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Gravity as a tool

Posted by: Ekkehard Augustin - Thu Nov 10, 2005 12:54 pm
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Gravity as a tool 
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Post Gravity as a tool   Posted on: Thu Nov 10, 2005 12:54 pm
The article "Gravity-Powered Asteroid Tractor Proposed to Thwart Impact" ( www.space.com/businesstechnology/051109 ... actor.html )
says that the gravity of an unmanned 20t-spacecraft could tow away a 650m wide asteroid from a collision path to Earth.

So it might be possible to use it for transportation of asteroids throught space to other places where they may be mined etc.

What about it?



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Post    Posted on: Thu Nov 10, 2005 3:59 pm
I saw that story:
Gravity Tractor for Moving Asteroids Proposed
http://www.bautforum.com/showthread.php ... t=Asteroid

Asteroid Mover
http://antwrp.gsfc.nasa.gov/apod/ap051110.html
http://www.spacedaily.com/news/asteroid-05za.html
http://www.bautforum.com/showthread.php?t=34660



I'm going to have to take Mr. Lu to task on this one:


*******************************
How NOT to Deflect an Asteroid

Impactors are the greatest threat this planet faces, posing a greater threat than any hurricane or tornado. And yet we have fewer numbers of researchers examining the sky for comets and asteroids than we do meteorologists, who derive their professions name specifically from the impacts of space rocks upon our atmosphere, often with catastrophic results, as in the Tunguska event.

We have yet another killer space rock inbound, whose strike potential has proved to be more likely than once thought:

http://www.space.com/news/051103_asteroid_apophis.html

As bad as this news is, we have even greater cause for concern in that the organization that has championed the cause of Asteroid Defense has described a deflection method that is highly questionable to say the least:

http://www.space.com/businesstechnology ... actor.html

I am not an anti-nuclear activist, though many in academia seem to be (Jay Melosh, Michio Kaku, etc.).
But neither am I the only one who has concerns with the nuclear electric approach:

http://www.newscientistspace.com/articl ... onomy.html
http://www.spacedaily.com/news/nuclearspace-04d.html

Yet this concern repeatedly falls upon deaf ears thanks to the infatuation many have with this technology coming from individuals who have no experience in space propulsion, and absolutely no interest in the launch vehicles needed to orbit them safely.

One only needs to consider the repeated failures of The Planetary Society and its schizophrenic penchant to place overthought payloads atop underperforming launch vehicles like Volna.

The safety of the Earth is simply too important to be left in the hands of such amateurs.

The problem stems from indifference on two levels:

A.) Indifference to the absolute necessity of a robust space program on the part of the general public, and worse:

B.) The even greater level of indifference and outright hostility shown to the launch vehicle development community on the part of scientists and payload designers who fail to appreciate big simple rockets because they aren't sexy enough.

The B612 Foundation's scheme for diverting asteroids consists of having a spindly NEP/JIMO contraption "draw off" an asteroid by perturbing its orbit using only the mere presence (no contact!) of the spacecraft...rather like how a mouse is supposed to intimidate an elephant I suppose...

"Maybe if we stared at it hard enough it'd..."

But if the actual high speed collision of the Deep Impact craft upon its target was likened to a gnat getting hit by a tractor-trailer rig, how are we supposed to take B612's concept seriously?

I have many questions about this technology:

What if a small meteoroid strikes the asteroid to be deflected, negating the already negligible influence this undersized JIMO type craft could ever have?

Is there any "surge" capability in such low thrust craft? (Hardly.)

What if a fragment of a meteoroid punches a hole in the craft's radiators --which are a must in order to shed heat?

Can such a first generation nuclear craft start with the same reliability of the latest generation of nuclear submarines?

Can such a low thrust craft even be built?

And most importantly...

Shouldn't you at least have a big chemical upper stage to push the craft out of Earth/Moon so you can responsibly pull the rods?

If the answer to the question above is "yes" (to save wear and tear during the slow spiral out), why not save a step--and a lot of development money--and just have a big chemical interceptor to start with?

A simple, rugged fuel tank loaded with propellants will also have the mass needed to draw an asteroid away from its threatning orbit. Plus, such a rocket stage could also make contact and use its own thrust in an emergency.

A nuclear thermal craft combines higher Isp and thrust.

The JIMO type craft described by B612 is too small and weak to have much influence, gravitational or otherwise.

So why not use the same HLLV architecture that Griffin and others have called for as part of VSE, and leave NEP at home for now seeing that Apophis is closing quickly and Congress moves slowly?

http://www.spacedaily.com/news/oped-04zc.html

Could it be the hostility on the part of the scientists to the real concerns of engineers and 'launch vehicle boosters' that I described?

Probably.

Isn't Earth's safety more important than such childishness?

Certainly.

Let me leave you with this:

What many NEP-apologists fail to admit is that JIMO was getting to be such a mass pig that it would have taken an HLLV to get it in space to start with--at least to do it right. Otherwise, you must use the same troubled ISS piece-at-a-time assembly process, with each load atop lousy no-engine-out EELVs like this one:

http://www.spacedaily.com/news/oped-05zi.html
http://www.spacedaily.com/news/oped-05za.html
http://spaceflightnow.com/delta/d310/04 ... ch/03.html

Now imagine a nuclear pile atop that.

Any Questions?

Good.

So write your congressman. Support HLLV and Mike Griffin.
And tell B612 to begone.


Last edited by publiusr on Fri Nov 18, 2005 11:29 pm, edited 2 times in total.



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Post    Posted on: Thu Nov 10, 2005 7:29 pm
To answer your question Ekkehard, I don't think this approach would have any special advantages for making asteroids easier to mine. To deflect an asteroid from a collision with Earth only a very small change in its orbit is required. To significantly change its orbit would require vastly more energy.

What is new in this proposal is using gravitational attraction to avoid having to bring the spacecraft in direct contact with the asteroid. It's sort of like the difference between pushing a car by getting behind it versus putting a magnet in front of it to pull it along without physical contact. You still need the same amount of force acting on the asteroid to change its orbit. It's just being applied indirectly.

The advantages of this approach are that you don't have to worry about stably attaching the rocket to a loosely aggregated asteroid, and you don't have to worry about keeping the rocket pointed in the right direction even as the asteroid tumbles in space. The disadvantage is that you have to have a massive spacecraft in order to exert much force on the asteroid. You must also carefully control the thrust to just counteract the gravitational pull between the asteroid and the spacecraft, while carefully keeping the spacecraft in the correct position relative to the asteroid.

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Post    Posted on: Thu Nov 10, 2005 8:13 pm
publiusr wrote:
Any Questions?
YES :twisted:

Since nothing in the idea says we don't need heavy lift, and since you yourself said it would almost certainly require heavy lift to launch it, why are you so hostile to the whole idea? Don't like NEP? Make it SEP! Mass too small? Make it bigger and use a bigger heavy lift launcher.

Personally I think it is really good out of the box thinking and a really cool idea. And the math looks right too.
http://arxiv.org/ftp/astro-ph/papers/0509/0509595.pdf


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Post    Posted on: Fri Nov 11, 2005 8:19 am
Hello, Enthusiast,

there is a factor implicitly involved but not considered explicitly: the time the vehicle tows the asteroid by gravitation. If the asteroid is very far away from the location where it is to be mined then the consumption of the energy required to change the orbit can be distributed over a longer period of time.

Another factor is the mass of the asteroid - to tow asteroids destined to be mined smaller asteroids can be selected, asteroids which are not 200 meters wide but 20 meters only for example.

The relation between time and required energy on the one side and the mass of the asteroid on the other side would be interesting. ...



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Post    Posted on: Fri Nov 11, 2005 1:13 pm
While it may make a vaguely interesting paper study, this has to qualify as among the worst schemes ever devised to deflect an asteroid. In the case of Apophis were talking specifically about 'towing' it by about its own diameter over a number of years and then invoking something akin to chaos theory in a close interaction with Earth to try to make it miss on the next pass. Mankind will survive Apophis but may well be lost to lethargy and indifference.


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Post    Posted on: Fri Nov 11, 2005 3:55 pm
Hmm.. lessee, have I got this straight?

On one side we have publi (whom bills himself as a "self-educated" "gadfly" -- his words, not mine) and nihiladrem (84 posts, none of which have said what his qualifications are, but at least he has demonstrated some technical proficiency)

On the other side we have two men whom work for NASA, have flown in space, have a formal scientific education, and whom have consulted with people doing real-world research in the field, and who have listed thier references:

1. Chapman, C.R. The Hazard of Near-Earth Asteroid Impacts on Earth. Earth and Planetary Science Letters 222, 1-15 (2004).
2. Schweickart, R.L., Lu, E.T., Hut, P., and Chapman, C.R. The Asteroid Tugboat. Scientific American, 54-61 (2003).
3. Scheeres, D.J., and Schweickart, R.L. The Mechanics of Moving Asteroids. AIAA paper 2004-1446. (2004).
4. Chesley, S.R., and Spahr, T.B. Earth Impactors: Orbital Characteristics and Warning Times, Mitigation of Hazardous Comets and Asteroids, editors Belton, M.J.S. et al. Cambridge University Press, 22-37 (2004).
5. Carusi, A., Valsecchi, G.B., D’Abramo, G., and Boatini, A. Deflecting Near Earth Objects (NEO) In Route of Collisions with the Earth. Icarus 159, 417-422 (2002).
6. JPL Sentry Impact Risk Page, http://neo.jpl.nasa.gov/risk/

...so what was the remark about amateurs?

I think that if you fellows have issue with points of funding or policy, you should argue those points; in the technical aspects of this debate you are most clearly out of your depth against these authors, unless you have some pocket aces you'd like to roll at this time?


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Post    Posted on: Fri Nov 11, 2005 5:12 pm
SawSS1, I believe I have said in one of my early posts that I'm a computational physics graduate, nothing to make a song and dance about...and as you shall see I wasn't attacking their technical credentials.

I'm not really sure if this is a veiled attempt to justify building JIMO technology, but if it was the timing of this news is terrible. In some respects I do kind of like wastefully high-tech proposals - but the end use of this one is a stretch.

The simple technical reason why this is a poor *scheme* to deflect an asteroid, which requires no great qualifications to arrive at is that it will consume rather enormous resources and at the same time not produce a demonstration of anything that will have a chance of saving us from the most unpredictable and dangerous kinds of asteroids. So if another asteroid comes round at relatively short notice on a real collision course(this threat is a indirect one and not as yet thought to pose a very significant chance of hitting us on later close encounters) a timely JIMO style rendezvous can't be made. And even if it could the roughly millionth of a meter per second delta-V will be about a million times too little to do anything useful. I can't be alone in thinking that there is something intrinsically wrong with trying to move NEA's with accelerations of about the same magnitude as the Pioneer anomaly.
Another reason I don't like it which is not technical is that the 'don't even dare to touch it!' approach helps reinforce an impression of man's increasingly less bold attitude to space exploration.


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Post    Posted on: Fri Nov 11, 2005 5:30 pm
nihiladrem wrote:
nothing to make a song and dance about


The hell you say. I would conclude that it gives you a very strong hand in this game, seeing as how it specifically relates to your field of study.

Quote:
So if another asteroid comes round at relatively short notice on a real collision course


It all comes down to that, of course... and I don't know any real good astronomers whom could weigh in on that count. At least Griffin's flyboys talked to the JPL guys whose job it is to count rocks, though, so presumably this factor was accounted for in thier work. Japan's asteroid landing project might give some insight into the real technical challenges in "landing" on an asteroid, and certainly the NEAR mission would be appropriate fodder for similar analyses, but niether is mentioned in the context of the report. This does suggest that the project is perhaps a trojan horse for some other purpose, but it does not necessarily negate the principle. The folks in earth-crossing object research have long been saying that early intercept is key, and the concept speaks directly to that idea... perhaps the real intent may be to increase funding for the NEO search project.

Only Griffin knows for sure.


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Post    Posted on: Fri Nov 11, 2005 7:27 pm
SawSS1Jun21 wrote:
I would conclude that it gives you a very strong hand in this game, seeing as how it specifically relates to your field of study.

Yowzers, that might be overstating it!
I don't think it really counts for much if you haven't done the sums, and in this case I really haven't. In fact I've even managed to misread their proposal - they potentially have much more delta-V than I thought from my rapid read of the pdf. It's still no use if you don't have huge advance warning but it does elevate the methods standing somewhat. Of course it is still very unmanly.


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Post    Posted on: Fri Nov 11, 2005 7:27 pm
Much as I like asteroid deflection ideas, this comment (which I recently encountered) seems relevant: “…we’re wasting out time trying to sell …(an asteroid) …insurance policy to a species that didn’t see the need for a tsunami warning network in the Indian Ocean until Dec 27, 2004.”


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Post    Posted on: Fri Nov 11, 2005 8:01 pm
Until we have a working asteroid defense there are always catchy phrases to distract us from the danger.

"How I learned to stop worrying and love the NEA"
"All's fair in war and planetary bombardment."
"Gravity your unseen enemy in space"

I'm in the wrong thread.


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Post    Posted on: Sat Nov 12, 2005 4:19 am
nihiladrem wrote:
I don't think it really counts for much if you haven't done the sums, and in this case I really haven't.
Well I have. And here they are.
In the actual report here:
http://arxiv.org/ftp/astro-ph/papers/0509/0509595.pdf
the premise is that very little velocity change is needed if it is applied soon enough. They say that the mean change in velocity required to deflect an asteroid from an Earth impact trajectory is about 0.035/t where t is the number of years before impact that you apply the change. Assuming we apply the change 10 years before impact, only a 3.5 mm/sec. velocity change is needed. Now you must calculate how long it would take to make that change.
Consider a hypothetical 200 meter diameter spherical asteroid with density 2 g/cc. It has a mass of 6.38E9 kg. A 20,000 kg tug hovering 50 meters above the surface pulls on the asteroid with a force of 0.5 newtons. This is the scenario used in the original report. Applying that force on the 6.38E9 kg. asteroid results in an acceleration of 5.93E-11 meters/sec^2. That acceleration applied for 2 years results in 3.7 mm/sec. velocity change, more than enough. Since their ion engines are angled out they are saying they need a full ONE newton of thrust to effect the deflection. The Deep Space One ion engine supplies 0.092 N of thrust, so eleven such ion engines would be needed. Eleven DS1 space craft have a mass less than 20,000 kg, so you would need to include a bunch of ballast too. Launching 11 DS1s and a bunch of ballast is clearly not beyond existing technology.

The report also mentions the real asteroid Apophis (2004 MN4), 320 meters in diameter. Which will pass near Earth in 2029. If it passes at EXACTLY the right distance on that date, then it will be deflected to a path that will hit Earth in 2036. They also say that a millionth of a meter per second velocity change applied a few years before the close approach in 2029 would prevent a later impact and that a 1 ton gravitational tractor with conventional chemical thrusters could accomplish this deflection mission since only about 0.1 Newtons of thrust are required for a duration of about a month. This also can easily be accomplished without any major new technology.

NO NEP is needed.

In short, if you just read the paper and did your sums, you would have to agree that it is a pretty good idea.


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Post    Posted on: Sat Nov 12, 2005 8:50 am
What about another effect that is merely a micro-saceled one?

Asteroids often or mostly if not allways are conglomerates of dust and rocks. So it might be that the 20t spacecraft attracts dust particles to itself.

Additionally the environment in space may cause rocks to split into smaller pieces and new dust. They might be attrcted to the spacecraft too

May be that the attrction needs to caused by small landers like the tiny robit of Hayabusa.

So it might be that the mass of the spacecraft is increased as years go by which should improve the gravitational pull of the spacecraft and simplify the whole tasl or project - slightly at least.

If that really is so then this again would be enabling mining by using gravitational pull.

What about the effect of gravitational attracting the dust, small stones etc.?



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Post    Posted on: Sat Nov 12, 2005 3:17 pm
campbelp2002 wrote:
They say that the mean change in velocity required to deflect an asteroid from an Earth impact trajectory is about 0.035/t where t is the number of years before impact that you apply the change.

Yes, I did rapidly do some sums after re-reading, the scheme isn't sized for the possibly unusual Apophis (it was this which tripped me up) but it is practically limited to deflections with very long advance warnings. We are still stuck around Pioneer anomaly accelerations or less and realistically a minimum deflection time after rendezvous of four years.

If you consider NEP a good way of doing rendezvous the scheme offers small benefits, only the gravitational interaction could be considered a significant new feature. It is suitable for some missions and actively limits accelerations in others. One plus point is that this scheme has already had significant work put into it (JIMO I mean, the rest is trivial). However, if the asteroid is very weakly held together it spectacularly fails to attempt use it as reaction mass, not a simple choice but the only hope for late deflections.

As you have noted, for Apophis it is not even a particularly appropriate scheme but I suppose it could be justified as a trial run for later on were there not other developments possible in the interim.

BTW your figure for the 200m sphere mass is slightly wrong, but the others are correct.


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