  The evolution of the radio emission from Sco X-1 is determined from a 56-hour
continuous VLBI observation and from shorter observations over a four-year
period. The radio source consists of a variable core near the binary, and two
variable compact radio lobes which form near the core, move diametrically
outward, then fade away. Subsequently, a new lobe-pair form near the core and
the behavior repeats. The differences in the radio properties of the two lobes
are consistent with the delay and Doppler-boosting associated with an average
space velocity of 0.45c at 44 deg to the line of sight. Four lobe speeds,
between 0.32c and 0.57c, were measured for several lobe-pairs on different
days. The speed during each epoch remained constant over many hours. The
direction of motion of the lobes over all epochs remained constant to a few
degrees.
  Two core flares are contemporaneous with two lobe flares after removal of the
delay associated with an energy burst moving with speed >0.95c in a twin-beam
from the core to each lobe. This is the first direct measurement of the speed
of energy flow within an astrophysical jet. The similarity of the core and lobe
flares suggests that the twin-beam flow is symmetric and that the core is
located near the base of the beam.
