I heard that solar sails are propelled by photons hitting the 'sails'. But photons have no mass and therefore no momentum, so in a collision they provide no impulse?
-- sodium
yes the photons do provide the impulse to the sails. According to the Planetary Society ho have built the first solar sail powered craft 99% of its propulsion should come from reflecting photons, only 1% from the solar wind. See
http://www.u3p.net/tipe/phot_jp.htm --rmhermen
What about future methods like matter-
antimatter propulsion? --
mike dill