QUESTION
A recent "Space Camp" poster I received stated that one square mile of 
sunlight weighs 3lbs. Can you explain this please?

ANSWER from Alan Federman on April 18, 1996:
The pressure of light (sunlight) is a measurable force. (Those little
radiometers that spin are examples of this.)  if we placed a flat mylar
"sail" 1 mile square in space (some certain distance from the sun) The
force would be equal to attaching a 32 lb weight to a (massless object) in
earth's gravity field. I am assuming this means a force of 32 lbs per second.

Since the force of sunlight is continuous, the solar sail ship could continue
to accelerate as long as the force of sunlight is significant. Theoretically
is is possible to accelerate to a sizeable fraction of the speed of light
using this method. It is also possible to "tack" ie sail at an angle to
the force of the light. By flying a zig-zag it would be possible to
go toward the sun.