The name "Pioneer" was applied to several different space missions and spacecraft designs. The first U.S. moon probes, launched by the Air Force in 1958-59, bore the name. Though Pioneers 0 through 3 failed, Pioneer 4 flew by the moon at a distance of 60,000 kilometers in March 1959. It became the first U.S. spacecraft to escape Earth's gravity and enter orbit around the Sun.
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 little noticed) missions.
The name regained star status when Pioneer 10 left Earth in March 1972. It became the first spacecraft to brave the Asteroid Belt and fly past Jupiter. Pioneer 11 launched in April 1973, bound for Jupiter and Saturn. It went silent in 1995. Pioneer 10 sent its last signal from beyond Pluto in 2003.
The final Pioneer launches occurred in 1978. The Pioneer Venus Multiprobe spacecraft dropped four instrumented capsules on Venus, while Pioneer Venus Orbiter (PVO) (image at top of post) surveyed the planet until 1992. The latter was informally designated Pioneer 12 and the former Pioneer 13.
If Hughes Aircraft had had its way, the Pioneer name also might have been applied to a Mars spacecraft. In a 1974 report, it described a Pioneer Mars Orbiter (PMO) derived from the Hughes PVO spacecraft design. PVO was planned to be a drum 2.5 meters in diameter and 1.2 meters long with a 3.3-meter antenna mast on top and a solid-propellant Venus orbit insertion motor on the bottom.
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 September 7, 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 last only 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 low-gain and dish-shaped high-gain antennas, 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 the 37° north latitude point. 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.
Penetrator deployment would occur near apoapsis (orbit high point). Hinged covers would open on the ends of the launch tube, then the solid-propellant deployment rocket motor would ignite so that the penetrator would leave the tube and fall from orbit. The dome-nosed penetrator would drop through Mars's atmosphere and implant itself in the surface up to 15 meters deep.
After emplacement, 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 have to impact on 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.
Reference:
Pioneer Mars Surface Penetrator Mission: Mission Analysis and Orbiter Design, Hughes Aircraft Company, August 1974.
