Mariner program

• 1. Introduction

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  The Mariner program gave humanity its first close look at three worlds beyond Earth. Between 1962 and late 1973, NASA's Jet Propulsion Laboratory designed and built ten robotic probes that traveled to Venus, Mars, and Mercury. No spacecraft had ever successfully visited another planet before. The Mariners would change that, one flyby at a time.

  The total cost of the program came to approximately $554 million. For that sum, humanity received the first planetary flyby, the first photographs of another planet's surface from close range, the first spacecraft to orbit Mars, and the first use of gravity assist. Seven of the ten vehicles succeeded. The three that failed were lost on the launchpad or shortly after liftoff, not in the void of space.

  The questions this program was built to answer were audacious for the early 1960s. What does Venus look like beneath its clouds? Is the surface of Mars anything like the Moon? Could a spacecraft use one planet's gravity to slingshot toward another? The Mariners were sent to find out, and what came back reshaped everything scientists thought they knew about the inner Solar System.

• 2. Naming the Fleet

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  In May 1960, JPL engineer Edgar M. Cortright suggested that NASA's new planetary mission probes be "patterned after nautical terms, to convey the impression of travel to great distances and remote lands." That single naming decision rippled outward. The same logic gave names to the Ranger, Surveyor, and Viking programs as well.

  The choice of a nautical theme was more than branding. JPL had been conducting mission studies since 1960, looking at how to build small-scale spacecraft that could visit the nearest planets frequently and cheaply. The soon-to-be-available Atlas launch vehicles made that ambition plausible. So did the expanding Deep Space Instrumentation Facility, a global network of ground stations being developed to keep spacecraft in contact even at interplanetary distances. It was later renamed the Deep Space Network.

  Each Mariner was designed around a practical philosophy. Solar panels pointed at the Sun to generate power; a dish antenna pointed at Earth to maintain contact. Scientific instruments rode along to study the target planet, some needing to point precisely at the surface and some working passively to measure magnetic fields and charged particles in all directions. Crucially, JPL engineers decided to make the Mariners "three-axis-stabilized," meaning they would not spin in flight as other probes did. That gave instrument teams fine pointing control the mission science depended on.

• 3. First Contact with Venus

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  On the 22nd of July 1962, Mariner 1 lifted off from Cape Canaveral. Approximately five minutes after liftoff, the Air Force range safety officer destroyed it. Its Atlas-Agena rocket had veered off course, and the standing protocol left no other option. The Venus mission that JPL had spent years preparing came down to a single backup spacecraft.

  Mariner 2, launched on the 27th of August 1962, began a three-and-a-half-month journey toward Venus. It was a copy of the destroyed Mariner 1, but it would write a new chapter in exploration. Along the way it made the first direct measurement of the solar wind, confirming the existence of a constant outflow of charged particles streaming from the Sun. It also found interplanetary dust to be far scarcer than scientists had predicted, and it detected energetic particles from several solar flares as well as cosmic rays originating from outside the Solar System entirely.

  When Mariner 2 swept past Venus on the 14th of December 1962, it scanned the planet using infrared and microwave radiometers. Because Venus is wrapped in bright, opaque clouds that hide the surface completely, the spacecraft carried no camera. What it found with those radiometers was striking anyway. The clouds were cool, but the surface of Venus beneath them was extremely hot. Mariner 2 had become the first spacecraft to fly by another planet, and it had brought back the first direct data about a world other than Earth or the Moon. It now drifts in a heliocentric orbit, no longer in contact with Earth.

• 4. Mars Up Close

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  Mariner 3 launched on the 5th of November 1964, but the shroud meant to protect it during ascent failed to open and the probe never reached Mars. Three weeks later, on the 28th of November 1964, its twin Mariner 4 tried again and succeeded. On the 14th of July 1965, it flew past Mars and collected the first close-up photographs ever taken of another planet.

  The pictures were stored on a small onboard tape recorder and played back slowly over a long period. What they revealed was a cold, cratered landscape. Lunar-type impact craters dominated the surface, some of them dusted with frost in the Martian evening. Scientists had long speculated about what lay beneath the Martian surface features visible from Earth; the close-range images offered no canals, no sign of life-friendly terrain. Mars, at least in the regions Mariner 4 photographed, looked far more like the Moon than like Earth.

  Mariner 4 outlived its expected mission of roughly eight months and survived for about three years in solar orbit. During that time it made coordinated solar wind measurements alongside Mariner 5, which JPL launched toward Venus in 1967. That collaboration between probes operating in different parts of the inner Solar System was itself a first. Mariner 4 eventually lost contact after being bombarded by micrometeoroids and now drifts as a derelict in heliocentric orbit.

• 5. Mapping the Red Planet

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  Mariners 6 and 7 flew to Mars together in 1969, launched in February and March of that year respectively. Identical in design, they weighed 413 kilograms each and carried wide-angle and narrow-angle cameras alongside infrared and ultraviolet spectrometers. They crossed over the equator and southern hemisphere of Mars, analyzing the atmosphere and surface and returning hundreds of pictures.

  Both spacecraft happened to fly over heavily cratered regions. By bad luck, they missed the massive northern volcanoes and the enormous equatorial canyon that later missions would discover. Their approach images did show the dark surface features that observers on Earth had mapped for generations, but no canals. The view from close range stripped away that particular myth once and for all.

  Mariner 9, launched in May 1971, went one step further than any of its predecessors. It entered Martian orbit on the 14th of November 1971, becoming the first artificial satellite of any planet other than Earth. Its launch mass was nearly 998 kilograms, nearly double what earlier Mariners had weighed, partly because of the onboard rocket propellant needed to slow it into orbit. When it arrived, Mars was in the grip of a global dust storm. The spacecraft, now programmable using onboard flight computers introduced since 1969, simply waited. When the dust settled, Mariner 9 compiled a global mosaic of high-resolution images. It also sent back the first close-up pictures of Mars's two small moons, Phobos and Deimos. Mariner 9 was deactivated 516 days after entering orbit and remains in Areocentric orbit.

• 6. Mercury and Gravity Assist

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  Mariner 10 launched on the 3rd of November 1973, the last Mariner ever to fly. Its destination was Mercury, the innermost planet, but reaching it required a trick no spacecraft had ever attempted at a planet. Mission designers sent the probe first to Venus, then used Venus's gravitational pull to bend the spacecraft's path and fling it onward toward Mercury. This was the first operational use of a gravity assist maneuver.

  At Venus, Mariner 10 gained speed and changed course without burning any onboard fuel. It then became the first spacecraft ever to encounter two planets in a single mission. At Mercury, a second fortuitous property of the gravity assist geometry meant the spacecraft returned at six-month intervals, making repeated close mapping passes over the planet's surface. Across those passes it photographed roughly half the globe. Mercury's slow rotation meant the other half was always in darkness when Mariner 10 swung back around, leaving it unphotographed.

  For 33 years after Mariner 10's final Mercury encounter, no other spacecraft photographed Mercury at close range. The images it returned remained the primary visual record of the planet's surface until NASA's MESSENGER mission arrived decades later. Mariner 10 now orbits the Sun as a defunct probe, still out there in the dark.

• 7. Legacy in the Outer Solar System

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  The Mariner program's influence did not stop at Mercury. In 1972, NASA approved the Mariner Jupiter-Saturn program after canceling the far more ambitious Grand Tour, which had proposed sending multiple spacecraft to visit all of the outer planets. The scaled-back replacement called for two Mariner-derived probes to fly past only Jupiter and Saturn.

  JPL engineers, however, built those spacecraft with more in mind. Trajectory planners selected flight paths that would allow one probe to swing past Saturn's moon Titan, whose substantial atmosphere made it a scientific priority. The other probe was aimed so that, if the first mission succeeded at Titan, it could continue onward to Uranus and Neptune. Just before launch in 1977, the program's name changed to Voyager. Voyager 1 completed its Titan flyby as planned, freeing Voyager 2 to extend its journey to the two ice giants.

  The Viking program's Mars orbiters were enlarged versions of the Mariner 9 spacecraft. Later probes including Galileo and Magellan drew on Mariner heritage as well. The second-generation Mariner Mark II series evolved into the Cassini-Huygens mission that later reached Saturn. A program that began with a 203-kilogram probe launched toward Venus in 1962 laid the engineering foundation for spacecraft that would reach the edge of the Solar System and beyond.