Wednesday, July 19, 2017
How might we steer objects from the Kuiper belt to achieve objectives in the inner orbits of the solar system? This question arose from my proposal to alter the rotation of the planet Venus for purposes of exploitation and habitation. That proposal is discussed in the article "Small Fetters" in which I express doubt that humanity has the right stuff to achieve any such objective. http://fullduplexjonrichfield.blogspot.co.za/2013/12/small-fetters.html
Some critics denied that it could be practicable to achieve anything of the kind as a realistic engineering project at all, even in principle. Some simply said that the scale of the project was too large ever to be done in principle. Some said that it could not be worth doing even if it were possible. Some of the more open-minded suggested that, assuming it were done when 'tis done, then 'twere well I should explain how it were to be done and why.
In a thread I pointed out that Kuiper Belt objects, that authorities assume to number in their millions — some estimates suggest billions, should bear kinetic energy more than adequate for practical exploitation. I did not at that time bother to discuss the detailed engineering, taking it for granted that navigating rocks from trans-Pluto orbit to intercept Venus suitably was not intellectually challenging in principle, however challenging the project might appear from political or engineering perspectives.
However, contrary to my expectations, some participants in the thread impugned this, and since some aspects of the topic are of intrinsic interest, I here discuss approaches to obtaining and navigating comets, asteroids, and assorted Kuiper Belt meteorites as required.
I still skirt currently irrelevant engineering issues of course.
Several other ideas and approaches might prove relevant, but here I concentrate on projects such as the engineering of Venus and Mercury and assume that the scale of the project would involve something of the order of 100,000 Kuiper Belt objects of perhaps 10 km diameter on average. Please do not bother to point out that not all Kuiper Belt objects of that order of magnitude are spherical; this is strictly a spherical cow exercise.
Obviously there are many types of approach to such potential projects, and in this discussion I ignore all that are not aimed at a long-term commitment (probably 1000 years or more, possibly even several thousand years; gaining a new planet with all its resources would be worth a good deal more than that in any rational scheme of things). The discussion also has nothing to do with steering any single object, of either gigantic or trivial size, nor bringing it to some sort of relative stop or storage trajectory at some arbitrary position in space. Instead I consider perhaps tens to hundreds of thousands of objects of worthwhile size, small enough to steer economically, and large enough to achieve particular objectives.
Nothing of the kind would be worth discussion without an appropriate infrastructure, but at such a distant remove, details of that infrastructure are hardly worth discussion. Anyone who doubts this should read Arthur C. Clarke's original proposal for communications satellites; he proposed that they should be manned! His proposal was in no way stupid; in fact, given our current applications of communications satellites, if they really had to be manned, they still would have been worth it; it is just our luck that our advances in technology since World War II free us from any requirement for such an extravagance. Similarly, it is practically certain that anything I propose here would seem ludicrous to engineers of the 22nd century.
Instead I merely brush over some plausible requirements of such an infrastructure in discussing principles.
It is well not to be too casual about beginning a highly technological 1000-year project. I suggest that for reasons of navigation and communication we should start by installing at the very least a few hundred permanent unmanned satellites in strategic orbits in the solar system. That no such project has yet been undertaken is a blot on our record already. However, the solar system equivalent of a GPS system plus communication relay system plus near space (meaning solar system in this context) astronomic and cosmological observation system, would be necessary as the first step in the project. Call such craft the Relay Satellites.
We should be able to install a profitable, workable and worthwhile foundation within a few decades, but the planned lifetime of the Relay Satellite system would be measured in millennia rather than years and there would be no question of committing to naive uniformity of design during such a period. That infrastructure would be an indefinite project, adapting to our needs and technologies for the foreseeable future. In practice of course, certain apparently arbitrary standards might remain constant, but such are merely practical details that are not immediately relevant to us. The principle is well understood and tolerated and there is a fair discussion of a traditional example at: http://www.snopes.com/history/american/gauge.asp
'Nuff said on that point.
As I see it the Relay Satellites should have modest navigational capabilities, just about enough for maintaining station and attitude in their appropriate trajectories indefinitely. Some of them in near solar orbit might use solar power of one sort or another, but by far the majority would occupy orbits beyond Mars, and some perhaps beyond Pluto. Whether to power them with beamed energy, isotope energy, or on-board fission or fusion power generators is an example of a question I leave to future generations. They should need considerable power for communication at least, plus very sensitive reception equipment, because they would be communicating partly with tiny craft that could not carry giant antennae to capture faint signals. Whether there would be relatively few multifunction satellites or relatively many specialist function satellites, and whether a satellite that outlives its fuel supply would be parked and catalogued, or scrapped, refurbished, recycled, or destroyed, I also leave to future interests. Personally I like the idea of large-scale, modular satellites to be serviced and upgraded by specialist unmanned craft, but I do not insist upon that.
Note that there is no suggestion that such a fleet of Relay Satellites should be dedicated to the Kuiper Belt object navigation project. There would be plenty of function for it without that. It is quite possible that a Kuiper Belt initiative would affect the scale and details of parts of the fleet, but that is not especially relevant here.
The second class of craft, prospecting craft, would in contrast be specialised for ranges of function dealing with exploration and prospecting in the Kuiper belt. To this end they would be capable of extremely long range, long-term navigation beyond the orbit of Pluto. Their communication and navigation capabilities would be powerful, but specialised for their role. They would rely on the permanent Relay Satellites for most of such functions, and partly for keeping track of the prospecting craft. Of course, each satellite would have its own intelligence and a very large memory, probably petabytes rather than terabytes, enough not only to manage the data that it accumulates, but the parameters of the infrastructural system. They would have sufficient intelligence for routine tasks, including some fairly complex ones, because apart from the question of how far robotics would have advanced in the next century or two, such tasks would be, if not highly stereotyped, at least confined to a small universe of discourse. They also would have great redundancy of function and capacity to ensure resilience in the face of predictable radiation and unpredictable accident. None of your single-drive hard disks and the like!
Prospecting craft would be exceptional in the amount of reaction mass and energy that they would have to carry, because they not only would have to reach the Kuiper Belt, but would have to do considerable amounts of unpredictable navigation within it. At such a distance from the sun, solar power would be practically worthless, probably including solar wind power. It also would not be practical for such craft to rely on isotopic thermal energy; they would need fission power generators at least, which is the good news; the bad news is that they also would need large amounts of reaction mass, even if they used ion thrusters. Much as they would rely on the relay satellites for control and communication, they might have to rely on rendezvous with tugs and maintenance vehicles for refuelling and upgrading.
Their function would be to locate and characterise as many Kuiper Belt bodies as possible, determining their nature, mass, trajectories and the like by any practical means, whether optical, radar, infrared, gravitational, theoretical or generic, to name but four... err... or so... Bodies that either pose a threat to the inner solar system, or that seem to be potentially valuable to the main project, probably would be visited physically to obtain all relevant information. For example, bodies that are rich in ice or ammonia might either be particularly valuable or not usable for the purposes of the project. Similarly, bodies that amount to aggregations of gravel might be valuable if their trajectories were particularly suitable for gentle manoeuvring, but hopelessly dangerous to use otherwise unless they could be cemented, say by combination with an iceberg or ammonia-berg.
This is a large subject, not worth exploring at this point in any depth. Suffice to say, some such functional craft would be needed to locate and evaluate the objects to be selected for navigation in the project.
The third class of craft would be the tugs and maintenance craft. They would be a job lot, and I do not discuss their design, which would be variable at all stages of the project. All the other craft would require updating, modification, refuelling, repair, and possibly even retrieval. The Relay Satellites might well require being transported to their stations as well. No one tug or maintenance craft would be suitable for all such functions. However, each one probably would be versatile and each one would have powerful thrusters of appropriate kinds. However, they might need less fuel than the prospectors, because they would have shorter missions, and more closely defined.
How many different classes of craft would be needed, I cannot say; the only other one worth discussing here would be the Kuiper Belt object navigation craft. Let’s call it the Rockrider craft. It is the one at the cutting edge, or the coalface if you prefer. It's job would be to rendezvous with a selected object, prepare it for transport, and steer it to the objective.
What possibly, just possibly, could be simpler?
A lot of things of course, but not many are as worthwhile.
Very well. Notice that I have said practically nothing so far about manned craft. I do not say there would be none such, but for the purposes so far discussed, I cannot see any being required, and frankly I cannot in the short term see any manned craft being practical for transient applications beyond say, the orbit of Mars. We are after all speaking of Rockrider craft undertaking voyages of decades at least, and commonly of centuries. Even if we condemned convicts to such voyages, it is hard to imagine what we would want them to do out there in space, even if we could trust them there. I leave such distasteful speculations to those with the appropriate distastes.
Now, each Rockrider craft would have the task of rendezvous with a nominated object that had been identified, located, and characterised by the prospector craft. Apart from a few prototypes, probably none would be launched before some thousands of target bodies had been selected as having suitable masses, constitutions, neighbours, and trajectories for the project. Many, possibly the majority of such Kuiper Belt objects, might be perfect for launching in a few hundred thousand years, but useless for short-term projects of 1000 years or so. However, there are assumed to be many millions of bodies out there, so I do not feel too defensive about assuming that we would be spoilt for choice of suitable objects.
A suitable object would have to be one that could profitably be adjusted in its attitude and trajectory, for a Rockrider craft to manipulate and navigate it down from say 40 astronomic units, to rendezvous as required with Venus or Mercury at less than 1 AU. For example, if an otherwise suitable 100 gigatonne object were spinning at a rate of several hertz, the very task of de-spinning it strikes me as discouraging; I would rather go on to look for something friendlier. Nor would we be interested in 10-tonne or peta-tonne objects, or at least that is what I assume. Again, we would prefer to deal with objects whose orbit we could adjust most economically in terms of energy and time. Exactly which variables would be most important in a given case, I do not much speculate upon.
I suspect that very circular orbits would be expensive to adjust, whereas elliptical orbits that approach, or could be coaxed near to Neptune, could be adjusted drastically at modest cost. Difficult decisions would be the business of the human orbital engineers, but generally most decisions could be handled programmatically. Possibly one could use nuclear explosions for crude preliminary adjustment of some kinds of orbits of suitable bodies, or even to persuade some bodies to collide usefully.
Using collisions might seem a bit optimistic, given that even millions of bodies so far out would have a considerable mean separation, but it’s just an example of the kind of consideration that might arise.
The point in general is that we would select bodies with orbits that could be adjusted with minimal investment of energy and material, whether by bombs or by thrust.
However, we could afford a reasonable investment, bearing in mind that a typical energetic profit for dropping from say 40 AU to the orbit of Venus would be about sixty-fold, and to Mercury, 100-fold. Those already are attractive figures. And if we could gain useful momentum by slingshotting past the major planets, we could increase that profit dramatically.
Slingshotting would be important for more than just the increased yield of energy; it would be vital for steering large bodies. Any adjustment of the trajectory of a billion- to trillion-tonne mass would be so expensive that we would care less about the factor of profit, than whether we could afford the project at all. As a result we might well be happy to work at a trajectory for a few centuries to get a finally profitable result by nibbling at the gravitational field of one planet after another. The computing load would be heavy, but routine. Much of it would be done Earthside many years in advance. There would be plenty of time to seek out the most obscure scenarios for each Kuiper Belt object, where each major improvement in handling a single rock would be worth billions of dollars.
Slingshotting would be important in two different ways: energetic gain, as mentioned, and steering. Energetic gain would work essentially by parasitising the orbital momentum of a planet. This is no novelty; it already is a routine technique of long standing in spacecraft navigation.
Steering is another aspect. Obviously passing a sufficiently large planet in a suitable trajectory can change the course of a body almost arbitrarily within the ecliptic. What is more, by passing the planet at greater or lesser distance, one can affect the angle of change practically as much or as little as one likes. In passing close to the planet, one is in a position to adjust one’s exit direction greatly by adjusting one’s incoming course by only a few tens or hundreds of kilometres. To achieve such a difference would require only the gentlest of nudges or persistent pressure a few years in advance. However, with such a delicate requirement one can see why we would want such an elaborate infrastructure of navigation satellites.
The other steering requirement that slingshotting offers, is one’s position relative to the ecliptic. By adjusting one’s position so that we pass to the north or south of a planet, we can adjust our approach so as to move out of the ecliptic. We could for example hit the north or south pole of Mercury practically vertically, or either limb of Venus grazingly, with an enormous bang in either case. We would deliver many times more energy and momentum than we had invested. It would achieve either excavation or adjustment of rotation as required.
Well then, we know that if we can steer a planet into a good starting position and apply a bit of adjustment at critical points, and can stop our rock against a target instead of having to stop it in space, we have it made.
This is critically important, please note. If we got no more energy out of a rock than we had put into it, we might as well save ourselves the trouble and go and shove at the planet directly. And if we did that, we could not nearly afford the energy. This whole exercise is predicated on the idea that we might manage to get away with three or four orders of magnitude less energy, by application of a little brainwork, commitment and patience. And being apes rather than termites, we can easily manage that can’t we?
The way we always do?
All the same, there still is the requirement to apply that fraction of a percent of energy, or nothing special will happen.
Let us then consider a hypothetical project. Imagine a typically peanut-shaped, teratonne, predominantly rocky body, spinning about a short axis, but not so fast that any part of it is travelling much faster than escape velocity for this rock. OK?
Still, the spin is unacceptably high. Our Rockrider selects a suitable spot, based on the prospector’s information, instruction from Earth, and its own calculations, lands there, using tethers as necessary, checks the details, and drills into the body of the rock near one end, probably using plasma or laser drilling for the most part. Together with adjustments calculated in the light of what it finds on the way down, it carefully places a multi-megatonne nuclear bomb ordered in advance in the light of the prospecting report decades before, covers the hole nicely like a cat, retreats to a safe distance a few hundred kilometres away in space and on the sheltered side of the rock, and when the attitude and position are right, it blasts a tidy slice off one end of the rock. The resultant vector of the blast both kills the rotation or very nearly, and accelerates the rock into an improved, more elliptical, trajectory. You see, the depth of the bomb was such that several thousand tonnes of material were blasted off at a modest velocity, imparting a really efficient delta-V in the desired direction.
Waste not, want not! Eat your heart out, Saturn V!
Having checked how well the blast had worked, perhaps while it waited for the blast site to cool, the Rockrider lands again, possibly on the blast site, and anchors itself nicely. It begins to run its nuclear generator and plasma drills and to excavate more material in the form of vaporised rocky or sooty material that it condenses as an impalpable powder in very intense atmospheres of energetic electrons and ions in separate chambers. The cooled particles become powerfully charged microscopic electrets. An electret wouldn't have to retain its charge for more than a few seconds, but in practice probably would do so for years or indefinitely.
Meanwhile the Rockrider has unlimbered its main thrusters, which are specialised twin (or multiple?) linear accelerators, whether electrodynamic, electrostatic or laser. Details, details... It feeds them charged dust particles that they accelerate to modest velocities, roughly two thirds of the delta-V of the whole system as calculated for the entire trip. Pretty well optimal for the energy utilisation, which is one of the limiting factors for the project. In principle such electret propulsion could be far more efficient than ion thrusters. The reaction mass is cheap. In navigating a teratonne rock we could afford to use thousands of tonnes as charged reaction mass without serious regret.
Right. The Rockrider and its Earth support have calculated not only the best things to do, but the best ways to adjust the intensity and direction of the acceleration in feedback to the response of the rock to the thrust. That is what we call steering, right?
Now it gets boring. That Rockrider is going to sit there for a long time, rendezvousing with a few planets for slingshot purposes during the next few centuries. Possibly it gets a few more charges of fuel from visiting tugs and maintenance craft. “Oh, it’s you again is it?” “Yeah chatterbox. Who did you expect? Goldilox?” A few days before impact, the Rockrider kisses its mount goodbye, and goes off for some maintenance and its next trip, which had been chosen for it before it even started on this one.
Coasting Rockriders could kill time by acting as incidental observers of conditions and events wherever they pass or pause, or by relaying signals wherever convenient.
Notice that there are major differences between any viable options for this kind of navigation and the Buck Rogers stories. There is no question of fast turns and dramatic accelerations (except for the occasional nuclear blast of course.) Everything is worked out years or centuries in advance and gently nudged for centuries en route. We cannot afford the energy or the risks of abrupt manoeuvres, but we can trade energy for time, which we have plenty of if we are to aspire to the dignity of termites rather than apes.
Another objection that might occur to cavillers is that if it is going to take us hundreds of years to ride a single rock home, and we need 100000 rocks, we will take a lot of millions of years for the project. But that is a blinkered point of view. We would have thousands of Rockriders, all working in parallel, and sometimes in teams. It might be centuries before more than a few of the first rocks began to splash down, but then it would be a flood of hundreds per year. Although it would generally be the intention that each Rockrider would bring in more than one rock, even one typical rock would pay a generous profit for its Rockrider.
A really serious objection for a race of monkeys is that we would be labouring for human benefits thousands or millions of years after we were multiply recycled dust and dregs, and monkeys don't do that sort of profitable venture; they want it fast, cheap, and now. But all is not lost. Though the ultimate bull market benefits are far in the future, there is plenty of bull for the developers en route. Whole dynasties of companies could profit hugely from running the projects, improving the technology, applying the information gained much as we have profited immeasurably from satellite communications, weather observations, and Earth science and mapping, even while moaning bitterly and persistently about the costs of space technology and research.
Preventing a single dino-killer collision with Earth (never mind turning it into a profitable collision with Venus or Mercury) would pay for the whole initiative.
What could be simpler? Or easier?
Or more gratifying?