Dyson Shells and the Future… Part 2

A comment on Transterrestrial Musings suggested I might be against space development. A quick browse through my earlier blog-posts would quickly correct such a misapprehension. What I was arguing was the race to bust up planets to make cybernetic “Islands of the Lotus-Eaters” might be as much a trap and a dead-end as the Luddite call to “return to the Earth” and thus effectively go extinct.

Spreading into the Cosmos is non-optional for Humanity – Humanity 1.0 or our future upgrades, whatever their architecture. But disassembling the planets isn’t needed – at least, not yet. Take Dvorsky’s “Dyson Shell” plan. For many reasons a Dyson Shell is unlikely as a habitat and Dyson (like Stapledon before him) meant a Cloud or Swarm of habitats to increase habitat size for Life. However a Dyson Shell isn’t non-sensical if we want something else – energy collectors. Wrapping the Sun in “statites” – optically levitated structures – is perfectly reasonable and avoids the issues of the science-fictional Shell Habitat. Such structures, however, have a vulnerability, from in-falling meteoroids and comets. Stuff is always falling into the Sun, or coming very close.

The simplest mitigation effort would be making the individual collectors small enough to manoeuvre out of the way of the in-falling matter. Given sufficient detection time the collector in danger can shift sideways. But that does have one important implication – the collectors need sufficient space between them to do so. Thus coverage of the Sun is likely to be less than 100%, more like 50% or less. This constraint lets us then compute the rough mass of the Dyson Shell.

For a perfect absorber the ratio between the outward force of sunlight to the inward pull of gravity is 1:1300. That means energy collecting statites need to be very thin. Interestingly, because the sunlight and gravity decline in intensity via the inverse square law, except in very close proximity to the Sun, a statite able to levitate near the Earth will do so at any radial distance from the Sun. The exception is when close to the Sun and instead of being a “point source”, the Sun is a great big wall of light. For materials purposes we’ll assume an operating temperature of 1000 K and 50% conversion efficiency, which puts our collector at about 0.1 AU. Here the sunlight is 100 times stronger than at Earth’s orbit.

To levitate the collector’s areal mass density is 0.00077 kg/m2, which is very thin. A possible design is large reflectors concentrating onto an energy converter, though the exact details we’ll leave for future engineers. What that figure lets us do is estimate the total mass required. At 0.1 AU the total area is 2.81 x 1021 m2, meaning the total mass of our 50% coverage Dyson Shell is 1.08 x 1018 kg. About a quadrillion tonnes. Being so thin each collector can solar-sail its way inwards to its operating position around the Sun. It also means the fraction of Mercury mined, as Mercury masses 3.3 x 1023 kg, is very small.

To transfer the energy collected, solar pumped and energised lasers, presumably solid-state, will be used. With half coverage of the Sun and 50% conversion efficiency, the total energy supplied to the Solar System civilisation is a staggering ~1026 W. Essentially a million tonnes of energy per second is available.

So what do we do with it all? One possibility, which would go a long way towards making a Dyson Swarm, is transferring the power to distant objects and terraforming them. Not just the planets we know, but the potentially thousands of planet-sized objects between the stars, the Nomads of the Galaxy which were recently in the science news. Again, the difficulties of managing so many planetary sized laser streams is an exercise for future engineers, but even with 100,000 Earth-sized worlds illuminated (the Sun’s output is equivalent to 2.2 billion times what Earth receives) the total amount of sky covered by each stream is minute so streams crossing planets will be rare and predictable, thus can be mitigated. Engineered eclipses?

A final thought, for the Worriers, is that power transfer lasers, on planet scale, don’t need to be very intense. Eventually the Earth will need a planetary shell to modify the Sun’s natural input, as its luminosity increases during its Main Sequence climb, but that’s not needed for laser defense just yet. A question worth pondering is just how thick a shell is needed and how high it needs to be, as well as how strong. That’s for a future posting.