Friday, August 01, 2008

Solar Sails

Sciam has a short article by Larry Greenmeier ("Voyaging to the Stars on a Solar Breeze: Space Sail to Take Flight") on space sails, discussing both radiation-pressure and solar wind plasma momentum-driven designs. NanoSail-D is a mini Solar sail (from NASA) soon to be tested in space.

Unfortunately, rather contrary to the title of the Sciam article, solar sails will have problems as a means of propulsion for interstellar (as opposed to interplanetary) probes. Nevertheless they are far more plausible and practicle than Bussard ramjets or Daedalus.

For a fixed sail size the force on the solar sail due to either the Sun's radiation or the Solar wind will diminish as the inverse square of the distance of the Sail from the Sun (*). Basically you get most of your acceleration on the sail when you're closest to the Sun, and your velocity very quickly asymptotes out to a terminal velocity and then you're coasting along with little control.

I find solar sails interesting for a number of reasons, but from a professional standpoint one reason is that the physics is very similar to the standard model of clouds enveloped and accelerated in superwinds, and its really quite simple and neat too. Sadly blogger (and the web) is horrible at displaying equations so I won't bother trying .

(*) Now if you use a giant laser or maser to accelerate your sail, so that your light beam has relatively little divergence, you can accelerate your sail for much longer. This scheme is used by the Moties in Niven and Pournelle's "The Mote in God's Eye", or by humanity in Greg Bear's "Queen of Angels." Both great works of speculative fiction well worth reading.

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