  The Schwarzschild solution is used to find the exact relativistic motion of a
payload in the gravitational field of a mass moving with constant velocity. At
radial approach or recession speeds faster than 3^-1/2 times the speed of
light, even a small mass gravitationally repels a payload. At relativistic
speeds, a suitable mass can quickly propel a heavy payload from rest nearly to
the speed of light with negligible stresses on the payload.
