|Pioneer Venus Orbiter (PVO) in Venus Orbit. The Pioneer Mars Orbiter (PMO) would have been based on this design. Image credit: Hughes Aerospace.|
Pioneer 5 (March 1960), a new-design spacecraft like no other Pioneer, was a pathfinder for interplanetary missions. It set a new record by transmitting until it was 36.2 million kilometers from Earth.
In 1965, NASA's Ames Research Center (ARC) took over the series name for its interplanetary "weather stations." The first, Pioneer 6, entered solar orbit between Earth and Venus in December 1965, where it monitored magnetic fields and radiation. Pioneers 7, 8, and 9 performed similarly prosaic (and generally little noticed) missions.
|Pioneer 4, last of the U.S. Air Force Pioneers, shown attached to its small solid-propellant upper stage. The spacecraft, the first successful Pioneer, measured about 30 centimeters from pointy top to flat bottom. Image credit: NASA|
|Pioneer 5, the first wholly NASA-operated spacecraft in the series, served as a pathfinder for subsequent U.S. planetary missions. Image credit: NASA|
|Pioneers 6 through 9 were drum-shaped spacecraft that measured "space weather" conditions in interplanetary space near Earth's orbit. Image credit: NASA|
The final Pioneer launches occurred in 1978. The Pioneer Venus Multiprobe spacecraft dropped four instrumented capsules on Venus, while Pioneer Venus Orbiter (PVO) surveyed the planet until 1992. The latter was informally designated Pioneer 12 and the former Pioneer 13.
|Nuclear-powered Pioneers 10 and 11, Earth's first outer Solar System probes, were virtual twins. Image credit: NASA|
|Pioneer Venus Multiprobe deploys its one large and three small atmosphere probes. Two survived landing and return data from the surface of Venus - the only U.S. spacecraft to land on Venus. Image credit: NASA|
The report cited differences between the PMO and PVO designs: for example, its orbit insertion motor would need to be larger since PMO would arrive at Mars traveling faster than PVO would arrive at Venus. In addition, PMO would operate in Mars orbit, about twice as far from the Sun as Venus, so solar cells for making electricity would entirely cover its sides. PVO would operate in Venus orbit, so it would be only partly covered with solar cells.
The most obvious difference between the PVO and PMO designs would be the Mars spacecraft's six 2.3-meter-long, 0.3-meter-diameter penetrator launch tubes. These would replace PVO's science instruments; apart from unspecified instruments in the penetrators, PMO would carry no science payload.
PMO, like PVO, would leave Earth on an two-stage Atlas-Centaur rocket. Because PMO would weigh more than PVO (1091 kilograms versus 523 kilograms), however, it would need a solid-propellant third stage to complete Earth escape. To make room for the third stage and penetrators, PMO's conical launch shroud would be 0.8 meters longer than its PVO counterpart.
PMO would need to reach Mars on 7 September 1980 so that its Mars orbit insertion motor could place it in its planned Mars orbit. To reach the planet on that date, PMO would need to depart Earth during one of 10 consecutive daily launch opportunities starting on 28 October 1979. 2 November 1979 would be the nominal launch date. The launch opportunities would only last from 10 to 15 minutes each.
The Centaur second stage would place PMO in a low-Earth orbit, then would ignite again 30 minutes later to begin pushing the spacecraft out of Earth orbit. The third-stage motor would then ignite to place PMO on course for Mars. PMO would weigh 1069 kilograms after third-stage separation. Launch on 2 November 1979 would yield a 310-day Earth-Mars transfer.
Following third-stage separation, PMO would use hydrazine thrusters to set itself spinning at 15 revolutions per minute (RPM) for stabilization. The antenna mast, bearing a low-gain antenna and a dish-shaped high-gain antenna, would revolve in the opposite direction at the same rate, so would appear to stand still. Controllers on Earth would use the thrusters to carefully target PMO so that it would not accidentally hit Mars and introduce terrestrial microbes. They would perform a final course correction 30 days before Mars arrival.
One day out from Mars, on 6 September 1980, PMO would orient itself for its Mars orbit insertion burn and increase its spin rate to 30 RPM. The spacecraft's high-gain antenna would not point at Earth during the insertion burn. Controllers on Earth could, however, send PMO commands through the low-gain antenna.
PMO would reach Mars late in northern hemisphere summer, when the planet's south polar cap would be near its maximum extent. Hughes Aircraft proposed two possible elliptical Mars orbits - south polar and north polar - each with a period of 24.6 hours (one martian day) and a periapsis (low point) of 1000 kilometers. South polar orbit periapsis would occur above a point on Mars's surface 72° south of the equator, while north polar orbit periapsis would occur above a point at 37° north latitude. The spacecraft's high periapsis altitude would serve to forestall orbital decay, helping to ensure that PMO would not drop living terrestrial microbes on Mars. PMO would have a mass of 741 kilograms after orbit insertion.
The PMO mission's Mars orbit phase would last one martian year (686 terrestrial days). During this mission phase, PMO would deploy its six 45-kilogram penetrators singly and in pairs using a penetrator deployment system based on the Hughes-built TOW missile launcher. Prior to launch from Earth, the penetrators would be sealed inside their launch tubes and heated to kill microbes.
|Pioneer Mars Orbiter deploys a penetrator slightly different from the one described in its August 1974 study. Image credit: Hughes Aerospace|
After impact, the penetrator would extend its antenna and begin transmitting data from its science instruments. Two antennas on PMO's mast would receive penetrator radio signals, which the spacecraft would record for relay to Earth through the high-gain dish. For their weak signals to be received, the penetrators would need to impact the surface not far from PMO's periapsis point.
PMO could maintain radio contact with a given penetrator for at least eight minutes at a time. A PMO in south polar orbit would initially place its penetrators between 63° and 87° south; a north-polar-orbiting PMO would place them between 56° and 80° north. Periapsis would gradually shift north or south, however, permitting placement at other latitudes. With all six penetrators deployed, PMO would have a mass of 412 kilograms.
Pioneer Mars Surface Penetrator Mission: Mission Analysis and Orbiter Design, Hughes Aircraft Company, August 1974
The Russians are Roving! The Russians are Roving! A 1970 JPL Plan for a 1979 Mars Rover
Prelude to Mars Sample Return: the Mars 1984 Mission (1977)