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Kosmos 300 (Russian: Космос 300 meaning Cosmos 300) (Ye-8-5 series) was the fourth Soviet attempt at an unmanned lunar sample return. It was probably similar in design to the later Luna 16 spacecraft. It was launched, on a Proton rocket, on September 23, 1969. The mission was a failure. The engines on the Block D upper stage failed, leaving the spacecraft stranded in Earth orbit.