The Pioneer Missions: Explorations of the Solar System

launch of Pioneer 10
Pioneer 10 launched from Cape Canaveral on March 2, 1972, on a one-way trip out past Jupiter. It is now the most distant spacecraft from Earth. NASA

People have been in the "explore the solar system" mode since the early 1960s, when the first lunar and Mars probes left Earth to study those worlds. The Pioneer series of spacecraft are a large part of that effort. They performed first-of-their-kind explorations of the Sun, Jupiter, Saturn and Venus. They also paved the way for many other probes, including the Voyager 1 and 2 missions, Cassini,  Galileo, and New Horizons

Pioneer 0, 1, 2

Pioneer Missions 0, 1, and 2 were the United States' first lunar attempts. These identical spacecraft, which all failed to meet their lunar objectives, were followed by 3 and 4, which succeeded in becoming America's first successful lunar missions. Pioneer 5 provided the first maps of the interplanetary magnetic field. Pioneers 6,7,8, and 9 were the world's first solar monitoring network and provided warnings of increased solar activity which could affect Earth orbiting satellites and ground systems. The twin Pioneer 10 and 11 vehicles were the first spacecraft to ever visit Jupiter and Saturn. The craft performed a wide variety of scientific observations of the two planets and returned environmental data that was used during the design of the more sophisticated Voyager probes. The  Pioneer ​Venus mission, consisting of the Venus Orbiter (Pioneer 12) and Venus Multiprobe (Pioneer 13), was the United States' first long-term mission to observe Venus. It studied the structure and composition of the Venus atmosphere.

The mission also provided the first radar map of the planet's surface.

Pioneer 3, 4

Following the unsuccessful USAF/NASA Pioneer Missions 0, 1, and 2 lunar missions, the U.S. Army and NASA launched two more lunar missions. Smaller than the previous spacecraft in the series, Pioneer 3 and 4 each carried only a single experiment to detect cosmic radiation. Both vehicles were planned to fly by the Moon and return data about Earth and Moon's radiation environment. The launch of Pioneer 3 failed when the launch vehicle first's stage cut-off prematurely.

Although Pioneer 3 did not achieve escape velocity, it reached an altitude of 102,332 km and discovered a second radiation belt around Earth. The launch of Pioneer 4 was successful, and it was the first American spacecraft to escape Earth's gravitational pull as it passed within 58,983 km of the moon (about twice the planned flyby altitude). The spacecraft did return data on the Moon radiation environment, although the desire to be the first man-made vehicle to fly past the moon was lost when the Soviet Union's Luna 1 passed by the Moon several weeks before Pioneer 4.

Pioneer 6, 7, 7, 9, E

Pioneers 6, 7, 8, and 9 were created to make the first detailed, comprehensive measurements of the solar wind, solar magnetic field and cosmic rays. Designed to measure large scale magnetic phenomena and particles and fields in interplanetary space, data from the vehicles has been used to better understand stellar processes as well as the structure and flow of the solar wind. The vehicles also acted as the world's first space-based solar weather network, providing practical data on solar storms which impact communications and power on Earth. A fifth spacecraft, Pioneer E, was lost when it failed to orbit due to a launch vehicle failure.

Pioneer 10, 11

Pioneers 10 and 11 were the first spacecraft to visit Jupiter (Pioneer 10 and 11) and Saturn (Pioneer 11 only). Acting as pathfinders for the Voyager missions, the vehicles provided the first up-close science observations of these planets, as well as information about the environments that would be encountered by the Voyagers. Instruments aboard the two craft studied Jupiter and Saturn's atmospheres, magnetic fields, moons, and rings, as well as the interplanetary magnetic and dust particle environments, the solar wind, and cosmic rays. Following their planetary encounters, the vehicles continued on escape trajectories from the solar system. At the end of 1995, Pioneer 10 (the first man-made object to leave the solar system) was about 64 AU from the Sun and heading toward interstellar space at 2.6 AU/year.

At the same time Pioneer 11 was 44.7 AU from the Sun and heading outward at 2.5 AU/year. Following their planetary encounters, some experiments aboard both spacecraft were turned off to save power as the vehicle's RTG power output degraded. Pioneer 11's mission ended on September 30, 1995 when its RTG power level was insufficient to operate any experiments and the spacecraft could no longer be controlled. Contact with Pioneer 10 was lost in 2003.

Pioneer Venus Orbiter

Pioneer Venus Orbiter was designed to perform long-term observations of the Venus atmosphere and surface features. After entering orbit around Venus in 1978, the spacecraft returned global maps of the planet's clouds, atmosphere and ionosphere, measurements of the atmosphere-solar wind interaction, and radar maps of 93 percent of the surface of Venus. Additionally, the vehicle made use of several opportunities to make systematic UV observations of several comets. With a planned primary mission duration of only eight months, the Pioneer spacecraft remained in operation until October 8, 1992 when it finally burned up in the atmosphere of Venus after running out of propellant. Data from the Orbiter was correlated with data from its sister vehicle (Pioneer Venus Multiprobe and its atmospheric probes) to relate specific local measurements to the general state of the planet and its environment as observed from orbit.

Despite their drastically different roles, the Pioneer Orbiter and Multiprobe were very similar in design. The use of identical systems (including flight hardware, flight software, and ground test equipment) and incorporation of existing designs from previous missions (including OSO and Intelsat) allowed the mission to meet its objectives at minimum cost.

Pioneer Venus Multiprobe

Pioneer Venus Multiprobe carried 4 probes designed to perform in-situ atmospheric measurements. Released from the carrier vehicle in mid-November 1978, the probes entered the atmosphere at 41,600 km/hr and carried a variety of experiments to measure chemical composition, pressure, density, and temperature of the mid-to-lower atmosphere. The probes, consisting of one large heavily instrumented probe and three smaller probes, were targeted at different locations. The large probe entered near the planet's equator (in daylight). The small probes were sent to different spots.

The probes were not designed to survive impact with the surface, but the Day probe, sent to the daylight side, did manage to last a while. It sent temperature data from the surface for 67 minutes until its batteries were depleted. The carrier vehicle, not designed for atmospheric reentry, followed the probes into the Venusian environment and relayed data about the characteristics of the extreme outer atmosphere until it was destroyed by atmospheric heating.