In this article, the New Scientist writes that the next generation of spacecraft might be propelled with the help of the sun.
A physicist is claiming that solar sailing -- the idea of using sunlight to blow spacecraft across the solar system -- is at odds with the laws of thermal physics.
Both NASA and the European Space Agency are developing solar sails and, although never tested, the concept is quite simple. A solar sail is essentially a giant mirror that reflects photons of sunlight back in the direction they came from.
Although photons do not have mass, they are considered to have momentum, so according to the law of conservation of momentum, the photon loses some of its energy to the sail as it bounces off, giving the sail a shove in the opposite direction.
But Thomas Gold from Cornell University in New York says the proponents of solar sailing have forgotten about thermodynamics, the branch of physics governing heat transfer.
And this is where it's becoming interesting. Let's see what writes Gold, in "The solar sail and the mirror."
The radiation pressure exerted by incoherent light on diverse surfaces is examined. The thermodynamic rule, first given by Carnot in 1824, describes the limitation to the amount of free energy that can be obtained from a source of thermal energy, and he gave the compelling reason for this rule, that if more free energy than he had prescribed could ever be extracted, then a heat pump could use that free energy and re-create all the heat energy that had been consumed. A perpetual motion machine could then be constructed. Now, 179 years later, it is proposed to fly a spacecraft that is expected to gain velocity from the radiation pressure the sunlight is expected to exert on solar sails, panels of thin plastic sheets, mirror surfaced on the side facing the sun. However a detailed examination of this proposal shows it to be in direct conflict with Carnot's rule, and no such pressure can be expected. Either Carnot's accepted rule is in error, or the solar sail proposal will not work at all.
So, as this large illustration from New Scientist shows, the real question is: "Can it really sail away?"
The dispute could be settled in September, when the Pasadena-based Planetary Society hopes to launch Cosmos 1, the world's first solar sail. The 100-kilogram craft will be sent into orbit around the Earth, before unfurling a set of reflecting blades in an attempt to boost its altitude. Louis Friedman, the project's director, is undaunted by Gold's criticism. "Solar sailing is possible," he insists.
Sources: Paul Parsons, New Scientist, July 2, 2003; Thomas Gold, Cornell University, June 5, 2003
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