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  #51  
Old 12-19-2017, 02:02 AM
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All the ones I've seen photographs of look pretty solid. Where is the evidence of all these "gravel pile" asteroids if every one we've imaged at high resolution looks like a rock?
Some of the medium-to-smaller asteroids do look quite loosely packed. Here's Itokawa asteroid, which is probably best described as a contact binary; it seems to consist of two fairly solid lumps joined together by a 'neck' of dusty material and boulders. I would expect this to change shape considerably if exposed to any propulsive blast from a nuke - maybe even split into two or more pieces.
https://upload.wikimedia.org/wikiped...id_Itokawa.jpg
  #52  
Old 12-19-2017, 04:55 AM
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.

As the article you cited described described, a "rubble pile" in astrogeological terms means the object is like a partially melted shot and wax slug projectile commonly fired from a shotgun. It is cohesively bound together, just not brittle.
Having just now heard of it, you are of course now qualified to correct the established consensus reached by the real scientists on the matter.

Another link.

Another link.

Another link

Another link.
  #53  
Old 12-19-2017, 07:16 AM
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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.
Let's assume - and this is unlikely, as LSLGuy has suggested - that we can identify a collision-course asteroid ten years out with sufficient accuracy to compel a national or international response. Let us further suppose that the chosen response is to launch a series of nukes to try to divert the asteroid with a sideways nudge.

If you want to make it happen, you have to deliver your nukes to the asteroid while it's still a long ways out. That means your missiles need a LOT of speed, on top of an escape velocity of about 25,000 MPH. These are going to be BIG rockets. The Saturn V could take 107,000 pounds to trans-lunar injection. A nuke weighs a lot less than that, but you're not stopping at the moon; you need to be hustling past the moon about as fast as you can go, if you want to get to the asteroid in time. Plus, if you want to consistently detonate your nukes on one side of the asteroid, you're going to need some amazing targeting hardware and software, and/or some maneuvering thrusters. Maybe even some braking thrusters so you don't require amazingly precise detonation timing. So your payload isn't just a simple 1,000-pound nuke. Bottom line, you're stuck with big rockets, which take a while to design and build.

So...ten years' warning? Even assuming sufficient motivation, it'll be at least two years before your first nuke leaves the earth, and it'll be several more years before it gets to the asteroid. At which point the asteroid is no longer ten years out, it's more like four years out, and now needs a pretty big nudge to assure a miss. But the rest of your nukes are going to arrive later, when the asteroid is even closer. I think it's pretty optimistic to believe that we could build/launch rockets the size of the Saturn V at intervals shorter than six months, so you're only going to get a handful of nukes up to the asteroid before it lands in your front yard.
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Old 12-19-2017, 09:50 AM
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Well, a near-Earth asteroid is likely to have an orbital period fairly close to the Earth's, and make many near misses before it makes a hit. For instance, Apophis has a period of 0.89 years. So in theory if you wanted to nudge Apophis out of a potential hit, you could send whatever to it during one of the scheduled near-misses, such as in 2029, when it will come within around 20,000 miles of the surface. You are therefore probably a tiny bit less screwed seeing a Near-Earth asteroid 10 years out than you are seeing a comet 10 years out.
  #55  
Old 12-19-2017, 12:46 PM
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Having just now heard of it, you are of course now qualified to correct the established consensus reached by the real scientists on the matter...
Actually I had heard of them for years, I just thought it was a different object category from the recently imaged asteroids and Martian moon Phobos.

The term "rubble pile" can be confusing and the comments on this thread illustrate that. People tend to picture it this way: https://photos.smugmug.com/photos/i-...-pWWDz5W-S.jpg

Such a loose pile of rocks would fly apart like cosmic billiards if struck by an object. Yet that clearly did not happen to Phobos (also described as a "rubble pile") during the massive Stickney impact:

https://photos.smugmug.com/photos/i-...PdR4jf5-X3.jpg

The reason for the cohesive strength of these "rubble piles" is still being researched by scientists, but a key 2013 paper determined it's because the fine material acts as a "cement", making a cohesive matrix that binds the aggregate into a body (Sanchez et al., 2013):

http://inspirehep.net/record/1237623...A1306.1622.pdf

So the first article you quoted which described these as "partially melted shot and wax" was correct from the standpoint of this paper.

While some of these objects do not look like nor behave like literal "rubble piles" (at least from a collision standpoint), they also don't behave like monolithic rock.

Regarding the thread topic of asteroid momentum transfer, the imagery of the Stickney crater on Phobos shows what happened when a huge impact caused momentum transfer to that object.

Much is unknown about the geology of these objects, but current information indicates they differ in internal composition. Some like Itokawa are more clearly in the "rubble pile" category. https://en.wikipedia.org/wiki/25143_Itokawa

Others like Lutetia are probably not "rubble piles": https://en.wikipedia.org/wiki/21_Lutetia
  #56  
Old 12-19-2017, 02:43 PM
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So...ten years' warning? Even assuming sufficient motivation, it'll be at least two years before your first nuke leaves the earth, and it'll be several more years before it gets to the asteroid. At which point the asteroid is no longer ten years out, it's more like four years out, and now needs a pretty big nudge to assure a miss. But the rest of your nukes are going to arrive later, when the asteroid is even closer. I think it's pretty optimistic to believe that we could build/launch rockets the size of the Saturn V at intervals shorter than six months, so you're only going to get a handful of nukes up to the asteroid before it lands in your front yard.
I think this is a reasonable analysis. Obviously this is a bad case scenario. My main point was really that each nuke you set off is getting you more push per kilogram of payload mass. Also, I had implicitly assumed you were going to spend the propellant to brake, but your idea is better - just fly by and set off the nuclear shaped charge at the right nanosecond. And yeah, you would have to pick the patch you set it off next to carefully, apparently the material type matters.
  #57  
Old 12-19-2017, 06:16 PM
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...I think it's pretty optimistic to believe that we could build/launch rockets the size of the Saturn V at intervals shorter than six months, so you're only going to get a handful of nukes up to the asteroid before it lands in your front yard.
Apollo 10 and 11 were launched just two months apart, and if Apollo 11 had failed, there were additional Saturn Vs in the pipeline for two more missions that year. So even when global survival was not at stake, the capability existed (at least then) to launch them every two months.

But the entire Saturn V production pipeline from the subcomponent level to the Vehicle Assembly Building and test facilities were designed from the outset to support that launch rate. Achieving that infrastructure took about seven years.

The SpaceX Falcon Heavy can lift about half the Saturn V payload, and in theory given enough time SpaceX's production rate might be increased but it would take years. OTOH presumably the NEO deflection scenario would be an issue of global survival so funding might dwarf Apollo. This implies the effort would be nationalized or a consortium of nations.

Considering a NEO deflection mission would be unmanned, the time-consuming aspect of man-rating the vehicle would not apply, so that would help some. The fastest spacecraft ever flown - New Horizons - was launched by an Atlas V to a solar escape velocity of 36,000 mph: https://en.wikipedia.org/wiki/New_Horizons

The Atlas V 551 has about 1/8th the payload capacity of a Saturn V and about 1/4 the capacity of Falcon Heavy.

Despite the relatively limited payload, it launched New Horizon to such a high velocity it passed earth's moon in eight hours, vs three days for Saturn/Apollo. It reached Jupiter in about one year, vs about two years for Voyager 2 (which was launched on a Titan III).

However to achieve that velocity on a relatively small booster, New Horizon only weighed about 1,000 lbs.

The actual real-world yield to weight ratio for a nuclear warhead is about 400 kg per megaton. So for a booster the approx. size of the Atlas V 551 that launched New Horizon, the warhead payload might be less than 200 kg (440 lbs) to allow mass for the spacecraft structure, power, communications, guidance, thrusters, etc. If it were only 100 kg that would equate to an approx. 250 kiloton warhead that could reach Jupiter's orbit in a year if launched from an Atlas V 551 or equivalent.
  #58  
Old 12-19-2017, 07:11 PM
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Probably the first thing you would launch is a tracking/guidance and imaging probe. Dual use, get pictures of the asteroid and land a guidance transmitter on its surface. Once you have the guidance transmitter in place getting the nukes into the right position and timing their detonation is going to be vastly easier. Without reasonably detailed imaging you probably have little chance of having any clue when and where to use the nuke(s).

I suspect a mass budget of 50:50 is a bit optimistic. It is a bit depressing just how much of any interplanetary probe is still mostly fuel and support infrastructure. In a crash build programme one would expect just about everything possible would be repurposed from existing designs, and possibly even existing hardware.

If you need an Armada of craft and nukes it is possible the pacing factor would be the tooling to make launch vehicles. Some things you can throw money and people at, some things can't go any faster no matter what you do.

Personally I suspect a huge issue would still be politics. You can guarantee that there will be a huge body of asteroid-deniers convinced that the entire thing is a fabrication/conspiracy. The notion of having a number of nukes in space with targeting and delivery systems capable of dropping them anywhere would not be a an easy thing to stomach for any nation. Even after the asteroid is deflected - have all the nukes been used? What assurances do we have about this? If an international programme was used (and we would assume it was) has every nation (ie US, Russia, maybe China) ensures in a verifiable manner that there are no nukes left? Etc, etc. Nothing is insuperable, but the technical difficulties might only be the beginning.
  #59  
Old 12-19-2017, 08:18 PM
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Probably the first thing you would launch is a tracking/guidance and imaging probe. Dual use, get pictures of the asteroid and land a guidance transmitter on its surface. Once you have the guidance transmitter in place getting the nukes into the right position and timing their detonation is going to be vastly easier. Without reasonably detailed imaging you probably have little chance of having any clue when and where to use the nuke(s).
There is no question a guidance and imaging probe would make things easier, and that would probably be done. However I don't think it's absolutely necessary. The Rosetta probe landed on a comet despite not having any prior close-up imagery or emplaced navigation aids, the NEAR spacecraft landed on Eros, same situation. The OSIRIS-Rex probe will land on an asteroid and return a sample, also without prior imaging or emplaced nav aids.

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...I suspect a mass budget of 50:50 is a bit optimistic. It is a bit depressing just how much of any interplanetary probe is still mostly fuel and support infrastructure. In a crash build programme one would expect just about everything possible would be repurposed from existing designs, and possibly even existing hardware...
Yes, you always need more vehicle mass which hurts payload fraction. However in this case (using New Horizon as a guide) the payload was 1,054 lbs (478 kg). If we use 200 kg for the warhead, that is a fraction of 42%. I also mentioned 100 kg which would be 21%. But if it were only 50 kg (10.4% payload mass fraction) that still equates to 125 kilotons.

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...The notion of having a number of nukes in space with targeting and delivery systems capable of dropping them anywhere would not be a an easy thing to stomach for any nation...
Actually they could not be dropped anywhere -- that would require a reentry vehicle with the associated heavy aeroshell and frictional thermal protection. When the vehicle is headed directly away from earth at 36,000 mph, every gram counts, so you'd never waste so much mass accelerating a useless atmospheric heat shield to the outer solar system.
  #60  
Old 12-19-2017, 08:48 PM
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How the hell do you make a atomic bomb go off in one direction?
Get Liam Payne to swallow it.
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  #61  
Old 12-19-2017, 09:28 PM
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There is no question a guidance and imaging probe would make things easier, and that would probably be done. However I don't think it's absolutely necessary.
No I was really thinking it would be more about accelerating the speed at which you could make decisions. Interplanetary probes and their research programmes run at a very slow measured pace. Here we want to be able to work out exactly where a nuke must be placed and do it very quickly - having already launched the nukes right behind the guidance and imaging probe. Given we have no good idea of the shape, rotation or composition of the asteroid, faffing about once the nukes arrive working out what to do is going to lose a lot of time.

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Actually they could not be dropped anywhere -- that would require a reentry vehicle with the associated heavy aeroshell and frictional thermal protection. When the vehicle is headed directly away from earth at 36,000 mph, every gram counts, so you'd never waste so much mass accelerating a useless atmospheric heat shield to the outer solar system.
Yeah, although you would need to trust the other nations that the nuke they launched, and just so happened didn't use, and is still up there, didn't just happen to also have a re-entry vehicle. OTOH, you don't absolutely need a re-entry vehicle. That is just the most efficient way of breaking - using the atmosphere. If you have a nice amount of fuel remaining in your system you can decelerate with that, and the nuke can just fall through the atmosphere with only trivial heating. Virgin Galactic doesn't need a heat shield.
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Old 12-19-2017, 09:32 PM
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How the hell do you make a atomic bomb go off in one direction?
You don't have to. You just have some tungsten on the side facing the asteroid.
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Old 12-19-2017, 09:40 PM
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How the hell do you make a atomic bomb go off in one direction?
You can't. Sadly you just take the hit that half the energy has basically vanished into space.

Inside the nuke things are as weird as one can imagine, and there are asymmetries in boosted and thermonuclear weapons. You have the notion of neutron reflectors and various other ways of modifying the physics to guide the process. However these things last nanoseconds before they turn into superheated plasma, and I very much doubt there is any sort of asymmetry in the energy of the fireball once it gets much more than a metre in diameter. Maybe selective asymmetry in the tampers will yield some difference in the nature of radiation in different directions. As I noted earlier you probably want to favour gamma radiation over neutrons to carry the energy to avoid useless deep heating of the rock and favour maximum energy going into vaporising the surface rock. OTOH, I don't know enough (like nothing really) about the relative energy deposition that would occur and is needed. Just seems like it would be a reasonable thing to do. We have a vast body of knowledge about the effects of nukes and such things as cratering and energy balances - many many tests before the bans came in.
  #64  
Old 12-20-2017, 05:48 AM
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You can't. Sadly you just take the hit that half the energy has basically vanished into space.
What about shooting muskmelons at the target?
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Old 12-20-2017, 06:26 AM
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Any chance we could win the game, at least with enough advance notice, using a billiard shot? Instead of trying to deflect the 5km asteroid, just find a much smaller asteroid already on a near-collision course with the big asteroid, and deflect the small asteroid enough to nudge it into a deflecting collision with the larger threat.
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Old 12-20-2017, 06:28 AM
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Any chance we could win the game, at least with enough advance notice, using a billiard shot? Instead of trying to deflect the 5km asteroid, just find a much smaller asteroid already on a near-collision course with the big asteroid, and deflect the small asteroid enough to nudge it into a deflecting collision with the larger threat.
You mean like using a moped to divert a semi?
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Old 12-20-2017, 06:46 AM
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Any chance we could win the game, at least with enough advance notice, using a billiard shot? Instead of trying to deflect the 5km asteroid, just find a much smaller asteroid already on a near-collision course with the big asteroid, and deflect the small asteroid enough to nudge it into a deflecting collision with the larger threat.
What if we use one guy on a trolley?
  #68  
Old 12-20-2017, 10:34 AM
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You mean like using a moped to divert a semi?
If you can get it to collide at all, the momentum transfer will be huge. Obviously time until Earth collision will be a major factor in determining what impulse is needed, but the impulse would be much greater than thrusting on the big asteroid itself. Remember that you'd be exploiting the smaller asteroid's existing momentum the human-provided impulse would be to redirect the small asteroid, not to increase its energy.
  #69  
Old 12-20-2017, 10:46 AM
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...just find a much smaller asteroid already on a near-collision course with the big asteroid...
The odds against this seem extremely high.
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Old 12-20-2017, 10:13 PM
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The odds against this seem extremely high.
Much more correct than #66 or #67. Thank you.

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  #71  
Old 12-21-2017, 12:02 AM
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If it's a nuclear explosion nearby, then as mentioned, the main impact is high-energy particles, photons and neutrons, some vaporised bomb components. Causing a collision using another high-energy rock is simpler, but then how do we give that rock the necessary momentum and direction. It seems to me the most effective diversionary tactic is to bury the nuke inside the asteroid deep enough (shallow enough) so it blows a substantial chunk of the asteroid off. preferably this would be from the side - or some fancy orbital math to determine which direction course change diverts the asteroid from collision course the most. It could be that speeding it up or slowing it down would work better. The only worry is where that chunk blown off ends up, but a few of those apartment-block sized meteors is preferable to a KT event. The only problem is - how many of these bombs do you have to set off to effectively change the course of the asteroid? A sphere say, 20m on a side, about the size of the recent Russian meteor vs. a 10km sphere, a ratio of 20^3 vs 10,000^3 - that chunk would have to fly off at one helluva velocity to divert the asteroid very much. Or, we need that huge fleet of heavy launch vehicles at the ready.
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Old 12-21-2017, 02:46 AM
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Others like Lutetia are probably not "rubble piles": https://en.wikipedia.org/wiki/21_Lutetia
Lutetia is unusual in that it probably formed in the inner solar system, perhaps from the Lunar formation event. So it is more solid, and drier, than most.

We probably don't have to worry about the big objects, anyway; it is smaller objects, around 100 metres or less, that could cause a credible threat. These often tumble so fast and erratically that they have negative gravity on their surface at times, so they are likely to be single objects not rubble piles. Nukes would probably work for these solid objects, though I'd prefer to use gravity tugs if they could be made to work.
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Old 12-24-2017, 09:43 PM
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SamuelA, not a slight, nor an insult; but is English your native tongue?

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I ask to help determine what you mean.
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Old 12-25-2017, 01:03 AM
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SamuelA, not a slight, nor an insult; but is English your native tongue?

Tripler
I ask to help determine what you mean.
Tripler, not a slight or an insult, but do you have a college degree? When Stranger wrote his little post, the main topic was about an effect that doesn't physically matter.

This was apparent in the original post.

Every poster in this thread who actually passed physics 1 has confirmed this. The only ones arguing against me...like you...just try to twist the words around or make personal attacks.
  #75  
Old 12-25-2017, 04:05 AM
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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. <snip>
People who use "homogenous" instead of the correct term "homogeneous" really get my goat. Has he ever actually had a science class?
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Old 12-25-2017, 07:51 AM
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SamuelA, I was not making a personal attack, I was asking a question that you have yet to answer as is your style.

I have already answered yours.

But if you want to turn my question into an attack on your logic, by all means let me know and I will let loose facts upon you.

Tripler
Your posts are disjointed, and if English is not your first language, it would help me understand where your argument is coming from.
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Old 12-25-2017, 04:14 PM
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People who use "homogenous" instead of the correct term "homogeneous" really get my goat. Has he ever actually had a science class?
Lol. You obviously didn't pay attention in yours to the topics about weighting of evidence. I neglected to type an e lol.
  #78  
Old 12-25-2017, 04:28 PM
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...
Every poster in this thread who actually passed physics 1 has confirmed this. The only ones arguing against me...like you...just try to twist the words around or make personal attacks.
Seriously, I haven't been keeping score. Are there actually posters in this thread agreeing with you?
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Old 12-25-2017, 05:18 PM
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SamuelA, I'm going to ask a very direct question; prepare yourself.

What are your credentials? Are you an Engineer? Are you a Physicist? What is your background and technical basis to make some of these assertions?

Tripler
Go ahead kid.
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Old 12-25-2017, 05:25 PM
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SamuelA, I'm going to ask a very direct question; prepare yourself.

What are your credentials? Are you an Engineer? Are you a Physicist? What is your background and technical basis to make some of these assertions?

Tripler
Go ahead kid.
It's going to be some version of "book-larnin" isn't the full extent of human knowledge.
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Old 12-25-2017, 10:30 PM
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I just wandered by with a big tub of popcorn and I think I'll stay for a while.
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Old 12-26-2017, 01:05 AM
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SamuelA are you able to produce an fact-based argument to back up your assertions, or is your argument "what college did you go to?". Responses to that are appropriately, I'd still like a fact-based argument explained coherently, followed by "what college did you go to", and finally, my belief that you are not to going to the like the answer.

Let's start with, please explain your position with citations.

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  #83  
Old 12-26-2017, 02:25 AM
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In fairness, this isn't a totally trivial problem and in order to short circuit a bit of too and fro, perhaps we can break it down.

The universe has a set of conservation laws. If we take our asteroid and the nuke sitting near it, and draw a box around the pair - say a box a few kilometres on a side (although it doesn't really matter) we can say that within that box a set of properties must be conserved. Critically we are interesting in momentum and energy, but there is charge, spin, and so on.) Kinematics is governed by momentum and energy.

So, the next thing we do is convert some of the colour force binding energy in some of the nuke into a mix of kinetic energy - mostly in the form of very fast moving neutrons, a lot of highly energetic photons in the form of gamma rays, and a small amount of very fast moving atoms in the form of a plasma. Because momentum is conserved wrt to the nuke, the matter will all tend to expand away from the nuke in a spherical fireball.

So, there is a bunch of stuff, photons, neutrons and plasma that hits the asteroid.

Now within the confines of our box, we have two conservations laws we want to observe: energy and momentum.

Energy is messy, we can covert energy between all sorts of forms, but for kinematics we are only interested in kinetic energy. Momentum is much nicer. You can't convert it into anything else, and the conservation laws provide an easy to work with set of constraints on the final outcome.

So, within our box, the sum of all momentum of all individual masses always just adds to the same value. Before we detonated our nuke we can define it to be zero. After, so long as our box is big enough to still contain everything that started inside the sum must still be zero. Momentum is a vector.

So, if the only thing that happens is that the momentum of the residue from the nuke that hits the asteroid starts the asteroid moving, the motion of the nuke is fully constrained by nothing more than this added momentum. We ignore the energy conservation question, and treat the interaction of the nuke and asteroid as an elastic collision. Total momentum of the system is conserved, as half the momentum of the nuke went out in the other direction into space, and so long as we take our snapshot quickly enough, the momentum inside the box remains zero. All good.

But we have ignored the energy, and essentially assumed it somehow was fully adsorbed by the asteroid, or vanished into space. However we have a huge range of options that might occur. The energy may be expected to vaporise a slab of the asteroid, and then as that expands, it pushes both on the asteroid and outwards into space. Momentum is conserved, the magnitude of the momentum transferred to the asteroid is the same as the magnitude of the momentum of the gas expanding out into space from the area of vaporisation. But note that we now have provided the asteroid with additional velocity, greater than the momentum that impinged from the nuke. We did this by moving mass on the asteroid with the energy we have from the nuke, but conserving momentum by having the mass move in different directions, some in some out.

Next we might notice that the vaporisation produces a massive shock wave that travels into the asteroid, this shock wave fractures the rock into small particles and the pressure of the expanding gas above will start to force the rock particles away from the centre of the blast impingement and they will naturally get forced at considerable speed is a curved path that eventually has them blast off the surface of the asteroid back in the the direction the initial blast came from, leaving a crater behind. Momentum remains conserved, the asteroid moves even faster away from the blast, with a velocity given by the balance of the mass of the ejecta and its velocity (the ejecta momentum) equalling he change in asteroid's momentum. The interesting trick is that if we wish to maximise the velocity change of the asteroid, we want to maximise the momentum of the eject, all the time making best use of the energy - so minimising energy used.
energy = mv2
momentum = mv
The upshot is that for a given energy budget we want to move the maximum mass with minimum velocity. But we can't have the velocity of the eject so low that the ejecta remains gravitationally bound to the asteroid, so we must minimally exceed the escape velocity of the asteroid.

The point so far? There is no one momentum change of the asteroid. It can vary dramatically depending upon how well your nuke excavates and ejects mass from the surface.

The excavation process is thus critical. You need to maximise the transfer of kinetic energy from the nuke impingement to the rock directly below the point of impingement with as little loss as possible. First up you need to fracture the rock into small particles that will flow in front of the blast. This means you want solid homogeneous rock that transmits the shock wave with little loss and fractures as the wave passes. And you want more solid unfractured rock underneath the area of fracturing rock to support it as it flows and is ejected. The worst case is going to be and asteroid made up of some sort of aggregate of muck, one that is very lossy and simply adsorbs the energy of the blast and heats up, possibly just melting a big slab of the interior of the asteroid. Energy remains conserved, but now uselessly in the form of heat. Momentum remains conserved, as there was no ejecta, and so the asteroid doesn't change velocity.

The question of maximising the momentum change of the asteroid is thus clearly one of maximising the efficiency of the ejection process. We need a process that ejects the maximum mass possible. Anything that reduces the mass ejected (even if the ejected mass contains a large amount of energy and is thus travelling fast) reduces the total velocity change of the asteroid. We can see that a critical part of this process is the manner in which the cratering process proceeds, and how this is governed by the properties of the shock wave in the asteroid. This can vary from high efficiency in a homogeneous solid asteroid, to very poor in an asteroid made up of an aggregate of junk.

A large asteroid may well be a gravitationally bound mess of junk, whilst smaller asteroids are probably more likely to be single entities, as they lack enough gravity to bind together a mess of junk. YMMV.
  #84  
Old 12-26-2017, 04:30 AM
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We don't have to worry about large asteroids, since none of them come near the Earth. Some medium-sized asteroids are rubble piles, others are not; and asteroids with diameters of around 100 metres are too small and rotate too fast to be rubble-piles. So the nuclear option would work in quite a large number of cases. Once again, SamuelA is partially correct, despite the skepticism on this board.

How do we maximise the momentum transfer in such a process? Well, Freeman Dyson did some work on this concept, in the context of designing a space drive using atomic bombs. The nuclear devices in his concept include a significant quantity of propellant, and designed so that the force of the explosion was focused in a particular direction. Here's a diagram from wikipedia.
https://upload.wikimedia.org/wikiped...pulse_unit.png
These propulsive bombs are a very old concept, and may not be feasible in thi configuration, but the concept could be probably be refined in many ways.

Last edited by eburacum45; 12-26-2017 at 04:30 AM.
  #85  
Old 12-26-2017, 05:17 AM
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The entire thread is however about large asteroids. Anything up to a 10km KT asteroid. The basic quote from Stranger on a Train that SamualA complains about is (bolding mine):
Quote:
Redirecting a much larger object, however, becomes a substantial challenge not only because of the additional mass but because the impulse will impinge upon only by a fraction of aspect of the body and would likely be absorbed as deformation within the body (inelastic transfer or liquification) rather than uniformly delivered to the entire aspect. Moving really large masses requires some fundamentally different method of propulsion than just pushing because at those scales a “solid” body doesn’t act very solid.
SamualA quotes this and goes on to say:
Quote:
As I understand it, momentum is conserved in this universe. Thus, I perform the following mental experiments ...
finishing with
Quote:
I don't see how a paper on this would have survived peer review.
SamualA does not seem to have ever looked as the questions of energy, and based his criticisms entirely on momentum conservation, with such comments as:
Quote:
Point remains. Even an undergrad engineer should understand conservation of momentum, even if they aren't in aerospace.
My point is simple. There is no, and has never been, an error involving conservation of momentum in any of what Stranger on a Train wrote. The discussion has been entirely about problems of maximising the impulse, and mechanisms by which energy goes into useless forms before being able to propel ejecta from the asteroid so that the momentum of that ejecta can add to the velocity change of the asteroid. This intermediate step has perhaps been missed in the condensed commentary that Stranger on a Train supplied, but it is somewhat unreasonable to jump from there to public accusations of professional incompetence without first putting in a few minutes background research to try to understand the topic.
OTOH, I have sympathy with an initial reading of the quote, as unless you start to look at the questions of underlying efficiency of using the energy from the nuke, it does read as if there is an error. But, like I say, it is always worth digging a bit further before accusing people of incompetence.

With the nuke as linked, that looks to be much the same device as the Casaba Howitser that Darren Garrison linked to earlier. This design is not intended to maximise momentum transfer. It is intended to create a highly focussed beam of accelerated matter for use as a space born weapon. No matter what, you have to use two equations to work out the momentum available for transfer. You have mass, and you have energy, and you have to conserve momentum as a vector. The idea that a directed matter nuke can deliver higher momentum ironically fails on exactly the same conservation law. Momentum must be conserved. The initial momentum of the nuke is zero, and no matter what you do with your directed mass jet, its momentum can only be as large as the momentum of matter ejected in the opposite direction by the nuke. Secondly, momentum is only m * v. Whereas energy is mv2. The nuke delivers a fixed amount of energy. The absolute best possible momentum change of the asteroid is not with a small amount of very high speed mass, but a very large amount of low speed mass. You need to optimise energy transfer to the asteroid in order to get maximum mass to blast off it back at you. Not deliver the minimum momentum by hitting it with a jet of super fast but low mass.
I would guess there is a lot more to the mechanics of optimising the mass ejection from the asteroid than I am talking about, but it is going to be pretty clear that the mechanical properties of the rock and energy loss mechanisms within it are going to be a very substantial part of the issue.
  #86  
Old 12-26-2017, 06:11 AM
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With the nuke as linked . . .
We haven't even gotten to the nuclear weapon part yet. I can speak to those.

Tripler
There is no such thing as a nuclear shaped charge.
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Old 12-26-2017, 06:35 AM
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I will say, though, Francis Vaughan, that yours are the most logical and reasoned posts I've seen in a long, long time. Thank you!!

Trip
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Old 12-26-2017, 07:58 AM
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We haven't even gotten to the nuclear weapon part yet. I can speak to those.

Tripler
There is no such thing as a nuclear shaped charge.
No, I remain highly sceptical of a whole range of these designs. The Casabla Howitzer seems to be something that is claimed to exist as a precursor weapon before X-Ray lasers, but everything you find on the internet is fanciful discussion of a sci-fi weapon, with the most hand waving arguments imaginable about how it works.

The claim seems to be that a jacket of depleted uranium is capable of reflecting the x-rays and gamma rays from the very young fireball enough to make the tungsten plate accept a significant fraction of the entire nuke's energy and produce what is indeed essentially a shaped charge. I call BS, but for the sake of the argument here I'm letting it slide.
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Old 12-26-2017, 08:07 AM
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I call BS, but for the sake of the argument here I'm letting it slide.
Agreed.

I've had to pull up the citations, but there are basic design features that are not addressed. I'd speak to them, but I like my job and want to avoid prison.

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  #90  
Old 12-26-2017, 08:32 AM
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No, I remain highly sceptical of a whole range of these designs. The Casabla Howitzer seems to be something that is claimed to exist as a precursor weapon before X-Ray lasers, but everything you find on the internet is fanciful discussion of a sci-fi weapon, with the most hand waving arguments imaginable about how it works.
If you do a Google Books search, you'll find other references, including in congressional reports on the Joint Committee on Atomic Energy. It may not be something that could actually work in the real world, but it is something that was taken seriously by the US government at some point.
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Old 12-26-2017, 08:44 AM
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If you do a Google Books search, you'll find other references, including in congressional reports on the Joint Committee on Atomic Energy. It may not be something that could actually work in the real world, but it is something that was taken seriously by the US government at some point.
I did just that, and found those references. The trouble is that except for use of the name, there is little to join the claimed device and its operation to anything that has any provenance or suggestion of actually working. Given that X-Ray lasers never worked either, I think I can be forgiven for my scepticism that the design has any change of being of any value.

There are "reflectors" in nukes, but they don't actually reflect, they sort of result in a short term increase in concentration during the scant nanoseconds before they vaporise. The idea that a weapon could be constructed that directs 80+% of its energy into a tiny thin beam is not on the cards (and the notion is clearly a compounded set of misreadings of the very thin material available.)

Last edited by Francis Vaughan; 12-26-2017 at 08:45 AM.
  #92  
Old 12-26-2017, 09:09 AM
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Yes. Thank you for taking the time to identify the details most of us (including me) were jumping over.

Once confusion & resistance to argument sets in the best cure is the one you took. Start with what's agreed, identify all the unstated implicit spherical cows, make each of explicit, then adjust them into real world cows in the real world, not the Physics 101 world.

Thank you.
  #93  
Old 12-26-2017, 09:20 AM
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If you do a Google Books search, you'll find other references, including in congressional reports on the Joint Committee on Atomic Energy. It may not be something that could actually work in the real world, but it is something that was taken seriously by the US government at some point.
Granted.

But in the early atomic age many cockamamie things were taken seriously by the US government. See https://en.wikipedia.org/wiki/Supers...titude_Missile & http://www.merkle.com/pluto/pluto.html for one "interesting" example.

Last edited by LSLGuy; 12-26-2017 at 09:20 AM.
  #94  
Old 12-26-2017, 11:37 AM
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....I remain highly sceptical of a whole range of these designs...The claim seems to be that a jacket of depleted uranium is capable of reflecting the x-rays and gamma rays from the very young fireball enough to make the tungsten plate accept a significant fraction of the entire nuke's energy and produce what is indeed essentially a shaped charge...
But what does this have to do with the feasibility of redirecting asteroids which are threatening earth? Most of the papers I've seen describing nuclear methods involve using a stand-off detonation which either causes spalling or vaporization of a thin surface layer of material. The papers discussed to what extent the asteroid geology would affect this. They generally report it may be workable whether the body is a solid rock or a rubble pile, although knowledge of the composition would help calibrate the detonation point and number of devices required.

"Deflecting Asteroids by Means of Standoff Nuclear Explosions" (Gennery, 2014): home.earthlink.net/~dgennery/2004_1439.pdf

"Interception and Disruption" (Solem, 1995) : https://www.osti.gov/scitech/servlets/purl/101350

"On the Efficiency of Nuclear Explosives in Deflecting the Orbits of NEOs" (Yabushita, 1996): http://adsbit.harvard.edu/full/seri/...00183.000.html

"Nuclear Explosion Near Surface of Asteroids and Comets -
General Description of the Phenomenon" (Shubin, et al, 1997): csc.ac.ru/news/1997_1/ae27.pdf

Many of these papers (inc'l various non-nuclear approaches) can be found in this single large PDF, "Proceedings of the Planetary Defense Workshop", 1995: https://e-reports-ext.llnl.gov/pdf/232015.pdf
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Old 12-26-2017, 12:25 PM
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The question of maximising the momentum change of the asteroid is thus clearly one of maximising the efficiency of the ejection process. We need a process that ejects the maximum mass possible. Anything that reduces the mass ejected (even if the ejected mass contains a large amount of energy and is thus travelling fast) reduces the total velocity change of the asteroid. We can see that a critical part of this process is the manner in which the cratering process proceeds, and how this is governed by the properties of the shock wave in the asteroid. This can vary from high efficiency in a homogeneous solid asteroid, to very poor in an asteroid made up of an aggregate of junk.

A large asteroid may well be a gravitationally bound mess of junk, whilst smaller asteroids are probably more likely to be single entities, as they lack enough gravity to bind together a mess of junk. YMMV.
Thank you. This seems to actually address what I was talking about.

There are 3 main mechanisms, then, not just 2.

1. The nuke itself has some mass, it's not just photons. If you put a tungsten plate on the side facing the asteroid, the mass of that tungsten is going to be converted to high speed gas that embeds in the asteroid and creates a small momentum change. But, like you say, that gas has very high velocity but little mass, so this effect is small. This is the principle by which the "orion drive" is supposed to work.

One mistake on this thread is mixing up the "casaba howitzer" nuclear shaped charge concept with the "project orion" nuclear pulse charge. They are very different proposals and one is science fiction and the other idea is solid.

The "project orion" charge is just an efficiently sized nuke, behind a some propellant, aimed at a plate on the back of a spacecraft. A diffuse cone of superheated gas is pushed by the explosion at the plate, and the spacecraft absorbs the shock smoothly and gains momentum. This is how I proposed redirecting asteroids, just without the shock absorber, since it doesn't affect the momentum transfer in any way. This seemed to be where Stranger was going wrong.

The casaba howitzer is some super secret concept where somehow the nuclear explosion creates a thin and narrow beam of particles that can hit an enemy spacecraft thousands of kilometers away. In theory, this would have been a USSR orion drive spacecraft vs one built by the USA, dueling it out by ejecting these nuclear beam weapon charges into space, each combatant maneuvering quite dramatically with their orion drives. It would make for a cool science fiction story but I don't know how you get that kind of focused, long range beam, maybe Tripler does. Both spacecraft would have been able to launch fully loaded from the ground, would each mass thousands of tons and be armored, have a crew of dozens to hundreds and lots of vacuum tube electronics...would have been pretty cool, shame about the nuclear fallout from the launches.

2. The radiation from the nuke can boil material off the surface. If you have reasonably efficient nuclear charges and you set the nuke off from 0 distance away, some significant fraction of the energy goes into boiling. This is not affected by the deformation or shock waves, either. It's just photons impinging on the surface, the material is now hot enough to exceed the vapor pressure in vacuum, it boils, and if the velocity is greater than escape velocity, it is lost forever.

Since you get a megaton for a few hundred kilograms of nuke, this is also pretty efficient. Basically a nuclear-(steam or metal vapor) rocket.

Both using the mechanism from Project Orion (high speed gas from a propellant package in the nuke) or using this boiling mechanism is much better than NASA's current proposals, where it's things like "park a solar-electric ion drive spacecraft real close and pull with gravity". That's going to be a glacially slow thing to do. Though, I suppose, over the long run if the ISP of the solar-electric spacecraft is high enough, it may work out to be the same actual effect on the asteroid.

3. The third mechanism, I didn't consider, is that apparently if you hammer a solid mass of iron hard enough, the shock waves create spalling. This can apparently let you create a really huge crater, sending solid fragments of the asteroid away into space. This is obviously a very efficient way to redirect an asteroid.

And, apparently, this maybe was what Stranger was talking about. This mechanism depends on the composition of the asteroid and certain asteroids won't create a nice crater, if you hammer them, they'll just soak the energy internally. This makes sense. Though even then, if you set a nuke off nearby, you'd get some momentum change because of effects (1) and (2)...but not enough to redirect a 10km+ asteroid in the small time you'd have before it ends most or all life on earth. And Stranger uses the word impulse to describe hammering the asteroid so it rings and creates this crater, which is actually defined as a momentum transfer. When it's not necessarily.

Last edited by SamuelA; 12-26-2017 at 12:30 PM.
  #96  
Old 12-26-2017, 12:31 PM
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Impulse has multiple definitions, one of which is p=mv...which is not necessarily even true. If you set off a bomb and a crater ejecting material is not formed, you do not get any momentum transfer at all. But the explosion itself is considered an "impulse" on the system.

Last edited by SamuelA; 12-26-2017 at 12:31 PM.
  #97  
Old 12-26-2017, 12:35 PM
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Could you please run through the math on your suggestions? Start with some basic assumptions about speed, mass, and distance, and then work out the equations for exactly what would happen at the time of impact and for any changes in orbit.
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Old 12-26-2017, 01:12 PM
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Could you please run through the math on your suggestions? Start with some basic assumptions about speed, mass, and distance, and then work out the equations for exactly what would happen at the time of impact and for any changes in orbit.
Asteroid is 1.410^12 kilograms, a number I googled for a 1 kilometer asteroid. In order to push it's impact probability down acceptably, a 1 cm/second change in velocity is needed. (I read that in a wired magazine article as being enough)

Nuclear pulse charge + small thrusters for guidance is assumed to be 300 kilograms, with a 150 kiloton yield. ISP is assumed to be 7500. (10k-20k from a paper on Project Orion, reducing it because you need a guidance system)

So how many nukes do you need?

(1.410^12)(0.01) = (7500*9.8)(300)(n)

n = 634. So 300 kilograms * 634 = 190,200 kilograms must reach the asteroid.

Falcon heavy stated payload to mars is 16,800. So you need 11 launches to do it.

Of course, you need to actually develop those nuclear pulse charges. Maybe you can't get a really efficient one in the time you have and you need 10 times as many inefficient ones. Not all the rockets will make it. You may need a lot more dV than a "mere" mars injection in order to reach the asteroid in time. All these push the numbers into the less favorable territory, but it feels like it might work from this napkin analysis.

Bigger asteroids - 10 kilometer spheres - yeah, it's not good. 1000 times the mass, this problem becomes 1000 times harder. Now you would need to hope you can eject craters in it, which in turn means it better be made of the right kind of material for that.

Last edited by SamuelA; 12-26-2017 at 01:13 PM.
  #99  
Old 12-26-2017, 03:13 PM
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But what does this have to do with the feasibility of redirecting asteroids which are threatening earth?
Exactly nothing. That was meant to be the point. It seemed that there was a background thread of talking about some sort of special nuke, that in the extreme was actually directional. Perhaps I should have ignored it. But it kept coming back.

You can modify the energy balance of a nuke between gamma rays and very fast neutrons, and as I noted earlier in this thread, this would seem to be a good idea as neutrons would be more penetrating and perhaps more apt to deposit their energy uselessly deeper in the rock. I suspect the use of tungsten wrappers or the like may have stemmed from this, and not from very dubious ideas about constructing a directional weapon. That was all.
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Old 12-26-2017, 04:08 PM
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A large asteroid may well be a gravitationally bound mess of junk, whilst smaller asteroids are probably more likely to be single entities, as they lack enough gravity to bind together a mess of junk. YMMV.
Depending on what you conciser "smaller." I found this interesting article suggesting that even 150m asteroids might be rubble piles.
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