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 ^ { \circ } to the line of sight .
Four lobe speeds , between 0.32c to 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 \beta _ { j } > 0.95 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 .