 They are of particular interest in exploration of the Moon, but suffer from gravitational perturbation effects that make most unstable, and leave only a few orbital inclinations possible for indefinite "frozen orbits", useful for long-term stays in LLO.The Soviet Union sent the first spacecraft to the vicinity of the Moon, the robotic vehicle Luna 1, on January 4, 1959.It passed within of the Moon's surface, but did not achieve lunar orbit.