Well, I hope everyone is having a good New Year! 2012 has been an amazing year for me, I've gotten to see a lot of things happen that I'd been waiting a long time for, both in my personal life, my professional career (well, to the extent that I can have one pre-graduation), and in the field of spaceflight. The pictures of Dragon berthing and the videos of Grasshopper...they give me chills, even on the tenth viewing. And next year...there's a bunch more where all that came from. But me rambling about my life isn't why you came to this thread, is it? Well, you're in luck, then, because it's that time again. When we last left the exploration of the Outer Planets, the four Voyager probes had completed their Grand Tour, flying by every major body in the outer solar system. However, a fly-by is only the beginning. To really learn in-depth, you have to get into orbit, and with that in mind we turn the focus of Eyes this week to the King of Planets, Jupiter.
Also, to briefly turn back to my ramblings before the post proper, a production update: work is now proceeding on both Part III and the couple of posts left to be written for Part II, to the extent that it's a bit of a minefield to navigate the Google Drive we use--I keep pulling up the documents for Part III when I want their Part II equivalents. That's just anecdotal, but things are coming along, and we should have a pretty smooth schedule through the end of this part, and we're hoping for a relatively short haitus. That last part will depend a lot, though, on how much we can power through over the rest of break. Anyway, without further digression...1076 replies, 132871 views
Eyes Turned Skyward, Part II: Post #19
The exploration of the Jovian system was in no way completed by the Voyager missions. Even before the first Voyager climbed off the pad--indeed, before the first Mariner had been launched--the Jet Propulsion Laboratory and Ames Research Center had been studying the logical next step to the flyby missions of the Pioneers and Voyagers; an orbiter designed to survive for years in close proximity to Jupiter, touring the moons and dropping a probe into its atmosphere to directly explore it. Over time, the designs developed by these two centers merged into a single probe, built by both Ames and JPL, which was finally approved in 1976, just after the successful touchdown of Viking 1 on Mars and the consequent (though short-lived) burst of enthusiasm for planetary exploration. Along with the Hubble Space Telescope and the Kirchhoff comet probe, it would be a "cornerstone" mission of the 1980s, and would not coincidentally define the design of a new upper stage being developed by the Lewis Research Center to allow NASA to retire its Titan IIIE fleet from service. It had always been intended that the Titan IIIE be an interim vehicle, with a new version of the venerable Centaur being developed by the Lewis Research Center, responsible for the original Centaur design, to allow the Saturn IC to serve as both the primary crew and probe launch vehicle for the 1980s and beyond. However, through most of the 1970s advocates of the "Big Centaur" and "Little Centaur" had been carrying out a constant battle over which concept would better serve NASA's needs, with "Big Centaur" supporters pointing to improved performance and "Little Centaur" promoters favoring simpler development. The needs of Kirchhoff and Galileo were such that the "Little Centaur" could not possibly provide them, finally providing the momentum needed for "Big Centaur," or Centaur E (as it became known) to win out and begin development. With an increased hydrogen tank diameter, Centaur E would be capable of directly inserting Kirchhoff into a heliocentric orbit, and sending Galileo directly to Jupiter. In a historical irony, Centaur had nearly been cancelled due to Saturn; now, it would bring to its zenith Saturn's capability for launching interplanetary missions.
With launch vehicle defined and program approval, the Galileo team buckled down to work. A joint project of Ames and JPL, Galileo would be managed by the latter, although the former would be responsible for the particle and fields instruments and the planetary probe, due to their great experience with both in the Pioneer program. Galileo would take full advantage of the technology developed for the Pioneer 10/11 and Voyager missions, particularly in the realm of hardened electronics, necessary for survival in the Jovian environment, but also in more mundane areas such as the CCD imagers developed for Voyager-Uranus, significantly modified for greater radiation resilience and higher quality images for use aboard Galileo. The greatest technical difficulty encountered during development was the probe, which would have to survive an incredibly hostile entry environment, sometimes compared to entering the Earth's atmosphere directly through a nuclear fireball, although the orbiter's complex spin-despin bus design, together with its large instrument suite and the exceedingly hostile near-Jovian environment added their own difficulties. Despite this, the main problems faced by Galileo were all financial, as the Reagan administration attempted to cut planetary science budgets and forced a delay in launch, already behind due to development difficulties, from 1983 to 1984. Despite being "all-American" and therefore without significant international components unlike its main competitors for research dollars, its advanced state of development and the defense implications present in any spacecraft which could survive nuclear fireballs or Jupiter's intense radiation fields protected it against further budget cuts, and it was able to proceed to its new launch date on schedule. The first launch of the Saturn-Centaur in early March sent Galileo speeding out towards Jupiter, with no difficulties experienced in deploying the high-gain antenna and performing initial post launch activities. For the next three years, Galileo explored nothing but interplanetary space, doing little other than collecting particles and fields data in a little-explored region of the Solar System and conducting occasional engineering tests as it slowly climbed towards the king of the planets. At last, in mid-1987 events began to speed up as Galileo close in on Jupiter, releasing its probe some five months before encounter, then performing a maneuver to ensure it did not follow the probe into Jupiter's atmosphere. As it got closer and closer, it detected modulation of the solar wind by the increasingly nearby Jovian magnetosphere, then the planet's influence itself. At the same time, the level of detail its cameras could resolve on Jupiter reached and then exceeded the best images possible from ground-based or Earth-orbiting telescopes. Finally, it was Arrival Day.
Streaking in at 60 km/s, the probe impacted Jupiter south of its equator, slamming into the planet like a subcompact hitting a speeding semitrailer. At that, however, it was lucky, as the winds at its entry point were blowing away from it, reducing by nearly a fifth its effective entry velocity. Nevertheless, as it entered the atmosphere over half of the heat shield burned away and acceleration peaked at over 250g, enough to quickly kill anyone on board. As a robot, however, the probe stoically endured its sentence, ejecting its heat shield and deploying its parachute just under three minutes after first encountering the atmosphere. As its instruments began collecting data, they immediately noted the presence of a thick ammonia cloud deck. Although only a few minutes passed before the probe had passed through and out into clearer skies, soon afterwards it entered another thick cloud deck, this time composed of ammonium hydrosulfide. As it continued to descend, it collected data on the density, pressure, and temperature of the Jovian atmosphere, along with recording multiple powerful lightning strikes in the surrounding clouds, suggesting intense and highly active storms. Fortunately, the probe itself was not struck by lightning during the descent! Over twenty minutes after entering the atmosphere for the first time, the probe encountered the expected water cloud layer, the thickest and most active of all. In fact, the clouds through which the probe passed were so thick and active that initially many scientists seriously questioned standard models of Jovian formation, suggesting that the planet might have a massive, icy core from which the evidently high volatile content of the planet’s atmosphere might have been liberated by the planet's extreme heat. Imaging by Galileo and Earth-based telescopes of the probe's entry site, however, revealed that the probe had passed through a powerful storm, a "white spot" near a zone-belt boundary. Like dropping a probe into an Earthly hurricane or cyclone and then trying to extrapolate to the rest of the Earth's atmosphere, this would give a highly distorted picture of typical conditions and compositions in Jupiter's atmosphere, with the unusual Galileo probe results being nothing more than that, unusual. While the probe itself was descending, however, all this was far in the future, and it cleared the water cloud layer several minutes later. Through the now-clear skies it continued to fall for nearly half an hour more, before finally the rising temperature caused the radio transmitter on board to fail. Eventually, the probe melted, then vaporized from the increasingly high temperature of the interior, becoming one with the planet.
While all this was going on, the orbiter waited, patiently retransmitting everything the probe beamed back to Earth while its own tape recorder carried data from the moon flybys it had performed before the probe's entry into Jupiter. At last, once the probe ceased transmitting, the orbiter began preparations for its Jupiter orbital insertion burn, located near the bottom of Jupiter's gravity well and therefore perfectly positioned for using the Oberth effect to maximum effect. While diminutive compared to the F-1A that had driven it aloft, the engine propelling Galileo could and would burn for nearly an hour to complete its mission, slowing Galileo enough for Jupiter to capture it, allowing Galileo to finally begin its primary mission. Galileo would sling around the Jovian system, repeatedly visiting the outer three Galilean moons even while observing the planet itself and the environment around it. During these flybys, Galileo confirmed the impression of Voyager researchers that Europa might have a subsurface ocean, providing strong evidence that not only was that the case, but some mechanism coupling the ocean and surface existed to resmooth the surface, eliminating hints of impact craters and making Europa look nearly as resurfaced as Earth or Io. Besides this, Galileo made the surprising discovery that both Ganymede and Callisto, the outermost and seemingly least active of the Galilean bodies, also possessed subsurface oceans, although located far deeper within their crusts than Europa's. In addition, the interaction of Ganymede with the powerful Jovian magnetic field showed that it had a core of metal, similar to the Earth, capable of creating a magnetic field in its own right. As with the earlier Voyager missions, Galileo was showing that the icy moons of the outer solar system were far more active and dynamic bodies than had previously been suspected. The same was true of Jupiter, as high-resolution imagery and videos made by the orbiter showed a welter of fine atmospheric details impossible to make out in Voyager or Pioneer imagery. Especially in combination with Voyager results, Galileo was able to show that, like Earth's atmosphere, the visible layers of Jupiter undergo significant long-term and seasonal changes, with major variation in cloudtop wind speeds, temperature distribution, and the fine structure of even long-lived storms and weather features.
The second phase of Galileo's mission was the extended mission. Free of the constraints imposed by the primary phase, and with a bevy of preliminary results to guide them, scientists could choose the most interesting available targets for study. While many proposals were made during the primary mission and even the transit phase, in the end, there was only one choice: Fire and Ice (as NASA promoted it). Or rather, a series of flybys of Europa from a variety of angles and altitudes, designed to probe the many intriguing aspects of the moon's icy crust, followed by close flybys of volcanic Io to further characterize not only the moon itself but also the surrounding space. A series of radiation-induced faults prevented data return from several passes, and limited the operation of several instruments on others, but the spacecraft was able to work through them with the aid of its handlers back on Earth and return a great deal of information about Europa, including a considerable amount of data supporting the ocean hypothesis, data that seemed to constrain the thickness of the crust, and data about the deep interior of the moon, below the ocean. Additionally, the probe collected data about Jupiter's atmosphere, which when compared to Voyager data and earlier Galileo data allowed the first analyses of Jupiter's seasonal cycle, and observations of the other Galilean moons. Later in the first extended mission period, just prior to the planned Io flyby, it also dipped increasingly deep into the Jovian magnetic field, exploring regions closer and closer to the planet that had previously just been browsed while also making relatively close flybys of Callisto and Ganymede to assist in lowering its orbit so that it could pass by Io. Finally, during the two close flybys of Io, Galileo returned the first ultra-high resolution imagery of the moon's surface, confirming the existence of silicate lavas on Io and providing information on Io's magnetic field during the second, polar pass. Unfortunately, radiation damage caused further issues with Galileo's systems and it was unable to return all the planned data from this series of flybys either. Meanwhile, Galileo's instruments had been collecting vast amounts of data about the electromagnetic and particle environment around Jupiter, recording how the planet responded to variations in solar behavior, and thereby providing a unique and impossible to duplicate perspective on the Sun.
The most exciting and unfortunately the last phase of Galileo's mission came about by chance during early 1992. During routine engineering test imagery of the space around Jupiter, the probe detected an object showing a distinct coma in one of its starfields. Interested, the imaging team scheduled further imagery of roughly the same area of space, hoping to catch the comet again and begin orbital calculations. These observations duly confirmed the discovery, and the comet was recorded by the International Astronomical Union's Central Bureau for Astronomical Telegrams as Comet 1992d, then Comet Galileo. It was quickly realized that Comet Galileo was no ordinary comet. In particular, it seemed to be orbiting Jupiter, not the Sun, something which had been predicted but never before seen, and by itself enough to inspire curiosity about the object. Furthermore, it was calculated that in July the comet would make a very close pass to Jupiter, possibly within the planet's Roche limit. If this occurred, the comet might break up from the stresses, something never before observed. While certainly interested, the Galileo science team made only a secondary effort to observe the flyby, as it was much too late to significantly change the probe's orbital behavior to ensure optimal coverage. Nevertheless, the probe was able to image the comet's close pass to Jupiter and its subsequent fragmentation, along with the Hubble Space Telescope and a number of other Earth-based observatories. This, however, was just the beginning, for after the comet broke up during the close pass revised orbital calculations showed that it would not merely make a close flyby in 1994, but instead the remains of the body would plunge into Jupiter itself. This sent the scientific value from "unprecedented" to "incalculable," as Comet Galileo was so large that it might be centuries or millennia before another such event occurred. Besides that, the 1980s had seen a great deal of speculation on the importance of cometary and asteroid impacts on the history of the Solar System, most prominently the theory that a large body had hit the Earth 65 million years earlier and caused the extinction of the dinosaurs. Actually observing such an impact could help constrain such theories by providing data about what really happened during such impacts. Furthermore, it was possible that the comet fragments might punch holes in the upper atmosphere large enough for Galileo to collect data about lower atmospheric levels, previously only explored briefly by the Galileo probe. Altogether, whatever plans for the extended mission had existed prior to 1992 no longer mattered, with the utmost importance instead being that Galileo would be in position to watch the fragments as they impacted, something which would not be possible from any other platform in the solar system. When the time came in 1994, Galileo was ready, and provided a spectacular front-row seat not just to scientists, but to the entire world, which had become fascinated by the impending plunge into Jupiter. While some of the more extreme suggested outcomes did not occur, the observation of gigantic fireballs and huge, dusty scars easily visible from Earth and persisting for months in the Jovian atmosphere lent an ominous plausibility to tales of impactors devastating Earth and causing the collapse of global civilization. After completing the return of its impact data to Earth, Galileo's orbit was reshaped to intersect Jupiter itself on its next perijove, to avoid possibly contaminating Europa's global ocean with Terran organisms. In a fiery plunge into the atmosphere, it--very briefly--continued the scientific mission its own probe had carried out some eight years earlier, reporting on the conditions of Jupiter's upper atmosphere, near-Jovian magnetic and electric fields, and the charged particle environment very close to Jupiter until it finally failed in the heat and stresses of Jovian entry.