Asteroid Orbital Decay Harvesting

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.