FYI,

"Solar Sailing 'Breaks Laws of Physics'"
New Scientist
http://www.newscientist.com/news/news.jsp?id=ns99993895

: 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.

: Solar sails are designed to be perfect mirrors, meaning that they
: reflect all the photons that strike them.

: 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.

Mark Reiff