Moving Asteroids

bobvanx · Jan 16, 2005

I'm a big fan of "less is more," so I have an inclination towards solar sails, pulsed laser plasma detonation, ion drives and the like. But moving an asteroid out of a collision course, that really does seem like it would take a large impulse. An Orion-class engine would be my favored tool. So far, we discover Earth approachers several orbits before the projected impact. So have a mission, sitting ready, to rendezvous with the rock and set up a pusher plate with a supply of low-yield nukes. Perform a TCM at perihelion, change the orbit by quite a bit! All we ever really want to do is change the period by a few days, so the intersection point is not coincident with Earth's orbit.

vogon13 · Jan 17, 2005

The farther away ( in space or time) an asteroid is from an earth impact, the less nudge needed to achieve near miss instead of splat. I suppose a really big object very far away might need a petty big swat but not too likely to happen. Orion concept seems like a real good way to put colonies on asteroids, and then perhaps colonists could decide on altering rotation, inclination, eccentricity, period of their world for convenience, or whatever reason they might have. That new book on the Orion project raises some very interesting ideas, don't recall asteroid deflection as being one, though. Many of the Orion designers and engineers wanted to go to Enceladus. By 1970. I'm not making this up. <div class="Discussion_UserSignature"> TPTB went to Dallas and all I got was Plucked !!So many people, so few recipes !!Let's clean up this stinkhole !! </div>

mental_avenger · Jan 17, 2005

I see no advantage to perfoming a trajetory correction maneuver at perihelion. The further from coincidence that an evasive maneurver is perfomed, the smaller the input of energy required. Many asteroids are “piles of rock” conglomerates, loosely held together by a very weak gravitational bond. It would take very little to shatter one of those rock piles, and the result could be deadly. Therefore, the “orion” concept would be not only impracticle, but potentially dangerous. <div class="Discussion_UserSignature"> Our Solar System must be passing through a Non Sequitur area of space. </div>

mental_avenger · Jan 17, 2005

Orion concept seems like a real good way to put colonies on asteroids, Colonies on asteroids? Reality check. On Phobos, the nearest moon of Mars, a person would weigh about 2 grams. On an average asteroid, a person would weigh less than half a gram. Don't let the Bruce Willis movie fool you. There won't be any colonies “on” an asteroid. On some asteroids, that rotation that you talk about could easily throw off anyone who tried to “land”. Forget the movie hype and come back down to reality. There are not going to be “colonists” on asteroids. It will be tough enough to get colonies on Mars. <div class="Discussion_UserSignature"> Our Solar System must be passing through a Non Sequitur area of space. </div>

qzzq · Jan 17, 2005

Mental_Avenger, What if we make the asteroid spin fast enough, couldn't the centrifugal force act like gravity? -- With our heads pointed at the asteroid of course. If we can seriously think about colonizing asteroids, it should be doable. Also, the ores would just be flying out of the asteroids core! (after a slight push of course) <div class="Discussion_UserSignature"> *** </div>

bobvanx · Jan 17, 2005

Post deleted by bobvanx. Thanks to Steve for proofreading. The corrected post now occurs further down the thread.

bobvanx · Jan 17, 2005

>> Many asteroids are “piles of rock” conglomerates, That's one theory, yes. Many are certainly solid bodies, too. Most (if not all) will be covered with loosely held boulders and regolith. So what? Even a relatively low-impulse Orion-class propulsion system will certainly free the main body of quite a bit of its detritus. These much smaller objects would spread out into a cloud. Heaviest objects would land back on the main body. Anything that retained enough of it original momentum would stay on the Earth-crossing orbit. Trading the certain impact of a large body for a shower of dust grains is a good thing.

bobvanx · Jan 17, 2005

By golly, you're right. Perihelion is closest approach to the sun. Well, I hate to let that incorrect graphic stay, so I'll delete the post and correct it.

bobvanx · Jan 17, 2005

<blockquote>In reply to:<hr />I see no advantage to perfoming a trajetory correction maneuver at perihelion.<hr /></blockquote>The advantage becomes apparent when you apply the physics of orbital mechanics. Your instinct is sound, to find the place in the orbit where the least energy is required to change the orbit. The spot in an orbit where you get the greatest "leverage" from your delta-V is at perihelion. The orbital geometry that makes a rock dangerous is not whether on a 2D map the tracks of its orbit and Earth's coincide. The criticality comes from where the two orbital planes intersect. IFF the planes intersect such that the orbital tracks coincide, then there is a risk of impact. So to "solve" the problem of an impact, you can do three things: change the asteroid's orbital plane, such that the point of its orbit where it crosses rotates away from Earth's orbit (this option is energy expensive); slightly change the period of its orbit, such that it arrives at Earth's orbit before or after the Earth passes (least energy, but merely moves the problem further into the future); increase or reduce its orbit such that the intersection point moves outside or inside Earth's orbit. It can be hard to visualize the true nature of orbital crossings with a 2D diagram, but I'm going to give it a try. I shaded the area that is "below" the plane of Earth's orbit to help indicate where the asteroid is actually crossing through. You can see that although the orbits appear to cross on the right side as well, this is an artifact of drawing the diagram in 2D rather than in 3D.

mental_avenger · Jan 17, 2005

That's one theory, yes. Many are certainly solid bodies, too. I don't beleive there is anything “certain” about it. But the fact remains, that we won't be able to choose which ones we attempt to deflect, we work with the one(s) that are threatening. Even a relatively low-impulse Orion-class propulsion system will certainly free the main body of quite a bit of its detritus. Unless it could be proven that the asteroid was a very solid body, using explosions to provide motive force would be foolhardy. Heaviest objects would land back on the main body. With explosive propulsion, not likely. Anything leaving the surface will probably exceed escape velocity, which would be extremely low. Trading the certain impact of a large body for a shower of dust grains is a good thing. Dust grains, perhaps. But creating multiple large fragments would probably be a lot more devastating to the Earth than one larger body. Again, ignore the TV and movie hype. It is unlikely that there is a way to reduce a 2km asteroid into harmless dust. Any attempt to do so could result in far worse consequences. I have yet to see a practical application for the “orion” concept, and this certainly isn't one of them. <div class="Discussion_UserSignature"> Our Solar System must be passing through a Non Sequitur area of space. </div>

mental_avenger · Jan 17, 2005

Perhaps "Inside", tho. Many have thought about using a small asteroid as hull of a space ship or space colony which is mobile. Have to be solid rock, tho many are not. True, that has been proposed. However, they seem to ignore several problems. It takes a lot of energy to move that much mass, so it isn't really mobile. Also, if the asteroid IS a pile-of-rocks conglamoration, then it would break up from the stresses. We have seen comets break up from the stress of merely passing close to a massive body. If an asteroid is very solid, then tunneling and hollowing out the center would be very expensive. Explosives would be out of the question, so cutting and abrasion would have to do all the work. In order to create a cavern large enough for artificial gravity, the asteroid would necessarily have to be very solid, with no weak spots that would separate when it is “spun up” to create the artificial gravity. In addition, the square feet required to mantain even a small colony would be larger than would be practical to build into an asteroid. I can't imagine where the payback would be. In some cases. But rotation can be stopped or slowed by applying a force to counteract it. The first ones there might have trouble stayin on without a tether, until they slowed down the rotation. Then it would be easier. Consider that if you weighted 1/2 gram on an asteroid, your 20 ton ship would weight about 5 ounces. As you were saying.............. <div class="Discussion_UserSignature"> Our Solar System must be passing through a Non Sequitur area of space. </div>

bobvanx · Jan 17, 2005

I'm a bit disappointed you didn't thank me for explaining why applying the orbital correction at perihelion was the most appropriate.

mental_avenger · Jan 17, 2005

The advantage becomes apparent when you apply the physics of orbital mechanics. Your instinct is sound, to find the place in the orbit where the least energy is required to change the orbit. The spot in an orbit where you get the greatest "leverage" from your delta-V is at perihelion. While that is true, there may be an overriding consideration. Depending on the orbit of the asteorid, the difference in orbital velocity between perihelion and aphelion may only be 15%. While this may give you the most “bang for your buck” (% deviation) at that point (perihelion), it won't necessarily provide the most total deviation to the orbit. IOW, if you back up halfway around the orbit and apply the deviation force there, at aphelion, by the time the asteroid has reach perihelion, it will already have deviated (over the course of 1/2 orbit), more than the same amount of force could deviate the orbit at perihelion. Example. 3361 Orpheus with an orbit of q=.819, Q=1.60 a=1.21 P=1.33 That gives us an orbit of 7.5995AU or 706,754,674.5 miles. Suppose we impart a deviation (perpendicular to the orbital plane) of .1 degrees at aphelion. By the time the orbit reached perhelion, it would already be deflected by 610,865 miles, IF it was a straight line. Orbital mechanics doesn't work that way. The actual deviation would be 305,432.5 miles in the opposite direction at perihelion. Still, that would be a big head start on deflection, compared to beginning the same deflection at perihelion. The 15% or so that you gained by deflecting at perihelion, would be overshadowed by the overall deflection started at aphelion. If this is unclear, let me know. <div class="Discussion_UserSignature"> Our Solar System must be passing through a Non Sequitur area of space. </div>

bobvanx · Jan 17, 2005

You're welcome!

mental_avenger · Jan 17, 2005

Thank you for the explanation. Even though that is something I do understand, I was distracted by some preconcieved ideas about asteroid deflection systems. I did forget one big advantage to perfoming an orbital change at perihelion. Since the orbit of any NEO will be closer to Earth orbit at perihelion, then a mission that is launced from Earth to nudge the orbit of a NEO will be closer, and therefore faster. It depends on which orbit the NEO is scheduled to intercept Earth orbit. Since the practical location for asteroid detection and tracking is Solar orbit at about 1.4AU, that may also be the practical location for proposed asteroid deflection stations. In that case, all orbital deflection would occur at aphelion. I am understanding that there is no way to discuss this in general terms, as the proceedures would be so highly dependent upon the individual case. <div class="Discussion_UserSignature"> Our Solar System must be passing through a Non Sequitur area of space. </div>

bobvanx · Jan 18, 2005

>>the practical location for asteroid detection and tracking is Solar orbit at about 1.4AU Absolutely! I have no idea what the expense would be to set up an observatory there, but we have to move the search off Earth! Since LINEAR does its job with a mere 1m 'scope, this sounds like it's in the same class as a Discovery Mission (or whatever they are called these days). The other place that's a good detection and tracking location is in a Venusian Trojan Halo orbit. The Atens and Apollos can be spotted from there really well.

Saiph · Jan 18, 2005

You sure it isn't an apahelion burn is the most energy efficient approach? E= 1/2*M*V^2 (or E~V^2 for our purposes) Lets consider a sample orbit: Va = 2 Vp = 4 We can apply .5 units of energy total through whatever means we require. Ea = 2^2 = 4 Ep = 4^2 = 16 Now, we boost it by .5 units: Ea = 4.5 Ep = 16.5 We can already see that the .5 boost is a bigger % change for the apahelion application. Lets look at the resulting velocities: Va2 = sqrt(4.5) = 2.12 Vp2 = sqrt(16.5) = 4.06 The % shift in velocity is: %a = 6% %p = 1.5% And the largest shift in velocity creates the largest shift in orbital trajectories correct? Or did I go wrong somewhere? <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- The Tenth Doctor, "Blink" </div>

Saiph · Jan 18, 2005

Remcook's reply in the "slingshot problem" thread from a week or so ago: <hr />OK. it's basically just conservation of energy. Whenever the engine is not burning, energy is conserved. Now, the total energy (per unit mass) is: Ek + Ep = 1/2 M V^2 - GM/r = const. it's basically because the kinetic energy increases with the square of the velocity that enables this. Say if your speed at perigee is 10. Your kinetic energy = 50. (1/2 V^2) and you increase it with 1, your kinetic energy becomes: 121/2 = 60.5 so, your increase is: 60.5 -50 = 10.5 Now, say that your speed at apogee is 5 (a lower number in any case). your kinetic enrgy is 12.5. increase with the same 1 gives a new kinetic energy of: 36/2= 18 Increase in energy is: 18-12.5 = 5.5 < 10.5 By burning, you can only change your kinetic energy, not your potential energy (you can't suddenly move yourself discontinuously), so you can better do it where speed is highest. <hr /> Which is basically saying the same thing. He, however, has a fixed velocity adjustment. So perihelion works better if you can create a fixed shift in velocity. However when you apply thrust to body, you only have so much energy (fuel) to support the endeavor, so i believe the fixed energy approach is a more valid one. Anyone correct me if I'm wrong. <div class="Discussion_UserSignature"> --------------------SaiphMOD@gmail.com -------------------"This is my Timey Wimey Detector. Goes "bing" when there's stuff. It also fries eggs at 30 paces, wether you want it to or not actually. I've learned to stay away from hens: It's not pretty when they blow" -- The Tenth Doctor, "Blink" </div>

bobvanx · Jan 18, 2005

hmm... What I notice about orbital dynamics from Galileo, Cassini, Stardust, MGS, and Messenger, it appears they apply large delta-V at the peri (-gee, -helion, -apses) to perform large changes in orbital periods and they apply small delta-V at the ap (-ogee, -helion, -apses) to fine tune a targeted spot in space. [blathering paragraph omitted] To move an intersection point so it is no longer coincident with the Earth seems like it would require both of these.

robnissen · Jan 18, 2005

"It is unlikely that there is a way to reduce a 2km asteroid into harmless dust. Any attempt to do so could result in far worse consequences." I have never understood this argument. Most would agree that at some point, an asteroid would become civilization destroying and destroy the vast majority of human life on the planet. The only issue is what size that asteroid must be, whether its 1km, 10km, 100km, whatever. But if that is true and you could break that asteroid into 4 smaller chunks, that would seem to me to be a good thing. For example, if it takes a 10 km asteroid to destroy civilization, it seems to me that breaking that into four 2-3km asteroids that would destroy four nations, rather than the entire earth, would be a good thing. Similarly, if a 2 km asteroid is nation destroying, it would seem to me that breaking that into four .5km city destroying asteroids would be a good thing, on down the line. As the gravity of an asteroid is too low to prevent the chunks from escaping from each other, it would seem that multiple nation destroying strikes, would be preferable to one civilization destroying strike. Since size and velocity create kinetic enery, it would seem to me that 4 strikes with .25 of the kinetic enery would be much better than 1 strike with all the kinetic energy (since the relative volocity of all four parts would not change), in terms of the earth's ability to absorb the strike. (And this ignores the fact, that one or more of the chunks might be pushed far enough away to miss Earth all together.) In any event, even if it didn't help, I can't imagine how breaking up a large asteroid, could ever possibly "result in far worse consequences." Am I missing something here?

igorsboss · Jan 19, 2005

In any event, even if it didn't help, I can't imagine how breaking up a large asteroid, could ever possibly "result in far worse consequences." Am I missing something here? The problem is that the total energy of the impactor remains the same. If the impactor is large and solid, it would completely obliterate one area, then kick up a bunch of hot ejecta, and the hot ejecta incinerates the planet. If the impactor is the same mass as above, but in small pieces, the sum of the energies of the little impactors equals the energy of the large impactor. The planet might not have the big crater, but it still gets incinerated. This time, the incineration may be over a wider area. The only way to reduce the total energy is to cause some or all of the impactor to miss the planet.

igorsboss · Jan 19, 2005

The only way to reduce the total energy is to cause some or all of the impactor to miss the planet. Hey, igorsboss, what an excellent point you have there. <img src="/images/icons/wink.gif" /> What if we planted an A-bomb deep inside the asteroid, then used thrusters to spin the asteroid as fast as we could before we detonated the explosive? If the spin were fast enough, the resulting bits of the rubble pile would take divergent paths, so that many of the bits of asteroid would miss the planet. The sum of the energy of the bits that still impact would be smaller than the original impactor. The number of crater-forming impacts or incineration events would be smaller. Just random musings...

mental_avenger · Jan 19, 2005

A large asteroid strike on land would vaporize both the asteroid and a great deal of the impact area. All that ejecta would be thrown up into the atmosphere. Much of that ejecta would be molten, and would start fires in a wide area. But the finer particles, the dust and gas, would remain in the upper atmosphere and spread out. How far that cloud would spread, and how thick it would be, would depend on the size of the asteroid. If the asteroid was large enough, the cloud could cover the Earth, which would create a “nuclear winter”, blocking out the light from the Sun, freezing the planet and killing most of the plant and animal life. The cloud from a smaller asteroid might not spread far enough to destroy the planet. However, if that smaller asteroid was broken up, and struck in widely spaced locations around the Earth, the cloud then might cover enough area to destroy the planet. For even smaller asteroids, trading the loss of one city for the loss of several is not a good solution either. There are many factors involved. However, one thing remains clear. Diverting an asteroid from impacting Earth is a much better solution than breaking it up. So there is really no point in even considering blasting a threatening asteroid with nukes, if there is any way to divert it. Like I said, forget the TV and Movie scenarios, where the asteroid is blown to bits and Earth is saved. <div class="Discussion_UserSignature"> Our Solar System must be passing through a Non Sequitur area of space. </div>

mrmorris · Jan 24, 2005

While I agree that diverting an asteroid is better than breaking it into pieces but leaving its course unchanged -- there are two things you're not accounting for in your argument that breaking the asteroid into pieces is not helpful. 1. The smaller pieces will have a much greater surface area proportionate to the original. On re-entry -- this equates to a higher percentage of the asteroid burning off and a smaller percentage of the original mass surviving to reach the ground. 2. Asteroids (actually at this point they'd be properly called meteors) often 'explode' in mid air. This happens for the same reasons rocks heated in a fire can crack explosively -- namely that rock is a poor conductor of heat and the outer layers get much hotter than the center -- which causes expansion stress and explosive release. This effect will be more pronounced on the smaller pieces than a single large one. The result will be that the smaller pieces will be more likely to break up into even smaller pieces -- becoming less destructive and magnifying the effect of #1. Everything depends on how large the original asteroid is, and just how much smaller the pieces it can be broken up into. If the asteroid is just a pile of rock conglomerates, loosely held together by a very weak gravitational bond, then breaking it up into thousands of pieces would seem to be very possible. In addition -- deflecting such an asteroid without breaking it up would seem to be very difficult.

Leovinus · Jan 24, 2005

I've got an idea for moving an asteroid: 1) Come up with a way to split the asteroid in two through the center of mass on command. Maybe what you do is dig the asteroid down to the point where there is a little column separating the two halves and this little column can be destroyed by explosives. 2) Spin the asteroid up 3) At the right time, split the asteroid. 4) The two halves fly appart off in different directions based on centrifugal force. <div class="Discussion_UserSignature"> </div>

Moving Asteroids

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