Pioneer program
Pioneer 6 and Pioneer 9 are orbiting the Sun at 0.8 AU, with orbits slightly shorter than Earth. Pioneer 7 and Pioneer 8 are in solar orbits with 1.1 AU distance to the Sun. Their orbits are slightly longer than Earth's. Since the probes' orbital periods are different from that of the Earth, from time to time, they face a side of the Sun that cannot be seen from Earth.