Mars is unique among the planets in being just close enough, and having a sufficiently clear atmosphere, for observers to make out vague surface details from Earth. Mars appeared to show seasonal variations and some observers, such as Percival Lowell, thought they saw evidence of “non-natural structures”, which Lowell took to be an elaborate canal network intended to distribute precious water from the polar regions to the equatorial cities.
By the 1960s this view of Mars was not widespread in scientific circles, but Mars did show what seemed to be seasonal variations and it was difficult to categorically exclude the possibility that Mars harbored some form of life. Perhaps not master canal builders, thoats or John Carter, but possibly vegetation or algae that came and went with the seasons.
In 1964 NASA launched the Mariner 4 probe to Mars. It was part of a general planetary science effort, but there was some hope that Mariner 4 would show, in addition to the general geography (areography, actually) of the planet, the reason for the seasonal variations, which many took to be the yearly advance and retreat of grass-like vegetation.
Mariner 4 was a product of the Jet Propulsion Laboratory and bore a family resemblance to the earlier Ranger series of lunar probes. It consisted of an octagonal equipment bus, four large solar panels for electricity, and a high gain antenna and various instruments mounted on the top of the bus. The primary science instrument was a camera, but Mariner was also equipped with sensors to measure the dust, radiation, solar wind and magnetic environment around it. The main “engineering objective” of the mission was to prove that interplanetary flights of long duration were not beyond the state of existing technology and technique.
Mariner 4 was designed as a so-called “fly-by mission”. This means that it was intended to pass relatively near Mars, but not go into orbit around it. It would sweep by Mars, conduct science for the brief interval of close approach, and then move on into a solar orbit of its own and leave Mars behind. This was done largely for reasons of simplicity. A fly-by mission is much less demanding on the spacecraft than a mission where the spacecraft is intended to go into orbit – the latter requires exceptionally fine guidance, not to mention a propulsion system of considerable vigor. A fly-by mission requires no propulsion system, and is somewhat less dependent on precise navigation and guidance.
Mariner 4 was launched on November 28, 1964 from Cape Canaveral aboard an Atlas-Agena D booster. The launch was successful and the probe settled down to an uneventful seven and a half month long cruise to Mars.
Closest approach to Mars was on July 15, where Mariner 4 took 21 complete pictures and a portion of a 22nd picture as it passed by Mars. It also sampled the dust, radiation and magnetic environment around Mars, but none of the data was immediately returned. Instead it was stored on a tape recorder for later transmission. Late on July 15 Mariner 4 passed behind Mars, allowing scientists on Earth to perform atmospheric occultation experiments with Mariner 4’s radio signal. It emerged from behind Mars about an hour later, and proceeded along its solitary solar orbit, now mowing away from Mars. The taped data and photographs were then transmitted to Earth, with each of the pictures being transmitted twice, just in case. The whole procedure of transmitting the data took almost three weeks, a performance that by today’s standards is positively glacial.
Mariner 4 continued to measure the solar wind and radiation environment for over two years before it finally ran out of attitude control system fuel in December, 1967. Dust impacts caused the spacecraft’s main communications antenna to point away from Earth and contact was lost; on December 21 the mission was formally ended.
The data returned by Mariner 4 were a shock, especially to those who had expected the probe to reveal some signs of life. Mars turned out to have no measurable magnetic field, meaning that the surface of the planet was bombarded by intense charged particle radiation from the sun. The air pressure was only a percent or two of Earth’s air pressure, and the average temperature seemed to be minus 100 degrees C. All of this looked bad for life, but the photographs clinched the deal: they showed a landscape that had been so pounded by impacts it looked like the moon, an ancient cratered wasteland that proved that Mars had no water, no erosion processes, and no tectonic activity. In other words, Mars appeared to be as dead, and as permanently dead, as the moon, and much of the impetus to explore Mars waned. Why waste time and money exploring a planet that was just a repeat of the moon?
But in a way, this “dead Mars” school of thought, driven by the detailed data provided by Mariner 4, was as inaccurate as the “living Mars” school of thought. Mariner 4, by a fluke, only photographed the oldest and most heavily cratered regions of Mars. It never saw the great water-carved outflow channels, or the smooth and uncratered northern lowlands, or eroded and complex landforms that, while ambiguous in their interpretations, at least prove that Mars isn’t as dead and changeless as the moon and strongly hint at the presence of large amounts of subterranean water ice.
Yet Mariner 4, despite this stroke of bad luck, remains a triumph. It was the first man-made probe to successfully fly by another planet and return photographs from it and remains a remarkable achievement in any epoch and by any standard.
William Hamman - I have been professionally involved in the aerospace field for 27 years, most of that time in the linked areas of guidance, navigation and ...
