Well, everyone, it's that time once again. Last week, we touched on the position of the Russian space program (and their alliances with other programs) as the commercial and international market became the key for the survival of their program. This week, we're taking a jaunt out to what the American program is up to at the same time as we track down the latest in their outer planet exploration missions.
Eyes Turned Skywards, Part III: Post #9
From Earth, Saturn is perhaps the most intriguing of the giant planets. While its own complex atmospheric systems are virtually invisible to ground-based observatories, unlike the glorious belts, storms, and zones of Jupiter, it more than compensates with its famous rings, the only set of giant planet rings easily visible from Earth. As with Jupiter, NASA planning for advanced missions to Saturn and its system of rings and moons, to follow flybys like the Pioneers or Voyagers, began early, almost before the planetary exploration program itself, although due to the greater challenges involved in exploring Saturn these missions tended to be granted a lower priority than missions to Jupiter or the inner planets. By the mid-1970s, these plans had coalesced into a family of “super-Voyagers” or “super-Pioneers,” beefed up with extra propellant tanks to handle orbit insertion and modification, and a modified instrument suite to better address questions specific to each planet. By using the Saturn-Centaur, these probes could be dispatched directly to Jupiter or Saturn; indeed Jovian probes could carry additional scientific equipment, such as an atmospheric probe. Alternatively, the Titan IIIE--the Titan-Centaur--with an additional solid “kick stage” could be used, although this would limit probe capabilities and require, for Saturn orbiters, the use of gravity assists to reach the destination. It would, however, be available earlier than the Saturn-Centaur, and possibly be cheaper as well. Although the major focus during these studies was exploration of Jupiter and the Jovian system, some attention was paid to the possibility of Saturn orbiters at a later date, a pattern that would repeat through the 1970s and into the 1980s; although Saturn was a decidedly lower priority than Jupiter, it would nevertheless benefit from the attention paid to the latter.
This was apparent in the next round of Saturn orbiter analyses, started after the approval of the Galileo Jupiter orbiter missions in 1976. Now, instead of being based on the Voyagers or Pioneers, Saturn orbiters would be based on the more capable but heavier Galileo platform, carrying an array of instruments and probes to explore not only the planet, but also its moons. Since the previous round of studies, observations of Saturn’s moons, especially the largest moon, Titan, had revealed them to be, as with Jupiter’s moons, more interesting than previously thought. In particular, evidence seemed to indicate that Titan might have an atmosphere, probably thicker and more dynamic than the Martian atmosphere which had been explored in detail by Viking, making it the only moon in the solar system with an appreciable atmosphere (although Voyager 4 later showed that Triton had a thin but perceptible atmosphere). Interest only grew after Pioneer 11 and Voyagers 1 and 2 flew past Saturn and its moons, revealing many additional scientific questions just waiting to be answered and confirming the thick and dynamic nature of Titan’s atmosphere. Although the greater mass meant that even Saturn-Centaur would require a kick stage to send the “Saturn Orbiter with Probes,” or SOWP, to Saturn, the trade off was felt to be worth it in the additional scientific return possible.
As these studies began to sharpen up the details of the notional Saturn probe, new opportunities began to emerge for the tentative SOWP program. While a few foreigners, particularly French scientists associated with CNES and its balloon programs, had been involved in discussion of possible Saturn and Titan missions, most of the discussion to date had taken place at Ames or JPL, with little involvement from non-Americans.
As part of their program to further develop a common European space science program, the European Space Agency encouraged a series of meetings between members of the National Academy of Sciences and the European Science Foundation in the early 1980s, largely before the Vulkan Panic, to discuss possible future areas of cooperation between the scientific programs of the European Space Agency and NASA. Given their previous and ongoing collaborations for Hubble, Helios-Encke, and Kirchhoff-Newton, much of the discussion focused on possible alliances in astronomy and planetary science, although sharing of data and possible joint missions for Earth science and helioscience programs were also discussed extensively. The growth of European planetary science over the past decade, coupled with significant interest from the Americans in involving international collaborators in SOWP (if for no more noble reason than protecting SOWP against any reappearance of budget-cutting enthusiasm within the OMB), led to substantial interest from European scientists in participating in SOWP. There were a number of components where European industry could clearly and easily make useful contributions, while European scientists had unique experience and advantages in certain possible instruments. Although no specific agreements were made, the consensus was clear that any future Saturn mission would be a joint mission--led by NASA, true, but including ESA as a critical partner.
At first, the Vulkan Panic and subsequent infusion of funds into NASA changed very little about the design of SOWP. Despite the advantages of significant budgets and improved performance from the Multibody, it was still much too early in planning to proceed to formal approval and the start of detailed design and manufacture. Instead, JPL continued formal studies into spacecraft configuration and design, while inviting ESA to participate more directly in defining SOWP, now tentatively named “Cassini” after the Franco-Italian astronomer Giovanni Domenico Cassini, who had discovered four of Saturn’s moons and the Cassini Division within the planet’s rings, besides a number of other contributions to science. Over the next several years, Cassini’s design became more and more well defined until, in 1985, it was finally submitted to Congress for a new start. Despite the years that had passed since the initial furor of the Vulkan Panic, Cassini fairly sailed through Congressional approval, with the costs balanced out by arguments about the need to maintain the unique American capability of exploring the outer planets, something which otherwise would atrophy and decay after Galileo’s end.
The Cassini Saturn System Mission approved by Congress would be a behemoth of a mission, the “cornerstone to end all cornerstones” as detractors said. The orbiter alone, equipped with an expansive scientific suite including a cloud-penetrating radar for mapping Titan, an improved version of the Galileo imaging system, and other modifications, would mass as much as the complete Galileo spacecraft, orbiter, probe, fuel, all, even when unfueled. Furthermore, it would carry two parasite probes, one that would be released before reaching Saturn and penetrate the atmosphere of the planet like Galileo’s probe, and another which would be released later to explore Titan. Altogether, and including the propellant needed for Saturn Orbit Insertion and other critical maneuvers, Cassini would set a new record for probe weight, at over six and a half metric tons at launch. In fact, Cassini was so heavy that even Saturn-Centaur with a substantial kick stage could not propel it directly to Saturn; instead, it would need to take a complicated path using multiple flybys of Earth and Venus before being able to speed on to the ringed planet, something which would increase the complexity of Cassini relative to Galileo still further. Consideration had been given to instead using a Heavy-Centaur, which
would be capable of directly injecting the probe onto a trans-Saturn trajectory, but although this would not significantly affect the lifetime cost of the probe, peak costs would be higher--too high for the science budget to support, especially given the already high projected cost of the program. Even then, substantial components, including the spacecraft’s entire propulsion system and the Titan probe--now nicknamed “Huygens” after the Dutch astronomer, mathematician, and physicist Christiaan Huygens--needed to be produced in Europe to prevent exceeding projected budgets.
With the program defined and budgetary authorization in hand, development quickly began. Although design and manufacturing would need to be relatively quick to meet the planned 1992 launch date, generous budgets and the problem being one less of entirely new development and more of integrating existing technologies like the advanced radioisotope thermal generators developed for Galileo or the thermal protection material invented for Galileo’s atmospheric probe into a single, coherent whole meant that scientists, engineers, and mission planners were optimistic about their ability to meet deadlines. As with virtually all large aerospace projects, however, these early assessments quickly proved inaccurate. No previous spacecraft had had to endure thermal and radiation environments ranging from the fury of the Sun around Venus to the cool and quiet of Saturn orbit. None had had to support so many parasite craft during such a long voyage from launch to probe delivery. None had needed such an endurance merely to complete their primary mission. Increasingly, as JPL and ESA engineers confronted these problems, it looked like Cassini’s launch might slip from 1992 to 1994, the next possible date when Venus could be used for a flyby.
In response, NASA returned to Congress asking for more money for the probe, hoping to throw enough resources at the spacecraft to complete it on time despite the difficulties. As the rapid budget growth that had characterized the agency’s funding through most of the 1980s was coming to an end, obtaining this supplementary funding proved more difficult than agency officials had anticipated. Despite failing to obtain these additional monetary resources, JPL leadership was still officially aiming for launch in 1992, hoping to simply push its existing personnel and technical resources harder to make up the difference. With the scientists, engineers, and technicians involved in the efforts to prepare the probe slowly coming to a consensus that the probe could not possibly be ready by that time, morale began a slow-motion collapse, strained by the disconnect between management and the workers actually in charge of implementing the program, further slowing Cassini development.
In the wake of Bush’s “constellations of exploration” speech, Cassini gained prominent billing as the largest and one of the most important NASA planetary exploration missions planned for the next decade. Increased funding followed increased attention, but by the time additional resources began to flow into the program, it could not realistically be ready by 1992. As 1990 wore on, management was finally forced to face this fact, officially delaying launch from 1992 to 1994. With two more years to build and test the spacecraft, more funding and resources flowing into Cassini accounts, and increased support by upper-level management, morale recovered and the program began to get back on schedule. Even when Gore was elected, his budget-cutting instincts and a more budget-conscious Congress found a riper target in the as-yet inchoate Ares Program than in the more concrete and nearly ready Cassini, sparing it significant pain. By mid-1994, the probe had been completed and shipped to Kennedy for final systems integration and mating with its booster, and in early September was rolled out to the launch pad atop a Saturn M02-Centaur. Launch went smoothly, easily inserting Cassini onto its planned trans-Venusian trajectory.
With launch behind it, Cassini commenced on its voyage to Saturn. Although few of its instruments could usefully operate during the voyage except in an engineering capacity, those few which could, such as particle and fields instruments, were left running to gather what data they could, while the others were periodically tested to assure their continuing functionality. During its decade-long voyage to Saturn, Cassini slumbered as it flew by Venus, Earth, and then Earth again before finally being slung into the outer solar system. Bypassing Jupiter because of its intense, deadly radiation, which it had not been designed to resist, it was not until August 2004 that the probe finally stirred itself for its arrival at Saturn, jettisoning its probe onto a Saturn-bound trajectory and then making a short rocket burn to prevent the main probe from impacting the planet. As Saturn swelled ahead of the probe, more and more instruments were activated, checked out, and set to work collecting early data, until finally, just before Christmas, the spacecraft’s two parts arrived at the ringed planet simultaneously.
Much like the Galileo probe before it, Cassini’s atmospheric probe slammed into Saturn’s atmosphere traveling tens of thousands of kilometers per hour, far above hypersonic speeds even in the thin hydrogen-helium upper atmosphere of the planet. Instantly enveloped in a plasma sheath stretching for kilometers, the probe was subjected to decelerations of hundreds of gees as it slowed to a more palatable speed. Once it slowed sufficiently far that a parachute would not be ripped apart instantly on deployment, it fired a drogue through its heatshield’s backshell; moments later, the backshell and drogue detached and the main parachute spread itself, slowing the probe even further. Freed of its heat shield, the probe was now able to look around itself, exploring its surroundings with a variety of scientific instruments. Unlike its sibling, however, what greeted it as it began peering at Saturn from within was not a turbulent storm but the relatively calm southern midlatitudes of Saturn’s atmosphere. Although a thin haze surrounded it, and the ubiquitous and powerful jet stream winds were bearing the probe along, little else disturbed the probe’s descent through the atmosphere. A few minutes after opening up, it passed through a thin, high-level layer of clouds, before emerging once again into the open sky. As it fell, it constantly sampled Saturn’s atmosphere, probing its composition in great detail. Much like Jupiter’s, it was made mostly of hydrogen and helium--but that was not what most interested scientists. Like Galileo’s probe, what they were after was heavier, less common stuff: carbon, oxygen, argon, and other massive volatiles. Surprisingly, given its position farther away from the Sun, in the more volatile-rich outer Solar System, Saturn’s atmosphere proved to have fewer volatiles than Galileo’s probe had indicated for Jupiter’s--although whether this was a real difference between the two planets or an artifact of the very different situations the two probes had found themselves in as they entered their respective atmospheres instantly became an ongoing point of scientific debate and argument.
As the probe continued to fall, though, those debates were months in the future. The data needed to write the papers and create the conference presentations that would spur them on was still being transmitted to Cassini high above Saturn, not enlivening the memories of computer systems back on Earth. In the moment, the probe was still falling through the atmosphere of Saturn, slowed by its main parachute. Unlike Jupiter, Saturn’s lower density and consequently lower gravity meant that the probe was falling more slowly than Galileo’s probe had done while passing through similar pressure levels on the giant planet, even though for the same reason those pressure levels were located deeper in Saturn than they had been on Jupiter. If the probe continued to fall at the same stately speed, it would run out of batteries, terminating further data collection, long before it reached the deeper areas of most interest to scientists. As it passed through the one bar pressure level, roughly equivalent to sea level pressure on Earth, the solution Ames engineers had developed to this conundrum made itself known with the detonation of pyrotechnic devices around each of the risers connecting the probe with the main parachute, severing them in a single explosive action. No longer burdened by the parachute, the probe plunged away, deeper into the atmosphere, diving into the patchy but deep water ice cloud layer. Nearly an hour after it first entered the atmosphere, it ceased to transmit data, just as it had begun to indicate the tell-tale signs of a third cloud bank, composed of a water-ammonia mixture. The probe itself, like its Jovian counterpart, continued to sink into the planet until it eventually melted, then vaporized. With the probe’s signals cut off, Cassini turned away from Saturn and prepared itself for the most critical part of its mission yet: Saturn Orbit Insertion. As it passed through perikrone, its main engine, largely silent since launch, ignited. After burning for more than an hour, it shut down again, having placed Cassini into a highly elliptical Saturn orbit. At last, more than a decade after launch, and almost two since the program had started, Cassini was ready to begin its mission.
High above the ringed planet, the spacecraft’s electronic eyes had a grand perspective from which to observe the changing, fickle nature of the second gas giant. Just as Galileo had shown Jupiter to be a world of vast, rapidly changing weather interleaved with longer and slower climatic cycles, so too did Cassini follow in showing Saturn imitated its larger sibling. Around the north pole, a vast and curious hexagonal pattern surrounded a great and endless storm, fodder for endless speculation about alien lifeforms somehow manipulating the planet (although scientists quickly determined it was most likely merely a result of some strange fluid dynamics). In the south, a gigantic hurricane, complete with the first eyewall seen outside of Earth’s own atmosphere, occupied the pole, churning away endlessly, fueled by the planet’s rotation. Away from the permanent storms of the poles, other atmospheric disturbances rose, stormed (often the accompaniment of powerful lightning bolts), and died away, none greater than one that struck nearly halfway through the probe’s mission. Quickly growing to enormous proportions, the “Great White Spot” wrapped itself around the planet’s northern hemisphere, attaining a behemoth span far greater than any other storm ever witnessed on any other planet, even the famous planet-spanning Martian dust storms--which this one would have swallowed whole a dozen or more times over. As Cassini watched, the planet’s temperatures and prevailing winds shifted with the seasons, just like the inner planets or Jupiter. Even though too little data could be collected to definitively explore every aspect of Saturn’s climate, what was collected was enough for a hundred theses and more papers, fueling academic investigation for years.
Although exploring Saturn’s weather and climate was an important part of its mission, it was not the only or even perhaps the most important subject of Cassini’s explorations. After all, Cassini was in space, and from space only a vanishingly thin outer layer of the planet could be observed; even its probe could only penetrate into a single tiny region of the planet. Like the other giant planets, however, Saturn has a vast collection of moons, ranging from tiny specks of dust floating in its famous rings to the gigantic Titan, the second largest moon in the solar system. Comparatively open to Cassini’s observations, these, not the planet itself, had been the primary focus on Cassini’s mission since early planning on SOWP had begun. Again like the other giant planets, these moons proved to be far more varied and active than astronomers in the middle of the twentieth century, before they could be observed from close range, had thought, and, despite the revelations of the Voyager probes, even more than had been suspected only a decade or two earlier.
Chief among the moons which Cassini was targeting was mighty Titan, by far the largest of the planet’s collection. The only moon in the solar system to possess an atmosphere of any significant thickness--indeed, thicker than Earth’s--Titan had also been a primary target of Voyager 1’s Saturn flyby, but had frustrated the probe’s observations through a thick layer of virtually opaque haze enveloping the entire globe. Scientists, although disappointed, had not given up their interest in the moon, and Cassini came prepared to pierce the haze through three methods. First, spectral analysis through a variety of methods had shown there were very narrow “gaps” in the haze at certain frequencies of infrared light, which Cassini’s optical instruments were sensitive to. By imaging the moon at those frequencies, pictures could be taken of deeper regions of the atmosphere, even the surface, a technique demonstrated by the Hubble Space Telescope in the late 1980s and early 1990s. Second, like the Venus orbiter VOIR, Cassini carried a radar capable of ignoring the moon’s clouds and hazes altogether to image the surface directly. This had been one of the highest-priority instruments for a Saturn mission since the discovery of the frustratingly opaque haze layer, despite its significant weight and power consumption, and its presence aboard Cassini had been a given since the very earliest SWOP design concepts. Finally and most dramatically, Cassini was not alone in its mission to explore Titan. It carried not just the single probe it had dropped in Saturn’s upper atmosphere, but a second, designed and built by ESA, intended for Titan and Titan alone.
When ESA took on this task in the 1986 Memorandum of Understanding which finalized the exact arrangement of American and European contributions to Cassini, they were confronting one of the trickiest tasks ever faced by the designers of a planetary entry probe. Despite observations from Earth, the Hubble Space Telescope, and Voyagers 1 and 2, very little was known about Titan’s surface. The most intriguing observations were those of methane and ethane, light hydrocarbons that would rapidly break down in Titan’s upper atmosphere. If they were being seen, that meant that there had to be some kind of source at the surface. Some models suggested that this source might be alien volcanoes, erupting liquid methane or even water from within the surface, while others indicated that the moon might be englobed in a vast, cold ocean of methane and ethane, a strange and unusual sea--but the first found off of Earth. Lacking certainty, ESA designed the probe against any eventuality. Huygens would be able to float on alien seas, survive landing on alien soils, endure atmospheric pressures half again or more as great as at Earth’s surface, cold and hot, transmitting useful data from them all. Building on experience from the Mars Surface Elements they had built for the Soviet Mars 12/13 missions and information from NASA Ames, which had constructed the Galileo entry probe and was constructing the Cassini Saturn entry probe, ESA quickly went to work on the spacecraft. Like the rest of Cassini, they quickly ran into problems. Titan, after all, was a considerably different environment from the surface of Mars or the atmosphere of Saturn, and many of the necessary requirements had little in common with the areas they had drawn experience from. Moreover, Huygens was intended to last not for the months of the Mars Surface Elements but for mere hours, perhaps transmitting some data from the surface after its dramatic plunge through the atmosphere. Although the Europeans had some experience with short-lived, battery-powered spacecraft, ensuring the necessary performance under Titantian conditions and after more than a decade in space was something else, and ESA welcomed the postponement of the launch with its own sigh of relief.
Several months after entering Saturn orbit, Cassini ejected its remaining probe onto a Titan-crossing trajectory, then, a few days later, carried out a brief burn to remove itself from the danger of encountering the moon personally. A few weeks later, Huygens hit Titan, screaming into its atmosphere at thousands of kilometers per hour. Despite traveling much more slowly, like its sibling a kilometers-long streamer tail of shocked plasma burst into existence around the probe as it hit Titan’s atmosphere, trailing away from the probe as it rapidly decelerated in the thin upper atmosphere. Within minutes, it had slowed enough for first the drogue and then the main parachute to deploy, allowing it to eject the rapidly cooling heat shield and begin collecting data. As it fell through thin haze, haze that would pervade the entire visible atmosphere throughout its mission, it detected winds rivaling those of the great Martian dust storms, though far from Saturn’s fury. Chemical samplers, greedily sucking up the Titanian atmosphere and putting it through complex equipment, found complex organic molecules throughout the atmosphere, already known from remote sensing but now sampled in greater detail. A thin ionosphere was detected at lower levels, probably the result of galactic cosmic rays hitting Titan’s atmosphere. As Huygens continued to fall, these instruments built up vertical profiles of wind speeds, atmospheric composition, and more, all the while radioing the data back to Cassini and to sensitive radio telescopes back on Earth, tracking Huygens’ signals to provide a back-up wind measurement and determine its position.
Nearly two hours after entry, as it neared the surface, its descent camera was finally able to penetrate the haze. Hindered not only by the haze but by Titan’s dim sunlight as it drifted downwards, it was nevertheless able to return the first actual pictures of Titan’s landscape ever seen on Earth. In its images were gently rolling, brightly-colored hills, etched with channels that appeared to have been carved by flowing liquids. Nowhere in sight were the extensive dark-colored areas that scientists had suspected of being lakes or seas, a disappointment for those who wanted a definite answer to their composition and state, nor any indication of liquid actually flowing through the channels Huygens was seeing, at least while it was descending. As it neared the surface, the photographs and surface data it was radioing to Cassini became ever more detailed and informative, culminating in a set of final images transmitted just as it was about to touch down, showing the Titanian surface in magnificent detail. Unfortunately, moments after touchdown, Cassini and Earth lost contact with Huygens, with no trace of the signal being detected between the projected touchdown time and when the orbiter finally would have descended below the horizon as seen from the landing site. A joint NASA-ESA board of inquiry determined that the most likely cause of the failure was inadequate rad hardening on the primary and backup radio transmitters during assembly, coupled with errors in the transmitter firmware that was supposed to oversee the transition from descent to landing operations. Both had been largely copied from the earlier Mars Surface Element probes, then modified to meet the different conditions that would be encountered at Titan in order to save money during Huygen’s development and assembly. However, the vastly different conditions encountered by Huygens during cruise and operations were not, in fact, fully insulated against errors that might be caused by those changing conditions. Although the inability of the board to examine flight hardware meant that this could only ever be a provisional finding, the fact that both transmitters had behaved erratically during descent supported their findings.
Despite the disappointment of Huygen’s failure on landing, however, the data it returned, together with the other data returned by Cassini during its many flybys of the moon, proved a vast and valuable source of information on Titan, greatly refining scientific knowledge of the body. And besides, much as with Saturn itself Cassini was intended to do more than just explore the largest of the planet’s many moons. All of the major moons of Saturn received their own flybys, from Mimas, at the outer edge of Saturn’s main rings, to Iapetus, the most distant and probably most famous, after Titan, of Saturn’s large moons. Of these flybys and the discoveries they represented, the most unexpected were the observation of great geysers of water erupting from certain areas around the south pole of Enceladus. This small, icy moon, previously thought to be of little interest, suddenly found itself catapulted nearly to the top of the shortlist of planets and moons thought most likely to harbor life, behind only Mars and Jupiter’s moon Europa. Although it had elicited relatively little interest in pre-mission planning, now scientists talked about a possible future mission dedicated solely to exploring the moon, perhaps even returning samples from its geysers to Earth. For the moment, with the Artemis program and other probes occupying the agency, this amounted to little more than idle discussion, but, then again, without idle discussion at some point no space mission would ever have been launched. In the meantime, Cassini continued to explore the Saturn system, being repeatedly extended to allow it to spend ever more time harvesting yet more data on the entire system--the planet, its captivating and beautiful ring systems, and its multitude of moons. As it entered its second decade of operation, Cassini had not only a storied career behind it, but much to look forwards to.
