The Soviet Zond spacecraft was not yet ready for piloted circumlunar missions in 1968, after five unsuccessful and partially successful automated test launches: Cosmos 146 on March 10, 1967; Cosmos 154 on April 8, 1967; Zond 1967A September 27, 1967; Zond 1967B on November 22, 1967. Where was the Zond 4 over when it was destroyed by an explosion?