Mariner 2 lifts off for Venus, five weeks after its twin veered into the Atlantic
Five weeks earlier, on July 22, Mariner 1 had veered off course and been blown up by a range safety officer; a missing hyphen in its guidance code was later blamed for the failure. At 1:53 a.m. Eastern on August 27, 1962, its twin, Mariner 2, rode an Atlas-Agena B rocket off Launch Complex 12 at Cape Canaveral. Within 44 minutes its solar panels had unfolded; about 18 minutes later, its Sun sensor locked onto the Sun. The spacecraft was now bound for Venus.
Mariner 2 gave the United States its first working interplanetary spacecraft at the height of the space race, JPL recovered from a public failure and reached another planet before the Soviets did. The mission's radiometer data, showing Venus's cloud tops were cool but its surface scorching, gave American planetary scientists the evidence they cited for the rest of the decade to rule out Venus as Earth's twin.
QWhy did NASA end up launching Mariner on the smaller Agena B instead of the rocket it was designed for?
Mariner was originally planned for the Atlas-Centaur, a rocket with far more lifting power, but the Centaur upper stage ran into serious development trouble on its own schedule. Rather than wait, NASA and JPL redesigned the mission around the smaller, already-flying Agena B second stage. That forced engineers to shrink and simplify the spacecraft, folding Mariner's design into a hybrid of the shelved Mariner A concept and JPL's Block 1 Ranger lunar probe, built in an eleven-month sprint. The substitution shaped the entire spacecraft's tight weight budget.
QHow precisely did engineers hit the spacecraft's target weight?
Mariner 2 weighed in within 3 pounds of its design target of 447 pounds, a margin of just over half a percent on a spacecraft assembled in under a year. Of that weight, 406 pounds went to non-experimental systems alone: fuel, maneuvering hardware, and the radio equipment needed to receive commands and transmit data back to Earth. The remaining weight covered every scientific instrument the mission carried, leaving JPL's engineers little room to work with once the rocket switch shrank the whole design.
QWhat kept Mariner 2's onboard computer from cooking itself during the flight?
The spacecraft relied on two separate systems to survive the temperature swings of deep space: passive protection from insulated, highly reflective surfaces, and active protection from a set of louvers built specifically to shield the case housing the onboard computer. The louvers opened and closed automatically as temperatures shifted. Without the louvers, the onboard computer running Mariner 2's automated sequences could have overheated before the spacecraft reached Venus.
