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

Planetary scientists have been in the "explore the solar system" mode since the early 1960s, ever since NASA and other space agencies were capable of lofting satellites from Earth. That's when the first lunar and Mars probes left Earth to study those worlds. The Pioneer series of spacecraft were 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 missions, Cassini, Galileo, and New Horizons. 

Pioneer Able spacecraft
The first in the Pioneer series of spacecraft was called Pioneer Able, and it studied the Moon. NASA 

Pioneer 0, 1, 2

Pioneer Missions 0, 1, and 2 were the United States' first attempts to study the Moon using spacecraft. These identical missions, which all failed to meet their lunar objectives, were followed by Pioneers 3 and 4. They were America's first successful lunar missions. The next one in the series, Pioneer 5 provided the first maps of the interplanetary magnetic field. Pioneers 6,7,8, and 9 followed up as the world's first solar monitoring network and provided warnings of increased solar activity which could affect Earth-orbiting satellites and ground systems.

As NASA and the planetary science community were able to build more robust spacecraft that could travel farther than the inner solar system, they created and deployed the twin Pioneer 10 and 11 vehicles. These 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.

Pioneer 10
Pioneer 10 was built at NASA Ames Research Center and included multiple detectors and instruments to study the planet, its gravitational field, and its magnetic field. NASA 

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. These were smaller than the previous spacecraft in the series and each carried only a single experiment to detect cosmic radiation. Both vehicles were supposed 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 design for the Pioneer 3 and 4 spacecraft
This is the configuration for Pioneers 3 and 4. NASA

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 fields, and cosmic rays. Designed to measure large scale magnetic phenomena and particles and fields in interplanetary space, data from the vehicles have 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 11
This artist's concept of the Pioneer 12 spacecraft (twin to Pioneer 11) at Jupiter. It, like its twin, measured conditions at Jupiter, including its magnetic field and radiation environment. NASA

Pioneer Venus Orbiter and Multiprobe Mission

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.

Pioneer Venus Multiprobe mission (artist's concept).
The Pioneer Venus Multiprobe was launched in 1978 and arrived in late autumn. The probes descended through the atmosphere and sent back information about the conditions. NASA 

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.

The Pioneer missions had a long and honorable place in space exploration history. They paved the way for other missions and contributed greatly to our understanding of not only planets but also the interplanetary space through which they move.

Fast Facts about The Pioneer Missions

  • The Pioneer missions comprised a number of spacecraft to planets ranging from the Moon and Venus to the outer gas giants Jupiter and Saturn.
  • The first successful Pioneer missions went to the Moon.
  • The most complex mission was Pioneer Venus Multiprobe.

Edited and updated by Carolyn Collins Petersen