http://arxiv.org/PS_cache/gr-qc/pdf/0505/0505099.pdf [Broken] Franklin Felber says in this paper that for the first time has obtained the solution of the orbits of a particle near an ultrarelativistic central field (in second order of aproximation). This leads to the gravitomagnetic aproximation and to some kind of gravitatory repulsión in certain circunstances. Theoretically if some favorable celestial body (star) would be found that would allow drastic restrictions in the fuel necessary to acerlerate a ship to near of light speeds. Well, ¿do you think it is really worthin or simply NASA publicity? (the author works for NASA).