway). Replace the rubber bullets by rocket exhaust and you have the easiest and most well-studied concept of asteroid deflection.

    Time is important - the more time you have, the smaller the necessary deflection.

    A shotgun of smaller objects is better than one massive impact, if you cannot fully deflect the asteroid this is still an option.

    Why? You just care about the total momentum.
  9. Apr 25, 2015 #8
    First of all thanks for pointing out the time issue; absolutely and can't believe I forgot to include that. Second I like your idea of a "shotgun" style blast a lot better than the original "machine gun". When I posted I was thinking of a single stream of rubber bullets hitting the asteroid one after the other in the same spot in rapid succession, in which case the point at which they hit relative to the asteroid's center of mass would effect the trajectory change.
  10. Apr 25, 2015 #9


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    That was meant as comment about an asteroid destruction.
    Only if take into account how the projectiles eject mass from the asteroid. Something you really want to have, it makes the impacts much more effective as you can eject huge amounts of mass.
  11. Apr 25, 2015 #10
    BTW, the "blow-up" approach isn't necessarily flawed. Yes, it creates more fragments, but if you get the fragments small enough, they will burn up in the atmosphere.
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