Letting the imagination roam has philosophical as well as practical benefits. From the interstellar perspective, consider the Daedalus starship, designed with loving detail by members of the British Interplanetary Society in the 1970s. The mammoth (54,000 ton) vehicle was never conceived as remotely feasible at our stage of technology. But ‘our stage of technology’ is exactly the point the project illustrated. Daedalus demonstrated that there was nothing in physical law to prevent the construction of a starship. The question was, when would we reach the level of building it? For as Robert Forward frequently pointed out, interstellar flight could no longer be considered impossible.
We can’t know the answer to the question, but recall that before Daedalus, there was a lot of ‘informed’ opinion that interstellar flight was a chimera, and that all species were necessarily restricted to their home systems. Daedalus made the point debatable. If a civilization had a thousand year jump on us in terms of tech, could they build this thing? Probably, but they’d also surely come up with far better methods than we in the 1970s could imagine. Daedalus was, then, a possibility maker, a driver for further imaginings.
Fortunately, the Daedalus impulse – and the broader concept of thought experiments that so captivated Einstein – remains with us. I think, for example, of Cliff Singer’s pellet-driven starship, one that would demand a particle accelerator fully 100,000 miles long. Crazy? Sure, but a few decades later we were talking about slinging nanochip satellites in swarms using Jupiter’s magnificent magnetic fields, finding a way to do with nature what was evidently impossible for us to build with our own hands.
Robert Forward used to conceive of enormous laser sails for interstellar exploration, sails whose outbound laser flux would be amplified by an even larger 560,000-ton Fresnel lens built between the orbits of Saturn and Uranus. But I discovered in a new paper from Greg Matloff (New York City College of Technology, CUNY) that it was James Early who introduced another extraordinary idea, that of using a gigantic sail-like structure not for propulsion but as a sunshade. Early’s 1989 paper in the Journal of the British Interplanetary Society specifically addressed the ‘greenhouse effect,’ which even then concerned scientists in terms of its effect on global climate. Could technology tame it?
Once again we’re in Daedalus country, or Forward country, if you will. Imagine a true megastructure, a 2000 kilometer sunshade located at the L1 Lagrange region between the Earth and the Sun, approximately 1.5 million kilometers from Earth. The five Lagrange points allow a spacecraft to remain in a relatively fixed orbital position in relation to two larger masses, in the case of L1 the Earth and the Sun. But L1 is not stable, which means that a structure like the sunshade would require thrusting capability for course correction to maintain its optimum position in relation to the Earth. Bear in mind as well the effect of solar radiation pressure on the shade.
Image: Physicist and prolific writer Greg Matloff, author of The Starflight Handbook (Wiley, 1989) and many other books and papers including the indispensable Deep Space Probes (Springer, 2005).
Would a 2000-kilometer shade be sufficient, assuming the intention of reducing the Earth’s effective temperature (255 K) by one K? We learn that solar flux would need to be reduced by 1.5 percent to reduce Earth’s EFF to 254 K. 2000 kilometers does in fact somewhat overshoot the need, reducing solar influx by about 2 percent. That’s a figure that changes over astronomical time, of course, for like any active star, the Sun experiences increased luminosity as it ages, but 2000 km certainly serves for now.
But how to build such a thing? Matloff looks at two versions of the technology, the first being a fully opaque, thick sunshade which would be constructed of lunar or perhaps asteroidal materials. Think in terms of a square sunshade with a thickness of 10-4 meters, and a density of 2,000 kg/m3, producing a mass of 8 X 1011 kg. Building such a thing on Earth is a non-starter, so we can think in terms of assembly in lunar orbit, with the shade materials taken from an asteroid of 460 meters in radius. Corrective thrusting via solar-electric methods with an exhaust velocity of 100 km/s adds up to an eye-opening fuel consumption of 400 kg/s.
But we have other options. Matloff goes on to consider a transparent diffractive film sail (Andreas Hein has recently explored this possibility). Here the sail is imprinted with a diffraction pattern that diverts incoming sunlight from striking the Earth. This is a sail that experiences low solar radiation pressure, its mass reaching 6.4 X 108 kg. But thinner transparent surfaces are feasible as the technology matures, reducing the mass on orbit to 107 kg. Such a futuristic sunshade could be built on Earth and delivered to LEO through 100 flights of today’s super-heavy launch vehicles. Presumably other options will emerge by the time we have the assembly capabilities.
Either of these designs would divert 5.6 X 1015 watts of sunlight from the Earth, energy that if directed to other optical devices would offer numerous possibilities. Matloff considers powering up laser arrays for asteroid mitigation, an in-space defensive system that would work with energy levels much higher than those available through currently envisioned systems like the proposed Breakthrough Starshot Earth-based laser array. A space-based system would also have the advantage of not being confined to a single hemisphere on the surface.
Other possibilities emerge. A laser near the sunshade could tap some of the solar flux and direct it to power stations in geosynchronous Earth orbit, where it would be converted into a microwave frequency to which the Earth’s atmosphere is transparent. You can see the political problem here, which Matloff acknowledges. Any such instrumentation clearly has implications as a weapon, demanding international governance, although through what mechanisms remains to be determined.
But let’s push this concept as hard as we can. How about accelerating a starship? Matloff works the math on a crewed generation ship accelerated to interstellar velocities, with travel time to the nearest star totaling about four centuries. The point is, this is an energy source that makes abundant solar power available while producing the desired reduction in temperatures on Earth, a benefit that could drive development of these technologies not only by us but conceivably by other civilizations as well. If such is a case, we have a new kind of technosignature:
If sufficiently large telescopes are constructed on Earth or in space, astronomers might occasionally survey the vicinity of nearby habitable planets for momentary visual glints. If these sporadic events correspond to the planet-star L1 point, they might constitute an observable technosignature of an existing advanced extraterrestrial civilization.
When considering technosignatures from ET sunshades, it is worth noting that a single monolithic sunshade might be replaced by two or more smaller devices. Also, an advanced extraterrestrial civilization may choose to place its sunshade in a location other than planet-star L1.
There is a Bob Forward quality to this paper that reminds me of Forward’s pleasure in delving into the feasibility of projects from the standpoint of physics while leaving open the issue of how engineers could create structures that at present seem fantastic. That quality might be described as ‘visionary,’ calling up, say, Konstantin Tsiolkovsky in its sheer sweep. Matloff, who knew Forward well, preserves Forward’s exuberance, the pleasure of painting what will be possible for our descendants, who as they one day leave our system will surely continue the exploration of their own ‘Daedalus country.’
The paper is Matloff, “The Lagrange Sunshade: Its Effectiveness in Combating Global Warming and Its Application to Earth Defense from Asteroid Impacts, Beaming Solar Energy for Terrestrial Use, Propelling Interstellar Migration by Laser-Photon Sails and Its Technosignature,” JBIS Vol. 76, No. 4 (April 2023). The Early paper is “Space-based solar shield to offset greenhouse effect,” JBIS Vol. 42, Dec. 1989, p. 567-569 (abstract).