Kosmos 300 ( meaning Cosmos 300) (Ye-8-5 series) was the fourth Soviet attempt at an uncrewed 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 due to an oxidizer leak, leaving the spacecraft to burn up in Earth's atmosphere.