The Moon spins on its axis once per Earth orbit, so the same side always faces Earth. To get information from the far side of the Moon relayed to Earth is difficult, because the Moon itself blocks the line of sight back to Earth. To get round this problem, SELENE has one satellite close to the Moon to collect data, and a companion sub-satellite in a much higher orbit to relay that information back to Earth. ??????? Why not just store the data and stransmit when non-obstructed? And two satellites orbiting the same object would still have that object obstructing their communication, unless they were BOTH in extremely high orbits.