h a l f b a k e r yStrap *this* to the back of your cat.
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There has been interest lately in spacecraft which move at great speeds, then whack into asteroids. An upcoming project aims to see if an asteroid can be diverted from its course in this way (linked).
It seems to me that such collisions must be fairly elastic and thus inefficient - a lot of energy
is likely taken away from the collision by fragments of asteroid and spacecraft which fly in all directions. This makes for spectacular fireworks but is an inefficient way to transfer energy. Ideally the collision would be perfectly inelastic: all of the spacecraft's energy would be translated into change in the angular momentum of the asteroid.
I propose that if the spacecraft blew up _before_ hitting the asteroid, it would be a better way of transferring energy. In this scheme, the incoming craft would convert into as large a volume of gas as possible as it neared the asteroid. Conservation of momentum states that the energy transferred to the asteroid will be the same, only spread over a longer time. A cloud of incoming gas molecules will more reliably transfer the kinetic energy, with less chance of large pieces ricocheting away and carrying kinetic energy with them.
(?) Don Quijote Asteroid Diverting Mission
http://www.cnn.com/....deflect/index.html
I do not know what is up with that j. Ok, now I do. [bungston, Mar 22 2006]
[bungston] your own link states that the 'j' is the Spanish spelling
http://www.donquijote.org/
[xandram, Mar 22 2006]
Check out the hammer and bungee cord annotation
Low_20budget_20spacecraft
I think that method would be superior [normzone, Mar 24 2006]
(?) Collision simulator
http://www.walter-f...ph11e/collision.htm
[bungston, Mar 26 2006]
What would a collision do to earth
http://janus.astro.umd.edu/astro/impact/
Sample various impacts here! [bungston, Mar 31 2006]
Three body problem
http://www.scholarp.../Three_body_problem
Not yet solved [csea, Jul 02 2009]
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I would think that the impactor would tend to vaporize it's self and a good portion of the asteroid appon impact, a thought which is barred out by nasa's impacting probe that interacted with that comet.
Wouldn't this tend to act a bit like a rocket engein, giving you more thrust than your vaulted inelastic collision? |
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Planting a rocket engine on the asteroid would be a great way to move it. For purposes of physics, I do not think that the spacecraft imparts any energy other than its kinetic energy. The rocketlike fireworks conceal the fact that the transfer of energy is inefficient. |
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You can see that a perfectly elastic collision would be pointless: the spacecraft bounces off and travels away at its original speed, and the asteroid is unchanged. |
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Imagine now that the spacecraft collides and knocks off a piece of the asteroid the same mass as the spacecraft, which travels away at the speed of the original spacecraft back along its original trajectory. Again, the mass and trajectory of the main bulk of the asteroid is unchanged. |
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Imagine now that the spacecraft collides and knocks off a bunch of asteroid fragments, which travel away at various speeds. You can see that this is similar to the second circumstance: small pieces speed up, but the main bulk is unchanged. We want the whole thing to slow down. Knocking little pieces off of it does not further that goal. |
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//You can see that a perfectly elastic collision would be pointless: the spacecraft bounces off and travels away at its original speed, and the asteroid is unchanged.// |
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Ah, but if the spacecraft bounces perfectly off back to where it came from, then it would impart double the momentum. Is that right? If so, then we need a really, really bouncy spacecraft. |
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Dear [Bungston], what a beautifull idea! Science, imagery, originality, all top notch+ |
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I'm assuming that you want the cloud of gas to be about the size of the asteroid when they meet, right? So all the particals are used in slowing down the space rock because they are traveling in the same vector path. But what if you are just trying to nudge it off to the side a little? Wouldn't you lose half of your cloud/ force going off in the wrong direction? |
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I'm not sure about the science on this one [bung], I think it may be highly suspect. Anyway, I don't really know what I'm on about, and the title's great so I'll sit on the fence. |
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I keep hoping some physics luminary ([supercat]? [scubadooper]?) will weigh in on this. I am pretty sure E = mv but I have some insecurities about physics. |
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The gas cloud would be better regardless of whether the goal was a head on collision and slowing down, a sidelong collision and directional change, or even a collision from the rear to speed the asteroid up. The other benefit of the gas cloud is that gas molecules which strike the asteroid and ricochet are likely to in turn be hit by other molecules in the incoming cloud behind them. Eventually the momentum will be imparted to the asteroid, as opposed to momentum wasted by speeding the particle off into space, away from the asteroid. |
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If you accelerate the spacecraft enough, it will eventually approach the speed of light and gain mass until it weighs as much as the asteroid. I'll leave it to someone else to figure out how to do this. |
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momentum=m*v -- conserved in inelastic collision (rocket & asteroid stick together) |
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kinetic energy = .5*m*v^2 -- conserved in elastic collision (rocket & asteroid bouce off each other) |
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Interesting. Maybe direct the asteroid-diverting nuclear explosion as a long series of small blasts to send "wind" at the asteroid, instead of one big blast that sprays in every direction. |
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Um, no. For the whole idea, I mean. The perfectly elastic collision described in the annotation above, that generated no waste heat, would be the ideal asteroid diverter. Think of it as two events--first the asteroid catches the mass, which diverts its course, then it fires the mass away, which further diverts its course. |
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Practically speaking, the ideal collision in real life would be for the impactor to drive into the asteroid and to create an impact crater shaped like a rocket nozzle. The gasses, liquids and chunky bits generated from the energy of impact would then blast out from the asteroid, backwards along the flight path of the impactor. The "all directions" mentioned in the idea is indeed a less-than-ideal situation, but the body of the asteroid probably restricts the directions to something closer to the ideal. |
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An impact by a gas cloud would not be in-elastic, and would increase the possiblity that some of the mass would simply blow by the asteroid. |
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I have to agree with your assessment that things that embed themselves in the asteroid would impart less momentum that things that rebound - assuming that things can "bounce" off an asteroid. It would be nice if Hyabusa had actually told us what these things are made of instead of getting cold feet when the going got tough. |
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I was hoping someone with some
credentials would wander in here. I did
not realize that the acceptance of this
idea would hinge on the concept of
elasticity.
Conservation of energy: if the
projectile rebounds with exactly the
same mass and velocity it had on the
way in, there is nothing left to alter the
direction or energy of the asteroid. I
have linked a nifty simulation. Make
the incoming cart small and the resting
one big, then watch the difference
between elastic and inelastic. |
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//if the projectile rebounds with exactly the same mass and velocity//
But the velocity was 'positive', before the collision, and 'negative' after? |
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Set wagon 1 to 0.1kg, and wagon 2 to 1,000,000kg, and compare momentum of wagon 2 with both types of collision (initial velocities set as default).
Elastic: 0.0909
In-Elastic: 0.045 (half)
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Oddly, if wagon 1 is changed to 1kg, then it doesn't bounce in the reverse direction (?). |
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OK, I should have realized back up there at [Ling]'s first anno that it was I who needed a review of physics. Bouncy is best. |
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Apropos perfectly elastic collisions and reversal of direction, I now submit that collisions between molecules of a gas are the most perfectly elastic and thus the Gust Of Wind model is still the best way to impart energy to an asteroid. |
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Interesting. Have a bun[+]. |
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Additional [+] is that if you collide with a big hunk, a lot of the energy will be converted into heat, deformation, and shockwaves, none of which changes direction. The wind would likely not have much waste energy in deformation, etc, and may more efficiently impact directional change. |
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Hmm, okay, I can see some advantages for a gas cloud as an elastic collider. Especially when impacting on a loosely-composed asteroid. |
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Bun for creativity and for adapting. |
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Unabubba, the problems with mounting rockets are several. Mainly, slowing up to land on the asteroid uses a lot of fuel. Also, asteroids are squishy, and they are usually rotating. |
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/mounting rockets/ plus, imagine your remote rocket mouter settles down onto the asteroid and manages to sink its bolts into a rock. You turn it on, only to find that the rock it is bolted to shifts under the force. Your rocket, now at a steep angle, serves to spin the asteroid faster and faster. |
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I suppose if you put enough english on it you might be able to skip it off the earth's atmosphere. If it then whacks into the moon, that is slop unless you call it first. |
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//spin the asteroid faster and faster//
That's it! If it is spun fast enough, then it breaks up into little pieces. An even if some of the pieces are big, they will fly off in odd directions. |
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Energy and momentum must both be conserved. There is no practical way to hit an asteroid of the suggested size with enough momentum to meaningfully divert its course. Breaking the asteroid in half so that both halves would miss Earth going in opposite directions might be ever-so-slightly more feasible, but getting the required amount of energy transferred in the required fashion would require an amazing amount of luck. |
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Surely if you could shatter the asteroid into enough
pieces it would burn up in the atmosphere as a meteor
shower (pretty thing) instead of making it through intact
(bad thing). |
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//It seems to me that such collisions must be fairly elastic and thus inefficient ...all of the spacecraft's energy would be translated into change in the angular momentum of the asteroid.//
The question is not of energy (units of mv2), but of momentum (units of mv). These are not the same. On is scalar and the other is vector. A perfectly elastic collision, if it were possible, would impart more momentum to the asteroid. So the assumption is wrong, and the idea (sorry, bung) stinks.
//There is no practical way to hit an asteroid of the suggested size with enough momentum to meaningfully divert its course.//
This is only true if the asteroid is going to hit the earth fairly soon. If it’s still years away, it wouldn't take much to divert its course enough to make it miss. |
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Would this "spaceship" be bean powered? That would explain the "Gust of Wind" |
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First I was grumpy because I thought the hypothetical Unabubba asteroid was too big. I converted it to 1000 cubic meters for simplicity and plugged it into the asteroid simulator. It turns out that collision with rocks this big are not infrequent (crater shown is off the coast of Australia), if I did my math right - the simulator (linked) predicts one every 200,000 years. |
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Then I decided to see if [supercat] was right about a collision being inadequate to move an asteroid. I am always ready to go head to head with [supercat] in a matter of physics. Assuming a 1000 kg projectile impacting at 30 km/sec onto Asteroid Unabubba. (30*1000)/10000000000000 = 0.0000108 km/hour as the end velocity of the asteroid. Assuming UB is moving at 20 km/second, it would take it about 1 year to reach us from the orbit of Jupiter. Over that time, this impact would cause the asteroid to change course 3.7 meters. Maybe enough to miss my car. |
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I grudginly concede that if asteroids are solid rocks, no single impact that we could reasonably generate will change its course. The skeptics win. The visionary trudges home, head hung low. |
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But what if a repeating railgun were set up on the moon, firing a computer-aimed stream of high-velocity moon rocks? One every 30 minutes! Yes! I'll show them! I'll show them all! |
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In case anyone was wondering, it would take 5 million 500 kg rocks flung at 20 km/second to move the UB asteroid 6000 km (the diameter of the earth). We might need more than one railgun. |
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//Over that time, this impact would cause the asteroid to change course 3.7 meters. Maybe enough to miss my car.//
I’m getting 370 meters for the UB asteroid. So that would take 16,000 impactors. And if you could get it 1000 years ahead of time...just 16. And probably less than that, because small changes in an orbit get magnified. |
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What's your wind velocity? Generate a high wind with a nuke shockwave? |
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Also, we could get lucky. Nudging it 1 meter off course by Jupiter could get it a bit closer to Saturn & Mars, which may alter it's trajectory much much more. |
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That UB asteroid is one stubborn SOB. |
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//Nudging it 1 meter off course by Jupiter could get it a bit closer to Saturn & Mars//
Nasa scientist jumps from his seat in the UB-asteroid mission room, screaming, "I told them it was Jupiter, Saturn, then Mars! God, what have I done? We're all going to die!" |
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3 years gone by: bung returns, older, wiser. Mmm, so much wiser. |
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/I grudginly concede that if asteroids are solid rocks, no single impact that we could reasonably generate will change its course. The skeptics win. The visionary trudges home, head hung low./ |
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The problem is a single impact of adequate force. But what about harnessing things that are already moving and whacking them into each other? The universe is full of things that are moving. This requires intimate knowledge of the position, mass, and velocity of local objects. But tweaking the direction of one thing so it whacks into another, bigger thing, then that whacks into another bigger thing and ultimately it whacks into Asteroid UB, saving the earth! Yes. Leverage. |
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So it's not about the asteroid belt, but now it's asteroid pool. |
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//asteroid pool//
4vesta in the corner pocket... ;) |
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//what about harnessing things that are already moving and whacking them into each other?//
This is surprisingly difficult for even 3 masses in 3D space. [link] |
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Let alone the fact that you had enough difficulty trying to move one of them 'anywhere but here' ... now you're trying to move lots of them 'exactly there'. Hmmm |
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"asteroid pool" is the opening sequence for the "3rd Rock from the Sun" TV Show as well as an episode of "Red Dwarf". And of course my "Orbital Garbage Snooker" post is somewhat related. |
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//asteroid pool" is the opening sequence for the "3rd Rock from the Sun" TV Show as well as an episode of "Red Dwarf".// All of course pre-dated by DNA's "intergalactic bar billiards" in Fit the Sixth of H2G2. |
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