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Light pollution: space elevator show-stopper?


Space Elevator

In the past few years two ideas that were once considered preposterous have begun to seem ever more reasonable: the space elevator, and solar power satellites. Many of the biggest engineering issues have actually been solved for both these projects. But one huge issue appears to have been neglected.

With the addition of its final set of solar panels, the international space station is slated to become the second brightest object in the night sky—brighter than Venus. Now, admittedly the ISS is the size of a football field, but it's also three hundred kilometers away from the Earth directly above the plane of its orbit—but much further away for most of the people who see it. Thousands of kilometers, for most of us.

Consider this: on any given night, you can look up (if you're not in a city that already drowns the stars) and see satellites. They're hundreds of kilometers away, and the biggest are no larger than a compact car—yet you can see them. Most are the size of a barrel, but perfectly visible.

Consider Bradley Edwards' ribbon design for the space elevator cable. This would be a meter or two wide and curved, so that it is effectively visible from all angles. So it's about the width of a barrel, but infinitely longer. Its reflective surface over one kilometer's distance would be at least as great as the ISS; but please multiply that light output by 35,000 because that's how many kilometers long it would have to be. A Hoytether (open meshwork) design would presumably reflect less, but how much less?

You could paint the ribbon black. Then again, how much would a coating weigh that had to cover 1 meter x 35 million meters of area? The black coating would heat the cable because the sun is so intense in orbit, so you wouldn't want it to be totally absorptive. But here's the thing:

The moon is black.

Actually, overall the moon's surface is about the shade of an asphalt highway. It absorbs almost all the light that hits it. The moon appears pearly white to us only because of the tiny fraction of light that's reflected off the lunar blacktop.

As if all this were not enough, the only practical means of powering the climber cars (which would be visible too) appears to be multi-megawatt lasers, aimed at solar cell arrays on the climbers. As Wikipedia puts it:

The proposed method is laser power beaming, using megawatt powered free electron or solid state lasers in combination with adaptive mirrors approximately 10 m wide and a photovoltaic array on the climber tuned to the laser frequency for efficiency.

So, the climbers are in the cross-hairs of, essentially, a set of a huge spotlights. Maybe you could use infrared or ultraviolet lasers, but if not, then even for the most efficient solar cells (40% or thereabouts) 60% of the laser light will be absorbed or reflected. Add to that light from the sun reflecting off the (presumably large) collectors, and you get something fiercely bright climbing the already bright cable.

This issue doesn't just affect the space elevator, by the way. It's also relevant to any substantial effort to place solar power satellites at geosynchronous orbit. Their immense surface area would pretty much guarantee that they'd shed a vast amount of light on the Earth.

But why should we care? Here again we can refer to Wikipedia, in its entry on http://en.wikipedia.org/wiki/Light_pollutionlight pollution:

Life exists with natural patterns of light and dark, so disruption of those patterns influences many aspects of animal behavior. Light pollution can confuse animal navigation, alter competitive interactions, change predator-prey relations, and influence animal physiology.

...Studies suggest that light pollution around lakes prevents zooplankton, such as Daphnia, from eating surface algae, helping cause algal blooms that can kill off the lakes' plants and lower water quality.

Lots of other life forms are affected—everything from birds to frogs. It doesn't take very much light to have a big effect. So, in the absence of any direct physical effects, the space elevator would still have a large, if not catastrophic, ecological impact.

I wish this weren't true. I'm a big fan of the elevator, and an even bigger fan of solar power satellites. But the devil, as they say, is in the details. If these structures cause the amount of light pollution I'm suggesting, then they are very far from being green options for energy and transport—regardless of how much carbon they may offset.

Comments

You have an absolutely valid point. PowerSat's aren't impact free. It is a matter of evaluating the impact of various energy sources on a 'per watt delivered' basis. Do we raise the temperature of the planet? Do we leave radioactive waste for centuries? Do we require wells, or mines, or leveling forests? Do we add light to the night sky? PowerSat's are one of the lowest impact solutions, but they're not zero impact. Thank you for pointing out one of the less obvious impacts.

WilliamManess (at) PowerSat (dot) com

Posted by: William Maness on March 16, 2009 3:36 PM

Now I might be missing something but wouldn't the lower part of space elevator -- the part that would cause most light pollution -- be enveloped by the Earth's shadow at night anyway? And thus the light pollution would be mostly from the part much higher up, and thus be a bit like the light pollution from a full moon?

Please correct me if I'm wrong, but I don't see this as a big problem.

Posted by: Jetse on March 18, 2009 1:24 PM

1. The lasers powering an Edwards elevator would likely be in the infared (i.e. invisible).
3. A technically more likely candidate for an elevator (because it doesn't need the carbon nanotubes to be so perfectly strong) is Kieth Henson's rolling ribbon-- and it needs no laser because the elevator is constantly moving in a loop.
4. There are "environmentalists" who are against off-shore wind power because they can see them. This seems like a similar concern.
5. This is less light than a full moon. Much less. Animals seem to have adapted pretty well to lunar cycles; they have adapted to our current bright skies pretty well. Those that don't... well 99.99% of all species that ever existed on this planet became extinct before proto-humans ever appeared on the scene. And if we don't move off this planet in a big way (taking our biosphere with us), then *everything* becomes extinct in a billion years or so when the sun goes into it's red cycle.
6. Global warming is the least of our problems. See the March issue of Nanotechnology Perceptions.

Posted by: Tihamer Toth-Fejel on March 22, 2009 10:10 PM

Sorting these issues out is a tricky business. Here's a piece by Hassan Masum about mass collaboratively figuring out the problems and possibilities with powersats: http://www.worldchanging.com/archives//007460.html

Posted by: Mark Tovey on March 23, 2009 2:11 AM

A couple of counterpoints:

The bottom 2000 km of the elevator are lit pretty much only when the surface is. (The higher the point, the earlier the sun rises, and the later it sets) - so they are dark most of the night.

The satellites fly at 200 km - if they were at 1000 km, they'd be only 1/25 as bright. if they were at 2000 km, they'd be 1/100 as bright...

The rest of the elevator is lit a lot more time, but because of the distance, it is so dim that what is left is a very faint hairline across the sky. Barely visible, if at all.

Finally, what you see in the sky is the solar panels of the satellites, not their bodies, so the sources are bigger than 1 m across...

And finally finally, I'd like to think that a thin hairline in the sky, even if it was moon-bright, would be a gorgeous site!

As for SSP, a 1 km x 1 km collector situated 40,000 km away is like a 10 m x 10 m collector situated 400 km away, so a Solar Power Satellite is more or less equivalent to a regular satellite.

Cheers!

Ben

Posted by: Ben Shelef on April 3, 2009 4:50 PM

Great comments, and very reassuring! I don't want this to be a show-stopper, and from the calculations some of you have made, it doesn't sound like it would be.

I just want to emphasize that light pollution is important not for cosmetic reasons, but because it can wreak havoc with the natural cycles of many plants and animals. So my concern with this issue was not frivolous.

Posted by: Karl Schroeder on April 22, 2009 1:44 PM

Sounds like the comments here have already captured the main points - I'd just like to add one more: geometry.

For the Moon, depending on the position within its orbit relative to Earth and Sun, what we see can be very bright (full moon), or considerably dimmer, or even almost invisible (new moon), because the sunlit side of the Moon is not necessarily visible from Earth's surface. Similarly, a space elevator ribbon could be oriented so it is fully sunlit on one side and dark on the other, or (and this might be useful for thermal management reasons anyway) it could be oriented side-on to the Sun, with relatively little sunlight actually impinging on the surface, and much less reflected away.

Moreover, depending on surface texture, much of the reflected light could be specular and so, just from the geometry of the situation, would bounce off the ribbon and head into deep space, never coming close to Earth's surface. The geometry for orbiting satellites is different since, when the Sun is directly behind the Earth, the solar panels naturally are aligned to reflect everything straight down, and satellites are as Karl mentioned "barrel-shaped" rather than ribbon-shaped, so that the body of the satellite also always has some component that allows specular reflection from Sun to satellite to Earth; that geometric requirement is not going to usually apply to a space elevator ribbon.

The other thing I'd like to go over is quantification. Ben Shelef above noted the inverse square law for radiation and its implications for solar power satellites. For the space elevator ribbon ignoring the geometry issues, the total light seen on the ground would be found by integrating the 1/r^2 rule over the length of the sunlit ribbon, to get the equivalent area/distance^2 (solid angle) that is relevant. Very roughly that gives you a solid angle equal to (width of ribbon) divided by (minimum distance to sunlit part of ribbon). If the ribbon is 1 meter wide and we're talking about distances 1000 km or more away, that's on the order of 10^-6 steradians.

The solid angle covered by the Moon is 6x10^-5 steradians, so considerably larger. It's possible just after sunset (and just before sunrise) that the geometry and arrangement of the lower part of the ribbon could give you lighting brighter than the full Moon within a distance of 10-20 km of the ribbon, but that would fade quickly as the sunlit portion rose while the Sun sets further.

For solar power satellites you can similarly look at solid angle - it's just the area divided by r^2, so in the case of a very large 16 km^2 satellite at 40,000 km altitude, that's 10^-7 steradians. To get to full-moon area levels you would need about 600 of those.

Posted by: Arthur Smith on April 28, 2009 7:20 AM

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