  Humans will launch spacecraft that travel at an appreciable fraction of the
speed of light. Spacecraft traffic will be tracked by radar. Scattering of
pulsed electromagnetic fields by an object in uniform translational motion at
relativistic speed may be computed using the frame-hopping technique. Pulse
scattering depends strongly on the velocity, shape, orientation, and
composition of the object. The peak magnitude of the backscattered signal
varies by many orders of magnitude depending on whether the object is advancing
toward or receding away from the source of the interrogating signal. The peak
magnitude of the backscattered signal goes to zero as the object recedes from
the observer at a velocity very closely approaching light speed, rendering the
object invisible to the observer. The energy scattered by an object in motion
may increase or decrease relative to the energy scattered by the same object at
rest. Both the magnitude and sign of the change depend on the velocity of the
object, as well as on its shape, orientation, and composition. In some cases,
the change in total scattered energy is greatest when the object is moving
transversely to the propagation direction of the interrogating signal, even
though the Doppler effect is strongest when the motion is parallel or
antiparallel to the propagation direction.
