The Planet Mars:
A History of Observation and Discovery

William Sheehan



Chapter 11
Spacecraft to Mars

The last of the oppositions of Mars that took place before spacecraft visited the planet and forever destroyed our innocence occurred in March 1965. It was an aphelic opposition, and it was also the first that I personally observed with a telescope; I was then a ten-year-old equipped with a modest 2-inch (5-cm) refractor. The instrument was too small to show much, but I was excited to be able make out a few of the dark patches, which at the time were still generally believed to be tracts of vegetation. Moreover, at that moment there was a mood of great expectation---two spacecraft were then headed toward Mars: the American Mariner 4 and the Russian Zond 2.

The Space Age officially began with the launch of the first satellite, Sputnik 1, by the Russians on October 4, 1957. The first successful probes to the Moon were launched in 1959, followed by rockets to Mars and Venus. The Russians had the clear edge in rocket development, and they attempted to launch a rocket toward Mars as early as October 1960; unfortunately, it failed to reach Earth orbit. Since the Russians made a point of shrouding their space program in secrecy, they published no information about it at the time.

The Russians launched a Venus probe in February 1961 but lost contact with it long before it reached its destination. They followed up with two more Mars probes in the fall of 1962. The first failed to reach Earth orbit, and again, nothing more was said about it; but the other, Mars 1, a spacecraft weighing 874 kilograms and equipped with television cameras, was placed in the correct orbit, and at first everything seemed to be going well. (Since we are now in the modern era, I propose henceforth to drop imperial equivalents and give values in metric units only.)

The basic principles involved in launching an interplanetary probe such as Mars 1 are straightforward enough. Recall that when an object is launched from the Earth's surface at relatively slow speeds, it simply follows a curved trajectory back to the Earth. At higher and higher speeds the curvature of the arc becomes more and more gentle, until there comes a certain point---at a velocity of 28,000 kilometers per hour, to be exact---when the rate at which the rocket is traveling forward and dropping downward equals that at which the Earth's surface below curves away from it. Thus, though it continues to fall freely, the rocket never reaches the ground (for the moment, we are ignoring air friction). At this point it has achieved Earth orbit. If the rocket is further accelerated to a speed of 11 kilometers per second, or 40,000 kilometers per hour---the escape velocity---it escapes from the sphere of the Earth's gravitational influence and become an independent body traveling in its own orbit around the Sun.

The fuel supply of a rocket is very limited, however, and the only practical way for it, or any spacecraft, to reach another planet is for it to be placed in what is known as a transfer orbit, so that it can coast, without using any fuel, for most of the journey. If it is to travel to the inner planets, Mercury and Venus, the spacecraft must be slowed down slightly relative to the Earth; in order to reach the outer planets, such as Mars, it must be speeded up.

The most energy-efficient trajectory between two planets is the so-called Hohmann transfer ellipse, named after the German engineer W. Hohmann, who first described it in 1925. If the orbits of the Earth and Mars were exactly circular, the Hohmann transfer ellipse would be a path in which the spacecraft left the Earth at an angle tangential to its orbit and arrived at an angle tangential to the orbit of Mars. This orbit would have its perihelion at the launch point (Earth) and its aphelion at the orbit of Mars; the spacecraft's period of revolution around the Sun would be 520 days, and in getting from the Earth to Mars it would travel halfway around this ellipse, so that the transit time from the Earth to Mars would be 260 days. Since in this time Mars would have moved a distance of (260/687) x 360° = 136° along its orbit around the Sun, it follows that in order for the spacecraft to reach Mars, the relative positions at launch must be such that the Earth-Sun-Mars angle is 180° - 136° = 44°.

Such conditions occur about fifty days before each opposition, and they define the "launch window." Of course, the actual orbits of the Earth and Mars are not exactly circular, nor do the two planets lie in quite the same plane; thus, the actual conditions will vary somewhat from launch window to launch window---in particular, less energy is required to reach Mars during launch windows at which a perihelic opposition occurs. Also, since the minimum energy requirement for the spacecraft to reach Mars is actually rather modest, it is possible for the trajectory to depart considerably from the Hohmann transfer ellipse. Without going into details, it is fair to say that in general, launch windows occur roughly two or three months before opposition. Thus, since there was to be an opposition in January 1963, the Russians were within the launch window in getting Mars 1 under way at the beginning of November 1962.

Mars 1 remained in contact with the Earth until March 21, 1963, by which time it was 106 million kilometers away from Earth. Unfortunately, radio communications were then suddenly lost, and the following June the spacecraft passed silently and uselessly by the planet at a distance of 195,000 kilometers; no photographs or other data were obtained.

The next launch window occurred in the fall of 1964. By then, the Americans had also become active. They had already sent the first successful interplanetary probe, Mariner 2, past Venus in December 1962, and it had made measurements showing the high surface temperatures (up to 477°C) and generally sinister conditions that prevail on that planet. In November 1964, the United States prepared to launch two Mars probes. The first, Mariner 3, set out on November 5, but the fiberglass shroud designed to protect it during its ascent through the Earth's atmosphere failed to eject, and the extra weight prevented the probe from achieving the proper transfer orbit. It was also unable to deploy its solar panels and soon ran out of power. The backup spacecraft, Mariner 4, was launched on November 28---the 305th anniversary, incidentally, of the day Christiaan Huygens drew a rough sketch of the Syrtis Major region. This time everything went well; the shroud was ejected, and the solar panels deployed. Two days later, the Russians followed with the launch of their latest Mars probe, Zond 2.

Mariner 4 led the way to Mars and was due to arrive there the following summer, some three weeks ahead of its Russian counterpart. But the Russians had not yet solved their problems with communications, and in early May 1965 their experience with Mars 1 was repeated---they lost contact with Zond 2, and it was never heard from again.

Fortunately, Mariner 4 continued to function as planned, and on July 14, 1965, made a close sweep past Mars. The American engineers had kept the design of the spacecraft simple; it weighed only 260 kilograms and carried a television camera and other scientific instruments, including a magnetometer and trapped-radiation detector (to measure the intensity of any magnetic fields and radiation belts around Mars). The first of twenty-two images was obtained when the spacecraft was 16,900 kilometers from the surface---it was a view of the limb, showing a section of the Amazonis desert near the dark patch Trivium Charontis. The rest of the series covered a discontinuous swath extending south and then eastward from Amazonis across Zephyria, Atlantis (the brightish region between the dark areas Cimmerium and Sirenum), Phaetontis, and Memnonia. The last three pictures were taken beyond the terminator of Mars from a distance of 11,900 kilometers. Although the images were of low contrast and rather murky---possibly because of a light leak in the camera system---they were good enough to show a distinctly cratered surface, of which the largest crater, in Mare Sirenum, measured 120 kilometers across. In all, about three hundred craters were recorded---but, I hasten to add, nary a canal!

In every way the Mariner 4 results came as a shock. The probe seemed to reveal a Mars that was, in a word, moonlike. The surface appeared old and dead, and apparently had not changed appreciably for billions of years. This dour impression was reinforced by the results of the S-band radio occultation experiment. Two hours after it made its closest approach to the surface of Mars (9,850 km), Mariner 4 passed behind the planet, at a point on the sunlit side between Electris and Mare Chronium. Its radio signal was distorted by its passage through the thin Martian atmosphere, and this was repeated when it emerged again from the night side, at a point above Mare Acidalium. By analyzing the shape of this distortion it was possible to calculate the surface pressure at the two occultation points. The result was distressingly low---only 4.0 to 6.1 millibars.1 When these data were combined with the earlier ground-based work of Kaplan, Münch, and Spinrad, who, as we have seen, showed the partial pressure of carbon dioxide on the surface of Mars to be around 4 millibars, it became clear that the Martian atmosphere must be made up of something like 95 percent carbon dioxide---thus it seemed only too likely that Ranyard and Stoney had been right after all in surmising that the polar caps must consist of frozen carbon dioxide instead of ordinary water ice.2 At that low pressure, liquid water, even in the relatively warm equatorial regions of the planet, would not be stable on the surface. In their official summary of the results of the Mariner 4 mission, R. B. Leighton and the other members of the television team argued that "the heavily cratered surface of Mars must be very ancient---perhaps 2 to 5 billion years old . . . [and] it is difficult to believe that free water in quantities sufficient to form streams or fill oceans could have existed on Mars since that time."3

In retrospect, it is somewhat surprising that the cratered surface of Mars evoked such great surprise. After all, impact is now known to be the dominant force in fashioning the early surfaces of the Moon and planets, and we have actually witnessed comet fragments crashing into Jupiter. In the early 1960s, however, the details of all this were still being worked out. Even the best known impact feature on Earth, the kilometer-wide crater near Winslow, Arizona, was not definitively shown to be the result of an impact until the late 1950s.4 The origin of the lunar craters was still hotly debated, and it was by no means certain that they had been formed by impact rather than by what can be described, broadly speaking, as volcanic processes. After Mariner 4 revealed the Martian craters, the debate extended to that planet as well. Only with the Apollo spacecraft missions was the issue finally resolved in the case of the Moon, and we can now say for certain that the craters, on both the Moon and Mars, were formed by impacts.5

As I mentioned earlier, a few farsighted astronomers such as Clyde Tombaugh, E. J. Öpik, and Ralph Baldwin had actually predicted the existence of craters on Mars. One, John E. Mellish, went so far as to claim that he had directly observed the Martian craters in November 1915, when the disk was only 7.8". He had used the 40-inch (1.02-m) Yerkes refractor just after sunrise with a magnifying power of 1,100x. Mellish did not publish an account of these observations until the year after the Mariner 4 flyby, though he had written earlier about them to other astronomers.6 He also implied that his onetime colleague E. E. Barnard had seen the Martian craters while he was at the Lick Observatory in the 1890s. Unfortunately, Mellish's drawings were destroyed in a fire; Barnard's, however, turned up at Yerkes a few years ago (they show some dark circular patches ["oases"], but no true craters).

Mellish's observation sparked controversy. Some astronomers were convinced that he had seen actual craters on Mars; others were frankly skeptical.7 Obviously he saw something; presumably it was some of the bewildering details---small masses and dark spots---into which the Martian surface resolves under the best viewing conditions. Never having seen anything like it before, he was understandably astonished and found intimations of strikingly unfamiliar aspect. But his specific claim to have seen craters can now be dismissed. It is quite impossible to observe any topographical relief directly from Earth; this the Hubble Space Telescope observations have now put beyond doubt. The presence of the Martian atmosphere smooths out the evidence of surface irregularities at the terminator, and moreover, though Mars does indeed have mountains, the tallest of them---including towering Olympus Mons, the tallest mountain in the solar system---are great volcanic shields rather than peaks; their slopes are very gradual, and they cast no visible shadows even at the terminator.

The Mariner 4 results spurred astronomers to completely revise their ideas about Mars; it was now obvious that earlier views had been very wide of the mark indeed. The vegetation theory was dead---the Martian environment seemed to be too hostile to support vegetation, and even the existence of the wave of darkening, which "took second place only to the Martian canals in historical development of the life on Mars hypothesis,"8 was doubtful. Though there could be no question that there were considerable changes in the form and intensity of the dark areas, the existence of an actual "wave" extending from the south pole to the equator each spring seemed at best only weakly supported by the observations. A study by Charles F. Capen of the Lowell Observatory showed that the greatest intensity changes actually seemed to occur in the light areas, and only the dark areas in the immediate vicinity of the south polar cap showed the classical pattern of appearing consistently lighter in summer than in spring. Systematic latitudinal changes in the intensity of features with the seasons, at least, did not seem to occur, and any changes that did take place were evidently due to windblown dust rather than vegetation.9

The changing perspective of Mars brought about by the Mariner 4 results is well illustrated by the before and after views of Capen, who was one of the most skillful Mars observers of that time. In 1964--65 he gave a description of Mars that was in many ways "classical." Thus he plotted numerous canals and noted vivid color changes; for instance: "The Syrtis Major was changing from a blue-green to a green-blue hue. . . . The Mare Acidalium changed from its winter shades of variegated gray and brown to its spring coloration of dark gray and blue-gray shades with gray-green oases."10 In May 1969, with Mars near opposition, Capen enjoyed a series of splendid views with the 82-inch (2.08-m) reflector at McDonald Observatory in Texas. "When Mars was first brought into focus on the nights of May 29, 30, and 31, its globe appeared to be draped in a dark gray spiderweb, resembling the shade and texture of iron-filings. When 1,000x was employed the global network . . . resolved into dark circular features and parallel aligned streaks, some of which were fortuitously aligned into canal-like lineaments." These details he afterward identified with large craters and composites of dark blotches and streaks shown on the spacecraft photographs.11

After Mariner 4, the next probes sent to Mars were Mariners 6 and 7 in 1969 (Mariner 5 had gone to Venus). Mariner 6 set out on February 24 and Mariner 7 on March 27. We now know that there were two unsuccessful Russian launches that spring as well. They were not even announced at the time, and there is no reason to say anything more about them.

The two Mariner spacecraft, like Mariner 4 before them, were designed as flyby missions, but they began imaging Mars well before their arrival. These far-encounter views had a resolution somewhat better than the best Earth-based images but were still too blurry to show the Martian surface features very well---for instance, Nix Olympica, which had first been seen by Schiaparelli in 1879 as a tiny white spot, appeared as a large, bright ring, interpreted at the time as the outline of a large impact crater.

The actual Mars encounters took place only a few days after the Apollo 11 flight, during which Neil Armstrong and Edwin Aldrin became the first men to walk upon the surface of the Moon. Mariner 6 arrived first, on July 31, 1969, and provided twenty-five close-up photographs of the equatorial region between longitudes 60° W and 320° W, including Aurorae Sinus, Pyrrhae Regio, and Deucalionis Regio. Mariner 7 followed on August 5 and obtained thirty-three photographs of a region along the edge of the south polar cap and another swath of the planet---again mostly in the southern hemisphere---covering Thymiamata, Deucalionis Regio (again), Hellespontus, Hellas, and Mare Hadriaticum. In all, the two probes increased the close-up coverage of the planet from Mariner 4's 1 percent to about 10 percent, and their photographs were much clearer than those of Mariner 4---mainly because of improved techniques, but partly because the spacecraft passed closer to the planet, within less than 3,500 kilometers.

The by now familiar craters were fairly ubiquitous in the photographs, and included a number of frost-covered features along the edge of the polar cap. For the most part, the moonlike Mars of Mariner 4 was very much in evidence, but there were also some surprises. One interesting area was Hellas, a large, circular bright area some 1,300 kilometers across. Classically it had been regarded as an elevated area, since it was a site where whitish clouds frequently developed, and it had often appeared to be frost covered in winter; however, we now know that it is a low-lying basin, and the Mariner photographs showed it to be surprisingly smooth. Another interesting area, located at 40° W, 15° S, consisted of jumbled ridges, or "chaotic terrain," which appeared to have formed by the withdrawal of subsurface material and collapse of the overlying sediments and rocks.

Apart from the television experiments, other instruments on board the two spacecraft confirmed that Mars's atmospheric pressures are very low---Mariner 6 measured 6.5 millibars in the Sinus Meridiani region, and Mariner 7 found only 3.5 millibars over Hellespontica Depressio, a region that had long been regarded as a depression, though in fact it proved to be elevated. The temperature at the south polar cap was measured at -123°C (-190°F), almost exactly what was expected for carbon dioxide ice. At the time most scientists believed that both of the caps consisted entirely of frozen carbon dioxide. Finally, the probes found no trace of a magnetic field.

All in all, the view from the flyby Mariners of 1969 was most discouraging. They had covered only a tenth of the Martian surface, but this had included several of the main dark areas of the planet---and it had been on the dark areas, after all, that observers had always focused their attention, ever since Christiaan Huygens had first sketched Syrtis Major in 1659. We now know that the dark areas of the southern hemisphere contain the most heavily cratered terrain on Mars. And although the cameras of the flyby Mariners took numerous photographs of craters, by sheer chance they missed all of the truly spectacular features of the Martian landscape---the volcanoes, canyons, and dry riverbeds.

As the 1960s ended, the drab and moonlike Mars seemed to have been confirmed by three flyby spacecraft, and the fascinating Lowellian world of dry sea bottoms, canals, lonely deserts, and dying civilizations had faded like a dream. In many ways 1969 was the nadir of Martian studies. But the moonlike Mars was to prove as much an illusion as the Lowellian Mars had been. This brings us to Mariner 9 and the next great year of Martian discovery---1971.

© 1996 The Arizona Board of Regents

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The University of Arizona Press, 2/2/97 2:15PM