Mariner Missions Conduct Mars Flybys Summary

  • Last updated on November 10, 2022

The Mariner flybys of Mars dashed dreams of Martian canals and ruined cities and replaced them with images of a barren, more lunar-like surface. Mariner 4, the first U.S. robotic probe to Mars, provided data on the red planet’s radiation density and cosmic-ray and cosmic-dust bombardment, and also provided data on the properties of the Martian atmosphere. Mariners 6 and 7 showed detailed images of the physical geography of Mars. Overall, the Mariner missions radically altered the images humans had of the solar system and its planets.

Summary of Event

Robert B. Leighton became fascinated with Mars when it came unusually close to Earth in 1956. Using the 60-inch telescope at Mount Wilson in the San Gabriel Mountains above Los Angeles, he became an expert on photographing Mars. Five years later, when the National Aeronautics and Space Administration National Aeronautics and Space Administration;Mariner program (NASA) solicited proposals for instruments to be flown on the Mariner Mars flyby missions, Leighton saw it as a priceless opportunity to photograph Mars up close. NASA accepted his proposal to include television cameras, and he became part of the Mariner team. Mariner program Space program, U.S.;Mariner program Mars (planet) [kw]Mariner Missions Conduct Mars Flybys (July 14, 1965-Aug. 5, 1969) [kw]Mars Flybys, Mariner Missions Conduct (July 14, 1965-Aug. 5, 1969) Mariner program Space program, U.S.;Mariner program Mars (planet) [g]North America;July 14, 1965-Aug. 5, 1969: Mariner Missions Conduct Mars Flybys[08430] [g]United States;July 14, 1965-Aug. 5, 1969: Mariner Missions Conduct Mars Flybys[08430] [c]Space and aviation;July 14, 1965-Aug. 5, 1969: Mariner Missions Conduct Mars Flybys[08430] [c]Science and technology;July 14, 1965-Aug. 5, 1969: Mariner Missions Conduct Mars Flybys[08430] [c]Astronomy;July 14, 1965-Aug. 5, 1969: Mariner Missions Conduct Mars Flybys[08430] Schiaparelli, Giovanni Virginio Leighton, Robert B. Van Allen, James

The mission of the Mariner series was to explore the inner planets and their space environments: Mariners 1 and 2 were to be Venus flybys; Mariners 3, 4, 5, 6, and 7 were to be Mars flybys; Mariners 8 and 9 were to be Mars orbiters; and Mariner 10 was to fly by both Venus and Mercury. Since this undertaking was in its infancy, NASA sent two spacecraft on most missions in case one failed. This proved to be a wise precaution given that Mariners 1, 3, and 8 did fail.

A radio telescope observation suggested that Mars might have Van Allen radiation belts Van Allen radiation belts[Vanallen radiation belts] (regions of trapped particle radiation above its atmosphere) one hundred times more intense than the Van Allen belts surrounding Earth. If this observation proved to be true, the operation of spacecraft near Mars would be severely limited. Space scientist James Van Allen, the leading expert on radiation belts, would design the instruments Mariners 3 and 4 used to search for the belts.

Mariner 3 was launched on November 5, 1964, but a fiberglass shroud failed to eject and the extra weight prevented the craft from reaching its proper orbit to Mars. Mariner 4 was launched three weeks later on November 28 with a 40-kilogram (88-pound) scientific payload. It flew by Mars on July 14, 1965, after a journey of 228 days and 523 million kilometers (325 million miles). Three primary objectives of the mission were to measure conditions in space, to make close-up images of Mars, and to provide experience in building and managing long space flights. Data on Earth’s Van Allen belts began to come in 45 minutes after launch. This was followed by seven months of data on the solar wind, solar magnetic field, dust, and cosmic rays. Mariner recorded twenty-four solar events, those times when the solar wind density or solar magnetic field suddenly changed.

The public and even the more optimistic scientists were completely shocked by the Mariner 4 flyby pictures. They had hoped to see the Mars that grew from the telescopic observations of nineteenth century astronomer Giovanni Virginio Schiaparelli—a Mars of canals, vegetation, and perhaps ruined cities. Schiaparelli’s observations inspired Mariner 4 engineers to program the probe to photograph several of the supposed canals and the dark regions that might composed of vegetation (spectra taken from Earth disagreed on whether chlorophyll was present in those regions). However, the Mariner 4 pictures showed nothing even remotely resembling canals or stands of vegetation; instead, images showed a Moon-like world, barren and cratered.

The Mariner 4 spacecraft.


The news from the occultation experiment of Mars was no better. In this experiment, Mariner 4 passed behind Mars as seen from Earth. Analysis of its radio signals indicated that the Martian atmospheric pressure was only 0.5 percent of Earth’s atmospheric pressure—so low that surface water would be unstable and would quickly evaporate.

Furthermore, Van Allen’s experiment found no trapped radiation belts. This meant that, at best, Mars could have only a very weak magnetic field; in fact, no global field was detected. Without a thicker atmosphere and a stronger magnetic field to protect it, the surface would be bathed in ultraviolet light from the Sun along with high-energy particles from space, a potentially lethal combination for unshielded life. The good news was that orbiting spacecraft would not need to survive the radiation of super-powerful Van Allen belts.

Building on the successful flight of Mariner 4, Mariners 6 and 7 used the more powerful Centaur rocket for a second stage, allowing the probes to carry115-kilogram (253-pound) payloads. In an advance over the technology of Mariner 4, Mariners 6 and 7 had new telemetry systems for communication that were two thousand times faster, and they had central computers and sequencers that could be updated during flight. (A sequencer controls the time in which a spacecraft would conduct various tasks.) Because Mariner 6 photographs showed intriguing features near the pole, the update capacity was used to reprogram Mariner 7 so that it would take extra pictures of the pole. Compared with Mariner 4, the larger payload also allowed a 50 percent increase in solar cells, a doubling of transmitter power, and the inclusion of two high-resolution cameras, one for wide angle and one for close-ups. Mariner 4 had a single low-resolution camera.





Mariner 6 was nearly lost ten days before launch while sitting atop the twelve-story Atlas/Centaur rocket. A faulty relay opened a valve, and the Atlas rocket began to crumple as it lost internal pressure. Two of the ground crew, Billy McClure McClure, Billy and Charles Beverlin Beverlin, Charles , at great personal risk, rushed under the rocket and started pumps to repressurize the Atlas. Both crewmembers later received the NASA Exceptional Bravery Medal. Mariner 6 and the Centaur were moved to a different Atlas and launched on schedule on February 24, 1969. Mariner 7 followed with a launch on March 27. Mariner 6 flew by Mars on July 3, and its identical twin Mariner 7, taking a shorter path, flew by Mars on August 5.

Most of Mariner 4’s instruments measured conditions in space between Earth and Mars, but the instruments of Mariners 6 and 7 focused on the planet Mars. Atmospheric pressure at the surface was estimated to be about 6 or 7 millibars (atmospheric pressure at Earth’s surface is 1,013 millibars). Ultraviolet and infrared spectrometers showed that the atmosphere and the surface of the south-polar cap are predominantly carbon dioxide. The infrared radiometers found a temperature of 17 degrees Celsius (63 degrees Fahrenheit) near the equator and -123 degrees Celsius (-190 degrees Fahrenheit) at the south pole.

While Mariners 6 and 7 confirmed the findings of Mariner 4, they also brought some surprises. The bright circular region named Hellas, considered to be a plateau because clouds formed above it, turned out to be a huge impact basin. Air-pressure measurements showed that the albedo feature named Hellespontica Depressio was actually an elevated plateau instead of a depression. Hinting at discoveries to come, an area of “chaotic terrain” was photographed and identified as a possible region of melted permafrost, suggesting water might have been present in the past.


The reprogrammable computer and continuous telemetry pioneered with Mariners 6 and 7 were used on subsequent missions, including the Viking orbiters and landers. Mariner images also were used in the selection of the Viking lander sites. Tracking the three Mariners allowed the most accurate determination of the masses of Earth and Mars and of their orbits up to that time. Monitoring Mariner 7’s radio beacon as it passed behind the Sun after the Mars flyby provided the most stringent test up to that time of the general relativity effect that the Sun’s gravity should bend the path of light.

Before Mariner 4, Mars was considered somewhat Earthlike; after Mariner 4, Mars was considered moonlike. Mariners 6 and 7 showed that Mars had its own character, neither Earthlike nor moonlike. Unlike the lunar far side, where ancient craters reach saturation density, the oldest craters on Mars have been eroded away. The featureless plain of the giant Hellas basin suggests that it might have once been a sea bottom, and the relative absence of craters on the northern plains suggests the same thing. Mars was again becoming a fascinating world. Discovery of the giant Martian volcanos, and of the 5,000-kilometer- (3,000-mile-) long Vallis Marineris canyon would have to wait until the Mariner 9 orbiter, but the space-age exploration of Mars began with Mariner 4. Mariner program Space program, U.S.;Mariner program Mars (planet)

Further Reading
  • citation-type="booksimple"

    xlink:type="simple">Boyce, Joseph M. The Smithsonian Book of Mars. Washington, D.C.: Smithsonian Institution Press, 2002. An authoritative summary of the exploration of Mars, written for general readers at all levels.
  • citation-type="booksimple"

    xlink:type="simple">Godwin, Robert, ed. Mars, the NASA Mission Reports. Burlington, Ont.: Apogee Books, 2000. An edited collection of press kits for the various missions. A goldmine of information by one of the most prolific writers on the space age.
  • citation-type="booksimple"

    xlink:type="simple">Hartmann, William K. Moons and Planets. 5th ed. Belmont, Calif.: Brooks/Cole, 2005. Provides detailed information about all objects in the solar system. Suitable on three separate levels: high school students, general readers, and college undergraduates studying planetary geology.
  • citation-type="booksimple"

    xlink:type="simple">Hartmann, William K., and Odell Raper. The New Mars: The Discoveries of Mariner 9. NASA SP-337. Washington, D.C.: Government Printing Office, 1974. One of the best research works on the past, present, and future of Mars and Martian explorations. “Early Mariners and the Profile of the Mariner 9 Mission,” as well as the first two chapters of historic data, presents a sweeping view of Mars as the planet of human hopes and fantasies.
  • citation-type="booksimple"

    xlink:type="simple">James, J. N. “The Voyage of Mariner IV.” Scientific American, March, 1966, 42-52. Discusses the overcoming of problems to get Mariner 4 to Mars. Includes descriptions of the experiments carried by the probe.
  • citation-type="booksimple"

    xlink:type="simple">Leighton, Robert B. “The Photographs from Mariner IV.” Scientific American, April, 1966, 54-68. In-depth discussions of the pictures from Mariner 4 and Mariners 6 and 7.
  • citation-type="booksimple"

    xlink:type="simple">_______. “The Surface of Mars.” Scientific American, May, 1970, 27-41. Continues with in-depth discussions of the pictures from Mariner 4 and Mariners 6 and 7.
  • citation-type="booksimple"

    xlink:type="simple">Morrison, David, and Tobias Owen. The Planetary System. 3d ed. San Francisco: Addison Wesley, 2003. Organized by planetary object, this work provides contemporary data on all planetary bodies visited by spacecraft since the early days of the space age. Suitable for high school and college students and for general readers.
  • citation-type="booksimple"

    xlink:type="simple">Sheehan, William, and Stephen James O’Meara. Mars: The Lure of the Red Planet. Amherst, N.Y.: Prometheus Books, 2001. Delves deeply into humanity’s fascination with Mars, including Orson Welles’s 1938 radio broadcast version of the book by H. G. Wells, War of the Worlds. The final chapters cover the scientific journeys to Mars, broken down into the mission of each spacecraft.
  • citation-type="booksimple"

    xlink:type="simple">Sloan, Richard K. “The Scientific Experiments of Mariner IV.” Scientific American, May, 1966, 62-72. Examines what was learned about Mars from the instruments used on Mariner 4.

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Categories: History