Mariner 1 was the first spacecraft of NASA's interplanetary Mariner program, built to conduct the first American planetary flyby of Venus. Developed by Jet Propulsion Laboratory and originally planned to be a purpose-built probe launched summer 1962, Mariner 1's design was changed when the Centaur proved unavailable at that early date. Mariner 1 and its sibling spacecraft Mariner 2 were then adapted from the lighter Ranger lunar spacecraft. Mariner 1 carried a suite of experiments to determine the temperature of Venus as well to measure magnetic fields and charged particles near the planet and in interplanetary space.