Alone in the Darkness: Mariner 4 to Mars, 50 Years Later

July 14 marks 50 years of visual reconnaissance of the solar system by NASA's Jet Propulsion Laboratory (JPL), beginning with Mariner 4's flyby of Mars in 1965.

Among JPL's first planetary efforts, Mariners 3 and 4 (known collectively as "Mariner Mars") were planned and executed by a group of pioneering scientists at Caltech in partnership with JPL. NASA was only 4 years old when the first Mars flyby was approved in 1962, but the core science team had been working together at Caltech for many years. The team included Caltech faculty Robert Sharp (after whom Mount Sharp, the main target of the Mars rover Curiosity, is named) and Gerry Neugebauer, professor of geology and of professor of physics, respectively; Robert Leighton and H. Victor Neher, professors of physics; and Bill Pickering, professor of electrical engineering, who was the director of JPL from 1954–1976. Rounding out the Caltech contingent was a young Bruce Murray, a new addition to the geology faculty, who would follow Pickering as JPL director in 1976.

"The Mariner missions marked the beginning of planetary geology, led by researchers at Caltech including Bruce Murray and Robert Sharp," said John Grotzinger, the Fletcher Jones Professor of Geology and chair of the Division of Geological and Planetary Sciences. "These early flyby missions showed the enormous potential of Mars to provide insight into the evolution of a close cousin to Earth and stimulated the creation of a program dedicated to iterative exploration involving orbiters, landers, and rovers."

By today's standards, Mariner Mars was a virtual leap into the unknown. NASA and JPL had little spaceflight experience to guide them. There had been just one successful planetary mission—Mariner 2's journey past Venus in 1962—to build upon. Sending spacecraft to other planets was still a new endeavor.  

The Mariner Mars spacecraft were originally designed without cameras. Neugebauer, Murray, and Leighton felt that a lot of science questions could be answered via images from this close encounter with Mars. As it turned out, sending back photos of the planet that had so long captured the imaginations of millions had the added benefit of making the Mars flyby more accessible to the public.

Mariner 3 launched on November 5, 1964. The Atlas rocket that boosted it clear of the atmosphere functioned perfectly (not always the case in the early years of spaceflight), but the shroud enclosing the payload failed to fully open and the spacecraft, unable to collect sunlight on its solar panels, ceased to function after about nine hours of flight.

Mariner 4 launched three weeks later on November 28 with a redesigned shroud. The probe deployed as planned and began its journey to Mars. But there was still drama in store for the mission. Within the first hour of the flight, the rocket's upper stage had pushed the spacecraft out of Earth orbit, and the solar panels had deployed. Then the guidance system acquired a lock on the sun, but a second object was needed to guide the spacecraft. This depended on a photocell finding the bright star Canopus, which was attempted about 15 hours later. During these first attempts, however, the primitive onboard electronics erroneously identified other stars of similar brightness.

Controllers managed to solve this problem but over the next few weeks realized that a small cloud of dust and paint flecks, ejected when Mariner 4 deployed, was traveling along with the spacecraft and interfering with the tracking of Canopus. A tiny paint chip, if close enough to the star tracker, could mimic the star. After more corrective action, Canopus was reacquired and Mariner's journey continued largely without incident. This star-tracking technology, along with many other design features of the spacecraft, has been used in every interplanetary mission JPL has flown since.

At the time, what was known about Mars had been learned from Earth-based telescopes. The images were fuzzy and indistinct—at its closest, Mars is still about 35 million miles distant. Scientific measurements derived from visual observations of the planet were inexact. While ideas about the true nature of Mars evolved throughout the first half of the 20th century, in 1965 nobody could say with any confidence how dense the martian atmosphere was or determine its exact composition. Telescopic surveys had recorded a visual event called the "wave of darkening," which some scientists theorized could be plant life blooming and perishing as the harsh martian seasons changed. A few of them still thought of Mars as a place capable of supporting advanced life, although most thought it unlikely. However, there was no conclusive evidence for either scenario.

So, as Mariner 4 flew past Mars, much was at stake, both for the scientific community and a curious general public. Were there canals or channels on the surface, as some astronomers had reported? Would we find advanced life forms or vast collections of plant life? Would there be liquid water on the surface?

Just over seven months after launch, the encounter with Mars was imminent. On July 14, 1965, Mariner's science instruments were activated. These included a magnetometer to measure magnetic fields, a Geiger counter to measure radiation, a cosmic ray telescope, a cosmic dust detector, and the television camera.

About seven hours before the encounter, the TV camera began acquiring images. After the probe passed Mars, an onboard data recorder—which used a 330-foot endless loop of magnetic tape to store still pictures—initiated playback of the raw images to Earth, transmitting them twice for certainty. Each image took 10 hours to transmit.

The 22 images sent by Mariner 4 appeared conclusive. Although they were low-resolution and black-and-white, they indicated that Mars was not a place likely to be friendly to life. It was a cold, dry desert, covered with so many craters as to strongly resemble Earth's moon. The atmospheric density was about one-thousandth that of Earth, and no liquid water was apparent on the surface.

When discussing the mission during an interview at Caltech in 1977, Leighton recalled viewing the first images at JPL. "If someone had asked 'What do you expect to see?' we would have said 'craters'…[yet] the fact that craters were there, and a predominant land form, was somehow surprising."

Leighton also recalled a letter he received from, of all people, a dairy farmer. It read, "I'm not very close to your world, but I really appreciate what you are doing. Keep it going." Leighton said of the sentiment, "A letter from a milkman…I thought that was kind of nice."

After its voyage past Mars, Mariner 4 maintained intermittent communication with JPL and returned data about the interplanetary environment for two more years. But by the end of 1967, the spacecraft had suffered tens of thousands of micrometeoroid impacts and was out of the nitrogen gas it used for maneuvering. The mission officially ended on December 21.

"Mariner 4 defined and pioneered the systems and technologies needed for a truly interplanetary spacecraft," says Rob Manning (BS '81), JPL's chief engineer for the Low-Density Supersonic Decelerator and formerly chief engineer for the Mars Science Laboratory. "All U.S. interplanetary missions that have followed were directly derived from the architecture and innovations that engineers behind Mariner invented. We stand on the shoulders of giants."