On May 8, 1962 after being delayed no less than ten times, NASA launched the first cryogenic Atlas Centaur rocket, designed to carry the canceled Mariner A and B probes to their planetary targets. Sixty-three seconds after lift off the launcher exploded in the Cape Canaveral sky. In parallel with an investigation on the causes of the explosion, NASA started a complete redesign of its Solar System exploration initiative, first delaying the first Mariner B launch to 1966 and then canceling it.
For the 1964 Mars launch window NASA considered two different proposals: the first by NASA's Goddard Space Flight Center, the second by Caltech's Jet Propulsion Laboratory (JPL). The Goddard proposal consisted of a mother ship and an atmospheric probe, designed to collect some data on the Martian atmosphere and possibly land softly on the surface. The JPL proposal was called Mariner C and envisaged two twin probes with a similar mission to the Mariner R which flew by Venus in 1962: flying by the planet collecting a few essential data and some picture of the Martian surface. Each probe's mass was to be of some hundreds kilograms, to enable it to be launched by the usual Atlas-Agena. In deciding between the two proposals NASA searched for the one better suited to achieve two goals: developing technologies for the future landing missions and optimizing the use of the launch window to develop the necessary scientific instruments. The chosen probe was also to use as much existing technology as possible. The Goddard proposal, although better suited to achieve the first goal, had to face two main problems: affordablity, as its mission comprised too many critical events (capsule separation, atmospheric entry, parachute opening and landing) each one of which could have spelled disaster for the mission, and a technical and ethical problem, as no one knew how to completely sterilize the capsule in order to minimize the chances of Martian environment biological contamination. This event, in the words of NASA, would have been a "scientific catastrophe".
At the end of 1962, after considering the launch of a Mariner B probe on top of an unproven Saturn-Agena combination and despite every indication that the Soviets were aiming at landing a probe on Mars as soon as possible, NASA approved the JPL sponsored probe, using two twin Mariner C probes, similar to the much heavier (850 kg instead of between 200 and 350) Mariner B.
In contrast to the Mariner R, Mariner C probes were to be real interplanetary probes, instead of an adaptation of an existing lunar probe design. This could prevent some problem of the Mariner-2 flight during which e.g. the electronic boxes internal temperature rose to a value 20 degrees Celsius above the design one. Mariner C included the standard octagonal structure common to every JPL probe. This structure had a diameter of 138.4 cm and was 45.7 cm tall. Inside the eight bays were carried the 10.5 W power communication system, the sequencer, the batteries and the hydrazine tank used for the course correction manoeuvre. The course correction engine was similar to the Mariner-2 one, having the same 225 N thrust and being restartable twice. Its nozzle, using four movable vanes for thrust vectoring, was mounted inside one of the eight bays. The attitude control system used twelve cold nitrogen thrusters and a group of Sun sensors and star sensors, aiming to Canopus (alpha Carinae) a bright star of the southern sky chosen for its high angular separation from the Sun. Unlike Mariner-2, no Earth sensor was carried, as our planet would always be quite close to the Sun, as seen from the probe. To the central structure were mounted four solar panels of 6.5 square meters area providing up to 700 W of power. At their tip each panel carried the cold nitrogen attitude control thrusters and a 0.16 square meter area "solar sail". The total span of the probe was 6.79 meters. Above the octagonal structure was mounted the high gain antenna, shaped like a parabolic ellipsoid of 117 cm major axis and 53 cm minor axis. Unlike Mariner R, carring a fully articulated antenna, Mariner C's antenna was fixed in a position dictated by the relative positions of the probe, the Sun and Earth at the time of the fly-by. Other than the high gain antenna, usable only for a few months centered on the fly-by date, the probe carried a low gain antenna, mounted at the tip of a two meters long boom and, under the central structure, the two degree of freedom (azimuth and elevation) camera platform.
The total launch mass of each Mariner C was 261 kg, of which 15.5 were taken by the scientific instruments.
These were divided into two groups: the instruments needing pointing control to aim at the planet, mounted on the movable platform and those needing no precise pointing, mounted everywere on the probe. The first group comprised only a single instrument: a vidicon camera equipped with a carousel carrying two red and two green filters. The camera was coupled to a Cassegrain telescope of 38 mm aperture and 305 mm focal length and was able to snap a maximum of 22 pictures of the planet. The pictures would be recored on a tape before being relayed to Earth at a speed of 8 bytes per second. Each pictures took 24 seconds to snap and 8 hours 20 minutes for Earth relay. The pictures were taken in couples with a short time delay between each couple. On the same articulated platform an infrared spectrometer or a three channels ultraviolet radiometer was to have been mounted to search for traces of Martian vegetation. In the end, neither instrument was mounted on the probe. The other scientific instruments were a plasma sensor, collecting data during the whole flight, a radiation sensor, a cosmic ray and a micrometeorite one. Completing the scientific payload were an ion chamber and a three axis helium magnetometer to take readings of the interplanetary magnetic field and to measure a possible intrinsic Martian magnetic field. To protect both sensors from the interference of the metallic probe structure, these were mounted on the same boom carrying the low gain antenna. For this mission D. L. Cain of JPL invented a scientific experiment requiring no additional instrument: by planning the fly by so that the planet would occult each probe, imediately after the picture taking sequence and before image relay, the atmospheric structure could be investigated and models of the pressure and temperature profiles could be constructed by simply measuring the refraction of a simple S band carrier signal transmitted by the probe. Altough the mission engineers would have preferred not risking losing track of the probe for a whole hour during such a critical moment of its mission, the experiment was approved a few months before launch, as it was able to collect extremely precious data in a simple and economic way.

The first of two Mariner C probes, now renamed Mariner-3 and -4, was ready for launch in early November 1964. The probe lifted off on November 5 in the afternoon, was briefly put into a parking orbit and finally launched towards Mars by the second ignition of the Agena stage. One hour after launch, contact was established with the outbound probe, but the telemetry clearly showed that something was wrong: the instruments and electronics were on but the solar panels were still in their stowed position and thus were not generating any power. It was soon established that the launcher aerodynamic shroud never deployed and was still enclosing the ill-fated probe. Because of the added mass, the stack could not be sent into an interplanetary orbit intersecting the orbit of Mars. Shroud separation was commanded from the ground several times and it was even envisaged to use the course correction motor to open it, but to no avail. Eight hours after launch the on board batteries failed and the probe was left into its orbit having a period of 448 days, a perihelion at 0.983 AU and an aphelion at 1.311 AU. A JPL-lead investigation was carried out and in just a few days the culprit was identified. To save some weight, NASA used a glassfibre shroud built by the Lewis Space Flight Center. This shroud was not put trough some of the required tests. In particular, the pressure difference between the inside of the shroud and the un-ventilated honeycomb cells led to the cracking of the inner wall of the shroud, which jammed the separation system. In less than two weeks a new magnesium shroud was built which was slightly heavier than the fiberglass one and, twenty-four hours after tests on it were completed, it was fitted around Mariner-4 on the launch pad.
On November 28, Mariner-4 left its mother planet and headed for a Type I interplanetary transfer orbit and a 246,378 km fly-by of Mars. This was required to minimize the chances of the unsterilized Agena rocket with Mars. On December 5, the probe corrected its trajectory and lowered its minimum distance from Mars to 9,600 km inside an optimal fly-by window dictated by many constrains (including: Mars occultation, Sun and Canopus visibility, angular distance between Canopus and the two tiny Martian moons). Many data were collected during the cruise phase of the mission, although the ion chamber failed in February. In particular, it was discovered that, the Sun being close to the minimum of its activity cycle, it was all but quiet, delivering no less than twenty flares. Some problem was caused by the attitude control system, vital for the success of the picture taking mission. The star sensor located Canopus after two days of searching and lost it twice in December, possibly fouled by the reflex of the Sun on some probe debris, ending up pointing at nearby gamma Velorum. This problem solved, everything was ready for the encounter.
As the day of the encounter approached, many ground based astronomical observatories started a patrol of the planet in support of the probe's mission, controlling in particular the weather over the regions to be observed by the camera. Adouin Dollfus, a well known French astronomer, observed some white cloud over the regions of Mare Sirenum and Phaetontis.
On July 14, 1965 the probe started the Mars data collection, commanding also the powering on of the camera system. The wide angle (50 degrees) planetary sensor mounted on the camera platform started searching for Mars, located it and, after stopping the platform movement, the control system waited for Mars to enter the field of wiew of the narrow angle (1.5 degrees) planetary sensor to start the camera and the tape recorder. This happened at 0.18 of July 15, Greenwich Mean Time. The probe reached the minimum distance of 9,846 km from the planet at 1.01 and this was followed, thirty minutes later, by 54 minutes of Mars disk occultation. This experiment delivered some exceptional data: based on terrestrial observation, scientists were expecting to find a rarefied atmosphere with a ground pressure of 80 hPa and a ground temperature close to zero centigrade. Mariner-4 showed the atmosphere to be extremely thin, having a ground pressure, depending on the composition (not to be measured by the probe), of between 4.1 and 7 hPa and a ground temperature close to -100 degrees Centigrades. This not only meant that future landing mission could not rely on parachutes only to achieve a softl landing but had to use a retrorocket package and that any future manned mission had to use bulky space suits instead of a simple breather mask. The magnetometer failed to measure any intrinsic planetary magnetic field having an intensity more than 0.1% of the terrestrial one and the particle detectors failed to detect any radiation belt.
Eight hours and a half after the probe was re-aquired after occultation, Mariner-4 started reading the images recorded on 100 meters of tape. Twenty-one complete images were recorded, in addition to 21 rows of the twenty-second, covering less than a hundredth of the Martian surface starting at 37 degrees North, 173 East to end in the night emisphere at 50 degrees South, 255 East. The first, historical image showed 365 km of the 17,600 km far Martian limb. It was the first surprise of the mission, for it showed a bright halo parallel to the limb. This could be haze, but it was far too thick for haze, or it could be a reflex in the camera optics. however, tests showed that the camera had to be seriously damaged to produce such a reflex. Starting from the third image, once contrast was stretched to compensate for the very high Sun (near the zenith), some object clearly related to Martian surface morphology started appearing, but only from image seven on they clearly showed what they were: hundreds of craters ranging in size up to 35 km. These first seven images covered the bright areas of Amazonis and Zephyria, but the next six images covered two dark maria: Mare Cimmerium and Mare Sirenum. Here the probe took its most famous picture, number 11, showing a huge 150 km diameter crater flanked by many smaller craters. Due to the geometry and illumination of the picture this was also the best resolution one, showing details as small as 1.4 km. The crater, located at 33 degrees South, 197 East, was later named Mariner crater to honour the probe which discovered it. Starting from picture 12 the image quality started worsening, both because the terminator between the night and day emisphere was close and because these were the areas where Dollfus had observed clouds up to a few days before the encounter. In particular, image 14 showed some dark streaks which could be the shadows of 3.5 km high clouds. Starting from image 14 to image 16, the probe imaged the clear area of Phetontis. Starting from image 17, Mariner-4 imaged an area on which the Sun had just risen and the contrast was very low. Starting from picture 19 the camera imaged the night emisphere on which no structure could be made out.
A total of 300 craters were found in the probe's images, with diameters ranging from 5 km, close to the camera resolution limit, to 120 km. None of the infamous "canals" was images, although the maps of Schiaparelli and Lovell showed few of them in the imaged areas. The data relayed by Mariner-4 took the scientific community by surprise, for they showed a planet with a much thinner atmosphere than previously tought and looking quite similar to the Moon.

The model emerging from the data, unfortunately quite reductive but destined to reign for the rest of the Sixties, could be summarized in five points:

•  The geological history of Mars is more similar to the one of the Moon instead of Earth.
•  As the cratering rate of Mars is probably similar to the Moon's one, their surface must have a similar age of between 2 and 5 billion years.
•  The very good conservation state of the craters mean that the Martian atmosphere did never exercize a significant erosive action and was thus always extremely thin.
• The absence of any geological structure other than craters and the absence of an intrinsic magnetic field mean that Mars did never possess any planet wide geological activity
• Mariner-4 data cannot prove wether life exists or has ever existed on Mars. However, previous points show this is unlikely.

Mariner-4 Orbital Elements:
Pre-encounter (reference frame unknown)

Semiaxis (AU)	Inclination	eccentricity	Ascending node	Perihelion longitude	Date of perihelion
1.27629	0.125699 deg.	0.227503	68.66553 deg.	352.56527 deg.	1964 Nov. 23.96629

Post-Encounter (reference frame unknown, summer 1965?)

Semiaxis (AU)	Inclination	eccentricity	Ascending node	Perihelion longitude	Date of perihelion
1.34085	2.53978 deg.	0.1732201	227.57053 deg.	200.53212 deg.	1964 Nov 16.309247

Bibliography
Chapman, C. R.; Pollack, J. B.; Sagan, C.: An Analysis of the Mariner-4 Cratering Statistics; Astronomical Journal, Vol. 74, p. 1039 (available on-line)
Corliss, W. R.: Space Probes and Planetary Exploration; Princeton, Van Nostrand
Ezell, E. C., Ezell. L. N. "On Mars", NASA SP-4212, available on-line.
Herriman, A. G.: Mariner IV Television - Spacecraft Photography in Planetary Astronomy; presented at the XVII International Astronautical Congress, Madrid, 1966
Koppes, C: JPL and the American Space Program, Yale University Press, p. 78-85
Mariner-Mars 1964 Final Project Report; Washington, NASA (available in Godwin, R.: Mars: the NASA Mission Reports; Burlington, Apogee Books)
NSSDC Master Catalog: Mariner-IV
Wilson, J. N.: Mechanical Design Evolution of the Mariner Spacecraft; presented at the XVII International Astronautical Congress, Madrid, 1966
Wilson, A.: Solar System Log, London, Jane's

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Mariner-4 Images (all 22 of them!)

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