Solar sailing

I just got back from vacation, which included some long plane trips that gave me a chance to catch up on my magazine reading.  So just a couple of months late, I read this article in the Atlantic on the Planetary Society’s attempts to get funding to build a prototype solar sailing spacecraft.  For those who don’t know, the idea is to propel the ship using big sails that reflect sunlight.  Since photons carry momentum, all of those photons bouncing off of the sail will impart momentum, making the craft go.

It’s a pretty good article, but there’s one bit of it that baffles me:

Not everyone concedes even the basics. The late Thomas Gold, of Cornell's Center for Radiophysics and Space Research, had insisted that solar sailing would never work, for the same reasons you cannot have a perpetual-motion machine: Carnot's rule and the iron second law of thermodynamics. No machine can extract an unlimited supply of free energy from any source; a certain "degradation" has to occur. And the problem is even more fundamental than that, Gold argued: the beautiful Mylar blades of Cosmos 1, or 2, will be too splendid to function, period. With "a perfect mirror, the two temperatures"€”of the sails and the sun€”"will be the same," Gold reasoned. "And it follows that the mirror cannot act as a heat engine at all: no free energy can be obtained from the light."

Any time you read a description of a technical argument in a nontechnical article, you have to reverse-engineer the details of the argument from the general description.  I can’t do that here: I can’t imagine any way that the argument imputed to Gold could make sense.  First, a “perfect mirror” is precisely the sort of thing that will not reach thermal equilibrium with the Sun, since it never absorbs thermal energy from the sunlight.  Second, even if you don’t have a perfect mirror, it’s not true that such a system will eventually reach the same temperature as the Sun.  For instance, the Earth has been absorbing 6000-degree sunlight for 4 billion years and is still at a relatively comfortable 300 K.  Something similar applies to the solar sail.  The point is that the Earth-Sun system is not a closed system: both are constantly radiating energy into the much colder deep-space environment.  The entire closed system (i.e., the Universe) is very gradually tending toward thermal equilibrium, but the idea that one small part of it (the Sun and the solar sail) will themselves reach equilibrium independent of the rest of the universe is nonsense.

If there’s a serious argument lurking in there, I’d love to know what it is.

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Ted Bunn

I am chair of the physics department at the University of Richmond. In addition to teaching a variety of undergraduate physics courses, I work on a variety of research projects in cosmology, the study of the origin, structure, and evolution of the Universe. University of Richmond undergraduates are involved in all aspects of this research. If you want to know more about my research, ask me!

3 thoughts on “Solar sailing”

  1. The “of the sails and the sun” interpolation by the Atlantic is wrong. Gold’s argument was that the incoming and reflected radiation have the same temperature, so it can’t extract any energy. More here:

    Looked at as an elastic collision, I think Gold’s argument only works if the solar sail is fixed to an infinite mass. For a real solar sail, the reflected
    temperature is slightly less than incident. As a Carnot engine, it isn’t very efficient, but it doesn’t have to be.

    There’s also some mention of this on the Wikipedia “solar sail” page.

  2. That helps a lot to clarify things! Viewed as a thermodynamic argument for why solar sails won’t work, Gold’s argument still looks utterly incoherent, but at least it’s a bit better than the notion that the mirror will reach the same temperature as the Sun.

  3. Fascinating concept, although I’m pretty sure I’m not smart enough to understand why it would or wouldn’t work. I would be interested in learning more about the project if it ever gets off the ground.

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