E=mc^2 is the equation for rest mass energy, once you start moving the equation become E= (1/(1-(v/c)^2)^0.5)mc^2, the faster you got the more your mass would increase, so more energy would be needed, so more mass etc in vicious circle. Oh and by the way, once one has left the immediate vicinity of a planet one is barely affected by the gravitational attractions as the force obeys an inverse square law. For something to have negative mass it has to be antimatter, which annihilates on contact with matter, for example humans, in an explosive manner.