  We present the enabling concept and technology for a dual spacecraft
formation-flying optical interferometer, to be launched into a deep space orbit
as Space Technology 3. The combiner spacecraft makes use of a nested cat's eye
delay line configuration that minimizes wavefront distortion and stores 20 m of
optical pathlength in a package of \sim 1.5 m length. A parabolic trajectory
for the secondary collector spacecraft enables baselines of up to 200 m for a
fixed 20 m stored delay and spacecraft separations of up to 1 km.
