Mariner Space Program

One of the most important undertakings of the U.S. space program. The Mariner interplanetary probes examined the three nearest planets, Venus, Mercury, and Mars, revolutionizing scientific understanding of those worlds.

Origins and History

During the late 1950’s and 1960’s, both the United States and the Soviet Union inaugurated programs to explore the Moon and other planets with automated spacecraft. The U.S. programs were called Ranger, Surveyor, Pioneer, and Mariner. The Jet Propulsion Laboratory (JPL) in Pasadena, California, which is jointly operated by the California Institute of Technology and the National Aeronautics and Space Administration (NASA), managed the Mariner space program. The Mariner spacecraft carried cameras and scientific instruments powered by solar panels to radio their findings back to Earth. Ten Mariner probes were launched on Atlas-Agena or Atlas-Centaur rockets. Two of these probes failed during launch, and another failed after reaching Earth orbit. Mariner 11 and Mariner 12 were renamed Voyager 1 and Voyager 2 and visited the outer planets during the 1970’s and 1980’s.

Venus

The first Mariner spacecraft was launched on July 22, 1962, using an Atlas-Agena B rocket. Normally, a guidance signal from the ground directed the rocket’s ascent; however, the signal failed, causing an onboard computer to take over. A programming mistake an omitted hyphen caused the rocket to veer to the left, toward shipping lanes and populated areas. A range safety officer was forced to detonate an explosive charge within the rocket, destroying it minutes after liftoff.

A second attempt on August 27, 1962, was successful, and Mariner 2 headed toward Venus, the planet nearest to Earth and almost the same size. Venus is covered with a thick layer of clouds that no telescope has ever pierced. Before the 1960’s, some speculated that a tropical world or a searing desert lay beneath the clouds. On December 14, 1962, Mariner 2, the first successful interplanetary probe, flew by Venus. The probe determined that Venus possessed neither radiation belts nor a magnetic field. Four days after passing Venus, the electronics on Mariner 2 failed because of the heat of the Sun.

In 1967, the Soviets achieved their first success with interplanetary probes. The Soviets launched many more probes than the United States did but were plagued by persistent failures at launch or during the long journey to other planets. Soviet probes were mass-produced, and the Soviets attempted to maintain Earth-like conditions within their large spacecraft. U.S. spacecraft were produced individually, and each component was rigorously tested and designed to work in the vacuum of space. As the Soviet Venera 4 flew by Venus on October 18, 1967, it dropped a probe into the atmosphere. Higher-than-expected atmospheric pressures crushed the probe before it reached the planet’s surface.

Mariner 5, launched June 14, 1967, arrived at Venus only a few days after the Soviet probe. Soviet and U.S. spacecraft often arrived at their destinations at about the same time because interplanetary probes had to be launched during a short window of time when Earth and the target planet were in the right positions. When Mariner 5 passed behind Venus, its radio signal passed through the planet’s atmosphere. U.S. astronomers measured the strength of the returning signal to learn the density of the atmosphere. Venus has a surface atmospheric pressure one hundred times greater than that of Earth. The surface temperature was a scorching 800 degrees Fahrenheit. On Venus, carbon dioxide clouds retain the sun’s heat, creating a runaway greenhouse effect. Later Soviet Venera probes succeeded in reaching the surface of Venus and transmitting scientific readings, as well as a limited number of television pictures. Venera probes were also the first to orbit Venus in the 1970’s.

Mars

Early astronomers imagined that Mars might be covered with canals. Seasonal changes in surface color were thought to indicate the waxing and waning of vegetation. Mars also had white polar caps that changed in size as the seasons passed, just as the polar ice caps on Earth do. However, by the 1960’s, astronomers had concluded that Mars was probably a barren planet, though the planet still captured the imaginations of astronomers and science-fiction fans. Scientists later determined that the northern polar cap is mostly water ice, and the southern polar cap is mostly carbon dioxide ice.

On November 5, 1964, Mariner 3, the first flyby mission to Mars, failed when the shroud protecting the probe during ascent did not eject from the probe. A backup probe, Mariner 4, was launched less than a month later on November 28 with a redesigned shroud. Mariner 4 flew by Mars on July 15, 1965, coming within 6,200 miles of the surface. A television camera took 21 pictures. The murky pictures transmitted to Earth revealed a cratered surface much like that of Earth’s moon. By analyzing the strength of Mariner 4’s radio signals as they passed through the Martian atmosphere, scientists determined the surface pressure of the atmosphere. Their analysis showed the atmosphere to be very thin, with a surface pressure equal to less than 1 percent of Earth’s atmosphere at sea level. The atmosphere was discovered to be mostly carbon dioxide, with only a tiny amount of water vapor.

Mariner 6 and Mariner 7 (launched February 24 and March 27, 1969) passed by Mars on July 31, 1969, and August 5, 1969, respectively, coming as close to the planet’s surface as 2,100 miles. Mariner 6 returned 75 pictures, and Mariner 7 returned 126 pictures. Even with these pictures, only one-tenth of the planet’s surface had been photographed. The probes confirmed the atmosphere’s low pressure and determined that Mars did not possess a magnetic field. A temperature reading of the southern polar cap revealed the temperature to be -190 degrees Fahrenheit. Though the surface of Mars was cold and cratered, scientists still hoped that Mars had once possessed a denser atmosphere and that life had thrived then.

Impact

The Mariner space program, which continued into the 1970’s, entirely remade scientific understanding of Venus, Mars, and later Mercury. Although the Mariner program helped advance study in astronomy and other sciences, the primary motivation for the program was the space race between the United States and the Soviet Union. The competitiveness that marked the space race in the early 1960’s gradually gave way to cooperation between the superpowers’ space scientists as relations between the two countries improved. In 1971, the United States and the Soviet Union agreed to share the information gained by the Mariner 9 and Mars 2 and 3 missions.

Subsequent Events

In the 1970’s, probes continued to investigate Mars and Venus, then flew by Mercury, the planet closest to the Sun. Mariner 8 and Mariner 9 were twice the mass of earlier probes and contained additional scientific instruments. On May 8, 1971, the second stage of an Atlas-Centaur failed to ignite during ascent, causing Mariner 8 to crash into the sea. Mariner 9, launched on May 30, 1971, entered orbit around Mars on November 13, 1971. However, a great dust storm obscured the surface of the planet, and ground controllers had to reprogram Mariner 9 to wait about a month until the storm cleared. Among the 7,239 pictures returned were some that showed what looked like ancient dry riverbeds and runoff channels. This indicated that free-flowing water must have existed on Mars in the past. Among the many discoveries was Valles Marineris, a great canyon that stretched a quarter of the way around the planet.

Shortly after Mariner 9 reached Mars, Soviet probes Mars 2 and Mars 3 arrived, dropping descent modules to examine the surface before going into orbit around the planet. Both descent modules failed, and the Soviet probes circled the planet, operating on automatic programs and taking useless pictures of the dust storm.

Mariner 10, launched November 3, 1973, flew by Venus on February 5, 1974, then went into orbit around the Sun, passing Mercury, the planet closest to the Sun, three times, on March 29, 1974, September 21, 1974, and March 16, 1975. Mariner 10 found that Mercury possessed a slight magnetic field and no atmosphere. The first flyby took the probe within 435 miles of the surface of the planet’s dark side. The second flyby went past the sunlit side of the planet. The final encounter passed only 125 miles from the darkside. Pictures returned by the probe showed a surface covered with craters and ancient lava flows.

In 1976, two U.S. Viking orbiters reached Mars, and each released a lander. These landers returned pictures from the surface of the planet that revealed a red, rocky landscape. An onboard laboratory tested the Martian soil and failed to find traces of bacterial life. Spacecraft from the United States and the Soviet Union have continued to explore Venus and Mars. Two other programs, Pioneer and Voyager, explored the outer planets.

Additional Information

Journey into Space: The First Three Decades of Space Exploration (1989), by Bruce Murray, a scientist at the Jet Propulsion Laboratory; Exploring Space: Voyages in the Solar System and Beyond (1990), by William E. Burrows; and The Illustrated Encyclopedia of Space Technology: A Comprehensive History of Space Exploration (1981), by Kenneth Gatland, provide a closer look at Mariner and other exploratory programs. Duncan Brewer’s Mercury and the Sun (1992) takes a closer look at the planet Mercury, and William Sheehan’s The Planet Mars: A History of Observation and Discovery (1996) focuses on Mars.