Transcription
Far beyond the reaches of the four rocky planets, lurking on the opposite side of the asteroid belt, we find our solar system's showpiece worlds: the two gas giants, Jupiter and Saturn. As the two largest planets, both exhibit their own range of fascinating phenomena, making them tantalizing targets for probes exploring the planetary neighborhood.
But with their tremendous gravitational influences, their irradiating magnetospheres, and above all, their vast distances, reaching these worlds with a probe presents a much greater challenge than traveling to one of the terrestrials. But in the early 1970s, the technology for more reliable probes capable of long voyages was primed, and missions to both Jupiter and Saturn ensued. And it is those missions that make up the topic of our video today.
In this second episode on our trilogy of probes that studied the solar system, we will reexamine their revolutionary findings over the years, from snapping breathtaking shots of their upper atmospheres to revealing seismic secrets about the moons that surround them. This is our journey to the gas giant planets.
The gas giants are the two principal planets of the solar neighborhood, both stunning, striking worlds with rich, extended atmospheres, each showcasing its own plethora of extreme weather processes. Any journey into their territory starts with Jupiter, the Godfather Planet. More than double the mass of all the other worlds combined, the clouds swirling in its upper atmosphere weave a vibrant tapestry of our solar system's oldest and largest planet. Saturn, on the other hand, is the solar system's Crown Jewel, with a smoother surface and an even greater number of moons, along with one of the most spectacular sites in space: its unparalleled planetary ring system. Furthermore, both of these Giants host their own collections of planet-sized moons, some with eyebrow-raising prospects for habitability.
As such, missions to study the gas giants have often found themselves at the top of NASA's to-do lists, with the first Jupiter-bound mission conceived as early as 1959. However, both worlds lie significantly farther from the Sun than the planets of our terrestrial neighborhood. The closer of the two, Jupiter, is around five times the distance separating the Sun and the Earth, while Saturn is around double that. As such, a voyage to either of these two worlds would take years instead of months, and for the planets lying beyond Saturn, whole decades would be required. It didn't seem feasible for the somewhat temperamental, short-lived probes of the late 1950s, which had barely begun to escape Earth's gravity.
And so, moving into the 1960s, as NASA set its sights on the bodies of the outer solar system, the scientists at their Jet Propulsion Laboratory began investigating ways to trim down this journey time, using both clever engineering techniques and natural means. As we covered in the last episode, humanity's maiden voyages to both Venus and Mars were launched during their respective windows of closest approach, i.e., when the orbits of either planets aligned them most closely with the Earth, enabling the simplest, shortest trajectory between the two. And in 1964, a study by the JPL revealed that a similar planetary alignment event involving all four of the large planets would take place sometime in the late 1970s, the likes of which only occurred once every 177 years.
This would present a once-in-a-lifetime opportunity to visit each of the giant planets, and perhaps even Pluto, with a spacecraft in only a fraction of the time required otherwise. By using each stop along the way for a gravitational boost, as we touched on last time, it had long been known by scientists that when icy comets, such as Comet Halley, passed by a planet, their velocities seemed to increase on the way out, as they received a gravitational assist upon exit. Applying this technique to a probe, specifically one flying by Jupiter, would induce a similar gravitational slingshot effect, which could effectively treble its speed and correct its course without the need to carry any extra fuel, pruning the journey time to Neptune down from around 4 decades to about 1 and a half.
As such, the JPL quickly conceived of an ambitious series of long-distance probes that would make use of this alignment by using Jupiter as a Stargate to visit all five of the then contemporary planets in the outer solar system. This project would eventually materialize as the world-famous pair of Voyager probes, but that is a story for another day. In the meantime, however, NASA sought to practice this gravitational slingshot maneuver before the Voyagers arrived, while also ticking off their now most pressing planetary targets: Jupiter and Saturn.
And so, in 1969, they signed off on a pair of probes as part of their Pioneer program, a dormant series of spin-stabilized probes which had some years earlier been used to study space weather. These missions, however, then designated Pioneer F and Pioneer G, would carry over old successful designs to create a new, more streamlined and resilient pair of probes fit for a much longer interplanetary voyage, one which would take them to the outer solar system. The former, today known as Pioneer 10, would become the first probe to cross the asteroid belt and reach Jupiter before receiving a gravitational boost and ultimately shooting off into deep space. Pioneer G, on the other hand, today known as Pioneer 11, would repeat these same steps but would then use its gravitational boost to carry on to the other gas giant planet, Saturn.
Hundreds of scientific experiments were proposed for both spacecraft, and their construction was estimated to have required more than 25 million working hours. Aboard their final set of mounted instruments were helium magnetometers, plasma probes, charged particle detectors, cosmic ray telescopes, radiation detectors, asteroid and meteorite trackers, Geiger tube telescopes, and finally, optical instruments for studying invisible ultraviolet and infrared wavelengths.
Pioneer 10 launched from Cape Canaveral on the 2nd of March 1972, spending about 4 months making its way towards the inner edge of the asteroid belt between Mars and Jupiter. It would enter this circumstellar fence of rocky projectiles on the 15th of July, but unlike the stuff of science fiction, its passage was smooth, barely straying within 10 million km of a large object during 7 months on the inside. And save for some negligible impacts with dust particles, Pioneer 10 emerged from the asteroid belt unscathed on the 15th of February in the following year, where it started to return the first heliophysics data from the space beyond the terrestrial neighborhood. It probed the solar winds and cosmic dust for their chemical constituents, while also detecting the first helium atoms from interstellar space.
After clearing the asteroid belt, Pioneer 10 maintained its outward passage for a further 9 months before eventually coming within 25 million km of Jupiter and initiating its observations phase on November the 6th. In the month that followed, the probe homed in on the Godfather Planet and studied its surrounding environment. In doing so, it amassed the first collection of more than 500 groundbreaking images of the giant planet and its Great Red Spot before making its closest approach on the 3rd of December.
But as the craft crossed Jupiter's magnetic equator, it was exposed to a huge electron flux, some 10,000 times stronger than the most corrosive surrounding Earth. This wreaked havoc on the probe's instruments by generating a series of erroneous commands which threatened to derail the mission altogether. While its engineers were able to rectify most of the troubling issues, only six of the spacecraft's 11 core instruments remained operational. As a result, an image of the moon Io, as well as some close-ups of Jupiter itself, were lost. And while the craft was able to capture two of the other Jovian moons, Europa and Ganymede, only images of the latter yielded any meaningful detail.
Upon starting its exit from the Jupiter system on December the 4th, Pioneer 10 became the first probe to receive a gravitational assist from another planet, catapulting the craft on route towards its final destination: interstellar space. Having accrued so much velocity from its trajectory through the Jupiter system, Pioneer 10 had now been set on an irreversible path, becoming the first man-made object to reach the solar system's escape velocity, i.e., the speed needed to overcome the gravity of the Sun. For the duration of the journey beyond, covering more than half a billion kilometers a year, Pioneer 10 crossed the orbit of Saturn just 3 years later in 1976, followed by the orbit of Uranus in 1979, and then Neptune in 1983.
It would continue along this path for another 15 years almost, before its vanishingly weak signal became too faint to transmit any readable data. But while the mission was officially concluded on the 31st of March 1997, Pioneer 10 would continue to beam back the occasional featureless telemetry signal for a further 6 years. In fact, the very last contact with the probe was on the 23rd of January 2003, from a distance of more than 80 astronomical units, finally capping off what was originally intended as a 21-month mission, after more than 30 years. A true testament to the craft's engineering and the time that was invested in future-proofing its design.
Just over a year after the launch of Pioneer 10, the subsequent follow-up mission, then known as Pioneer G, was prepared to follow. Like the former, this spacecraft would float across the asteroid belt en route to the Jupiter system before passing the large planet to receive a gravitational boost. But unlike its predecessor, which was slingshotted straight into deep space, Pioneer 11 would carry on towards the other gas giant, becoming the first probe to visit the solar system's Crown Jewel. There, it would study its atmosphere, magnetosphere, and its planetary rings, as well as the many intriguing moons in orbit around the Giant. And upon completion of those pre-programmed objectives, Pioneer 11 would be used to test a slingshot maneuver of its own, flying through Saturn's ring plane.
The probe lifted off from Cape Canaveral on the 5th of April 1973, embarking on an 18-month voyage to Jupiter through the asteroid belt. It encountered the giant planet in early November 1974 and made its closest approach a month later, on the 2nd of December. It came within just 43,000 km of Jupiter's cloud tops, three times closer than its counterpart the year before, but it did so at a much higher velocity, resulting in less prolonged exposure to damaging radiation. As such, all of the craft's 11 instruments this time survived their encounter, snapping over 200 even sharper images of the planet, including the first shots of its polar region and more of its swirly outer atmosphere. The probe also enjoyed more success in studying the Galilean moons, snapping shots of Io and making measurements of Callisto, where Pioneer 10 was unable.
Upon receiving its bespoke gravitational boost, Pioneer 11 would venture onwards for another 5 years before encountering its next target, Saturn, on the 29th of August 1979. Pioneer 11 entered the Saturn system and spent 3 days sinking towards the planet, en route to its closest approach, just 21,000 km from its outer edge, snapping increasingly close-up images all the while. It then became the first probe to cross Saturn's ring plane as a dry run for the voyage of spacecraft which were already on route to the gas giant.
In order to receive the kind of gravitational boost from Saturn required to push further out into the solar system, it is necessary to fly close enough to the planet that the extended ring plane of particles must be breached. Therefore, Pioneer 11 would practice flying through a gap in Saturn's rings in order to map a sufficiently clear route for the Voyagers. And if it had sustained any serious damage upon colliding with unseen moonlets, Voyager 2's ice giants mission to Uranus and Neptune would probably have been scrapped. But Pioneer 11 glided through smoothly with no problems and received a gravitational assist from Saturn that sent it hurtling onwards on the interstellar phase of its journey.
The Pioneer probes maintain their outward passages to this day, both now over a 100 astronomical units from Earth, verging on the very periphery of the Sun's extended atmosphere. And their journeys will continue for millions of years to come, first entering into stellar space and then eventually passing by other stars in the Taurus and Scutum constellations. One can only guess at what or who these probes might encounter given enough time. Perhaps the Pioneers will pass by planets hosting intelligent life.
And though both probes are now defunct, we did make alternative arrangements for this optimistic eventuality. Attached to both spacecraft is a postcard from humanity to anything that might find them in the distant future. The Pioneer plaques were designed by Carl Sagan and his peers. They are gold-finished aluminum slabs inscribed with general information about humanity, including a depiction of the human body overlaid on the probe for scale. It also depicts the order of the solar system and features a pulsar map for triangulating the position of our planet using neutron star emissions for reference. However, each one of these symbols has been criticized for being much too anthropocentric. Not to mention the fact that the pulsar map is now obsolete. But if nothing else, these plaques and their accompanying probes are unmistakably engineered. And if intelligent life does exist around other stars, as Sagan predicted, then perhaps they may one day see these probes streaking across their skies. Imagine if we found something like this cruising around the solar system. The repercussions on society, culture, and politics would be unprecedented.
With both gas giants reached by the dawn of the 1980s, the next step would be to return to these worlds with more persistent probes capable of not just flying past, but orbiting the gas giants. In the decade just gone, Mars orbiters had gone from science fiction to reality, most notably through Mariner 9, a groundbreakingly successful mission to study the Red Planet for more than 8 months through 1972. Therefore, when work began on a Jupiter orbiting probe later that decade, successful designs from the Mariner program would be carried over.
But by the time this spacecraft had materialized almost a decade later, its administrators felt that its technology had evolved sufficiently beyond its Mariner bases, warranting its own mission classification, and was thus reassigned as the Galileo spacecraft, named in honor of the discoverer of the four large Jovian moons, Galileo Galilei. This probe consisted of both an orbiter and a detachable module that would be dropped into Jupiter's upper atmosphere. However, the technology to both develop and test this Galileo entry probe would not become available until during the 1980s, and other unforeseen factors also hampered and delayed the Galileo mission. Everything from bad weather, test failures, lawsuits targeting the probe's nuclear technology, and even the Challenger disaster led to its launch being delayed all the way through until 1989.
But at last, the probe did finally lift off from Earth on the 18th of October, aboard the Space Shuttle Atlantis, which delivered it to a low Earth orbit. It would then execute a series of engine burns and thrusts to guide it into a heliocentric orbit, where it would receive both slingshots and course corrections from flybys of the Earth and Venus. After acquiring enough momentum, the probe then embarked on a long, arcing trajectory through the asteroid belt before coming within range of the Jupiter system in mid-July 1995.
Almost immediately, the Galileo entry probe was released and slowly drifted towards the cloud tops. Alongside its orbit, it would take another 5 months before the entry probe finally breached Jupiter's atmosphere on the 7th of December, traveling at around 50 km/s on impact. It then spent around an hour sinking beneath the clouds' edge, plunging to a depth of more than 180 km, hardly scratching the surface in the grand scheme of the giant structure, but still returning invaluable insights on the increasingly hellish conditions unfolding below. It measured chaotic wind speeds of more than 600 km per hour with temperatures climbing to a scorching 16,000 Kelvin before the probe's heat shield finally succumbed.
Back in space, the orbiter prepared to fire its engines to decelerate and achieve orbital insertion, making it the first probe to be captured by a world in the outer solar system on the 8th of December. Over the next 7 years and 9 months, it would take no less than 35 long laps around the Jupiter system, spending only short periods of time up close with the planet to give its instruments time to recover from exposure to radiation. Over the course of its mission, it assembled an extensive, comprehensive dataset for the Godfather Planet and its surrounding environment, making astounding discoveries on both counts.
It captured an active volcanic eruption on the torrid moon of Io and, on closer inspection, revealed lava flows on its surface for the first time. It also mapped Ganymede's surface in much greater detail, identifying complex magnetic field activity around this almost Mars-sized moon. And on Europa, it uncovered the strongest evidence yet for a long-speculated ocean of liquid water atop which the moon's icy crust is thought to be suspended upon.
Exhausting its fuel reserves in September 2003, the Galileo spacecraft's engines were fired for the last time, as the probe was intentionally steered into Jupiter in order to avoid crashing on one of the moons and contaminating a potential subsurface ocean. It then followed its entry probe to a crushing demise and concluded its mission on the 21st night of September 2003.
But as one door slammed shut, another swung open, because the end of the Galileo mission to Jupiter coincided with the start of the first Saturn orbiting mission by the spacecraft Cassini-Huygens. Cassini-Huygens is one of the flagship missions of the 21st century. Like Galileo, the spacecraft consisted of an orbiter accompanied by a detachable probe, but not an atmospheric probe this time around, rather a landing module for plunging into the thick, dense clouds that shroud the surface of Titan, Saturn's largest moon.
This mission was conceived in the early 1980s, only shortly after the success of Pioneer 11, as the US's National Academy of Sciences formed a coalition with European partners, eyeing future collaborative missions. A Saturn orbiter to study both the gas giant and its principal moon quickly emerged as the general consensus, but its development was stunted by financial disagreements and squabbles on the American side. Eventually, however, both NASA, the European Space Agency, and the Italian Space Agency agreed to the project, and the mission was designated Cassini-Huygens, after two prominent 17th-century European astronomers who made seminal contributions to the study of the gas giant.
Giovanni Cassini was an Italian astronomer who studied Saturn and its surrounding surroundings and was the first to identify gaps in its ring structure. Christiaan Huygens, on the other hand, was the first to correctly conclude that these rings are not part of the planet itself, while also spotting the moon of Titan in 1655. And like its namesake astronomers, the Cassini-Huygens spacecraft would perform extensive analyses of Saturn, its rings, and its moons, targeting more than 20 distinct objects during the primary phase of its mission.
During this time, it would release its landing module, the Huygens probe, which would descend upon Titan's frozen surface, giving us our first look under the hood at this fascinating, complex atmospheric oddity. After around a decade in development, Cassini-Huygens lifted off from Cape Canaveral on the 15th of October 1997, setting out on a 7-year voyage towards the solar system's Crown Jewel. After entering a heliocentric orbit, the craft gathered pace with two flybys of Venus in the two years that followed. This gave it the kick it required to embark on its long voyage to Saturn, calling at Jupiter along the way for a gravitational boost.
It encountered the Godfather Planet in December of the year 2000 and, after spending 6 months in the Jovian system, snapped over 26,000 stunning images, among them the iconic shots of the Galilean moons eclipsing the clouds of their host. After receiving its gravitational slingshot, the probe pressed onwards towards Saturn for another 3 and a half years, all the while beaming radio signals towards the Sun to test the curvature of spacetime and the predictions of general relativity.
It then started its approach towards Saturn in June 2004, executing the first in a series of complex, risky maneuvers in order to achieve orbital insertion. The craft had to roll back on itself and fire its engines along its path to decelerate until it was captured in orbit by Saturn. This required briefly misaligning Cassini's antenna with the Earth, but the craft came through the maneuver flawlessly nonetheless and reconnected to the Deep Space Network to commence with its 4-year primary mission.
First up on the agenda were flybys of a number of Saturn's smaller icy moons. It passed by Phoebe on the 11th of June 2004 and studied its bright patches to infer the existence of water ice. Soon after, the moons Methone, Pallene, and Polydeuces were identified lurking in early Cassini images of the system. But the spacecraft was soon focused on its principal satellite target: Titan, the unique planet-like moon with a dense atmosphere unlike anything else in the solar system.
As such, Titan has been viewed as an essential component of missions to Saturn, with Cassini carrying out no less than 45 flybys of the moon during its initial four years of operation. The first came in October 2004, when it passed just 1,200 km from the tip of Titan's clouds. Its infrared and ultraviolet instruments then began peeling back the layers of this atmosphere for the first time, compiling preliminary maps of Titan's surface with a view to identifying a suitable landing site for the Huygens probe.
And on Christmas Day 2004, Cassini's gift to Titan was sent hurtling down towards its clouds, finally penetrating their outer edge about 3 weeks later. It recorded and relayed its 2 and a half hour descent, beaming back around 350 images of the approach, though a further 350 were lost in a software failure. But on the 14th of January 2005, the Huygens probe landed in Titan's Aireti region and continued to broadcast what it saw for a further 90 minutes. In that time, it captured the first and still only glimpse we have from the surface of a body in the outer solar system.
These images show boulders of reflective water ice, frozen as hard as rock by Titan's brutal surface temperature of minus 180°, but what makes these images particularly interesting is that these boulders appear to have been smoothed by another flowing liquid, though certainly not water at this low temperature. Rather, Titan is home to a network of lakes and rivers of liquid methane, which Cassini found indisputable evidence for in 2006. In fact, Titan has a methane cycle which bears striking resemblance to the water cycle on Earth. And if we were to warm Titan up, this methane would evaporate, and the water ice mountain ranges would melt to take its place. Thus, in the very distant future, when the Sun has swollen to Red Giant stature and the Earth has been swallowed whole, it is Titan that may have the best hope for a future awakening of life, with rivers, streams, weather, and precipitation to mix up its surface-dwelling abundances of organic compounds.
Another of Saturn's moons with intriguing prospects for habitability is Enceladus, though it bears a quite different set of potentially life-giving characteristics. Enceladus is smaller than Titan and lacks a substantial atmosphere for supporting life on its frozen surface. But like Jupiter's moon Europa, Enceladus is thought to have a buried subsurface oceanic layer, which is warmed and tidally heated by gravitational resonances in the Saturn system. These motions cause friction within the moon's interior, resulting in energetic upwellings in the liquid layer which press against the icy surface and refreeze, creating faults and sometimes even causing cryovolcanic eruptions.
In 2005, around 2 months after the Huygens probe had touched down on Titan, Cassini saw the outline of an ice geyser eruption at Enceladus's South Pole, as it spurted out plumes which regularly fall into orbits along Saturn's ring plane and replenish its frozen material. A few years later, and Cassini went one better, as it flew straight through one of these water-rich plumes during a flyby of Enceladus, sampling its chemical constituents and ultimately confirming the presence of a tidally heated, salty liquid ocean beneath its surface.
With so much exciting potential seemingly locked away within these moons, it's easy to lose sight of Cassini's other studies, including its extensive analyses of Saturn's rings using radio signals and occultation. It determined their 3D structure, dynamics, and consistency by identifying both fine structures and more complex gravitationally bound clumps, clearing gaps in the rings' material. The probe also studied a number of extreme weather events playing out in Saturn's upper atmosphere, starting in 2006 when a cyclone was seen near one of its poles. It also studied one of the planet's most jaw-dropping features: its iconic North Pole hexagon, another enormous eyed storm extending almost 30,000 km, which was originally found by the Voyager spacecraft.
Between 2008 and 2017, Cassini observed gradual color changes in this cloud pattern in response to seasonal changes in its exposure to sunlight. And then in 2010, it saw something visible only once every 30 years: Saturn's Great White Spot. This storm bears resemblance to Jupiter's Great Red Spot, but unlike the latter, it did not persist for very long or stay rooted in one place. Instead, it whipped around the face of the planet, leaving long trails of suspect white discharge in its wake, which slowly faded away over a number of years.
By this time, Cassini had completed the primary phase of its mission, which lasted four years up to July 2008. But with plenty more fuel in the tank and its battery power still well conserved, its engineers were able to extend Cassini's operational life by an additional 2 years. This enabled the probe to study Saturn during its equinox event in 2009, thus coining the mission extension's title. And upon the conclusion of the equinox phase in June 2010, Cassini's mission was renewed again for another 6 and a half years, taking it all the way through to Saturn's summer solstice in 2017. This extension allowed for another 155 revolutions of Saturn, 54 of Titan, and 11 of Enceladus.
But by the time these were completed, the craft really was starting to run on fumes, and thus, as its ultimate act, Cassini engineers directed it to execute its so-called Grand Finale routine. The last part of the mission, which would entail the riskiest maneuvers yet. In April 2017, Cassini's orbit guided it through a gateway gap in Saturn's rings, passing just 300 km from their visible inner edge. It then made a further 22 orbits through this gap, enabling it to attain the closest observations yet of Saturn's atmosphere, weather, and rings. It took its final images of the giant planet in September before being intentionally flown into Saturn once again to avoid contaminating a potentially life-bearing moon.
And there is a rumor that this breathtaking shot was Cassini's final glimpse before it plunged to its crushing demise. But alas, this is just an artist's impression, often passed off as the truth on social media. In reality, Cassini's last view of Saturn was actually this much less interesting image. It then finally dropped into the gas giant's clouds on the 15th of September 2017, bringing to a close more than a decade of revolutionary discoveries and stunning shots of the Saturn system. And it may be decades before we see another orbiter that reveals as much to us as the Cassini spacecraft did.
The question is, what is actually on the table for the near future? Well, in recent years, missions to study the gas giants have become more aligned with the study of their moons, with a central focus on habitability around Europa of Jupiter and Titan of Saturn. As the closer of the two, the former has enjoyed more attention from scientists in the years since the Galileo and Cassini orbiters. In that time, we've reestablished a permanent presence around Jupiter with the Juno spacecraft, the second installment of NASA's New Frontiers program. It is the culmination of a number of restructured projects targeting Jupiter and Europa in the 2000s before finally getting off the ground in August 2011.
Among its scientific aims was to gain further insights into Jupiter's internal structure, shoring up our theories on its formation and evolution over time. And Juno continues to study the Godfather Planet's weather, magnetosphere, and chemical composition to this day, with its current mission extension taking it through to at least 2025. And as the 2030s approaches, a number of next-generation Jupiter probes will take to the skies to take Juno's place. The first of these, belonging to the European Space Agency, is the Jupiter Icy Moons Explorer probe, which launched earlier this year in April 2023.
As of the making of this video, the JUICE probe is currently trailing the Earth in a heliocentric orbit, due to make flybys of both our world and Venus as it gathers pace for its trip through the asteroid belt. It won't arrive at its destination until 2031, but when it does, it will study the moons Ganymede, Callisto, and Europa, making new maps of their surfaces while also searching for signs of subsurface oceans and organic molecules. And shortly after, it will be joined in orbit around Jupiter by NASA's own dedicated Galilean moon explorer, the Europa Clipper, which will also maneuver around the Jovian system, performing detailed reconnaissance of the habitable moon while also searching for a plume of erupted seawater to fly through, the way Cassini needed on Enceladus. The mission is set for launch in October 2024, though it won't take as long to reach Jupiter as the JUICE probe, and so will probably begin its mission around a similar time in the early 2030s.
Plans for a landing module extension, like the Huygens probe, for dropping down onto Europa's surface have also been tabled, nicknamed the Europa Lander. But this proposal did not make it into NASA's finalized plan for the Clipper and thus is unlikely to materialize anytime in the next decade. But something we may have a better glimpse of by then is Titan. While the resounding success of Cassini-Huygens has resulted in Saturn probes taking more of a back seat, there is still a date penciled in for 2027, which will see a probe launched that implements successful designs from NASA's Mars Ingenuity helicopter on the surface of Titan: the Titan Dragonfly. It will land in the Shangri-La fields sometime late in the next decade and will take dozens of controlled atmospheric flights spanning 175 km, more than double the distance of its Martian counterpart.
This mission will be the fourth installment of the New Frontiers program, the successor to Juno, which currently circles Jupiter. They are both part of a program for developing medium-range, high-reward missions selected by NASA for ticking off the remaining targets in the exploration of the solar system. And the first installment of this program, New Horizons, is one of the few glaring omissions we've yet to cover in this series on probes, along with the revolutionary Voyager spacecraft. And so, we have surely saved the best for last, because next time, in the final chapter of this trilogy, we will follow those spacecraft out on their longest ride yet, beyond the edge of the solar system, into the depths of interstellar space.
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Space.