"Mariner A", a spacecraft to be launched using the yet undeveloped Atlas-Centaur. By August 1961, it had become clear that the Centaur would not be ready in time. JPL proposed to NASA that the mission might be accomplished with a lighter spacecraft using the less powerful but operational Atlas-Agena. A hybrid of Mariner A and JPL's Block 1 Ranger lunar explorer, already under development, was suggested. NASA accepted the proposal, and JPL began an 11-month crash program to develop "Mariner R" (so named because it was a Ranger derivative). Mariner 1 would be the first Mariner R to be launched followed by Mariner 2. Spacecraft Three Mariner R spacecraft were built: two for launching and one to run tests, which was also to be used as a spare. Aside from its scientific capabilities, Mariner also had to transmit data back to Earth from a distance of more than , and to survive solar radiation twice as intense as that encountered in Earth orbit. Structure All three of the Mariner R spacecraft, including Mariner 2, weighed within of the design weight of , of which was devoted to non-experimental systems: maneuvering systems, fuel, and communications equipment for receiving commands and transmitting data. Once fully deployed in space, with its two solar panel "wings" extended, Mariner R was in height and across. The main body of the craft was hexagonal with six separate cases of electronic and electromechanical equipment: Two of the cases comprised the power system: switchgear that regulated and transmitted power from the 9800 solar cells to the rechargeable 1000 watt silver-zinc storage battery. Two more included the radio receiver, the three-watt transmitter, and control systems for Mariner's experiments. The fifth case held electronics for digitizing the analog data received by the experiments for transmission. The sixth case carried the three gyroscopes that determined Mariner's orientation in space. It also held the central computer and sequencer, the "brain" of the spacecraft that coordinated all of its activities pursuant to code in its memory banks and on a schedule maintained by an electronic clock tuned into equipment on Earth. At the rear of the spacecraft, a monopropellant (anhydrous hydrazine) 225 N rocket motor was mounted for course corrections. A nitrogen gas fueled stabilizing system of ten jet nozzles controlled by the onboard gyroscopes, Sun sensors, and Earth sensors, kept Mariner properly oriented to receive and transmit data to Earth. The primary