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Asteroid Orbital Decay Harvesting

mostly as a contribution to terraforming Mars - or "killing an indeterminate number of birds with an inderminate number of stones"
  (+5, -1)
(+5, -1)
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Gravitational potential energy is what we want to feed off here.

Our solar system has a lot of rocks that are falling towards the sun, but keep missing it because of their tangential velocity. If some of them could be nudged so that their velocity was no longer tangential, but pointed a bit more inward, then that potential energy would become ... more accessible, albeit in slightly scary ways.

Of course, the nudge would need some energy itself, but less than the kinetic energy that would ultimately become available.

Meanwhile, Mars is no fit place for human colonisation. Why not? Well the gravity is so weedy that a human child born there would (I hear) be fatally deformed. Also, the gravity is so weedy that the place might struggle to hold on to an atmosphere. Also, there's not much atmosphere - needs a whole lot more oxygen, much of it bonded with hydrogen. Also, it's cold.

Apparently, the current plans to put a human base on Mars have a rule about not impacting the existing Martian environment. I find this quite funny. In the past, European colonists disgraced themselves by treating inhabited lands as if they were uninhabited. That was a Bad Thing. Now, however, we seem to be treating an uninhabited planet, Mars, as if it were inhabited. I mean, to whom, exactly, are we showing respect and consideration by leaving it pristine? Anyway, enough of that rant.

Meanwhile, follow me to the asteroid belt. Yes, now you mention it, it *does* occupy the next orbit out from Mars. Unfortunately, there's not enough of it* to do more than pilot this idea, but once we've proved the concept, we can move on to the Kuiper belt and the Oort cloud to get some scale. Notwithstanding any carping from NIMBYs on the outer planets.

Now, if we hoik our POV out of the ecliptic plane for a moment, we see that most of the asteroids are orbiting anticlockwise (on the arbitrary assumption that we hoiked north, not south). A few, however, have retrograde orbits. That is, they are orbiting clockwise, from where we now sit. Whether these are interlopers from another star system, or just maladjusted, narcissistic rocks that don't play well with others, let us regard them for the time being as our special favourites.

Imagine one of these retrograde-orbiting rocks dressed up as a picador. Imagine the normal, anticlockwise rocks as on-rushing bulls. We will need an initial investment of energy and reaction mass to get the picador-rock close enough to a bull-rock to initiate a gravitational interaction. That's why, earlier on, we sent up a small spaceship with some robots and some innocent-looking equipment in crates, to park on the picador-rock and make a few modifications.

The picador rock now packs some sensors and, with the aid of these, it divides on-coming bull-rocks into two kinds, namely, rocks to throw and rocks to tap.

If it is going to throw a rock (i.e., alter its course so that its orbit starts to decay), then we do one of those sling-shotty things which have already been demonstrated by earlier real-life space probes, the difference being that, the thing we're sling-shotting around is of a similar order of magnitude to the picador-rock itself, so its course is also altered.

If it is going to tap a rock, it uses its picador-pike. Imagine a giant steel knitting needle wedged right through the middle of the picador-rock. The purpose of this is that it should *only just* touch the bull-rock as it rushes past. If there is more than the lightest touch, then the pike will snap off. However, if the touch is judged just right, it will set the picador-rock spinning. (The pike might be constructed in sections, connected by a slack internal tether, so that, if the end section does snap off and go spinning away into space, the picador can reel it back in using that tether).

That pike tether is different and separate from the *other* tether, which connects the picador-rock to the little spaceship which arrived to fit it out. The attachment of this other tether is very precisely related to the axis of spin implied by the direction of the picador-pike, so that, after a tapping contact, the tether does not get tangled by the resulting spin.

You see, we need a non-spinning object alongside the spinning picador rock, in order to convert that spin into electrical power. And we need the electrical power to charge up the gimbal-mounted mass drivers which, together with a modest amount of reaction mass, we use to position and re-position the picador-rock between encounters with bull-rocks.

You remember that there were sensors contributing to a decision about which rocks to throw. Well, those sensors are looking for two things, namely, evidence of high density and evidence of water ice. Any approaching object with one or the other (or both?) of those things is to be sent in the ultimate direction of Mars (possibly by way of a longish spiral path).

It is very important that they not strike Mars at a funny angle. Two moons are quite enough. We are trying to make Mars bigger and denser, not break bits off it. A matador-vehicle based either on Phobos or Deimos would therefore be used to land temporarily on the bull-rock during its final approach to Mars, adjust its course so that it bull's-eyes the red planet, and jump clear before it actually does. Repeat this until Mars is heavy and watery enough, (and with a bit of Martio-thermal energy) to support human habitation. Then stop. That part is important. If we forgot to stop before commencing colonisation, that would be bad.

Now, bearing in mind that much of this activity (after the initial pilot) is going to be based right out in the Kuiper Belt and Oort Cloud, we'll have plenty of time to insert other devices to capture energy from objects slingshotted past them, and to store it so as to provide a future source of energy for all those human-initiated activities which need to be carried on sort-of within the solar system but too far from the sun for solar panels to be much use. Early plans envisage a Nickel-Metal Hydride double-A cell scaled up volumetrically by a factor of the Japanese national debt and parked in orbit off Pluto.

*According to Wikipedia, //99.9% of the asteroid belt's original mass was lost in the first 100 million years of the Solar System's history//. However, [8th of 7]'s solicitor has advised him not to take questions on how *that* happened, so we'll just have to work with what we've got.

pertinax, Apr 02 2016

apparent retrograde impact from 1939 https://www.researc...Washougal_Meteorite
[pertinax, Apr 04 2016]

Hole Making_20a_20little..._20more_20habitable
[Skewed, Sep 17 2019]


       Mars has not yet been proved to be uninhabited, although the definition of "uninhabited" might have some bearing on the discussion.   

       Inhabited by bacteria? Distinctly possible, underground. Widely known to exist underground on Earth. Plus we also know giant meteor impacts have spread Earth surface-debris throughout the Solar System and beyond. Some lucky Earthy bacteria might have been sent to Mars millions or even a couple billions of years ago, and if they arrived gently enough, their descendants could be all over, not far beneath the surface.   

       As for more-complex life-forms inhabiting Mars, that seems rather less likely. They're generally not as tough as bacteria, and Mars certainly does have a very harsh environment. Nevertheless, if they exist, they have adapted to that environment, and if we mess it up, thinking that they don't exist, we could be making a Big Mistake....
Vernon, Apr 02 2016

       I'm convinced. [pertinex] and [Vernon] are one and the same.
blissmiss, Apr 02 2016

       Is there a Reader's Digest version ?
FlyingToaster, Apr 02 2016

       [blissmiss], I disagreed with something [pertinax] wrote. I'm definitely a different person.
Vernon, Apr 02 2016

       [pertinax], are you and [Vernon] married?   

       So, if I guess correctly from my skim-read, the idea is to slingshot useful asteroids into Mars to make it a warmer, gravitier, wetter place?   

       Sounds good to me. [+]   

       But why not also use some of the impacts to slow down Mars' orbit and bring it closer to the sun? Otherwise the heating bills are going to be crippling when we move in.   

       However, as soon as they find life there (and they will), I think this idea becomes bad. Apart from anything else, any life which is either orthogonal to terrestrial life, or which has a couple of billion years of independent evolution, is going to be immensely valuable, and probably worth delaying any Martian terraforming for a few decades.
MaxwellBuchanan, Apr 02 2016

       //Mars has not yet been proved to be uninhabited//   

       OK, this is true. However, we might say (with a little poetic licence) that Loch Ness has not yet been proved not to have a monster. My point is, how many "haven't found anything yet" results do we want to accumulate before we are willing to conclude "there's probably nothing there"?
pertinax, Apr 03 2016

       //I disagreed with something [pertinax] wrote. I'm definitely a different person.//   

       Non sequitur. I sometimes do that myself, given the passage of a little time.
pertinax, Apr 03 2016

       // [pertinax], are you and [Vernon] married? //   

       What happens in Vegas stays in Vegas.
pertinax, Apr 03 2016

       //the idea is to slingshot useful asteroids into Mars to make it a warmer, gravitier, wetter place?//   

       Yes. But you forgot the bit about the picadors. If there's no hat, we're not going.
pertinax, Apr 03 2016

       //probably worth delaying any Martian terraforming for a few decades.//   

       I have no problem with that.
pertinax, Apr 03 2016

       [pertinax], we've barely started looking, and what we've found so far pretty much excludes the surface from having life-as-we-know-it. Underground, however, perhaps only a decimeter or so, solar UV can't reach and the perchlorates at the surface may not be down there. But nothing so far sent to Mars can reach that depth to see what's there.   

       [MaxwellBuchanan], the reason Mars lost its atmosphere was because its planetary magnetic field stopped getting generated in its core. That let the Solar Wind literally blow most of its atmosphere away. If we decide the existing environment can be destroyed, then what we should do is collide something BIG with Mars, so as to re-melt the interior and let its magnetic field generation system start up again. Or we could wrap it in superconductors. and do the field-generating ourselves.   

       With a thick enough atmosphere, Mars can be plenty warm enough. Mars doesn't have Earth's problem of being too close to the inner edge of the Goldilocks Zone. (In about 300 million years the prediction is that naturally increasing Solar radiance will make Earth uninhabitable, long long before the Sun turns into a red giant.) To give Mars a thicker atmosphere, well, Venus has plenty to spare (90 times as much atmospheric pressure as Earth).   

       Regarding pertinax, I've never met that person that I know of. And I've never been married, either.
Vernon, Apr 03 2016

       OK, I'm happy to wait while bacterium-miners have a poke around. But, if they come up empty ...
pertinax, Apr 03 2016

       //But why not also use some of the impacts to slow down Mars' orbit and bring it closer to the sun?//   

       Of course, the elegant thing to do would be to swap it with Venus - without damaging, you know, that other rock that orbits between them. Then, to terraform Venus, we'd have to raise its pH by bombarding it with those Oort Cloud objects made mostly of soap flakes.
pertinax, Apr 03 2016

       //swap it with Venus//   

       No no no. That would still leave you with two inconveniently distant holiday homes. We should either:   

       (1) put one or both of them into Earth orbit, at a decent distance so that tidal effects aren't too great or   

       (b) Put them both in solar orbits close to earth (ditto above regarding tides), so that at least we can hop across easily once every few orbits when things line up.
MaxwellBuchanan, Apr 03 2016

       // [8th of 7]'s solicitor has advised him not to take questions on how *that* happened, //   

       You can ask all the questions you like, it's just that we've been advised not to answer.
8th of 7, Apr 04 2016

       Retrograde orbits sounds like trouble - prograde collisions between orbiting bodies are fairly tricky, because Dinosaurs. I'd be nervous about introducing retrograde objects into the inner solar-system which presumably is not just a cataclysmic eruption of ash and fire resulting in a global winter for a thousand years, but more a planetary vaporisation style event, should the calculations turn out to fail to take the metric system into consideration (for example). Poking with long sticks might be appropriate, but what speeds are we talking about with retro-vs-pro grade objects? Pretty fast, I'd warrant.
zen_tom, Apr 04 2016

       // I'd be nervous about [...] a planetary vaporisation style event //   


       But, since you ask, a quick Google suggest the order of 50km/s. Apparently, it's happened before (see link), and here we still are.
pertinax, Apr 04 2016

       You do realize, [pertinax], that everybody boos the picador?
lurch, Apr 04 2016

       In space, no-one can hear you boo.
pertinax, Apr 06 2016

       I am willing to pay a premium for fresh orbitals.
bungston, Apr 06 2016

       //Also, there's not much atmosphere - needs a whole//   

       Sp. 'hole' [linky] ;P
Skewed, Sep 17 2019

       Would this not destroy the training ground for interstellar travel? 1) get to a another object (the moon) 2) take enough camping gear. (Mars) 3) Make sure your camper is robust enough to take the belts.
wjt, Sep 20 2019

       Trying to generate energy from the difference in spin between the picador-rock and its associated spacecraft will necessarily reduce that difference in spin. In other words, the spacecraft will be spun up to the same rate of spin as the rock, before much energy can be generated that way.
notexactly, Sep 21 2019

       It's been a long time since I wrote this, but I think I was imagining the spacecraft as a stator to the rock's rotor. If a stator develops some spin around its own axis (not the rotor's axis), does that prevent power from being generated by the spinning of the rotor?
pertinax, Sep 21 2019

       A generator is a type of brake; a brake is a type of clutch. All clutches either allow or prevent (or something in between) the relative motion of two parts. They care nothing for the collective motion of those two parts relative to their surroundings (unless the surroundings seem to be one of the two parts, which just means you're thinking about the situation incorrectly, and need to step back one level).
notexactly, Sep 24 2019

       //need to step back one level//   

       You almost got me there, but then I remembered that if I stepped back I'd fall off the rock.   

       Do you think a flywheel-like approach would be more promising (so, not trying to store the energy of the impact as electricity at all)? In that scenario, the spacecraft would function as the flywheel, while the rock tried to get itself tidally locked again in time for the next impact.
pertinax, Sep 25 2019

       I don't know what you mean there. What's the rock trying to get tidally locked to?
notexactly, Sep 26 2019

       All right, I probably don't really mean tidally locked. I mean that the ideal state of the rock between impacts would probably involve it always presenting approximately the same face towards the sun. That may not be indispensable, but it certainly makes the rest of the thought-experiment easier.   

       So, not stepping back as such, but pirouetting awkwardly and starting again from the top, what I thought was something like this:   

       1. Imagine there's no spaceship, but only a modified rock (with pike). 2. Imagine a foreign body approaching at speed and striking the pike.   

       Now, if this foreign body had struck the rock near the rock's centre of mass, then I would expect the rock to be either smashed or displaced sharply. However, because the distal end of the pike is quite remote from the rock's centre of mass, I would expect most of the energy of the impact to be converted into spin, not linear acceleration.   

       Checkpoint A: is that sane so far?   

       3. If so, then we have the foreign body zooming away through space and, unless there's a crash, a tinkle and a sound of cursing from a nearby cube, the foreign body has now departed the scope of this thought experiment. (Anyway, I did warn him not to park there).   

       4. Remaining in the scope of the gedankenetc., there is a rock spinning on its axis. Because it's spinning, it has more energy than if it were in the same orbit, but not spinning.   

       Checkpoint B: still not barking yet?   

       5. If it keeps spinning like that, the spin is going to make it hard to steer. Besides, it won't have any energy to go anywhere (viz., to vary its orbit) if all the energy remains in that form. So, if I wave my arms hard enough, maybe there's a way to store that energy in a different form. My first thought was to store it as electrical energy in a battery.   

       6. Now, I thought the usual way to turn spin into electrical potential was to set up a stator near the rotor and make some awkward hand movements (was it left hand or right hand?) to work out where to put the terminals of your battery, and then magic happened. That was plan A.   

       7. Regarding your anno about the spacecraft being spun up to the same speed as the rock before energy could be generated, I'm not sure how to picture that. 7.a Did you mean that a pair of objects, one spinning clockwise and the other anticlockwise at the same speed, could somehow be used to generate power (i.e., a generator with no stator)? 7.b Did you mean that the two objects together would simply spin around their combined centre of mass? I can't see how to extract energy from that without some third object (and so ad infinitum) 7.c Did you mean that the whole rotor/stator model can't work in space, because there's nothing to keep the stator stationary? 7.d Something else?   

       8. Supposing you might mean something like 7.c, I floated plan B, compared to which plan A may look like a marvel of rigorous forethought. The essence of plan B is that the spin could somehow be transferred from the rock to the spacecraft, until the orientation of the rock towards the sun became stable, somewhat like orientation of a tidally locked object.   

       Regrettably, I have not brought any somehow, but I would appreciate your opinion about whether, in principle, flywheel-in- space is more unfeasible, less unfeasible or about as unfeasible in comparison with rotor-stator- generator-in-space.
pertinax, Sep 26 2019

       Oh dear. Am I going to have to tow that anno all the way back to the inner solar system to get a response to it?
pertinax, Sep 28 2019

       // Checkpoint A: is that sane so far? //   

       I think so. The rock with the pike will be set spinning, and that will account for most of the energy transferred in the collision. It will also have its trajectory changed slightly, as will the other rock, which will also be set spinning, but not as fast.   

       // Checkpoint B: still not barking yet? //   

       I think so. It has more energy due to spinning than if it was in the same orbit without spinning, but keep in mind that that's not the same orbit as it started in; that original orbit could have had more energy in total.   

       7a: No.
7b: Exactly.
7c: Exactly.
7d: Not that I've thought of yet.

       Plan B: Oh, I see. You have the rock spinning, and use that to spin up the spacecraft. Then:   

       a. You can slow down and stop the rock using external forces (light pressure from sunlight or something—maybe the YORP effect?). Now you have the spacecraft spinning, and can generate power by slowing it down relative to the now-held-stationary rock. (But you can't do it so fast that you'd break the hold of whatever's holding the rock stationary.) That should work, I think, but it will only be able to capture as much energy as is stored in the spacecraft's spin as kinetic energy. Even though the spacecraft will be spun up to the same speed as the rock, it's a lot lighter than the rock, so only a small amount of the rotational kinetic energy of the rock–spacecraft system will be in the spacecraft, and that in the rock will be wasted when you allow it to slow down against external forces, I'm pretty sure.   

       b. You can't slow down the rock by transferring its energy to the spacecraft without something external to push against. To be able to do that, you'd need some sort of external frame (by which I mean both a reference frame and some kind of structure made of materials), within which you'd slow down the rock. Then you'd use the energy that came out of slowing down the rock to speed up the spacecraft. But if you have that external frame, what would be the point of putting that energy into the spacecraft just to take it out again? You already have it out of the rock. If the spin axis of the rock and spacecraft is perpendicular to the line between the Sun and the rock/spacecraft, a suitable frame might be something like a solar wind keel, maybe, but that's yet to be invented. If the spin axis is radial relative to the sun, you might be able to use a heliogyro. But if you have a heliogyro (or, better yet, two counter-rotating heliogyros), you might as well just use that for everything, and not worry about trying to hit other rocks glancingly with a pike.   

       It might be more practical to catch the energy from the tapped bull-rock as orbital energy rather than rotational kinetic energy. If you have the spacecraft tethered to the picador-rock, with the tether attached to a rail car on a track encircling the picador-rock, and fly past the bull-rock in such a way that its gravity pulls the spacecraft into a higher orbit about the picador-rock (reeling out tether as this happens), then you could harvest energy by regeneratively braking the rail car. This will convert some of the spacecraft's orbital energy about the picador-rock into rotational kinetic energy of the picador-rock itself, and some into electricity. If you do this in alternating directions (which would be tough, because it would mean that the bull-rock's gravity would have to fully reverse the spacecraft's orbit in one pass), then you could do it repeatedly without spinning the up whole system on average. It would be very roughly analogous to extracting energy from a one-terminal AC electrical source without a ground, using a huge capacitor and a huge inductor to create a phase-lagged voltage that you can then use the input AC voltage relative to (not that I'm saying that's possible).   

       What's the point of tapping bull-rocks anyway, though? Is that energy needed for throwing other bull-rocks, somehow?
notexactly, Sep 30 2019

       Good question. What I was thinking was, the picador rock is going to be pottering around the Kuiper belt for a long time, trying to intercept some objects while dodging others. I wanted it to be self-sufficient in energy, and I assumed that, so far out, solar energy would not do much for us.
pertinax, Oct 13 2019


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