Arthur C. Clark and Robert Forward wrote science fiction stories
about solar sails, spacecraft that require no fuel to move around
in interplanetary space. NASA is working on a flight demonstration
of such solar sail crafts. Since they have no fuel on board, what
propels these solar sail crafts, the solar wind, solar storms, or
Sun light?

It is true that solar sails have huge very thin surfaces, like sails
on boats and they fly around by tacking in space. But it is not
the solar wind or solar storm particles that provide the force on
the sail. It is the plain Sun light. While solar wind particles fly
at over 1 million mph, there are so few of them (~10 particles/cm^3)
that the pressure they exert on a solar sail is 10 times less than
the pressure exerted by reflected Sun light.

To see pictures of ground testing of a real solar sail, check out:
http://www.nasa.gov/vision/universe/roboticexplorers/solar_sails.html

To learn more about solar sails, go to:
http://www.solarsails.info/

This week's question comes from Dr. Adam Szabo, a scientist in the Geospace Physics Branch of the Laboratory for Solar and Space Physics at Goddard.