Suppose we have developed an Earth-Moon industrial system, one that lets us use an electric launch system on the Moon to upload mass for chemical processing and the extraction of raw materials. What’s the next step toward extending it to the entire system? One idea, as Joseph Friedlander has been explaining on the NextBigFuture blog, is to do interesting things at the L4-L5 points, where stable gravitational pockets exist. Friedlander is thinking about building solar sails in space, and in this regard he echoes nanotechnology maven Eric Drexler, who wrote about sail technologies in The Engines of Creation (1986). Here’s Drexler on the subject:
To build lightsails with bulk technology, we must learn to make them in space; their vast reflectors will be too delicate to survive launch and unfolding. We will need to construct scaffolding structures, manufacture thin-film reflectors, and use remotely controlled robot arms in space. But space planners already aim to master construction, manufacturing, and robotics for other space applications. If we build lightsails early in the course of space development, the effort will exercise these skills without requiring the launch of much material. Though vast, the scaffolding (together with materials for many sails) will be light enough for one or two shuttle flights to lift to orbit.
The term ‘lightsails’ needs clarification. What Drexler means by it is a solar sail of higher performance than early sails. The latter will be made on Earth in sturdy enough fashion to survive not just launch but deployment. Drexler is interested in how to make sails bigger, and thinner. Why not use the lunar materials, then, to create what Friedlander calls a ‘gigantic solar sail loom,’ one that would stretch reinforcement wires in loom fashion over a framework that could reach 10 by 10 kilometers in size. The idea for this space shipyard is to create vast solar sails, spreading a volatile material on the framework, vaporizing thin amounts of aluminum onto it, then removing the volatile and support structure to create an ultra-thin, 100-square kilometer sail.
Sails as Asteroid Catchers
Friedlander’s interesting take on all this is well worth reading, and I’m especially pleased that he points to Al Globus’ idea of an asteroid-retrieval project called AsterAnts. Globus and colleagues Bryan Biegel and Steve Traugott (all working with MRJ Technology Solutions at NASA Ames) came up with the notion back in the late 1990s, presenting it as a NASA technical report and developing its ideas in a presentation at Space Frontier Conference 8. The notion is to retrieve small (1/2 to 1-meter) Near Earth Objects for orbital processing, and to do all this with solar sails that could be constructed and tested near the International Space Station.
Globus and team make the point that these small NEOs have a mass roughly equal to recent spacecraft (the paper cites Deep Space 1 and NEAR), some 500 kg, and thus should be manipulable with propulsion systems like solar electric. Small NEOs are numerous — the only problem may be detection — but it’s clear they could be useful because they include a wide variety of materials, including water, volatiles, and metals in large quantities. Globus mentions the never-built JPL design of an 820×820 meter sail meant to rendezvous with Halley’s Comet, and then runs the numbers on using square sails to retrieve a 500 kg object.
The result: Even with low acceleration, the sails need to be 200 meters to the side. But he also comes early to the realization that building a sail in an orbit as low as the ISS’ could be problematic:
It should be noted that solar sails cannot operate below about 1000 km since atmospheric drag exceeds the acceleration due to sunlight. Orbits between approximately 1000 km and 20,000 km are subject to high radiation… Thus, solar sails built at the ISS would probably need to be moved to a 1,000+ km orbit by chemical, tether, or solar electric propulsion or construction must take place in a 20,000+ km orbit. Worse, aerodynamic pressure on large sails may pose a hazard to the ISS. Detailed engineering would be required to choose a proper site for sail construction, but a design where spars and rigging are assembled at the ISS then moved along with rolled sail material to a teleoperated facility in high orbit for final assembly may be advantageous.
What is intriguing about this project is the idea of using existing space installations like the ISS for early experimentation on meteoroid processing and solar sail construction. Although we want to construct full-size sails in higher orbits, smaller sails could be built in a low-Earth orbit facility to develop the needed construction techniques. Deployment of a solar sail is perhaps the trickiest aspect of the technology, but ground-built sail material delivered to orbit and unrolled onto spars and rigging there would teach us much about how to build larger sails, with the eventual goal of working with thinner sail materials actually produced in space.
We need, then, to understand how thin films behave in weightless conditions and what sort of manufacturing techniques we can develop to make them thinner still. From the report:
To build solar sails on-orbit, tens of thousands of square meters of thin-film aluminum must be produced. Clearly, this would require a large, dedicated facility outside of the pressurized ISS volume. However, experiments to understand the behavior of thin-films in weightlessness and to develop manufacturing techniques could be conducted in the pressurized volume. For sail making, one approach is an electroplating technique, where a large drum is continuously plated with evaporated, charged aluminum on one side, and the solidified sheet is peeled off the back side of the roller. A more elaborate mechanism, credited to Eric Drexler, appears in [Wright 1992].
The reference is to Jerome Wright’s Space Sailing (1992). Friedlander, meanwhile, extends the AsterAnts idea to the kind of lightweight sails Drexler writes about, going after 1000-ton asteroids with an infrastructure encompassing entire fleets of sails, one that would begin to produce a billion tons a year of returned materials in high-orbit. In such a scenario, solar sails obviously reach a high level of development, enabling interplanetary travels that help us create the kind of system-wide infrastructure that may one day lead to interstellar missions. If you want to think big, check out Friedlander, who concludes with a discussion of a sail the size of Mercury.
I can just imagine Robert Forward’s eyes dancing at the thought of such a sail. The patron saint of so many interstellar concepts loved big ideas. It was Forward, after all, who envisioned an enormous Fresnel lens in the outer solar system to focus an inner system laser that would drive a huge lightsail to Alpha Centauri and other nearby stars. Here I’m using the term ‘lightsail’ in Forward’s terms (not Drexler’s), meaning a sail that uses not solar photons but beamed propulsion from a man-made installation to get up to speed. Forward even worked out a way to use laser beaming to decelerate a mission at the other end, but that’s a tale for another day.
Meanwhile, AsterAnts intrigues me because we’re already doing many of the necessary first steps, including surveying for ever smaller Near-Earth objects and developing sail materials that will help us produce thinner and thus more effective sails. We’re also developing the needed computational tools to attack both sail performance and trajectory issues, autonomous operations and orbital material-processing questions. We may have but one functioning solar sail (IKAROS), but we can investigate many of these issues with high-power simulations.
And there are practical benefits. Globus notes that solar sails could increase the number of geosychronous satellite orbital slots by a factor of three. This is based on another Forward idea, to use a sail to hold a geosychronous satellite out of plane. Today’s geostationary satellites orbit at an altitude where they can revolve around the Earth at the same rate that the Earth rotates, and they have to be spaced 2-3 degrees apart to avoid radio interference. That amounts to 120 to 180 satellites for any particular frequency band, creating congestion over crowded areas.
A solar-sail ‘statite’ of sufficient size could ‘hover,’ tilted so that light pressure is equal and opposite to the pull of gravity, thus increasing the number of direct broadcast slots dramatically. All of this is by way of making the case that space can pay off in multiple directions. Developing the needed sail technologies to make some of these things happen points not only to a supply of interesting materials from captured asteroids but also to economic benefits that are closer at hand. It also points to a space future in which sails take us into the Kuiper Belt and beyond.
I’ve never seen anyone consider the idea of a mixed mode use for sail material. Pure photonic propulsion, but also shaping the sail into a concentrator for a solar thermal rocket for higher thrust maneuvering. The design requirements might be incompatible, but it seems to me that a fresnel mirror design is possible that in one configuration concentrates the solar energy to a focus to heat a working fluid, and in another is flat for optimal photon reflection for propulsion.
With the abundance of volatiles in the solar system, this might seem to be an attractive proposition.
Alex: I agree, and I think that a sail could very well double as a concentrator, for the purpose of energy production as well as propulsion. It may be troublesome to get an exact parabolic shape, but even a rough approximation can concentrate sunlight many times for much better power/weight ratio in solar cells.
I am not sure that moving asteroids around is going to be worthwhile, compared with bringing the extraction equipment to the asteroids and shipping only the extracted raw material. Much depends on the relative mass of extracted material vs. unused tailings. If the tailings themselves are valuable, say for shielding, it would make more sense.
Also, why does Friedlander presuppose a moon mining operation? It seems more logical to start with asteroids right away and not bother with the moon.
I wonder if we can estimate electric current induced over a lightsail by plasma from a coronal mass ejection.
It could be worth sending a conducting tether out past the magnetosphere as an observatory.
Amazingly versatile stuff that sail material, not only can you use it to move stuff around, you can also use it to make any space or object in the solar system, even planets, (but excepting the sun itself) as hot or cold, or as illuminated or shadowed as you like.
@Eniac I have to agree; there are a good number of known NEOs that require less delta-v to reach than the Moon. Perhaps the Moon is considered useful because it’s always about three days away with Apollo-era technologies, and windows to reach those low delta-v NEOs are much more sporadic?
I’d really like to know if the bright chaps at SpaceX are motivated at all by concepts such as AsterAnts. There’s a marriage made in translunar space if there ever was one.
Another application for lightsails is the Rubbia’s thruster idea. Photons from a blackbody heat source such as a large pile of fissionables is redirected aft for thrust. Sail sizes are a few kilometres across, although they will get quite hot from the flux. This isn’t to be confused with Rubbia’s other interstellar propulsion idea which is to use fission fragments for direct thrust.