To add a little more detail, if you are receding from the light source the photons will be red-shifted and thus will have less momentum and produce less thrust on a solar sail.
On Thu, 7 Aug 2014 10:32:08 -0400 Carlo Vaccari <airplaniac2002@xxxxxxxxx> wrote:
A photon based propulsion system fails to generate free power because the force is not invariant with relative speed.On 07/08/14 21:38, Keld Laursen wrote: I do still wonder where a photonic system fails and I'd really like to hear an explanation for why it would, though I fear the answer involves actual numbers not just abstract thought to work. ;->