An advanced spacecraft streaking towards the most distant planetary encounter to date finally reaches its destination. As the spacecraft nears its closest approach to its target, it executes a preprogramed sequence of observations during a brief encounter using a suite of capable scientific instruments including a state-of-the-art digital imaging system that provides the best images ever of this distant world. After the frenzy of events surrounding the closest encounter, the probe continues on into deep space and starts transmitting its findings back to Earth over the weeks that follow.

This almost sounds like a description of the encounter of NASA’s New Horizon spacecraft with the dwarf planet, Pluto. But it also perfectly summarizes events that took place exactly a half of a century earlier to the day when Mariner 4 reached the planet Mars. While a veritable fleet of spacecraft are currently studying Mars from orbit and its surface, in July 1965 we had yet to reach the Red Planet despite numerous unsuccessful attempts over the previous five years. Much as New Horizons has done with Pluto, Mariner 4 provided our first close up look of a world that had only been observed with difficulty using Earth-based telescopes forever changing our view of our near neighbor.

The Mariner-Mars Spacecraft

The Mariner-Mars 1964 project was officially approved by NASA in early November 1962 – over a month before NASA’s first planet-bound spacecraft, Mariner 2, successfully encountered Venus on December 14 as part of the earlier Mariner-Venus 1962 project. While originally NASA wished to send advanced one-ton spacecraft to Mars using the Atlas-Centaur launch vehicle, the Centaur’s continuing development issues meant that it would not be available for the 1964 Mars launch window (see “The Launch of Atlas-Centaur 5”). Just as they did with the Mariner-Venus missions, NASA would instead be forced to rely on a much lighter weight spacecraft like Mariner 2 which was originally derived from the Ranger lunar spacecraft (see “The Prototype That Conquered The Solar System“). Like the Ranger and earlier Mariner-Venus spacecraft, the Mariner-Mars spacecraft would be the responsibility of Caltech’s Jet Propulsion Laboratory (JPL) in Pasadena, California.

For the Mariner-Mars 1964 project, NASA decided to launch a pair a spacecraft, designated Mariner C and D, to perform observations of Mars during a flyby including securing the first close up images of its surface. In order to mitigate potential schedule slips and improve the reliability of the spacecraft, a new spacecraft built using as much proven technology as possible was designed. The heart of the Mariner-Mars spacecraft consisted of an octagonal magnesium frame 1.38 meters across and 0.46 meters tall. Seven of the bays supported by this framework housed electronics and power supplies for the spacecraft’s various systems while the eighth held a monopropellant, 220-newton course correction engine. Capable of making two burns with a total delta-v of 81 meters/second, the hydrazine propellant for this engine was stored in a tank in the center of the spacecraft.

Power for the Mariner spacecraft was supplied by a quartet of 1.81-by-0.90-meter solar panels covered by a total of 7,056 solar cells. These panels supplied 310 watts of electricity when at Mars to power the spacecraft’s systems and charge a bank of silver-zinc batteries with a total storage capacity of 1,200 watt-hours. Two redundant sets of a half dozen nitrogen gas thrusters mounted on the ends of the solar panels provided attitude control for the three-axis stabilized probe. Also mounted on the ends of the solar panels were experimental 0.16-square meter solar pressure vanes that could be steered to control the spacecraft’s attitude using the force of sunlight reflecting off of their surfaces. Attitude references were supplied by Sun and star sensors along with gyroscopes to be used during mid-course maneuvers.

Mounted on top of the spacecraft bus was an elliptical 1.17-by-0.53 meter parabolic high gain antenna. When Mariner’s star sensor was locked onto the bright star Canopus, which is conveniently located near the south ecliptic pole, the fixed-positon high gain antenna was oriented to produce a long and narrow beam pattern for the S-band transmissions focused on the ecliptic plane where Earth would be during the mid- to late-July 1965 encounter with Mars and subsequent post-encounter data transmission. A broad-beam low gain antenna was mounted on the top of a 2.2-meter tall mast on the top of the spacecraft which permitted communication when the spacecraft was not pointing the high gain antenna towards the Earth.

The primary instrument for the Mariner-Mars mission was a camera system to secure the first close up images of the Red Planet. Earlier television systems flown on NASA’s Ranger lunar missions streamed live television images back to Earth 385,000 kilometers away in an analog format using a high powered transmitter (see “The Mission of Ranger 7”). Unfortunately this approach could not be used by Mariner given the power restrictions and much greater 217 million kilometer distance to Mars during the encounter which resulted in too weak of a signal for real-time analog television transmissions to be practical. Soviet engineers solved this problem on their early interplanetary spacecraft by using highly advanced photo-television systems that recorded images on film that was subsequently developed automatically then slowly scanned one line at a time with the data transmitted back to Earth using powerful pulsed C-band transmissions through a two-meter high gain antenna (see “You Can’t Fail Unless You Try: The Soviet Venus & Mars Missions of 1962“). Given the limits of their spacecraft, engineers at JPL instead opted for a imaging system that could transmit data in a digital format which was less susceptible to noise. This would be the first digital imaging system to be used beyond the Earth.

The imaging system on the Mariner-Mars spacecraft employed a single slow-scan vidicon camera fitted with a 305 mm focal length, f/8 Cassegrain telescope yielding a 1.05° field of view. The images were exposed through alternating red and green filters onto the camera’s vidicon tube and then slow scanned over the course of 24 seconds. The analog signal from the camera was converted into a digital format yielding a 200×200 pixel image with the brightness values encoded into a 6-bit word for each pixel. An automatic gain control system ensured that each image had a minimum of five brightness levels recorded regardless of the scene illumination and contrast. The 240,000 bits for each image were then recorded onto a digital tape recorder which had a 21-image storage capacity on its 100 meters of tape. While this seems primitive by today’s standards, this was cutting edge technology in 1964.

At a nominal 11,000 kilometer range, these images were about 200 kilometers on a side with a one-kilometer pixel footprint. Because of the limitations of the imaging system, no “far encounter” imagery was possible and only about 1% of the Martian surface would be imaged – sufficient to get at least a sample of the appearance of the surface of Mars. After the encounter, the digital images were transmitted back to Earth at the glacial rate of just 8⅓ bits per second. Including engineering data transmitted along with the images, each frame required about ten hours to transmit back to the Earth using the spacecraft’s 10.5-watt S-band transmitter.

The camera was mounted on a moveable platform on the underside of the spacecraft fitted with wide and narrow-angle sensors. As Mariner approached Mars, the platform would slew until the wide angle sensor detected the planet with the narrow angle sensor providing the optimum pointing for the imaging session. The images were then acquired with the scan platform in a fixed position with the motion of the spacecraft allowing a swath of images to be taken.

Originally the scan platform was also to carry either a infrared spectrometer or a three-channel ultraviolet radiometer to study Mars’ atmosphere. These instruments were eventually dropped as a result of delays in their development. Instead, the science team developed an occultation experiment that measured how Mariner’s S-band transmissions were altered by the atmosphere as the spacecraft passed behind Mars during the encounter. This allowed the temperature, pressure and electron concentration of the atmosphere to be measured as a function of altitude without the need of additional instrumentation. Mariner’s 16-kilogram instrument complement was rounded out by sensors to measure micrometeoroids, radiation and magnetic fields during the interplanetary cruise and in the vicinity of Mars.

Getting Underway

Launching a pair of spacecraft like had done for the 1962 Venus opportunity provided some measure of insurance against launch vehicle or spacecraft failures to improve the chances that at least one Mariner spacecraft would survive to reach Mars. Even with advancements in spacecraft systems design since the Mariner-Venus mission, the Mars mission requirements pushed the mass of the Mariner-Mars spacecraft up to 261 kilograms. This exceeded the payload capability of the Atlas-Agena B rocket used to launch the 204-kilogram Mariner 2 by about 45 kilograms for the 1964 Mars launch window.

In order to get the needed performance, the General Dynamics Atlas D was mated with a modified version of the improved Agena D built by Lockheed Space and Missile Company (which, after decades of corporate mergers, is now part of the aerospace giant Lockheed Martin along with what was General Dynamics Space Systems Division). This would be the first time NASA would use the Atlas-Agena D for one of its missions. Since the Ranger-style nose shroud used by Mariner 2 was too small to accommodate the Mariner C and D spacecraft with their larger solar panels and new antenna configuration, a new fiberglass shroud was developed for use on this and NASA’s future Atlas-Agena D missions.

For the first launch attempt, Mariner C, Atlas 289D and Agena 6931 lifted spacecraft serial number MC-2 from Launch Complex 13 at Cape Kennedy on November 5, 1964 at 19:22:05 GMT. While the launch appeared to be nominal, initial tracking showed that Mariner 3, as it was now called, was travelling too slowly to reach Mars. Apparently the new launch shroud had failed to separate from the Agena D during ascent sending the spacecraft off course and preventing it from deploying its solar panels. The spacecraft’s batteries were exhausted less than nine hours after launch before corrective action could free it – the mission was a total loss. A crash program was immediately started to determine the cause of the failure and fabricate an all-metal replacement shroud for Mariner D before the close of the Mars launch window at the end of November (for a complete description of the launch shroud design, the Mariner 3 failure and the effort to recover in time to launch Mariner 4, see “The Launch of Mariner 3”).

After a herculean effort by NASA and its contractors, a new launch shroud was delivered to Cape Kennedy on November 22, 1964. Modifications were made to the Atlas 288D/Agena 6932 launch vehicle to improve its performance and extend the launch window for Mariner D to December 3 even with the new, heavier shroud. Mariner spacecraft serial number MC-3 was finally launched from LC-12 at 19:22:01 GMT on November 28, 1964. The new shroud worked as intended this time with the Mariner spacecraft entering Earth orbit after a brief burn of the Agena D upper stage. After a 32-minute coast in a temporary 172 by 184-kilometer parking orbit, the Agena D fired its engine for a second time. The 95-second burn increased the velocity of the spacecraft to 11.50 kilometers/second and Mariner 4 was on its way to Mars.

Two days later, the Soviet Union launched Zond 2 towards Mars generating speculation that its was in competition with Mariner to reach Mars. It was only recently revealed that Zond 2 was instead launched on an engineering test flight towards Mars with little expectation of actually reaching its target in operating condition. In the event Zond 2 managed to beat the odds, it would be redirected to impact the Martian surface to deliver commemorative pennants. In the end, Zond 2 experienced a series of malfunctions and was officially declared lost by Soviet authorities on May 5, 1965 – three months before it was scheduled to reach Mars (see “The Soviet Zond Missions of 1963-65: Planetary Probe Test Flights”). Mariner 4 was on its own.

Initial tracking of Mariner 4 showed that it would miss Mars by 246,378 kilometers. The spacecraft had been purposely aimed away from Mars to minimize the chances that its unsterilized Agena D upper stage would impact Mars and contaminate it. At 16:09 GMT on December 5, Mariner 4 fired its course correction engine for 20 seconds at a distance of 2.034 million kilometers from the Earth to change its velocity by 17.3 meters per second. This single burn was enough to place Mariner 4 on course for a flyby 9,600 kilometers above the surface of Mars. The aim point was carefully selected to avoid passing into the shadow of Mars or block Mariner’s view of Canopus while at the same time pass behind Mars as viewed from Earth while avoiding the moons Phobos or Deimos during the photography session.

The Encounter with Mars

During the 228-day cruise to Mars, Mariner 4 was in nearly constant contact with ground controllers on the Earth returning a wealth of scientific data along with engineering information in its stream of telemetry. During the interplanetary cruise, Mariner’s instruments recorded the effects of 20 solar flares and the impact of 235 dust particles. Despite the overall success of this part of the mission, there were some problems. The Canopus star sensor was twice distracted by sunlight reflecting off of insulation shed by Mariner 4 in December requiring a couple of days for the sensor to lock back onto its guiding star. The ionization chamber experiment, designed primarily to record galactic cosmic rays, failed in February 1965 after a strong solar flare while problems with the solar plasma probe power supply starting on December 6 rendered half of the data being returned by this instrument unusable. In addition, the actuators on two of the four solar pressure vanes stopped functioning only 16 hours after launch and it was found that the forces generated by them were too small to be of much practical use.

Despite the issues, Mariner 4 continued towards its destination in good condition. In order to avoid a repeat of problems with the star sensor from sunlit particles shed by the spacecraft, the protective cover over the camera was opened a full five months before the flyby instead of just days before it. Originally it was hoped that Mariner 4 could image what is today called Syrtis Planum but the requirement that the spacecraft be visible to NASA’s primary tracking station in Goldstone, California to observe important flyby milestones and the occultation forced observations far to the east.

Late on July 14, 1965 the encounter sequence was started with the imaging system and its recorder powered up. After the scan platform slewed and acquired Mars, the 25-minute long imaging sequence started at 00:18 GMT on July 15 at a range of 17,600 kilometers. The first image captured the limb of Mars at 37° north latitude in Amazonis Planitia and swept south and eastward towards Terra Sirenum at 55° south latitude just past the terminator at a range of 12,000 kilometers. In all, 21 images and part of a 22nd were recorded by the end of the imaging session. Mariner 4 reached its closest point of 9,846 kilometers from Mars at 1:01 GMT (July 14 at 6:01 PM PDT at Mariner’s control center at JPL). An hour and a half later, Mariner 4 passed behind Mars as viewed from Earth and reappeared 54 minutes later. Mariner 4 had successfully completed the first close encounter with the Red Planet.

About 8½ hours after emerging from behind the Red Planet, Mariner 4 began it long playback of digital data from its tape recorder. The first image clearly showed the limb of Mars indicating that the camera system had operated as intended. Over the course of the next several days, more and more images were returned first showing indistinct light and dark markings. But before long, the images clearly began to show impact craters. Although many of them had been modified by erosion, they gave the impression that Mars was more Moon-like than Earth-like. A total of 70 craters in various states of preservation ranging from 5 to 120 kilometers in diameter were observed but there were no signs of the “canals” that so many astronomers had reported observing over the previous century.

Data from the other instruments indicated that Mars, like the Moon, also had no magnetic field or trapped radiation belts. Finally the occultation experiment confirmed recent observations from Earth that Mars had a thinner atmosphere than previously believed with a surface pressure maybe in the 10 to 20 millibar range (for a full discussion of the changing view of Mars’ atmosphere in the early 1960s, see “Zond 2: Old Mysteries Solved & New Questions Raised”). Later ground-based observations combined with the Mariner results drove down these estimates closer to the 6 millibar value accepted today. After decades of speculation, the widely held belief that Mars was an Earth-like abode for life had been squashed by the equivalent of less than a megabyte of data. Planetary scientists would not realize for almost another decade that, by chance, Mariner 4 had imaged some of the oldest and most cratered terrain on Mars missing geologically younger and more interesting features that may have shaped perceptions of the Red Planet differently.

Post Encounter and Beyond

In order to calibrate and understand better the images Mariner 4 had acquired, the camera recorded 11 additional images of black space well after the encounter and subsequently transmitted five of them back to Earth. No degradation of the imaging system was noted and the results confirmed that the bright feature seen above the limb of Mars in the first image was not an artifact, but was real indicating thin clouds or dust in the atmosphere. After the last telemetry of the encounter were received on October 1, 1965, Mariner 4 continued on its new 1.107 by 1.561 AU solar orbit moving behind the Sun and out of touch with the Earth at a range of 305 million kilometers.

Contact with Mariner 4 was reestablished in March 1966 using the newly commissioned 64-meter DSS-14 tracking antenna at Goldstone and continued intermittently into early 1967 as the probe continued in its solar orbit far from Earth. Continuous contact was finally reestablished in July 1967 as Mariner’s orbit carried it within 50 million kilometers of the Earth. As an engineering test, parts of images 16 and 17 were retransmitted back to Earth showing no degradation of the tape recorder. On September 15, Mariner 4 passed through a meteoroid shower 47.6 million kilometers from the Earth. The 17 hits recorded by the probe’s detector might have come from the long-lost periodic comet known as D/1895Q1 Swift whose remains were an estimated 20 million kilometers away. Additional recorded hits and an attitude perturbation suggests that Mariner 4 passed through the edge of the Leonid meteor stream in December.

In October 1967, Mariner 4 was used in some attitude control system tests in support of the Mariner 5 mission which was using a modified version of the Mariner-Mars backup spacecraft to flyby Venus (see “The Return to Venus: The Mission of Mariner 5“). On October 26, Mariner’s course correction engine was fired for 70 seconds for a 62 meter/second delta-v after almost three years in space. On November 22, the camera system imaged black space once again as a further test of the imaging system. Finally, on December 7, the spacecraft’s attitude control gas supply was exhausted. Unable to keep its solar panels pointed at the Sun to recharge its batteries, contact with Mariner 4 was finally lost on December 21, 1967 ending the most successful planetary mission to date.

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Related Video

Here is a NASA-produced documentary from 1965 about the Mariner 4 mission entitled “Men Encounter Mars”.

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Here is a JPL video from their ongoing von Karman Lecture Series entitled “1965: Discovery at Mars” from July 16, 2015.

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Related Reading

“The Launch of Mariner 3”, Drew Ex Machina, November 5, 2014 [Post]

“The Prototype That Conquered the Solar System”, Drew Ex Machina, September 15, 2015 [Post]

“What If Mariner 3 Had Reached Mars?”, Drew Ex Machina, July 17, 2015 [Post]

“Zond 2: Old Mysteries Solved & New Questions Raised”, Drew Ex Machina, July 17, 2014 [Post]

“The Soviet Zond Missions of 1963-65: Planetary Probe Test Flights”, Drew Ex Machina, April 18, 2019 [Post]

General References

Michael M. Mirabito, The Exploration of Outer Space with Cameras, McFarland, 1983

Paolo Ulivi with David M. Harland, Robotic Exploration of the Solar System Part 1: The Golden Age 1957 – 1982, Springer-Praxis, 2007

Mariner Mars 1964 Project Report: Mission and Spacecraft Development Volume I. From Project Inception Through Midcourse Maneuver, JPL Technical Report No. 32-740, NASA, March 1, 1965

Mariner-Mars 1964 Final Project Report, SP-139, NASA, 1967