So, this week we once again turn our sights to the Red Planet, this time a little less Red in our focus, to look in detail at American exploration of Mars, with Pioneer Mars, the Mars Reconnaissance Pioneer, and the Mars Traverse Rovers. This one goes a bit further into the future of the TL than most of our updates have, so I may be limited in what clarification I can provide without spoiling other aspects of the timeline.
Eyes Turned Skyward, Part II: Post #9
Although the Soviets were the most active in exploring Mars, the American space program had certainly not abandoned the planet. Even after the disappointing biological results of Viking, there were still many interesting geological, meteorological, climatological, and geophysical questions that could be asked about the terrestrial planets, and Mars remained in many ways the ideal planet (aside from Earth) to study those questions. After all, it lacked the hellish surface conditions or opaque atmosphere of Venus, and was far easier to reach than Mercury. Therefore, even as the Viking probes were touching down at Tritonis Lacus and Utopia Planitia, and the orbiters were settling into their routine, NASA was planning further missions. Among the leading candidates for launch at the next feasible opportunity was the so-called "Viking '79" mission. This would recycle much of the hardware designed and built for Viking, particularly remaining flight spares on Earth, to perform an even more ambitious mission such as delivering the first rovers to another planet or following up the hoped-for discovery of life at one or the other Viking sites. Even further in the future, Viking hardware derivatives might be used to conduct increasingly ambitious missions, leading up to a sample return in the late 1980s or early 1990s. However, with the non-discovery of life at the Viking 1 and 2 landing sites the public lost interest in expensive Viking-class missions, and the idea of a Viking '79--or a Viking '81, or a Viking '84--receded farther and farther into the distance. The final blow was dealt by Voyager-Uranus, whose approval came at just the moment that funding for a Viking '79 mission needed to start. Combined with the subsequent approval of the Galileo Jupiter orbiter and then the Kirchoff Halley flyby/comet rendezvous probe, it was obvious that another mission of the size and complexity of Viking could not be flown until perhaps the late 1980s.
Into the breach stepped the Ames Research Center. Like the Jet Propulsion Laboratory long involved in planetary exploration, Ames had previously proposed to use a derivative of their Pioneer Venus orbiter design to study Mars, mostly focusing on aeronomy--the study of the upper atmosphere--and spectroscopic imaging of the surface. Now these proposals were revived as a low-cost method to continue NASA's study of Mars. Such an orbiter would be cheap, perhaps $100-150 million, and could explore many interesting questions left unanswered by Viking. Approval of the "Pioneer Mars" mission was relatively speedy, although a planned second orbiter carrying penetrators, hard landing probes which could conduct a number of surface studies, was dropped due to cost and schedule concerns. The Pioneer Mars orbiter, unlike its Cytherean counterpart, would be a lone traveler to Mars. While it was a secondary priority at Ames during the preparation of its Cytherean siblings, once they launched in 1978 preparations stepped into high gear, and Pioneer Mars was launched by an Atlas-Centaur in 1979 for its date with the Red Planet. Once it reached Mars, it settled into a highly elliptical orbit, dipping down to just one hundred kilometers off the surface before popping back up to over thirty thousand kilometers altitude. Such an elongated orbit allowed it to skim the atmosphere relatively deeply, allowing measurements of its properties which would otherwise be impossible. At the same time, it allowed the spectroscopes carried by the probe to observe the planet from a close vantage point, giving them a better view of the planet than had heretofore been achieved. After an Earth year of this atmosphere-surfing, during which the probe burned a great deal of propellant and suffered significant changes to its orbit, Pioneer Mars executed a long burn to bring it to a higher Martian orbit, where it would not reenter until long after any microbes that might have survived the sterilization process it had undergone would die of old age and starvation. Even from this vantage point, though, it could continue to perform useful scientific observations, and so it did until finally running out of propellant and becoming uncontrollable in the late 1980s, a few years before its older sibling at Venus. After Pioneer Mars, there was a long gap in American Mars exploration. While the Vikings continued to operate for a few more years, and Pioneer Mars kept faithfully sending data, the focus had turned towards other bodies, away from the Red Planet. But Mars would not be so easily and quietly abandoned, and the faint echo of his war that sounded from the challenger of Vulkan rekindled NASA's interest in the planet. After the Vulkan launch, anything the Soviets did in space seemed threatening, and the dispatch of two probes in 1983 to Mars was no exception. Shortly after they arrived in early 1984, President Reagan proposed to add a pair of new missions, the Mars Reconnaissance Pioneer from Ames and the Mars Traverse Rovers from JPL to the list of scheduled American planetary missions, for launch in perhaps 1990. Congress approved the missions without debate when they came up for consideration, and the next pair of American Mars missions began to roll forward.
The first, the Mars Reconnaissance Pioneer, was in many ways the more straightforward of the pair. Designed to further the studies of the Viking orbiters and Pioneer Mars, MRP would conduct in-depth studies of the Martian atmosphere and weather systems for an entire Martian year, hopefully improving Earth's knowledge of Martian seasonality. Additionally, it would carry a suite of spectrometers to further refine the compositional data provided by the Pioneer Mars and Mars 9 missions, and a laser altimeter to refine height estimates provided by the radar altimeter aboard Pioneer Mars. The cameras intended to map weather features could also be used to obtain medium to high resolution imagery of planetary surface, giving the MRP a broad range of scientific objectives. Development proceeded smoothly; in truth, so far as any planetary exploration development program can be simple and straightforward, this was it. Many of the instruments that would be flown by the MRP had been pioneered by earlier planetary flights, or by Earth orbital missions, and could be had virtually off the shelf. While accommodations would need to be made for the unique target and environmental conditions, adapting existing instrument designs was still cheaper and easier than building them from scratch. Thus, the MRP proceeded to its 1990 launch date with little trouble, riding a Delta 4000 into orbit and then on to Mars. When it reached Mars, it undertook a completely novel technique to reach its planned low-altitude circular mapping orbit (itself a departure from the norm for most previous planetary missions). Rather than use its on-board rocket to perform a series of burns to circularize its orbit, the MRP would instead make a series of very low passes through the atmosphere, just over the 100 kilometers of Mars Pioneer. This would slowly drain energy from the probe and lower its maximum orbital altitude, saving hundreds of kilograms of propellant that would otherwise be needed. The extremely well-characterized nature of Mars' upper atmosphere, and Pioneer Mars' own inadvertent demonstration of the technique, was a key factor behind the approval by NASA administration of the otherwise risky aerobraking maneuver. After months of these low passes, the MRP finally settled into its final mapping orbit, beginning its intensive scientific investigation of the planet. Over months, then years of work, the MRP slowly built on, and occasionally demolished, the view of Mars that had been created by previous mission to the planet. In addition to a vast array of high-resolution imagery, the MRP also produced a highly detailed global spectroscopic map, a topographic map surpassing that of any other planet in the Solar System, and produced the first view of the entire yearly weather cycle of a planet besides Earth.
The Mars Traverse Rovers were altogether the more ambitious of the two responses to the Soviet Mars challenge. The surface counterpart to the MRP's orbiter, the mission would deliver, as the name indicates, a pair of rovers designed by JPL to the surface of Mars for a long-duration (perhaps one Earth year long) traverse of the Martian surface in conjunction with each other. Besides the obvious scientific returns that could be had by having a mobile imaging and scientific platform, the rovers would also serve as an engineering test for larger and more complex rovers, which could either serve as useful mobile platforms in their own right or be used to undertake the Holy Grail of Mars science, the Mars Sample Return mission both NASA and the Soviets had been chasing for many years. More complex and novel than the MRP, the Mars Traverse Rovers were plagued with issues from the start of the program. Even the question of how they would be propelled--wheels, tank-treads, or a complex multi-legged walking system were all serious contenders--was not resolved until a year into the project, with the simple and proven wheels coming out on top. While the original proposal used a heavily-modified Viking lander as essentially a sort of mothership for the pair of rovers, complete with its own suite of scientific instruments, constant weight and cost growth forced the capabilities of the lander to shrink in tandem, until it was little more than a delivery platform. Already, the problems encountered in designing rovers of this size and capability to operate in an alien environment, with no experience at JPL to temper the design process, had led to the launch date slipping to 1992 from the original estimate of 1990. The program continued in much the same vein right up to its launch date, constantly encountering problems and constantly finding a way around them, although in the process the launch slipped again, now to 1994. Finally the launch window arrived, and the rovers, tucked safely under their lander, itself encased inside an aeroshell for direct Mars entry, were dispatched on their way by another Delta 4000.
Targeted at the mouth of Ares Vallis, a vast water-carved channel on Mars that had long been fingered as an interesting site for Mars exploration, the Mars Traverse Rovers--now named Independence and Liberty from an elementary-school essay contest organized by the increasingly PR-conscious NASA--reached Mars in September 1995. After an anxious descent through the Martian atmosphere, the two rovers touched down safely only a dozen kilometers off target, with preparations for deployment starting almost immediately after touchdown. While the deployment of Independence proceeded smoothly, that of Liberty failed during the last step--lowering the rover to the surface of Mars from its resting place underneath the lander. Inspection by Independence showed that a locking pin, supposed to be removed prior to launch, had accidentally been left in place, preventing the rover's release. Several attempts to use Independence to remove the pin failed, and it was eventually decided to abandon Liberty in place, converting it to a "stationary scientific platform" with those instruments that could be productively used while the rover was hanging under the lander bus powered on. Meanwhile, Independence would continue with the primary mission, an attempt to make its way up Ares Vallis while studying the geological properties of the soil and rocks along its path. So thorough were JPL scientists in doing so, in fact, that a month after departing Independence could still view its lander on the horizon, having taken the time to inspect not only a large number of rocks but also a series of trenches dug by its own wheels in the Martian soil during its slow and meandering journey. This pattern of slow but scientifically productive movement continued for years as the rover made its way around the mouth of Ares Vallis, usually moving during the day and collecting spectroscopic and soil properties data at night, when optical navigation was impossible. Liberty continued to return barometric and temperature data from the landing site, although its own cameras were largely unusable and its other instruments hung uselessly far from the soil and rocks they were meant to investigate.
Independence was only finally done in by its wheel motors; while the RTG power source could potentially provide power for decades, as with the Voyagers and Pioneers venturing out into interstellar space, other components were not so durable. First, some 23 months after landing, nearly a whole Martian year, the motor on the right rear wheel failed. While easy enough to work around, as the remaining wheels had more than enough power to continue moving the vehicle--indeed, the slight increase in available power compensated the slight decrease from the RTG so far experienced--it was nevertheless a herald of things to come. Almost three years after that, another wheel failed, this time the left center. The resulting asymmetry made the rover difficult to control, slowing its movement to almost zero as controllers laboriously repositioned it after each short traverse. The rover's will to continue must have left it at this point, for only six months later a final wheel motor--the one on the left rear wheel--stopped operating. With three wheel motors out of commission, the rover could no longer move, and like its sibling before it would live out its days as a stationary scientific platform, albeit capable of inspecting not only the weather but also the rocks and soil around it. As with Liberty, this continued until demands on the Deep Space Network from a new generation of probes forced NASA to shut down some of the less productive older vehicles still operational. While a difficult decision to make, both Independence and Liberty were commanded to power down in October 2003. Perhaps some future expedition, whether by robots or humans, will find the vehicles and be able to power them back on; their RTGs will continue providing usable amounts of energy into the 2020s. However, despite their early demise, the rovers were spectacularly successful, returning even more scientific data than the Vikings, and from a much more diverse area.
