Terry Blanton wrote:
From: Harry Veeder
Solar sails work on a relatively simple principle -- that beams of light
bouncing off a reflective surface will transmit a push to the surface,
driving it forward.
Are you sure of this? A photon's relative mass is given by the ratio of
momentum to light speed. To impart momentum to the solar sail, how is this
ratio altered?
The ratio isn't altered. The sail is a mirror; when photons bounce off,
their momentum vectors "flip" -- they change direction by roughly 180
degrees. The momentum of each photon changes from +p to -p when its
direction is reversed. That's a net change in momentum of -2*p, so the
sail must pick up momemtum of +2*p to balance it.
Point the sail at the sun, and the vehicle accelerates straight outward,
away from the sun. Point it at an angle, and the sail will accelerate
(more slowly!) on a diagonal. But the sun's gravity can act similarly
to a "keel"; by accelerating back along the Earth's orbit the sail can
follow a (spiral) path that takes it closer to the sun. So ultimately
all directions of flight are possible for a solar sail vehicle. But,
you do need patience; they are not fast...
