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Gravitational potential energy is what we want to feed off
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
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
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
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
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
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
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
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
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.
apparent retrograde impact from 1939
[pertinax, Apr 04 2016]
[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
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
|I'm convinced. [pertinex] and [Vernon] are one and the same.
|Is there a Reader's Digest version ?
|[blissmiss], I disagreed with something [pertinax] wrote.
I'm definitely a different person.
|[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?
|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.
|//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"?
|//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], are you and [Vernon] married? //
|What happens in Vegas stays in Vegas.
|//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.
|//probably worth delaying any Martian terraforming for a few decades.//
|I have no problem with that.
|[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
|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.
|OK, I'm happy to wait while bacterium-miners have a poke around. But, if they come up empty ...
|//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.
|No no no. That would still leave you with two
inconveniently distant holiday homes. We should
|(1) put one or both of them into Earth orbit, at a
decent distance so that tidal effects aren't too
|(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.
|// [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.
|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.
|// 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.
|You do realize, [pertinax], that everybody boos the picador?
|In space, no-one can hear you boo.
|I am willing to pay a premium for fresh orbitals.
|//Also, there's not much atmosphere - needs a whole//
|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.
|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.
|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?
|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).
|//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.
|I don't know what you mean there. What's the rock trying to
get tidally locked to?
|All right, I probably don't really mean tidally locked. I
the ideal state of the rock between impacts would
involve it always presenting approximately the same face
towards the sun. That may not be indispensable, but it
makes the rest of the thought-experiment easier.
|So, not stepping back as such, but pirouetting awkwardly
starting again from the top, what I thought was something
|1. Imagine there's no spaceship, but only a modified rock
2. Imagine a foreign body approaching at speed and
|Now, if this foreign body had struck the rock near the
centre of mass, then I would expect the rock to be either
smashed or displaced sharply. However, because the distal
of the pike is quite remote from the rock's centre of mass,
would expect most of the energy of the impact to be
into spin, not linear acceleration.
|Checkpoint A: is that sane so far?
|3. If so, then we have the foreign body zooming away
space and, unless there's a crash, a tinkle and a sound of
cursing from a nearby cube, the foreign body has now
the scope of this thought experiment. (Anyway, I did warn
not to park there).
|4. Remaining in the scope of the gedankenetc., there is a
spinning on its axis. Because it's spinning, it has more
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
to steer. Besides, it won't have any energy to go anywhere
to vary its orbit) if all the energy remains in that form. So,
wave my arms hard enough, maybe there's a way to store
energy in a different form. My first thought was to store it
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
awkward hand movements (was it left hand or right hand?)
work out where to put the terminals of your battery, and
magic happened. That was plan A.
|7. Regarding your anno about the spacecraft being spun up
the same speed as the rock before energy could be
I'm not sure how to picture that.
7.a Did you mean that a pair of objects, one spinning
and the other anticlockwise at the same speed, could
be used to generate power (i.e., a generator with no
7.b Did you mean that the two objects together would
spin around their combined centre of mass? I can't see
extract energy from that without some third object (and so
7.c Did you mean that the whole rotor/stator model can't
space, because there's nothing to keep the stator
7.d Something else?
|8. Supposing you might mean something like 7.c, I floated
B, compared to which plan A may look like a marvel of
forethought. The essence of plan B is that the spin could
somehow be transferred from the rock to the spacecraft,
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,
space is more unfeasible, less unfeasible or about as
in comparison with rotor-stator- generator-in-space.
|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?
|// 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.
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 somethingmaybe 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 rockspacecraft 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
|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?
|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.