Let us suppose that we have an electrical launch facility on the Moon to export mass to the L4-L5 points in co-orbit with the Moon around the Earth.
My caption-- the Sun is in the middle of the picture, the Earth is at the 3 o'clock position relative to the Sun, the Moon is in the 2 o'clock position relative to the Earth. Note L4 and L5 at the 1 and 5 o'clock positions relative to the Sun. The Moon's mass makes these gravitational pockets in space stable orbits along the contour, like a steel ball circling a curved horn funnel in a museum display. Even a dumb mass will follow the path of least resistance. As simply as possible, you speed up to slow down (orbit expands, more distance to cover, less gravity) and you slow down to speed up (orbit contracts, less distance to cover more rapidly). The gravitational distortion of space (by the gravity fields of Sun, Earth and Moon) controls your exact direction.
Wiki caption: A contour plot of the effective potential of a two-body system. (the Sun and Earth here), showing the 5 Lagrange points. An object in free-fall would trace out a contour (such as the Moon, shown).
Relavant Links (orientation on celestial mechanics issues)
Now we suppose (for example) the lunar electric launcher exporting say one-ton (to cut down the number of packages that need interception) cast basalt sling weights to the L4/L5 regions. Each one ton package may have a few hundred milligrams of transponder to aid in capture but simple optical brightness aids (crumpled foil, like the Lunar Module but of aluminum) may suffice.
We then capture these flung payloads, at first with ordinary ion tugs or other low-thrust drives. (A steam rocket using lunar water would suffice as per the plans of Dr. Zuppero) A few meters a second should suffice, which indicates that with an exhaust velocity of one to ten thousand times that, each 1,000 or 10,000 tons of captured mass will use a single ton of water. In the case of a lunar steam drive the ratio is 500:1 which simply means that some of those cast basalt weights might contain shells of ice within (when lunar polar mining starts).
Wikipedia on Lunar Water: , the LCROSS mission flew its 2300 kg impactor into a permanently shadowed polar crater, and detected at least 100 kg of water in the plume of ejected material. A later, more definitive, study found the amount of detected water to be 155 ± 12 kg (or "5.6 ± 2.9% by mass").
In March 2010 NASA reported Mini-SAR radar aboard the Chandrayaan-1 detected what appear to be ice deposits at the lunar north pole, at least 600 million tonnes in sheets of relatively pure ice at least a couple meters thick.
Moon has a liter of water for every tonne of soil — [Times Online] This is a misconception-- it appears to be a liter per ton not of random bulk regolith ('every ton of soil') but of the very surface layers that can capture solar wind hydrogen.
Carle Pieters, Brown University, Chandrayaan-1 observation team “When we say ‘water on the moon’, we are not talking about lakes, oceans or even puddles. Water on the moon means molecules of water and hydroxyl that interact with molecules of rock and dust specifically in the top millimetres of the moon’s surface.”
It would take about 730 square metres of dirt (top few mm only) to produce a single liter of water...
But this is available all around the Moon at undisturbed sites, not just at polar regions...
To cut down on the delta-v (velocity change) budget –which means fuel expended-- we
build capture structures. (This is a modified version of Gerard K. O'Neill's plan-- O'Neill used a 'catcher's mitt' of Kevlar to accept low velocity impacts of incoming lunar-fiberglass woven bags filled with lunar regolith We instead choose to send cast basalt with an iron eyelet for ease of capture. (A latching hook or D-ring mechanism is sufficient.) But we will need a place to put them between capture and meltdown, and the solar furnace to melt them down and process them for metals and oxygen.
All this uploaded mass from the Moon is chemically processed to extract needed materials.
The goal: Build a space shipyard, a gigantic solar sail loom the size of a city (but only weighing a few thousand tons)
We have seen those scaffolding like shed-roof-frame structures around various floating spaceships in the Star Trek movies in free space (dry docks, and that, in vacuum, they certainly are)--
But really, it would be a framework of beams constrained only by tidal stresses, perhaps 10 by 10 kilometers in size, to stretch out thin reinforcement wires like a loom, then to systematically place a volitile ('wax' in K. Eric Drexler's phrase) material with support in spot X, vaporize thin amounts of aluminum onto it, then to remove the wax and then the unbonded support. What is left is superthin foil embedding ripstop fiber. Keep this up over 100 million square meters and you have a 100 square kilometer solar sail. (The actual shape would be more hexagonal).
Details on Drexlerian light (weight) solar sails.
JPL sail design from 1977 could reach 1 km/sec delta v (velocity change) per 8 days of unshaded continuous acceleration (above 1000 km height) at 1\7000th G acceleration. But Drexler had the idea of making them 40 times as thin, and therefore, 40 times as light and fast.(1/175th G acceleration for the same payload, or 40 times the payload for the same acceleration)
Hexagonal, under ~100 square kilometers weighing 20 tons.
To quote Drexler there, The 20 ton solar sail mentioned above could take 180 tons of payload to any place in the solar system, stop (not orbit, but stop) and hang there on light pressure. With 800 tons of payload to slow it down, it would finally have the same acceleration as a plastic film sail with no load at all. With 6 tons of payload, it could fly to Pluto in one and a half years.
Costs are given there at 10c a lb, in 1977 dollars, presumably 35c now. $1 a kilogram of payload weight can be taken as an approximation of costs per trip. With a large enough sail, you could move a 1 million ton prepackaged colony/refinery kit to the asteroids with population for $1 billion per trip. At a bare bones 10 tons of supplies a person to get started that is 100,000 people out of the Earth-Moon industrial system. Note that this REQUIRES an Earth-Moon industrial system to begin with!
Lightsails and their potential in Engines of creation Drexler
Google Books Deep-space probes By Gregory L. Matloff p 52
Matloff calculates 30nm sails (about the thinnest fully opaque ones) at 8.1 x 10e-5 kilograms a square meter of sail surface-- a sail lightness factor of 18.5. This is about the best all aluminum sail you can make-- by comparison today's sails are several microns thick, with one micron possible. But if we can build in space, in microgravity and no breeze in the factory, we should be able to make this level of sail.
In the same place, Matloff estimates peak sail performance near the Sun at 60 g of acceleration and a possible Alpha C mission in 2,500 years--
Henry Spencer and Jordan Kare discuss sails at this link
JPL's fairly conservative sail design for the 1986 Halley rendezvous mission got about 90m/s/day (in open space with continuous sunlight). Aggressive use of current technology might double or triple that. Assembly in high orbit (so the sail doesn't have to unfold) and considerably improved materials might give 500-700m/s/day.
Several km/s/day is conceivable with advanced space-assembled designs, but
that's well beyond what we can build now.
Let us assume now we have the Moon industries, the mass in L4/L5, sail looms and we can make the sails. The high performance sails such as Drexler Matloff and others have outlined. (with a high 'lightness factor' number)
The next step?
Well, consider the AsterAnts strategy by Al Globus
1. mm/s2 acceleration, almost 200 meters square sail. This retrieves 500 kg objects.
His idea is to send small solar sails to bring back these celestial crumbs---under 1 meter diameter natural planetoids with low encounter velocities. All this with unmanned craft. (Presumably with a human mission control but with high levels of automation) The problem would be identifying targets of such small size and low brightness at sufficient distance to plan missions.
However, consider bigger sails-- specifically, using Drexlerian light (weight) solar sails for the Globus AsterAnt strategy. The detection problem is gone-- we are detecting 1000 ton asteroids and in fact far less right now (not over the whole Solar System, but we are adding to the database as they randomly pass close by one by one...)
Number of known large near earth asteroids N = 7523 as of this writing. listed by delta v for rendezvous
listed by absolute magnitude (good guide to size—but darker ones on this list are bigger to reflect the same light)
List of close approaches past and future, encounter distance and sizes
http://neo.jpl.nasa.gov/ca/ Remember that this list is just the known ones...and for very recently...
If one 1000 ton asteroid (10 meters or less) passes within the Moon's distance each day, 20 million km allowable mission control distance (speed of light problem—67 second delay in and 67 second delay out-- about one light-minute distant) means 365,000 such could go by in a year. 365 million tons of captured mass (assuming no uninterceptable velocities-- which probably at least 10% will have) It depends on your sail technology. The faster you can accelerate, the quicker you can match velocities empty, then capture, then you can take your time returning. It would be doubtful that even a very fast sail could get in more than 4-6 missions a year because of orbital mechanics issues, and the missions per year number could easily be under unity with slower sails. So to do this would take tens of thousands of sails. A thousand ton payload 100 square kilometer sail with 20 tons of empty mass will need (for a remote controlled 100,000 sail fleet) 2 million tons of lunar aluminum-- probably about 28 million tons if maria basalt, less if highland anorthosite http://en.wikipedia.org/wiki/Anorthosite
Chemical composition of the lunar surface regolith (derived from crustal rocks) Compound Formula Composition (wt %) Maria Highlands silica SiO2 45.4% 45.5% alumina Al2O3 14.9% 24.0% lime CaO 11.8% 15.9% iron(II) oxide FeO 14.1% 5.9% magnesia MgO 9.2% 7.5% titanium dioxide TiO2 3.9% 0.6% sodium oxide Na2O 0.6% 0.6% Total 99.9% 100.0%
Aluminum oxide is about 53% aluminum
Bigger asteroids coming in for close flybys are next. This is normal mission profiles--teleoperated-- within a light minute or two—at 100 times the Moon's distance. For example, the next opportunity will be on January 7th, 2011, at 30 million kilometers out. Every month or two another big one swings by. At this stage we could not retrieve it but we could certainly drop mining equipment off and start seeing (if it is a conglomerate of various kinds of rock from previous collisions and agglomerations) if one part is more promising than the other. In which case the job of the teleoperated mining equipment is to mine rock quickly when close (with minimal lightspeed teleoperating delays) from random surface to a bin of known geometry (and predictable scenarios for removing the regolith to a cargo bag automatically)
Once that happens-- fleets of sails descend on that big asteroid whenever it is in convenient range. When not, it mines. When another big multikilometer asteroid nears a close flyby, we get more data and another mining base. After 10 years we would have hundreds of bases and access to most kinds of asteroidal materials and a predictable and massive influx of known regolith types for different industrial purposes.
Each sail encounter with a big mining-based asteroid is obviously the equivalent of swallowing a tiny asteroid whole in terms of mass. So if you can manage an encounter an hour for a few weeks (typical closeness period) you have the equivalent of 300 small captures. 1000 larger ones thus can yield as much as the random smaller objects. This is an argument for outposts on surface of asteroid to preload payloads, shuttle to remote dock (analogy to Kharg Island off Iran, where really huge tankers avoid the shallowness close in to the actual coastline.)
To further limit sail traffic during the key encounter periods with larger asteroids may have pre-refineries to extract concentrate from raw asteroidal regolith. (ship to final refinery in L4/L5)
We now are closing on most of a billion tons a year of returned materials in high-orbit, which implies that if we had no colonies to support and it could all be done with teleoperation, we could double our production of sails and retrieval of materials every few months (within limits—say a factor of 1000 over a decade) after which we are eating out most things that come our way on our whole side of the Solar System)
But all this will have the positive effect of removing (in effect) space junk. Congratulations, our proud solar empire is now a garbage picker.
Actually some of that garbage is quite valuable. The iron-nickel ones are basically gold ore (.5 parts per million, gold and similar silver, typically 20 ppm platinum group metals )
Still, we are talking tens to hundreds of billions of tons a year coming in to the L-4/L-5 points. The night sky is awash with new mobile constellations of sails...
Now we encounter- the sail slot problem (can't cast shadow on brother sails—think about it) so orderly rows of light in the sky in a super-crowded system. But it can't last...
in the Server Sky plan (computer satellites on quite a small scale in unbelievable numbers—supplying beamed electrical power and computing to Earth) , Keith Lofstrom has detailed the dangers of too much light in the night sky near the Earth-Moon system.
His figures for light pollution are relevant to a solar empire--
Since the Earth is cooled by black body radiation, proportional to temperature to the 4th power, a 1% increase in incoming energy will cause only a 0.25% increase in absolute temperature. If an acceptable Earth temperature rise is only 0.1 Kelvins above the average black body temperature of 255K, the temperature rise is caused by 4E-4 of the solar input. ...
Corals may be triggered to incorrectly spawn with as little as 10% of full moon illumination (which is 1E-5 full sun), or 1E-6 of full sun illumination in the night sky...
This is much lower than the thermal limit. If each server-sat, configured as a power-sat, could deliver 1 watt each to the electric grid, then "Power Sky" could deliver only 850 gigawatts from GEO distances. To provide 50 Terawatts ( the Smalley Terawatt challenge for 2100 ) we will have to go out to the moon.
Note that this same limit applies to any other kind of space solar power system with the same efficiencies Space solar power systems may need to be at lunar distances to provide large amounts of power in normal operation, while avoiding the "coral problem" after failure.
These figures are also relevant to a solar-sail based empire.
The natural full Moon is of apparent magnitude –12.6, far fainter than the blinding –26.73 apparent magnitude of the Sun—about 449,000 times less bright. So the Moon is 14.13 apparent magnitudes fainter than the Sun. As each unit of apparent magnitude is 2.512 times fainter or brighter than its’ neighbor (five magnitudes of difference are, by the Pogson scale of 1856, exactly 100 times brighter or fainter)-- we can therefore conclude that the Sun is 2.513 to the 14.13th power (449032.16 times) brighter than the Full Moon. The Moon’s area in the sky is about 0.2 square degrees. The sky itself has an area of 41253 square degrees. Thus the Moon’s area is 1/206265 of the sky!
It would be no huge trick, with 200,000 10 kilometer sails, to have an apparently isotropic (all directions equally) distribution over the sky so that every Full Moon-sized patch had its own sail. The relevant number then becomes how far away the sail is to calculate worse-case brightness (which will be rare-- see the factors in the Server Sky wiki link above. Briefly, it involves the angle of the sail both to the Sun and the observer on Earth.) A 10 kilometer sail dropping a 1000 ton comsat cargo off in geosynchronous orbit might well be as bright as -4 magnitude Venus.
the non-stellar objects in our Solar System that have maximum visible magnitudes less than or equal to +2.50 are the Moon (−12.92), Venus (−4.67), Jupiter (−2.94), Mars (−2.91), Mercury (−2.45), and Saturn (−0.49).
At the Moon's orbit-- at L-4 or L-5 it will be 1/100th as bright (or about as bright as a star 5 magnitudes fainter, such as Capella or Archenar or Hadar or Betelgeuse) But since L-4/L-5 will be key commerce points we can confidentally predict there will be absolute swarms of them there. It will be the most spectacular star cluster ever seen from Earth in three dimensions, slowly morphing. Light pollution, but beauty too. Too bad it won't be very varied in color, though even that might be possible at some loss of efficiency--- However, it may be too bright for the coral. (Alternative: perhaps a third of a million square kilometers of plastic nightshades for the coral undersea—but if society needs to do that by that stage, it will be wealthy enough. Estimated cost-- $1 square meter (300 billion US dollars)
Coral reefs are estimated to cover 284,300 square kilometers (109,800 sq mi) .
But really, you have to regard the coral like the canary in the coal mine. It basically is saying that it's time for humanity to move before other unintended effects hit.
So we shall have to move. The sails will enable fantastic levels of interplanetary colonization, especially in conjunction with the materials they have retrieved.
Consider the current ideas about cyclers (orbits and spaceships that Dr. Buzz Aldrin, second man on the Moon, investigated-- cyclers involving amortizing a spaceship over many encounters that 'cycle' between the two target planets or destinations) , and then contrast the reality that Drexlerian lightsails carried to a large-scale can in fact literally haul along a space colony, the cargo and payload being the 90% of it that is the shielding mass. The payload mass would be between the people and the outer hull.
Cyclers need radiation protection at minimum few thousand tons for a small module. So a 20 x 20 kilometer solar sail would be enough for a minimum solar-storm safe system.
the trajectory of a Mars Cycler.
Contrast the original 'big' Drexler sail of 6 miles across—800 tons slow sailing, 180 tons hover capability, 6 tons 100 km/sec solar escape trajectory -- with my suggested
Mercury (the planet) sized sail---. 3000 miles across, 250000 times the capacity-- 200, 000,000 tons slow sailing 45,000,000 tons hovering or rapid transit. 1.5 million tons to Pluto in 6 months (~85-100 km/sec)--but you need to slow down unless this is a one way mission. Of course you could always drop off a hydrogen-bomb half megaton payload Orion spacecraft for the slowdown part of the mission—once you establish a 50,000 person colony there future spacecraft can be slowed down by slinging gas (probably boiling liquid oxygen timed to evaporate right before slowdown encounter) in the hit path where it will impact the Orion-like blast shield and retro the craft without further nuclear pulse units.
In an interesting calculation in a related context http://www.walthelm.net/inverted-aerobraking/PulsedInvertedAerobraking.htm (gasdynamic conversion of fierce to less fierce momentum transfer) Dr. Axel Walthelm writes,
A bit of gas on top of the pusher plate seems to be able to distribute the kinetic energy evenly enough to avoid damage to the pusher plate. Say the gas has a density that causes on average ten collisions of an incoming kinetical fuel molecule with that gas. That causes a chain reaction, or rather a binary tree reaction. See it in three dimensions and it becomes a cone reaction. The original molecule hits another. Those two hit two more, each of them an average of 1/4th of the original energy. So what do we get after 10 collisions? 2 to the power of 10 is 1000. So the kinetic energy of the incoming molecule is distributed to about 1000 gas molecules before it hits the pusher plate. Terrifying 50 km/s become an average speed of 1.6 km/s, which is well below the speed of sound for many solid materials. Make the gas twice as dense, i.e. double collision probability, and 50 km/s become 50 m/s!
A Drexlerian lightsail the size of the planet Mercury-- larger than any Jovian Moon- would be visible, like those bodies, with mere binoculars out to the orbit of Saturn. Within 100 million miles of Earth you could see this thing with your naked eye on a lit city street at night—would be, shall we say, conspicuous. Imagine hundreds of these things cycling between the worlds, carrying cargo and loads of say 500,000 people at a time to their otherworldly settlement destinations. This would be the emigrant trail to thin the traffic out here through draining Earth of traffic in order to preserve Earth's environment against the light pollution-- night lightfall from the sail traffic.
There is a misconception that you cannot use solar sails for outer system missions can but you can--thrustingin close to sun where it pays just like an ordinary chemical propulsion--orbital tug-- boost, then immediately retro once you cut the load loose so you can do another job (3 a day instead of 1 in 3 years yields 1000 times faster amortization). Stuff in 'the pipeline' using the analogy of G. Harry Stine, continues on the transfer orbit to be picked up at journey's end. If it is an entire colony hull full of people (and shieldied by inanimate cargo) then they are already at home, and would be able to endure voyages enduring years in a way that the ordinary double occupancy cabin/train compartment/airliner seat model would not support.
The lightsail could take on people and cargo brought by shuttle from L4 say at the Moon's orbit, at a spot near the Sun-Earth null point L1 on the Sunward side, then dive in toward the Sun to change it's orbit to a comet-like one of extreme eccentricity, begin the long outward, slowing as it rose, the people living life in a space colony for years, until being dropped off for capture in the Outer System.(the much lightened sail would quickly dive back to pick up another load)
The advantages of an Outer System empire-- The cold of the Outer System can be terrifying to contemplate if you are a lone astronaut with your heater running broken-- but in large communities and with large space industries—waste heat is a real problem in Inner System spacecraft design (recall the moon mission spacecraft doing barbecue rolls to stay cool). In the Outer System cold is an industrial asset., as is remoteness (from a hypothetical lethal cosmic event affecting the entire Inner System, which both Bolonkin and Turchin have written about) human outer system settlement in heavily shielded colonies would leave a human remnant to rebuild civilization.) Another asset-- CHON. CHON—people and organics are made chiefly from Carbon Hydrogen, Oxygen and Nitrogen—so there is mass there for plenty of population growth.
In fact, in the Server Sky plan, Keith Lofstrom has detailed that to fully utilize the Sun's power, you have to go out to at least the Saturn- Uranus distance (100—400 x less insolation) if you intend having near-Dyson sphere coverage to utilize the Sun's full output. Otherwise the night side of Earth does not have a cool sky to radiate to, but a sea of star points radiating heat that actually keep Earth temperature much higher than tolerable. This would be true anthropogenic global warming.
Another job for huge solar sails-- boosting to 100 km/sec in a low G way elaborate observatories for use in the gravity focus opposite the key target stars closest to us (candidates would have to be 1) close 2) with detected planets in positions and of temperatures where liquid water could endure and with detected atmospheres thick enough for that to happen and 3) of stable dynamics and stellar behavior. Those three filters would easily eliminate, at a guess, 99% of all candidate systems—but still one can imagine hundreds of gravity focus observatories being shot out exactly opposite their target stars (the technique uses the Sun's gravity to focus light, so the Sun must be precisely opposite the target star). So to get data for Alpha Centauri you would need to shoot to the anti-Alpha C spot in the celestial sphere.
Claudio Maccone has written extensively of this technique, which can obtain 10e8 magnification of EM signal strength relative to not being in the focus. The spot size at 2200 AU has been calculated (by a Dr. Eshelman) as around 11 km wide. (The observatory travels exactly in that 11 km corridor opposite the target star and will not slow down but continue at 100 km/sec for long ages) The focal line, however, extends to infinity so once you shot a device that way to observe, it could track its target indefinitely. Even a 'small' 8 meter scope could get huge results. But imagine using an 11 km wide superscope array . (They would need to be radioisotope powered in the outer darkness). To use all the bandwidth allowed—this array could see milli-arcsecond or better resolution in any direction other than the gravity focus--.note that Venus might appear 61 arcseconds across at maximum size in the heavens from Earth and Pluto .1 arcsecond, so Pluto would be 100 x 100 pixels across from earth orbit with a milliarcsecond telescope. But pointing such an 11 km array toward the gravity focus, and obtaining another factor of 10 million, you would be able to see things unknowable today, (weather patterns on distant planets across the Galaxy?) although owing to the peculiar conditions at the gravity focus (images would display Einstein Rings) considerable experience might be necessary in interpeting the images.
The huge advantage of a sail boost mission is that we could send (with large sails) nuclear power units and complex observatories that were NOT G-hardened, instead of being limited to billions of networked micromechanical devices proof against high G. It is hard to believe that a space hardened version of the Keck Observatory say would not be unspeakably great to have at the sun's gravity-focus. So the ultimate use of the solar sails of a new solar empire,
For I dipped into the future, as far as human eye could see,
Saw the Vision of the world, and all the wonder that would be;
Saw the heavens fill with commerce, argosies of magic sails,
Pilots of the purple twilight dropping down with costly bales;
might be to launch observation stations to scout the outposts of a yet newer interstellar empire...
Gravity focus reading
At 550 AU, electromagnetic radiation from the occulted object is boosted by a factor of roughly 10e8. Secondly, gravity-focused radiation does not behave like light in a conventional optical lens in one important sense. The light does not diverge after the focus as the spacecraft continues to move away from the Sun. Indeed, the focal line extends to infinity.
The distance is 550 AU as a minimum, with continuing use of the instrument after that. There is no upper limit on the distance for FOCAL because, unlike the case with optical lenses, the gravity-focused radiation stays on the focal axis after 550 AU. In other words, the focal line extends to infinity. The disadvantage of the distance is offset by the fact that this proposed observing platform can do things no other telescope could handle. Ponder this, from Gregory Matloff’s Deep Space Probes book: For a FOCAL mission at the gravity lens, EM radiation from the occulted object is amplified by a factor of 10e8.
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