3.4: Space based sun-shades

Somewhat less attention has been paid to a space-based sun-shade. The most popular version of the ideaAlthough to be clear, no version of this idea is particularly popular  appears to be a version by Roger Angel proposing to deliver sun shades to theDid you know that we have a spacecraft at L1 already.

diagram of L1 orbit point
  L1 Lagrange Point, a point which remains at a constant relative position in between the Earth and Sun as they orbit. Angel’s paper proposes three things:

  1. "An optical design for a very thin refractive screen with low reflectivity, leading to a total sunshade mass of 20 million tons.”
  2. “A concept aimed at reducing transportation cost to $50/kg by using electromagnetic acceleration to escape Earth’s gravity, followed by ion propulsion.” (Hmm…)
  3. "An implementation of the sunshade as a cloud of many spacecraft, autonomously stabilized by modulating solar radiation pressure.”

There are some obvious and enormous practicality issues:

  1. Why “low reflectivity” and why would it need to be “stabilized”? Apparently in part because solar radiation exerts a radiation pressure on the spacecraft, which tends to push them away from the sun, and in part because orbits around L1 are inherently radially unstable.
  2. “Very thin” means <1 micron thick, significantly complicating fabrication and deployment as far as I can tell, e.g., can films much thinner than saran wrap really survive the environment of space? It turns out that this range of thickness is widely considered in the solar sail literature. Not that this makes it easy.
  3. In total, the sunshade would need to be a couple of thousand kilometers on a side. That would take a long time to assemble.

How do the costs then work out? SpaceX’s proposed/forecast launch costs to low-Earth orbit using the BFR rocket are $100/kg. So if the refractive screen weighs 20 million tons as proposed, the launch cost would be 2e7 tons * 1e2 $/kg = 2 trillion dollars, which is hefty. Angel’s novel electromagnetic launcher concept helps but the cost is still at best above $100 billion.

Interestingly, the earlier paper with Lowell Wood proposes a potentially lower-cost variant:

Now, 1e14 grams = 1e8 tons, which is similar to the 2e7 tons for the Roger Angel proposal. But the Teller/Wood/Hyde proposal suggests 1e5 smaller mass requirement than that: see footnotes 23, 24 and 25 in that paper. So that seems more economically feasible, though I am wondering about the challenges in actually building and maintaining such a system in space.

Positioning a sunlight-shade or Snell's Law-refractor (i.e., a 1-D Fresnel phase plate) composed of 10¹⁴ gms of lunar glass near the Earth-Sun interior Lagrange point (L1) has been suggested in Early JT, Space-Based Solar Shield To Offset Greenhouse Effect, J. Brit. Interplanet. Soc., 42, 567-9 (1989). The present proposal positions a metallic small-angle-scatterer of sunlight of comparable area but ~10⁵ -fold smaller mass Sunward of L1. A system of this type likely would be assembled quite close to the Earth, e.g., in LEO, and then rapidly "flown" into its deployment location as a solar sail, exploiting its very small mass-to-optical cross-section and its active radiation momentum management capabilities.

The other idea mentioned in the above footnote is using lunar regolith as a construction material, which has a much shallower gravity well to escape versus stuff launched from Earth; granted, this approach requires moon mining and manufacturing machines, probably with regolith-to-fuel processing powered by photovoltaics, and ferry rockets!

Clearly, we’d need to massively up our game in space technology to make something like this work. Reminds me of Jeff Bezos saying “We Have to Go to Space to Save Earth“, although this is not the context in which he meant it. A long shot at best, to be sure.

The National Academies report on solar radiation management concluded:

“…these ideas require the ability to manufacture in space, making them impractical at the current time. Overall, the committee has chosen to not consider these technologies because of the substantial time (>20 years), cost (trillions of dollars), and technology challenges associated with these issues (GAO, 2011; The Royal Society, 2009).”

But perhaps this hasn’t been studied to its absolute logical limit — for instance, there are other cool ideas for reducing mass.

MIT seems to have taken up the research on this area. You'd still construct your sun shade at the L1 point but this time instead of a screen in the more normal sense of the word they propose to use types of bubbles to do the same kind of job. One big benefit of this type of approach is that if decide you want to stop the effect you can simply burst the bubbles. This is still very much in the research stage but with launch costs falling and falling it is starting to look less completely crazy. ____________________________________________