To Mod: I didn’t know where to post this as it’s based on questions raised by the show “Bad Universe” on the Discovery Channel but also other areas. Relocate where you wish.
OK, I watched the new show “Bad Universe” hosted by Dr. Phil Platt, the creator of “Bad Astronomy” book and website. This week’s topic, killer asteroids. Two different segements raised questions for me. In one part of the show, they showed how a nuclear missile couldn’t stop a asteriod using a “bullet” stand-in against stand-in for various types of asteriods such as rocky ( a basalt rock) and rock pile( an asteroid made up of several asteroids held together by gravity, represented here by a bunch of rocks in a burlap sack). The rock pile had damaged to one rock, but the rest
Later, they introduced the idea of a kinetic impactor to damage and move the asteroid to a different path. So, they bring out their stand-in, a large rock sphere (I missed what it was made of, unfortunately, and the video isn’t online ). They shot their kinetic impactor. It not only causes a lot of damage to the sphere, but move it a bit. The test is declared a success.
Finally, to my questions. How could these tests be considered similar? Their stand-ins are completely different. Would the kinetic impactor be successful against the burlap sack asteroid and vice-versa? Also, the rock sphere was stationary. Dr. Pliatt stated an asteroid could travel at 15 miles per second. Wouldn’t shooting a kinetic impactor be the equivalent of shooting a BB at a moving train?
If you shoot a sack of gravel with a bullet or whatever, most of the energy of the bullet will end up being absorbed by the pieces of gravel rubbing, bouncing, and scraping against each other, and ultimately turn to heat. But that’s OK: For deflecting an asteroid, what you want to transfer is the momentum, not the kinetic energy, and the nice thing about momentum is it can’t “hide” in any other form like energy can hide in heat. You transfer momentum to the target, and that’s it, the target has a different momentum now.
Now, any momentum anything we build will have will be much smaller than the momentum of an asteroid, so we won’t be able to change its course very much. But that’s OK, if we have enough of an advance warning: A small change in course will eventually add up, and can make the difference between a hit and a miss.
Finally, what you saw on TV should be regarded as illustrative, not as an actual experiment. We know what sorts of things would be able to deflect an asteroid enough because we’ve done the calculations, not from experiment. The bags of gravel and such are mostly just to make the calculated answers seem more reasonable, and because they make for more entertaining television.
I saw that too and one other thing raised my eyebrows. They said the KT extinction event was caused by an asteroid that was so big that the top of it was still in space while the bottom of it hit the earth. In reality, the size is estimated about 6 miles diameter, making that statement ridiculous. It made me wonder if I heard it right, but the graphic seemed to match what they were saying.
Anyway, as for the kinetic impactor, I think it would be shot to the side of the asteroid to nudge it off course slightly. The idea being that it hits when it’s still months or years from earth and that slight nudge, over time, translates to a large change of course.
So that test was a success, I suppose, but those shot with the standard “nuke” also moved the rocks so they should work just as well.
That obviously didn’t clear anything up. I was a little confused about their demonstrations as well.
Phil tweeted that he should have said the top was above “most of the atmosphere”… once you are 6 miles up, I think you are above 95% of the volume of the atmosphere or so…
I haven’t seen the show in question, but it is conceptual possible to deflect a hazardous object applying only a small fraction of the kinetic energy of it as described in [post=12773678]this thread[/post], so long as you apply it at a point in the trajectory that is sufficiently distant from the intercept point. The trick is to distribute the impulse such that the momentum transfer is spread across the entire aspect of the object, so whether it is one object or many it will still accept and transfer momentum to the entire mass.
I believe the idea of the “nuke” tests was to show whether or not firing a projectile at the asteroid would be able to destroy it, not deflect it. The latter test showed that it’s possible to alter the momentum of the object, the former tests showed that destroying it would not be practical.
Of course, both series of tests were performed on Earth, where gravity and the atmosphere are both interfering as well. In a frictionless environment with no gravity, the resulting change of momentum would have been more evident than the “slight wobble” we saw on TV.
If an impact created a big explosion on the the asteroid’s surface, jetting some material into space, wouldn’t the recoil of the asteroid be greater than if it just absorbed the kinetic energy of the impact?
Well, standard pressure at that altitude is around 35% of what it is at sea level.
Volume is a bit harder to pin down, as it depends on where you decide the atmosphere stops. It’s often claimed that an altitude of 100km puts you in outer space, which means that at 6 miles you’d be above not quite 10% of the volume of the atmosphere.
I think he picked the wrong word with “volume” but 3/4 of the gasses in the atmosphere do reside in the 36k feet above the ground, so that’s probably what Phil meant by above “most of the atmosphere”. I still think that’s innacurate but I guess “as high as 3/4 of the gasses in atmosphere” just doesn’t roll off the tongue.
Whatever he meant, it seems to be a completely useless factoid at best, and I’m pretty sure the animation showed an asteroid hitting earth which appeared to be as big as the moon, but I’d have to see it again to be sure.
Anyway, it’s just a nitpick on a pop-sci program, but Plait made his name nitpicking pop-sci and I hate to see him join the dark side.
I believe that you would want to hit the object directly through its center of mass such that the majority of the energy goes towards changing its orbit rather than changing its spin. Virtually any orbital change is bound to be an improvement over one that encounters Earth at some point. Any Δv applied in any direction will change its orbit (east takes you out; out takes you west; west takes you in; in takes you east. north and south brings you back.") but the experts would determine the optimum direction in order to maximize long term safety. The show did briefly touch on the gravitational keyhole phenomenon where an object could be deflected only to return on a subsequent orbit.