Author Topic: Close Call with Asteroid 2006 XG1 in 2041  (Read 16137 times)

Offline HarryM

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Re: Close Call with Asteroid 2006 XG1 in 2041
« Reply #20 on: 01/12/2007 08:05 pm »
The Kursk was mostly hollow, which makes it alot easier to slice with a cable. It's the difference between cutting a pipe and a solid metal rod, considerably more difficult/more energy required. Nickel iron is tough stuff, no small problem when it comes to harvesting a large asteroid or chunks of one.

Offline TyMoore

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Re: Close Call with Asteroid 2006 XG1 in 2041
« Reply #21 on: 01/13/2007 02:06 am »
I don't know about capturing the asteroid into an Earth orbit, but is it even worth trying to mine? What kind of asteroid is it? Is it nickel-iron, metallic silicate, silicate, or carbonaceous?

One of the misconceptions with the idea of capturing an asteroid is that you spend a lot of effort moving stuff you don't want--like what will eventually become slag tailings. While this may prove useful in the long run as shielding material for space colonies, in the short run it would cost us a lot more in terms of not just delta-v, but in expended propellant to create that delta-v.

Asteroid mining may make more sense in taking a rudimentary processing station to the rock in question and loading up the 'ship' with valuable products like: water from ice, liquid ammonia, liquid methane, and liquid hydrogen, with a smattering of platinum group metals, etc... Now those products will be extremely valuable for continuing space efforts...


Offline publiusr

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Re: Close Call with Asteroid 2006 XG1 in 2041
« Reply #22 on: 02/03/2007 07:41 pm »
A Cable would be held away at one side of the asteroid thru some kind of yoke that rises on a tower as the loop passes underneath. If it has ice--that might help with the lubrication of the smaller cuts.

You just need to bring a lot of cable for breaks.

Offline TyMoore

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Re: Close Call with Asteroid 2006 XG1 in 2041
« Reply #23 on: 02/03/2007 11:15 pm »
Are you referring to some kind of giant cable saw like they use in quarrying large blocks of marble? Interesting idea...

Offline publiusr

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Re: Close Call with Asteroid 2006 XG1 in 2041
« Reply #24 on: 02/07/2007 07:57 pm »
Something like that.

A tower would be bolted/melted into the surface of a fractured nickel-iron 'berg and the fissures exploited using various methods. You spin up the tower and work the asteroid and the tower rises with space rated hydraulics to have the cable bite deep into the material. It would need a thick lube compound. The tower is the gravity tractor. It has big flywheels that can be used for other purposes.

An enclosed asteroid with a large fresnel lens to heat an asteroid (or some other way to melt portions) would allow the vacuum of space to pull blobs/ingots of material without the use of a fancy mass-driver. Fewer moving parts.

Offline Tom Ligon

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Re: Close Call with Asteroid 2006 XG1 in 2041
« Reply #25 on: 02/08/2007 11:05 pm »
The really interesting capture possibilities involve something called a keyhole ... a gravitational node at which a very small amount of impulse can cause large orbital changes.  These are the points being used to sling the Cassini probe to all those nifty flybys of Saturn's moons.

One of the possible near miss asteroids I read about a year or two back will pass within 10 km of a keyhole, and a possibility is that it could possibly (just barely possibly) hit Earth on the following path.  I think that keyhole was about 10000 km out.  But one might also use such a trick to either kick it into a long-term safe orbit or possibly do something fancy to capture it.

I'd be inclined to try a mass driver approach for any asteroid we wish to do this with.  Put a nuclear-powered dust gun on it and use its own material as reaction mass.  But most asteroids tumble, and that makes attitude control a problem with any thrust system attached to the asteroid.  One idea floated not long ago is to park a large spacecraft very close to the asteroid, and use thrusters to keep the spacecraft hovering off the asteroid in the direction you want to apply force.  The gravitational force acting on the ship also acts on the asteroid, so you get a very gentle force.  Kept up long enough, the asteroid can be affected considerably.  

We don't have the technology to do this yet, but we do have the knowledge of orbital mechanics.  I see no reason why it could not be done safely as long as we have a propulsion technology that will work and won't break up the asteroid.

It is very gratifying to see how quickly this bunch jumped on the idea of utilizing the asteroid rather than worrying about it.  I've always thought that any technology that could save us from an asteroid impact was not far removed from being able to capture one.

Offline TyMoore

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Re: Close Call with Asteroid 2006 XG1 in 2041
« Reply #26 on: 02/09/2007 05:18 am »
Mass drivers are really good for putting material into orbit about the moon, or on a path that allows it to intersect a materials transfer/processing station probably at one of the Earth-Moon liberation points. However, to use it for propulsion--this at first sounds like a good idea, but infact it isn't for a couple of very good reasons.

1) Because the 'muzzle' velocity of the mass driver is 'small,' (by this I mean compared to the 'exhaust' velocity of a good rocket engine like a high performance LH2/LOX engine, or a NTR,) then an appreciable mass fraction of the asteroid must be converted into 'pellets.' This is because the mass driver system and the asteroid act like a giant rocket--then the rocket equation applies.  To change the velocity of a 1 billion ton rock by 100 m/s with a mass driver that shoots at 2 km/s, requires expending about:

deltaV=Ce*ln(mi/mf) solving for mf gets us:

mf=mi*EXP(-deltaV/Ce) where mi = 1 billion tons, deltaV=100 m/s, Ce=2000 m/s which on substitution gives:

mf=950 million tons. So mi/mf=1.05 not too bad!

However, we're talking about 50 million tons of material which is a lot.

2) What happens to all the billions of little bags, pellets, chunks, that have been accelerated by the mass driver? Unlike regular rocket exhaust which may condense into very tiny ice particles, we now have billions of multikilo projectiles zipping around in a whole "range" of new orbits. These will be  impossible to track becuase of their number, and if this is done many times, will steadily result in serious navigation hazards to any planetary craft passing through them. Congratulations, you've just moved an asteroid. But now you've just 'polluted' a vast swath of near interplanetary space with a new belt of debris similar to the way a comet might jettison small rocks, dust, and sand. This may or may not pose a navigation hazard. But I know this, the more times you move big rocks around this way, then the more junk that is going to be flying around.

I'd make a couple of suggestions to prospective mass-driver propulsion advocates: make the accelerators really long so that you can really crank up the muzzle velocity. The higher the exhaust velocity, the less the mass is needed to be expelled. I would shoot for 25km/s exhaust velocity since at this speed, for asteroids dropping down into orbits from outside Earth orbit, this will put most mass driver exhaust into trajectories that tend to drop right into the sun. No problem there.  Raising an asteroid's orbit from inside Earth's orbit will put the pellets on a path that will eventualy exit the solar system. Again no problem there. It's much better to get rid of the stuff than have it flying around the inner solar system until decades later some poor sop's pressure hull is torn apart by collision with a 5 kilgram fibergalss bag of asteroid surface regolith. That would suck!




Offline meiza

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Re: Close Call with Asteroid 2006 XG1 in 2041
« Reply #27 on: 02/09/2007 01:36 pm »
Remember though, the bigger pellets you shoot, the lesser fraction they get of the kinetic energy, and more is given to the asteroid, so it is more efficient. So it goes away from the rocket equation the bigger the chunks are.
If the mass driver shot just one huge pellet of 50 million tons at 2000 m/s then the other 950 million tons would move in the opposite direction at a bit over 100 m/s.  m1v1+m2v2=0 yields actually 105 m/s.
If you have infinitesimal size pellets (rocket engine), then it's the ln(m0/m1)=deltav/vex or the 50 million reaction mass shot out at 2000 m/s, and we get about 103 m/s.
The results are close because the mass fractions are so tiny and the ln curve has a derivative close to one at one.

If you have a smaller asteroid, say 100m diameter sphere, it's about 3 million tonnes (m1). If you have puny mass drivers that only give 200 m/s vex, and you want a 100 m/s delta vee, if you shoot the whole deltam piece in one chunk (unrealistic), it gives a mass of (note that vex is a negative number):
m2*deltav + deltam*vex = 0
(m1-deltam)*deltav+deltam*vex=0
m1*deltav=deltam*deltav-deltam*vex
deltam=m1*deltav/(deltav-vex)
deltam=3 Mt * 100/300 = 1 Mt

(Mt = megatonne = million tonne = 10^9 kg)

With rocket equation you get (we have to add minus since vex is negative)
m1/(m1-deltam)=exp(-deltav/vex)
1-deltam/m1=exp(deltav/vex)
deltam=m1(1-exp(deltav/vex)) = 3 Mt*(1-exp(-0.5))= 1.18 Mt.

Of course, in reality the mass drivers are something in between these numbers since the chunks are of finite size but smaller than one megaton. :) I'm surprised from the calculations that the advantage with bigger chunks is so small.

I don't think a mass driver with a muzzle makes sense even if it is the most common starting point. A sling would be much better and would require very little infrastructure. The asteroid's rotation could be managed by accelerating the payloads clockwise or counterclockwise every other time, or a bias could be introduced to reduce/gain angular momentum for the asteroid.
The speed of rotation is dependent on the specific strength (modulus) of the sling arms and the payload per hardware fraction per shot. (At least)

Offline meiza

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Re: Close Call with Asteroid 2006 XG1 in 2041
« Reply #28 on: 02/09/2007 02:10 pm »
I drew some nice curves.

alpha = deltam/m1 = The fraction of expelled mass
beta = deltav/vex = The "challenge" of the mission, how much delta vee is needed compared to mass driver exit velocity
For single chunk:
deltam/m1=deltav/(deltav-vex)
alpha = 1/((deltav-vex)/deltav)
alpha = 1/(1-1/beta)

For infinitely small chunks:
deltam/m1=1-exp(deltav/vex)
alpha= 1-exp(beta)

The blue curve is the mass fraction needed for the mission when the reaction mass is shot out in one chunk. The red curve is a "rocket engine" type where the chunks are infinitely small. Real capabilities lie somewhere here in between. X axis is mission challenge beta and Y axis is required expelled mass fraction alpha.

Offline Tom Ligon

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Re: Close Call with Asteroid 2006 XG1 in 2041
« Reply #29 on: 02/09/2007 02:31 pm »
My personal fantasy is that we can get R. W. Bussard's p-B11 reactors to work, and use the megavolt output to produce a relativistic electron beam.  REB-heating is not particular about what it blasts to plasma.  Rock won't give the highest exhaust velocity, but it is probably the least valuable material in the asteroid.

I've always been a bit skeptical of the wisdom of slinging huge numbers of 1-ton masses out a long conveyor system, and I don think slinging a thousand times as many 1 kg masses is an improvement.  Space is big, but chronically doing that does not help the business of dealing with flying rocks, unless the Vex is enough to carry them out of the system entirely.  Really fine dust (I mention a "dust-gun" mass driver) might not be so bad ... we can make effective micrometeorite shields, and it might be easier to get the Vex of that high enough to leave the system.  An elecrostatic accelerator might be enough.

Looking at the total mass of anything that rates the name asteroid, it is difficult to imagine we can move them a useful amount without in some way using the body itself as reaction mass.  If we had enough rocket fuel available to squander on that kind of enterprise, we wouldn't be moaning over how difficult space travel is.  There might be enough volatiles on some asteroids to use as reaction mass, but that will be one of their most valuable resources.

Offline TyMoore

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Re: Close Call with Asteroid 2006 XG1 in 2041
« Reply #30 on: 02/10/2007 12:56 pm »
Meiza, thank you for the nice computations. Yes, if you throw momentum around in bigger chunks then Newton's Laws become more efficient, but how do you propose to sling larger masses? Only somekind of explosive mechanism can do it in one shot--and the reaction shock will probably shatter the rest--which would turn an asteroid into a really big shotgun! The mass that you end up saving is offset by controllability of the momentum. And mathematically speaking moving from continuous (like a rocket engine) to discrete (billions of 10kg packages to millions of 1 ton packages) doesn't penalize you as much since you also have very much more mass available as 'propellant.' Still, I'm a bit concerned about dumping so much mass in a range of new solar orbits. If I had the ability to simulate it, I would. I suspect that there might even by a thesis in it for someone who could do it--the results ought to be very interesting to see simulated with something like STK or such. My suspicions are that ejecting so many (billions) small, discrete masses on slightly different trajectories will tend to create some kind of 'belt' of debris that may eventually become a navigation hazard. This is why I suggested that such a high-deltav accelerator be used. Another thing--which I forgot to mention before--is that the packages should be composed of fine powder which could be mechanically dispersed or electromagnetically dispersed at the end of the acceleration phase. Slightly dispersing the powder will allow solar photons to disperse the powder even more, and hopefully will sweep it away (or sweep it up if going the other way.)

Otherwise, I am afraid that moving asteroids will involve creating belts of debris which may eventually become a serious navigation hazard.

Tom, the issue of volatiles is very much at the forefront of any large scale space transportation infrastructure issue.  Without the volatiles from asteroids, it all has to come from Earth itself, and that will make it too costly to do anything on a big scale. I suspect that large scale deep space transportation will have to involve volatile extraction/transportation to a high orbit materials processing station/storage depot. From there other vessels would be refueled and sent to other asteroids to extract metals, push asteroids etc. Asteroid impact mitigation can thus proceed quite naturally from an asteroid resource utulization industry already there. You would already have all the necessary infrastructure: the telescopes, the radars, the prospecting probes, and the means to mitigate collisions, and the means to resupply it all.


Offline refsmmat

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RE: Close Call with Asteroid 2006 XG1 in 2041
« Reply #31 on: 02/10/2007 02:13 pm »
How about mounting a large turntable on the asteroid surface, and then starting to spin it up?  The torque would build up very slowly, but left to its own devices it may spin up the asteroid fast enough to start to shed mass and change velocity.

Offline publiusr

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Re: Close Call with Asteroid 2006 XG1 in 2041
« Reply #32 on: 02/10/2007 06:24 pm »
With the cable plan above--I use the same mechanism needed to slice up a body--and with some changes, the flywheels cables can sling matter outwards. Thus the same device is used for both cutting and slinging.

Offline TyMoore

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Re: Close Call with Asteroid 2006 XG1 in 2041
« Reply #33 on: 02/12/2007 03:08 am »
That would make it extremely difficult to control both slicing and propulsion at the same time. Thrust Vector control would have to take priority of cutting--so while it seems like a good idea--I don't think it is. Also, it is very difficult to get enough delta-v even from something like a rotary pellet launcher to give worthwhile performance--without chewing up the entire asteroid. Of course, if you sent along a processing station, you can mine and convert an asteroid into useful materials, and then use the slag as reaction mass for a mass driver. Although in another post, I explained why I think that using a mass driver or rotary pellet launcher wouldn't do: it has to do with the relatively low delta-V which requires using most of the mass of the asteroid for reaction mass; and also creating navigation hazards from the belts of expelled debris...

Offline meiza

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Re: Close Call with Asteroid 2006 XG1 in 2041
« Reply #34 on: 02/12/2007 12:26 pm »
Yeah as the chunks you expel become bigger, it is worth looking at where the chunks themselves go to - they might impact earth. :( But probably even 10 meter diameter chunks aren't that hazardous? One can toy around with this: http://www.lpl.arizona.edu/impacteffects/

You don't need explosives for big chunks.
I was picturing some kind of big sling system with parts of the asteroid bagged and slinged around. The sling is accelerated veery slowly so it doesn't need that huge power (it still needs a lot).

Many things of course depend on the scale of the project. If it's a huge asteroid and there is a century of time, maybe one could use in situ production of glass fiber to make the tethers and bags for the sling. That way even big asteroids can be coped with.

In a very far looking ponderance, one can think about the solar system's neighbors. In tens of thousands of years, stars coming close to the solar systems could destabilize objects in the Oort cloud and some of them might head to the inner solar system. From the moon we can see that the orbital bombardments have come in "waves". http://www.centauri-dreams.org/?p=1033

Offline TyMoore

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Re: Close Call with Asteroid 2006 XG1 in 2041
« Reply #35 on: 02/12/2007 02:34 pm »
The ejected 'propellant' chunks might not be a hazard from the standpoint of iimpact with a planet, but from the standpoint of a navigation hazard to other spacecraft,  without doing any orbital dynamic modelling, I'd still have to say that several hundred million tons of reaction mass is definately going to be a hazard to other spacecraft. These would be newely created belts of debris; their solar orbits will possess a range of values (that's why I think of them as a 'belt') and since they are newely created, they won't have had time to be swept up by resonance interactions with the planets. If we must move an asteroid, then perhaps it would be better to use something like a very large mass driver with an exit velocity in the range of 25 km/s. For Prograde thrust (in the direction of orbit,) pellets will be fired retrograde which will dump most of them into the sun (or very close to it;) for Retrograde thrust (against the direction of orbit,) pellets will be fired Prograde: as long as the velocity addition from the mass driver exceeds the sqrt(2)*solar orbital velocity relation, the pellets will escape the solar system--and will not present a future navigation hazard to interplanetary spacecraft.

Offline publiusr

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Re: Close Call with Asteroid 2006 XG1 in 2041
« Reply #36 on: 02/18/2007 08:31 pm »
Quote
TyMoore - 11/2/2007  10:08 PM

That would make it extremely difficult to control both slicing and propulsion at the same time. ...

Many asteroids do rotate. And if all I need is one fragment to head towards Earth with enough ore, I can choose to expend one part of the asteroid bola if need be. It might be that multiple cuts are needed to create a chain perhaps? A dual use tether/cutting-cable will keep payloads down. You still need an Ares V or 3 to do it right--but perhaps not Sea Dragon. The cuts are made ahead of time. We need no mass driver.

It might be that you release one part of the bola with less ore materia with the cutter on the side returning to Earth. In this way the spin of the asteroid can be translated into a way to get more than small bits Earth bound in that you are adjusting rotational inertia into forward progress with but one cut of a cable. Math does the rest. Once in the vicinity of Earth other cuts are made ALMOST though the main body of the asteroid.

These can be detached at leisure with smaller automated craft (with enough fuel) and a ballute/parachute attachment. These smaller bits can travel the lessened distance with greater precision. With the right amount of distance, small burns are all that is needed.

 Perhaps a Molniya orbit once the main body of the Space rocked is halved. This big half loops around the moon in such a way that fragments can have that last little bit cut off and released such that it will pass by Earth with small burns--and tethers can bite into the atmosphere to drag the smaller bits in. Parachutes and ballutes just need a big enough desert and the fragments allowed to further break apart within the ballute.

Thus a piecemeal approach is superior to a line of debris with Mass drivers. As mass decreases with each little bit--the main body (ever smaller) will move away from Earth as its speed is steady and its mass reduces.

Look, I'm not just some big rocket lover. I also want to reduce the mass needed for these kinds of missions at least early on until such time as asteroid profits can make Sea Dragon and other ventures do-able. I just think Ares V is the min required to do such a mission right during the crucial first phases.

A rich nickel iron asteroid already headed for Earth could only be diverted in such a fashion--with no need for bombs.

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