Sorry if you didn’t get my point, which I admit I made obliquely.
My point was one of scale, the same problem with scale that made SamuelA mischaracterize Stranger’s point. Here’s the full post from the other thread.
He is specifically referring to an extreme example of much larger than 1 km, on the order of 10 km, like the K-T asteroid. On that scale a single nuke trying to affect the asteroid has about as much chance as the bird affecting the truck.
SamuelA is just wrong in his scenario and his criticism. Stranger is correct that a different strategy must be found.
Please re-read Stranger’s post instead of blindly assuming it’s correct. You obviously didn’t read it. Highlight for me the words where he says it is a problem of scale.
I’ve read in other sources that you need to adjust the velocity vector of the asteroid by less than 1 cm/second 5 years ahead of time to prevent it from hitting the earth. A nuclear shaped charge is also known as an orion drive, without the pusher plate since we don’t care what happens to the asteroid. You would use thousands of charges to do it, launched from numerous robotic spacecraft. (each warhead is a few hundred pounds at most, and obviously you mass produce heavy lift rockets to do this. This does assume you have 5-10 years of warning)
If the asteroid breaks up into smaller pieces, you obviously send your robotic spacecraft loaded with shaped charges to the most massive and coherent pieces and redirect those.
Basically, instead of a bird, it’s a bowling ball, and instead of traveling at 30 mph, it’s traveling at 3000 mph. And the truck has no engine. And you do this hundreds of times. And you don’t try to stop the truck, just push it slightly so it doesn’t hit a target it was heading for that is 1000 miles away.
Stranger used very aggressive numbers of 10-100 m/s deflection velocity. This is based on a distance of 30,000,000 km and some safety margin for missing Earth. But that is only about 14 days for an asteroid going at 25 km/s. If the timeframe is that narrow, we are screwed anyway; there’s no way we could get something up there that quickly. It’s a bad movie plot scenario and not worth considering.
In reality, either we can do something about it years in advance or we all die. If we have 10 years, then (very roughly) we can get the same deflection with a 4 cm/s delta-V. And potentially much less is needed depending on the geometry and orbital parameters.
Fortunately, the asteroids in the civilization-killing range are more easily detectable than otherwise. So there is more time to plan and a longer period over which to apply the forces.
As you say, scale matters. And the scales here are tremendous, since it is, after all, space. Any amount of deflection is sufficient if you start early enough. A 40 t truck inelastically colliding with a 0.2 kg bird slows it from 60 mph to 59.9997 mph. That is enough to miss by 80 feet after 50 hours. The same principle applies to asteroids, just with an even larger scale.
You don’t make that easy since you never cited the original thread or post.
The thread is here, and quick perusal shows that your OP objection was already raised in that thread, and answered there, by Stranger and by others. So it’s not clear to me what purpose is served by starting yet another thread to ask the exact same question a second time. ISTM to be mostly trying to hammer on the fact that Stranger “claimed many times to be a rocket scientist” yet in your mind he’s wrong and you’re right. But my impression here is that it’s you who is wrong, and indeed is completely misconstruing what the question even is.
Anyone going to Stranger’s post in that cited thread with the relevant quote can clearly see that he’s replying to a question about the scenario of a standoff nuclear explosion to divert an asteroid. The simplistic elementary notions of how momentum works that you seem so anxious to raise have essentially nothing to do with it. The surface and internal composition and other physical characteristics (yes, even spin) of the body in question have everything to do with it. And in that respect you are totally wrong over here when you claim that “The sudden flash is what generates most of the impulse. I dismiss it [importance of internal forces and composition] cavalierly because I know what I’m talking about.” No, you do not. It is absolutely not the particulate or radiative emissions (“the sudden flash”) that create the impulse, it’s the secondary thermal effects on the body itself which are entirely dependent on its physical characteristics:
A standoff nuclear explosion produces a change in the momentum of an asteroid primarily by means of material that is evaporated or spalled (by rapid thermal expansion) from its surface due to the deposition of energy. This energy is transmitted from the explosion mostly by means of neutrons and X rays. http://research.dynamicpatterns.com/wp-content/uploads/2011/03/Deflecting-Asteroids-by-Means-of-Standoff-Nuclear-Explosions.pdf Stranger tells us that he’s worked professionally on a study of exactly these problems. In the past he provided some additional detail here. I’m quite sure that Stranger understands the physical principles of momentum without your assistance, but that’s not what this is about.
Quote where he mentions scale because the impulse isn’t enough and I’ll concede the argument.
Oh, right, he doesn’t.
You’re damn straight I’m right.
As for ablative emission, first of all, during the flash is when most of that emission will happen. Almost immediately as the fireball from the nuke expands, the surface will no longer be hot enough to emit vaporized rock. You can see this effect clearly in high speed footage from nuclear explosions and this is the principle upon which ablative laser propulsion works.
Basically a standoff nuke delivers a huge amount of energy, in the form of gamma radiation to the asteroid. Note, there is essentially no useful momentum here. The nuke bathes the asteroid in photons. What we hope happens is that there is a nice amount of asteroid that is vaporised explosively, the bathed area is depressed down by the reaction to this vaporisation, and then bounces back, and a big slab of asteroid reacts back towards where the nuke was. The conservation of momentum between the remaining asteroid and that part of the asteroid that has reacted is where you get the momentum change in the asteroid. The nuke did not deposit any meaningful momentum into the system itself. It enabled the splitting of the asteroid into parts going in opposite directions - and the momentum is conserved there.
What Stranger is noting is that if your asteroid doesn’t behave as a rigid body there is a good chance that the energy will end up being dissipated in the asteroid’s structure, and rather than causing a nice high velocity reaction of a part of the surface, may just end up heating large amounts of rock to melting point and otherwise uselessly adsorbing the energy.
I missed the edit window, to make the point about scale is that larger asteroids are less likely to be homogeneous and thus the internal mechanics of the structure to reflect the depression of the surface reacting to the nuke are doing to be dispersive, and you won’t get the needed coherency to usefully spray back a huge pile of material. Nice small, especially mostly metal asteroids should be much better. Big amorphous rocky things, not so good.
I had that idea myself a few years ago. Fill a bag up with rubble from the asteroid, then dangle the bag as close to the asteroid as possible on a long tether; this moves the centre of gravity of the gravity tractor as close as possible to the asteroid, while still allowing the drive system to be relatively distant from the surface.
The thrusters need to be as far away as possible from the asteroid so that they can be oriented at a relatively shallow angle (of course, the exhaust from the thrusters must not hit the asteroid, or you get a reduction in net acceleration).
I gloss over the problems associated with landing on a rapidly-rotating asteroid and collecting rubble, which are considerable (as Philae demonstrated).
Ah, a classic SamuelA post where magical robotic armadas of easily manufactured heavy lift rockets and high yield nuclear device exists at the whim of the author.
If it’s an iron asteroid, how about The Mother Of All Magnets? It shouldn’t be that hard to build subspace sensors that can determine the composition from a lightyear or two away.
And this analysis is wrong. That’s why I posted this in General Questions. You’ve made the same fundamental physics error that Stranger has.
There are 2 effects here.
a. You set the nuke off near a homogenous patch of asteroid material. (ideally mostly the same color and material). You choose your detonation distance so that the intense heat of the fireball will momentarily cause the surface of the patch to turn to gas. This is because a light source of billions of degrees, in an environment with no background pressure, it’s pretty easy to get even iron to boil.
This is a surface phenomenon. Even if the asteroid is rotating, as long as it’s not rotating absurdly fast, the fireball will cool within microseconds and the outgassing will stop.
b. The nuke has kilograms of some solid on the side facing the asteroid. When the warhead goes off, the intense heat and radiation pressure sends that metal, which is now gas, at the asteroid at hundreds of kilometers a second. The original ‘project orion’ paper design used a tungsten plate.
Both (a) and (b) cause a momentum transfer. The asteroid now either has lost mass, which was escaping in a direction away from it, or gained mass at high velocity from the opposite direction. Both (a) and (b) affect the asteroid’s vector very slightly. You then have to keep doing this again and again.
It doesn’t matter what the asteroid does. If the asteroid is a giant spring that springs back and forth afterwards, it can no more negate the laws of conservation of momentum than anything else. If the asteroid does fragment into multiple pieces, some of those pieces will now have new velocity vectors and can be dealt with independently.
Remember WW2? If we’re talking about a 10 kilometer asteroid on a collision course, 10 years in advance, you can pretty much use ww2 numbers to guess what the response would be. There would be a herculean effort to produce the pulse charges and rockets.
Or not. We are already facing civilization altering levels of AGW and sizeable fractions of at least the US populace cannot agree the problem is real. Despite multiple decades of supporting data and PR effort to educate them.
If we have 10 years of warning, we have 10 years of perturbational uncertainty. So we will not have “scientists predict certain doom for Earth on 12/18/2027 at 6pm Eastern.” That is unrealistic as a scientific fact.
What we will have “Scientists have identified an error ellipse in time and space 30x the cross section of Earth. The Earth is within the ellipse, and near its centroid.” That’s the scientific fact we’ll have.
Accompanied of course by “<This> list of scientists thinks this is a serious problem. <This> list of scientists are hemming and hawing and nitpicking parts of the calc. And <this> list of con men are fomenting craziness against the idea purely for personal fame and profit. <This> list of religious leaders say its time to repent by joining their particular churches and praying mightily. <This> list of religious leaders say it’s time to accept our collective guilt and doom as <some god>'s punishment.”
At about 3 weeks before impact the situation will become statistically unequivocal. As shown in the other thread, at about 15 minutes the impactor will become visible to the general public. And that’s when unanimity will first dawn. Net of the “it is <some god>'s will.” crowd.
Maybe so. The giant asteroid is more tangible and it only takes a single nation’s effort to redirect it. If the entire USA switched to EVs in 5 years, it wouldn’t make a dent in the AGW problem. (it might buy a few more years until the 2 degrees C limit is reached, but then again, it would lower the market price for automobile fuel worldwide and people in other nations would respond by using even more of it)
