In this article on MSNBC.com:

http://www.msnbc.msn.com/id/3217961/

"At the start of the trip, the [near-] Earth station would focus its
particle beam on the magnetic sail of a Mars-bound space taxi, pushing it to
speeds of tens of thousands of miles an hour. During the approach to Mars,
the Red Planet station would fire its own beam to decelerate the
spacecraft."

Nowhere is mention made that this particle beam causes any "equal and
opposite" action at the particle-beam source. If this near-Earth station is
blasting a particle beam into space that causes a spaceship to accelerate
(momentum transfer) to "tens of thousands" of mph ... what is the source
doing to maintain position or orbital velocity? Am I missing something?

Jon