Most space exploration enthusiasts are familiar with the story of Mariner 4: On July 15, 1965 (Universal Time), Mariner 4 made the first close encounter of the planet Mars. The close up images of the Martian surface it returned revealed a cratered landscape more reminiscent of the Moon than the Earth. Combined with a Moon-like lack of a magnetic field and belts of trapped radiation as well as measurements that confirmed the emerging scientific view that the Martian atmosphere was less than 1% as dense as Earth’s, Mars was instantly transformed in the minds of scientists and the public from being a possible Earth-like abode for life into a dead Moon-like world with poor prospects of supporting life as we know it (see “Mariner 4 to Mars” for complete details of this mission).

While the view of the hostile environment of Mars has been strengthened over the last half a century as a result of a couple of dozen subsequent missions to our near neighbor, it was not until the first global maps of Mars became available in the early 1970s that it was realized that the 1% of the Martian surface Mariner 4 imaged most clearly by chance happened to be among the oldest and most cratered terrain on the Red Planet. In addition to the ancient Moon-like regions, Mars also sports a host of other geological features like water-cut channels, huge volcanoes and a gigantic equatorial rift system that provide evidence of a much more active planet with a richer geological history than the Moon. These features hint at a possibly more habitable past which drives much of our exploration of Mars today. Still, I can not help but wonder what would have happened if Mariner 4 or, had it not failed, Mariner 3 had imaged some of the younger and more geologically interesting parts of Mars instead in 1965? How would this have affected our views of Mars in the wake of the first Mariner Mars mission?

Variations of the Mariner 4 Trajectory

With hindsight, we now know that it is unlikely that Mariner 4 would have seen anything much more interesting than it had observed even under different circumstances. Because of the nature of the 1964 Mars launch window and the desire to simplify the Mariner spacecraft design by using a fixed-mounted high gain antenna to transmit data back to Earth, mission designers opted to limit the range of potential arrival dates at Mars with July 15, 1965 being the assigned as the encounter date for a Mariner launch taking place from November 13, 1964 to the end of the window in early December. While the precise approach trajectory during the encounter would have varied by several degrees relative to Mars depending on the actual launch date, the possible range of encounter geometries were all very similar owing to the limited low energy trajectories and the fixed arrival date.

Once the spacecraft reached Mars, there were a number of requirements which restricted the choice of trajectory past Mars: Mariner could not follow a trajectory where Mars or its moons would interfere with the view of the star Canopus or the Sun which were used as attitude references by the spacecraft. Mariner had to follow a trajectory that appeared to pass behind Mars as viewed from the Earth so that the spacecraft’s S-band radio beacon could probe the atmosphere of Mars. In order to secure images of sufficient quality, Mariner could flyby Mars at a distance no greater than 40,200 kilometers and within a specific range of angles in relation to the ecliptic plane to allow the scan platform to slew to bring Mars into view of the camera system. At the same time, the trajectory choice had to keep the probability of an accidental impact to less than 10-6. All these requirements limited Mariner 4 to trajectories that passed through an imaginary window measuring roughly 14,000 by 9,000 kilometers. While this aim point is huge by today’s standards, it pushed the limits of interplanetary navigation just seven years into the Space Age.

While these requirements limited primarily the range of latitudes that were observable by Mariner 4 as well as longitudes relative to the point of closest approach, there was one more important constraint on the Mars encounter. In order to maximize the visibility of the encounter from the primary tracking station at the Goldstone Deep Space Communications Complex in California (part of the forerunner of today’s Deep Space Network), the closest approach had to occur about one hour after the middle of Goldstone’s viewing period. The combination of all of these requirements limited the image swath of Mariner 4 to a fairly narrow range across Mars not all that different than the one actually flown. While a different mix of images could have been acquired with slight differences in approach trajectory and timing, they would have still revealed ancient cratered terrain especially later in the image sequence in the southern hemisphere where the lighting and resolution were best. The post-Mariner 4 view of Mars would have most likely been the same no matter which allowable trajectory was followed.

What About Mariner 3?

Mariner 4 was not the only spacecraft launched by NASA towards Mars in November 1964. It was preceded by the launch of Mariner 3 on November 5, 1965. But because its new lightweight fiberglass launch shroud failed to separate, the spacecraft ended up in the wrong solar orbit with a velocity deficit of 255 meters/second. Even if Mariner 3 managed to break free and deploy its solar panels, the shortfall exceeded the 81 meter/second delta-v capability of Mariner’s course correction propulsion system and it would have missed Mars by many millions of kilometers. At best, Mariner 3 would have returned data on the interplanetary environment to complement that gathered by a Mars-bound Mariner 4. Instead, the batteries of Mariner 3 were depleted 8 hours and 43 minutes after launch resulting in a total loss (see “50 years Ago Today: The Launch of Mariner 3” for a complete description of this mission). In the aftermath of this failure, an all-metal replacement shroud was hurriedly constructed and delivered in time for Mariner 4 to be launched towards Mars on November 28.

But what if Mariner 3 had not suffered from this shroud separation failure and managed to make it to Mars like Mariner 4? Mariner 3 was targeted to follow a slower trajectory that would reach Mars on July 17, 1965 after a flight of 254 days – the longest interplanetary mission ever attempted up until that time. It also was the first Mars-bound spacecraft ever launched into a Type II trajectory where the spacecraft travels through an angle greater than 180° before reaching its target (the Soviet Union had attempted to launch an engineering test flight of their 3MV planetary spacecraft into a Type II trajectory towards Venus in February 1964 but it fell victim to a launch vehicle failure – see “Trajectory Analysis of the Soviet 1964 Venus Missions”). Even though it was launched 23 days later, Mariner 4 followed a faster Type I trajectory that was targeted to reach the Red Planet two days earlier on July 15.

Despite the differences in the trajectory towards Mars, the approach geometry of Mariner 3 would have been broadly similar to that of Mariner 4 especially since they had similar encounter dates and both followed Class I trajectories that encountered Mars before the probes reached aphelion. In addition, both spacecraft would have had similar target windows at Mars and the same requirement on the timing of the encounter as viewed from the Goldstone tracking facility. The biggest difference would have been in the longitude range overflown by the two spacecraft. Since the Martian day is 38 minutes longer than that on Earth, Mars would have rotated through a smaller angle than the Earth during the two days that elapsed between the Mariner 4 encounter on July 15 and the planned encounter date of July 17 for Mariner 3. This alone would have caused the ground track of Mariner 3 to be 20° in longitude farther to the east than that of Mariner 4.

If the only differences between in the ground tracks of Mariner 3 and 4 were the result of this 20° difference in longitude, Mariner 3 would have likely imaged roughly the same sort of ancient cratered terrain as Mariner 4. But, if Mariner 3 passed closer to one edge of its aim window than Mariner 4, it could have caught a glimpse of some the younger and more geologically interesting terrain at the edge of Mars’ Tharsis region. If for some reason the mission constraints on the timing of the flyby of Mariner 3 had been relaxed somewhat by mission planners to allow a closest approach an hour or two earlier, Mariner 3 might have been able to catch an oblique view of the solar system’s largest volcano, Olympus Mons, in the first couple of images it acquired and possibly even the western most of the Tharsis Montes, Arsia Mons, along with the more geologically complex terrains to the south.

Based on my initial estimates with simple calculations, it seems more likely that had Mariner 3 successfully made it to Mars on July 17, 1965, it would have most likely viewed ancient cratered terrain not all that dissimilar to that viewed by Mariner 4. Even if it happened to have viewed something more interesting like a linear fault, wrinkle ridge or similar geological feature more common to the east of the ground track of Mariner 4, it may or may not have been recognized as such given the lighting conditions and kilometer-class resolution of the Mariner images. Even Mariner 4 managed to image the edge of the Martian volcanic feature Orcus Patera in Frame 3 but it was not recognized as being anything but maybe a potential crater until years later. Only under the some special (and potentially improbable) circumstances would Mariner 3 have had any chance of clearly observing something spectacular like Olympus Mons – a finding that would have had a clear impact on scientists’ view of Mars in 1965. That sort of discovery had to wait until the first Mars orbiter reached Mars in November 1971 and began the first systematic mapping of the Red Planet.

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Related Reading

“Mariner 4 to Mars”, Drew Ex Machina, July 14, 2015 [Post]

“50 years Ago Today: The Launch of Mariner 3”, Drew Ex Machina, November 5, 2014 [Post]

General References

V.C. Clark, Jr., W.E. Bollman, R.Y. Roth and W.J. Scholey, “Design Parameters for Ballistic Interplanetary Trajectories Part I. One-way Transfers to Mars and Venus”, Technical Report No. 32-77, JPL, January 16, 1963

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