Planet Is Intensely Active More Than Different Mission Concept Proposals, 4 Billion Years After Its Formation

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January 2020 • Astronomy.com RETURN TO PLUTO! Researchers already are planning an ambitious spacecraft that will orbit the mysterious world. BY S. ALAN STERN

THE EXPLORATION OF worth it, though: When New Horizons’ PLUTO WASN’T EASY, BUT data arrived on Earth, it revealed vast IT SURE WAS WORTH IT. As nitrogen glaciers, methane- and water- David Grinspoon and I recount in our ice mountains, a complex atmosphere, book, Chasing New Horizons, it took and a range of terrain ages that prove 14 years (1989 to 2003), about a dozen the planet is intensely active more than different mission concept proposals, 4 billion years after its formation. But and the weight of the first National there was more — including evidence Academy Planetary Science Decadal for organic compounds, liquids on Survey just to unleash the funding. Pluto’s surface in its past, ice volcanoes After a fierce competition among rival that erupted onto the planet’s surface, teams, NASA ultimately selected New and an ocean of liquid water in Pluto’s Horizons as its Pluto flyby mission. interior. That was followed by a breakneck Pluto’s moons didn’t disappoint, four-year schedule to design, build, and either. New Horizons found new evi- test the spacecraft in order to meet the dence that they formed together after an time-critical 2006 launch window ancient, giant impact between Pluto and needed to use Jupiter for a gravity assist. another dwarf planet. Pluto’s largest Once that was achieved, New Horizons moon, Charon, also revealed a puz- had to undertake the 9.5-year journey zlingly reflective surface and never- across our solar system to reach Pluto before-seen dark polar caps seemingly and its system of five moons. The entire made of methane that had escaped from effort took 26 years and the dedication Pluto’s atmosphere. of literally thousands of individuals, and Those are just a few of the discover- was done entirely with no backup, no ies that New Horizons made. And I can plan B, no Voyager 2 in case Voyager 1 tell you, now four years after that flyby, failed. the New Horizons science team and many other members of the planetary SPECTACULAR RESULTS science community have concluded that In a real sense, I think the solar system a new mission to explore Pluto in more saved the best for last. The wait was depth is required to unravel these puzzles. A proposed Pluto orbiter, seen here flying above the This is needed in part because much canyon system that dominates the planet’s large moon, Charon, could explore the Pluto system for of Pluto and its moons couldn’t be at least two years. RON MILLER FOR ASTRONOMY mapped in sufficient detail with a single

2 ASTRONOMY INSIGHTS • JANUARY 2020 RETURN TO PLUTO! Researchers already are planning an ambitious spacecraft that will orbit the mysterious world. BY S. ALAN STERN

WWW.ASTRONOMY.COM 3 The New Horizons Pluto is a dynamic world, with a roiling spacecraft, which had nitrogen glacier, avalanches, and a com- its closest encounter with Pluto on July 14, plex and time-variable atmosphere. But 2015, revealed the New Horizons’ flyby reconnaissance of planet in spectacular Pluto all took place in one roughly detail. 24-hour period, July 14, 2015. It gave us a wonderful snapshot first look, but no chance to see the daily, seasonal, and geological changes that an orbiter can probe but a flyby cannot. Third, we learned that new kinds of instrumentation will be needed to answer the many questions that the New Horizons data sets raised. Going back with an orbiter will allow us to use instruments to peer through Pluto’s glaciers with radar to determine how deep they are. The spacecraft also will carry Just 15 minutes after its closest approach to Pluto, NASA’s New Horizons spacecraft looked back toward the a mass spectrometer to sample Pluto’s planet and captured this near-sunset view of its rugged, icy mountains and flat ice plains. The smooth expanse of Sputnik Planitia (right) is flanked to the west (left) by rugged mountains up to 11,000 feet atmosphere, inventory all the com- (3,500 m) high. ALL IMAGES BY NASA/JHUAPL/SwRI UNLESS OTHERWISE NOTED pounds there, and determine the nature of Pluto’s hazes. And there is so much more. Only by radio tracking a vehicle, orbit after orbit, can we probe the interior of Pluto to determine if the suspected ocean is really there. And we need to send a thermal mapper to determine how Pluto’s vast glaciers are powered and if its ancient ice volcanoes are still active. Without flying a magnetometer, we will never know if the planet’s core is alive and churning or dead and frozen solid. Finally, we want to send lidars and other active instruments that can make maps even in the darkness of polar shadows. The aim is to inspect the night sides of Pluto and its moons to detect changes as the temperatures plum- When New Horizons was 11,000 miles (18,000 km) past Pluto, it took this wide-angle image that shows met when the Sun goes down. the deep haze layers of the planet’s atmosphere. The left and upper parts of the disk are dark because So, we need an orbiter to complete Pluto is casting its shadow on its atmosphere. The backlighting highlights more than a dozen layers of haze. The horizontal streaks in the sky beyond Pluto are stars, smeared out by the motion of the camera the job that New Horizons started so as it tracked Pluto. well. That orbiter should be designed to operate for years, not days, returning fast flyby. It’s also needed because Pluto’s MORE GOALS FOR A data week in and week out as it closely surface and atmosphere are evolving RETURN TO PLUTO inspects all of Pluto’s moons and watches with time and beg for an orbiter to Why do we want to go back? There the planet’s surface and atmosphere CHARON AND THE SMALL MOONS OF PLUTO observe those processes. But the main are many reasons. First, limited by the evolve. This composite image shows part of Pluto’s reason for a new mission is that many nature of its fast flyby, New Horizons Hydra largest moon, Charon, and all four small Nix of the mysteries New Horizons found could map only 40 percent of Pluto’s DESIGNING THE MISSION moons, as resolved by the Long Range Reconnaissance Imager on the New Horizons require new types of instrumentation to geology and surface composition at good With all this in mind, I led an internal spacecraft. Charon has a diameter of 751 miles Styx Kerberos be brought to bear — instruments New resolution. The same is true for Charon. research project from mid-2017 to mid- (1,212 km). Nix and Hydra have comparable Horizons did not carry. And we learned even less about 2019 at the Southwest Research sizes, approximately 25 miles (40 km) across in their longest dimension. Kerberos and Styx Whether that orbiter mission is Pluto’s four small moons Institute (SwRI) to study are much smaller and have comparable sizes, funded depends almost entirely on the because the craft didn’t how to implement roughly 6 to 7 miles (10 to 12 km) across in 10 miles findings of the next planetary decadal fly close to any of We need an a Pluto orbiter. their longest dimension. survey, whose committee will begin its them. orbiter to complete The study team 10 km work next year. The survey results will Second, we included Charon appear in 2022. learned that the job that New Horizons my SwRI started so well. 4 ASTRONOMY INSIGHTS • JANUARY 2020 CELESTRON PREMIER SELECT DEALERS B&H Photo – 800.947.9970 – bhphotovideo.com Woodland Hills – 888.427.8766 – telescopes.net visit High Point Scientific – 800.266.9590 – highpointscientific.com Adorama – 800.223.2500 – adorama.com Optics Planet – 800.504.5897 – opticsplanet.com Focus Camera – 800.221.0828 – focuscamera.com celestron.com/holiday Astronomics – 800.422.7876 – astronomics.com Agena AstroProducts – 562.215.4473 – agenaastro.com for all our latest holiday deals OPT Telescopes – 800.483.6287 – optcorp.com WWW.ASTRONOMY.COM 7 The New Horizons Pluto is a dynamic world, with a roiling spacecraft, which had nitrogen glacier, avalanches, and a com- its closest encounter with Pluto on July 14, plex and time-variable atmosphere. But 2015, revealed the New Horizons’ flyby reconnaissance of planet in spectacular Pluto all took place in one roughly detail. 24-hour period, July 14, 2015. It gave us a wonderful snapshot first look, but no chance to see the daily, seasonal, and geological changes that an orbiter can probe but a flyby cannot. Third, we learned that new kinds of instrumentation will be needed to answer the many questions that the New Horizons data sets raised. Going back with an orbiter will allow us to use instruments to peer through Pluto’s glaciers with radar to determine how deep they are. The spacecraft also will carry Just 15 minutes after its closest approach to Pluto, NASA’s New Horizons spacecraft looked back toward the a mass spectrometer to sample Pluto’s planet and captured this near-sunset view of its rugged, icy mountains and flat ice plains. The smooth expanse of Sputnik Planitia (right) is flanked to the west (left) by rugged mountains up to 11,000 feet atmosphere, inventory all the com- (3,500 m) high. ALL IMAGES BY NASA/JHUAPL/SwRI UNLESS OTHERWISE NOTED pounds there, and determine the nature of Pluto’s hazes. And there is so much more. Only by radio tracking a vehicle, orbit after orbit, can we probe the interior of Pluto to determine if the suspected ocean is really there. And we need to send a thermal mapper to determine how Pluto’s vast glaciers are powered and if its ancient ice volcanoes are still active. Without flying a magnetometer, we will never know if the planet’s core is alive and churning or dead and frozen solid. Finally, we want to send lidars and other active instruments that can make maps even in the darkness of polar shadows. The aim is to inspect the night sides of Pluto and its moons to detect changes as the temperatures plum- When New Horizons was 11,000 miles (18,000 km) past Pluto, it took this wide-angle image that shows met when the Sun goes down. the deep haze layers of the planet’s atmosphere. The left and upper parts of the disk are dark because So, we need an orbiter to complete Pluto is casting its shadow on its atmosphere. The backlighting highlights more than a dozen layers of haze. The horizontal streaks in the sky beyond Pluto are stars, smeared out by the motion of the camera the job that New Horizons started so as it tracked Pluto. well. That orbiter should be designed to operate for years, not days, returning fast flyby. It’s also needed because Pluto’s MORE GOALS FOR A data week in and week out as it closely surface and atmosphere are evolving RETURN TO PLUTO inspects all of Pluto’s moons and watches with time and beg for an orbiter to Why do we want to go back? There the planet’s surface and atmosphere CHARON AND THE SMALL MOONS OF PLUTO observe those processes. But the main are many reasons. First, limited by the evolve. This composite image shows part of Pluto’s reason for a new mission is that many nature of its fast flyby, New Horizons Hydra largest moon, Charon, and all four small Nix of the mysteries New Horizons found could map only 40 percent of Pluto’s DESIGNING THE MISSION moons, as resolved by the Long Range Reconnaissance Imager on the New Horizons require new types of instrumentation to geology and surface composition at good With all this in mind, I led an internal spacecraft. Charon has a diameter of 751 miles Styx Kerberos be brought to bear — instruments New resolution. The same is true for Charon. research project from mid-2017 to mid- (1,212 km). Nix and Hydra have comparable Horizons did not carry. And we learned even less about 2019 at the Southwest Research sizes, approximately 25 miles (40 km) across in their longest dimension. Kerberos and Styx Whether that orbiter mission is Pluto’s four small moons Institute (SwRI) to study are much smaller and have comparable sizes, funded depends almost entirely on the because the craft didn’t how to implement roughly 6 to 7 miles (10 to 12 km) across in 10 miles findings of the next planetary decadal fly close to any of We need an a Pluto orbiter. their longest dimension. survey, whose committee will begin its them. orbiter to complete The study team 10 km work next year. The survey results will Second, we included Charon appear in 2022. learned that the job that New Horizons my SwRI started so well. 6 ASTRONOMY INSIGHTS • JANUARY 2020 SAVE ON 50+ OF OUR MOST POPULAR ACCESSORIES When your telescope is equipped with the right accessories, you get more out of each night under the stars. 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B&H Photo – 800.947.9970 – bhphotovideo.com High Point Scientific – 800.266.9590 – highpointscientific.com Optics Planet – 800.504.5897 – opticsplanet.com Astronomics – 800.422.7876 – astronomics.com OPT Telescopes – 800.483.6287 – optcorp.com Woodland Hills – 888.427.8766 – telescopes.net Adorama – 800.223.2500 – adorama.com Focus Camera – 800.221.0828 – focuscamera.com Agena AstroProducts – 562.215.4473 – agenaastro.com SALE ENDS 1/31 This enhanced color mosaic combines some of the sharpest views of Pluto captured by New Horizons. The telescopic camera They are part of a sequence taken near closest atmosphere for sampling multiple times. on the New Horizons approach and have resolutions of about 250 to And it will fly over Pluto’s poles many spacecraft took this 280 feet (77 to 85 m) per pixel, which reveal surface high-resolution image features smaller than half a city block. Scientists times, as well as out into its distant of an intricate pattern then added lower resolution color data. The images “magnetospheric tail” for specialized of pits. Scientists form a strip 50 miles (80 km) wide, trending from studies. The tour could continue much believe these the northwest of Sputnik Planitia, across the Al-Idrisi indentations form Mountains, onto the shoreline of Pluto’s “heart” longer, but after two years, the spacecraft through a combination feature, and just into its icy plains. will have met all of the scientific objec- of ice fracturing and evaporation. The tives we set for it. scarcity of impact craters in this area may mean that these pits assists. When we simulated the idea, it THE SPACECRAFT formed relatively worked beautifully, eliminating most of Most of the capabilities needed for the recently. By studying the fuel an orbiter needs to carry. orbiter spacecraft are similar to those for them, researchers hope to learn about the ice New Horizons. But the resulting flight flow and the exchange THE TIMELINE system, which features twice the payload of nitrogen and other volatile materials The mission timeline we created is as mass of New Horizons, fully redun- between the surface follows: Launch in December 2028, a dant spacecraft systems, and electric and the atmosphere. This enhanced color view shows the southeastern portion of Pluto’s great ice plains. At lower right the plains Jupiter gravity-assist flyby in October and chemical propulsion systems, has a border rugged, dark highlands named Krun Macula. (Krun is the lord of the underworld in the Mandaean 2030, braking into Pluto orbit begin- mass of 5,156 pounds (2,339 kilograms). religion, and a macula is a dark feature on a planetary surface.) Researchers think the dark red color comes from tholins, complex molecules found across Pluto. ning in 2046, and concluding in 2059. That’s the current estimate. However, We could get there faster if high-power the launch vehicle we chose will be able nuclear systems are available to increase to lift 30 percent more than that into colleagues Tiffany Finley, Mark Tapley, Pluto orbiter to follow up on the discov- braking thrust so the orbiter has enough space, in case the payload changes. That completely untenable for an orbiter, same way that the Mariner, Pioneer, and John Scherrer, and Amanda Zangari eries of New Horizons. power to slow down at Pluto, but a weight is nearly five times more than which needs to send its data to Earth Voyager missions conducted the first (who is now at Lincoln Labs). One of the top breakthroughs was a slower trajectory is feasible with current New Horizons, but that’s the nature of an every 15 to 30 days to be effective. That reconnaissance of all the terrestrial and The mission design we created put game-changing solution to the propul- systems. orbiter mission because it must carry so in turn requires a much larger dish giant planets decades ago. a New Horizons-scale spacecraft with sion requirements to tour the Pluto sys- The resulting two-year tour of the much fuel to brake into Pluto orbit. antenna and a transmitter nearly 10 However, the spectacular complexity new instruments into Pluto orbit. It tem, which originally added a huge Pluto system begins with a Charon grav- There are three primary differences times more powerful, along with smarter found in the Pluto system created con- requires one of the massive new, Saturn amount of mass to the spacecraft. We ity assist and a braking maneuver to between the proposed orbiter and New software to remove background noise tention in the Kuiper Belt, dwarf planet, V-class launch vehicles now in develop- discovered that virtually all of the pro- complete the Pluto orbit injection. Then, Horizons. The first is the need for either from images and spectra. Fortunately, and planetary origins communities. ment, a Jupiter gravity assist, and a pulsive needs to explore many aspects using orbits that are typically just a cou- more onboard fuel or spacecraft reaction none of these changes requires new tech- Many, myself included, argued that we nuclear electric stage augmented by a of the planet and its satellites could be ple of weeks long, the craft will explore wheels to perform the thousands of nology, and the mission looks entirely needed instead to return to Pluto to chemical propulsion braking stage to eliminated using targeted close flybys of each of Pluto’s small moons half a dozen pointing maneuvers during the two-year feasible with existing flight systems. study it in depth via an orbiter mission, reach Pluto orbit. Pluto’s Texas-sized moon Charon times in close flybys — and tour. The second is the need for larger rather than reconnoitering the diversity We studied what kind of payload for repeated gravity assists. Charon itself with more onboard data storage than the 16 giga- GO WIDE OR GO DEEP? of Kuiper Belt dwarf planets with new should be flown on such an orbiter, what This is analogous to than 30 close flybys. It bytes that New Horizons carried. But the Before the exploration of the Pluto sys- flyby missions to several of them. kind of orbital tour would be possible, how the Cassini The tour also will make both biggest single change is the need for a tem in 2015, most planetary scientists But, in our second breakthrough, the what the main orbiter spacecraft attri- orbiter toured the could continue polar and equato- more capable communications system. thought the natural follow-up to New Pluto orbiter study that I led at SwRI also butes need to be, and how to get such a Saturn system much longer, but after rial flybys of The communications system built to Horizons would be the flyby recon- showed that it is possible to have both spacecraft from Earth to Pluto. This using the large Pluto and dip return all the data from the single flyby naissance of other dwarf planets in the — i.e., to combine the two missions into study constituted the first, and presently moon Titan two years, the spacecraft will into Pluto’s that New Horizons performed took Kuiper Belt. This way, the diversity of one, and to do so at an affordable cost. the only, investigation of how to build a for gravity have met all of the scientific upper 16 months to complete its job. That’s dwarf planets could be explored in the Because that breaks the tug of war objectives we set for it. 8 ASTRONOMY INSIGHTS • JANUARY 2020 This enhanced color mosaic combines some of the sharpest views of Pluto captured by New Horizons. The telescopic camera They are part of a sequence taken near closest atmosphere for sampling multiple times. on the New Horizons approach and have resolutions of about 250 to And it will fly over Pluto’s poles many spacecraft took this 280 feet (77 to 85 m) per pixel, which reveal surface high-resolution image features smaller than half a city block. Scientists times, as well as out into its distant of an intricate pattern then added lower resolution color data. The images “magnetospheric tail” for specialized of pits. Scientists form a strip 50 miles (80 km) wide, trending from studies. The tour could continue much believe these the northwest of Sputnik Planitia, across the Al-Idrisi indentations form Mountains, onto the shoreline of Pluto’s “heart” longer, but after two years, the spacecraft through a combination feature, and just into its icy plains. will have met all of the scientific objec- of ice fracturing and evaporation. The tives we set for it. scarcity of impact craters in this area may mean that these pits assists. When we simulated the idea, it THE SPACECRAFT formed relatively worked beautifully, eliminating most of Most of the capabilities needed for the recently. By studying the fuel an orbiter needs to carry. orbiter spacecraft are similar to those for them, researchers hope to learn about the ice New Horizons. But the resulting flight flow and the exchange THE TIMELINE system, which features twice the payload of nitrogen and other volatile materials The mission timeline we created is as mass of New Horizons, fully redun- between the surface follows: Launch in December 2028, a dant spacecraft systems, and electric and the atmosphere. This enhanced color view shows the southeastern portion of Pluto’s great ice plains. At lower right the plains Jupiter gravity-assist flyby in October and chemical propulsion systems, has a border rugged, dark highlands named Krun Macula. (Krun is the lord of the underworld in the Mandaean 2030, braking into Pluto orbit begin- mass of 5,156 pounds (2,339 kilograms). religion, and a macula is a dark feature on a planetary surface.) Researchers think the dark red color comes from tholins, complex molecules found across Pluto. ning in 2046, and concluding in 2059. That’s the current estimate. However, We could get there faster if high-power the launch vehicle we chose will be able nuclear systems are available to increase to lift 30 percent more than that into colleagues Tiffany Finley, Mark Tapley, Pluto orbiter to follow up on the discov- braking thrust so the orbiter has enough space, in case the payload changes. That completely untenable for an orbiter, same way that the Mariner, Pioneer, and John Scherrer, and Amanda Zangari eries of New Horizons. power to slow down at Pluto, but a weight is nearly five times more than which needs to send its data to Earth Voyager missions conducted the first (who is now at Lincoln Labs). One of the top breakthroughs was a slower trajectory is feasible with current New Horizons, but that’s the nature of an every 15 to 30 days to be effective. That reconnaissance of all the terrestrial and The mission design we created put game-changing solution to the propul- systems. orbiter mission because it must carry so in turn requires a much larger dish giant planets decades ago. a New Horizons-scale spacecraft with sion requirements to tour the Pluto sys- The resulting two-year tour of the much fuel to brake into Pluto orbit. antenna and a transmitter nearly 10 However, the spectacular complexity new instruments into Pluto orbit. It tem, which originally added a huge Pluto system begins with a Charon grav- There are three primary differences times more powerful, along with smarter found in the Pluto system created con- requires one of the massive new, Saturn amount of mass to the spacecraft. We ity assist and a braking maneuver to between the proposed orbiter and New software to remove background noise tention in the Kuiper Belt, dwarf planet, V-class launch vehicles now in develop- discovered that virtually all of the pro- complete the Pluto orbit injection. Then, Horizons. The first is the need for either from images and spectra. Fortunately, and planetary origins communities. ment, a Jupiter gravity assist, and a pulsive needs to explore many aspects using orbits that are typically just a cou- more onboard fuel or spacecraft reaction none of these changes requires new tech- Many, myself included, argued that we nuclear electric stage augmented by a of the planet and its satellites could be ple of weeks long, the craft will explore wheels to perform the thousands of nology, and the mission looks entirely needed instead to return to Pluto to chemical propulsion braking stage to eliminated using targeted close flybys of each of Pluto’s small moons half a dozen pointing maneuvers during the two-year feasible with existing flight systems. study it in depth via an orbiter mission, reach Pluto orbit. Pluto’s Texas-sized moon Charon times in close flybys — and tour. The second is the need for larger rather than reconnoitering the diversity We studied what kind of payload for repeated gravity assists. Charon itself with more onboard data storage than the 16 giga- GO WIDE OR GO DEEP? of Kuiper Belt dwarf planets with new should be flown on such an orbiter, what This is analogous to than 30 close flybys. It bytes that New Horizons carried. But the Before the exploration of the Pluto sys- flyby missions to several of them. kind of orbital tour would be possible, how the Cassini The tour also will make both biggest single change is the need for a tem in 2015, most planetary scientists But, in our second breakthrough, the what the main orbiter spacecraft attri- orbiter toured the could continue polar and equato- more capable communications system. thought the natural follow-up to New Pluto orbiter study that I led at SwRI also butes need to be, and how to get such a Saturn system much longer, but after rial flybys of The communications system built to Horizons would be the flyby recon- showed that it is possible to have both spacecraft from Earth to Pluto. This using the large Pluto and dip return all the data from the single flyby naissance of other dwarf planets in the — i.e., to combine the two missions into study constituted the first, and presently moon Titan two years, the spacecraft will into Pluto’s that New Horizons performed took Kuiper Belt. This way, the diversity of one, and to do so at an affordable cost. the only, investigation of how to build a for gravity have met all of the scientific upper 16 months to complete its job. That’s dwarf planets could be explored in the Because that breaks the tug of war objectives we set for it. WWW.ASTRONOMY.COM 9 A GEOLOGICAL WONDERLAND The nitrogen-ice plain Sputnik Planitia and surrounding terrain feature in this geological map. All of the terrain in this map has been imaged at a resolution of approximately 1,050 feet (320 m) per pixel or better. The blue and green that fill the center of the map represent different textures, from cellular terrain in the center and north to smooth, pitted plains in the south. The black lines represent Pluto’s north polar the troughs that are the boundaries of cellular regions in the ice. Purple represents the area shows a diversity mountain ranges that line Sputnik’s western border, and pink represents the scattered hills of geological and at its eastern edge. The possible cryovolcano Wright Mons appears in red in the southern compositional features corner of the map. Rugged highlands are dark brown along the western edge and in enhanced color. Long contain many large impact craters, mapped in yellow. canyons — the largest is about 45 miles (75 km) wide — run vertically across this region. Roughly parallel subsidiary canyons to the east and west are approximately 6 miles (10 km) wide. The degraded walls of these canyons appear much older than the more sharply defined canyon systems elsewhere on Pluto, perhaps because they are made of weaker material.

between going wide in studying other dwarf planets and going deep in Pluto studies, we dubbed the resultant all-in- one concept the “Gold Standard” Pluto- Kuiper Belt exploration mission.

THE GOLD STANDARD The game-changing Gold Standard mission works like this: First, after an orbital the Dawn propulsion system can even profound impact for a broad suite of closer to home, are too complex and tour of Pluto, a final pair of close gravity- place the Pluto orbiter around a second planetary science studies including planet too compelling to be left to a single first assist flybys of Charon will free the dwarf planet for another orbital mission. and planetesimal formation, ocean flyby reconnaissance. Pluto beckons, and orbiter from the Pluto system to explore These breakthroughs crush the debate worlds, exotic atmospheres, the workings we must return with an orbiter. The Gold the Kuiper Belt without any need for about whether the next exploration step of dwarf planets, and potentially even Standard can solve the tension between any propulsion from the orbiter. Next, after New Horizons should be to fly by astrobiology. This proposal creates a pow- scientists wishing to return to Pluto and using only the existing capabilities of the other Kuiper Belt objects and dwarf plan- erful mini-flagship mission for the next others who think it wiser to explore the NASA Dawn mission electric propulsion ets or to instead orbit Pluto, because planetary decadal survey to consider. broader Kuiper Belt.

This is the highest-resolution color shot of Pluto’s receding crescent, taken when New Horizons was system, the craft will conduct a flyby we’ve shown that this debate is moot — 120,000 miles (200,000 km) past the planet. The image is dominated by atmospheric layers of blue haze, tour of up to a half-dozen small Kuiper a single mission can do both. CONCLUSIONS Planetary scientist S. Alan Stern is the which scientists believe is a photochemical smog. It may form when sunlight interacts with methane and Belt objects and any one of a number of Such a mission could be launched in A key finding from New Horizons is that principal investigator of NASA’s New other molecules, producing a mixture of hydrocarbons. These, in turn, accumulate into small haze particles a fraction of a micrometer in size, which preferentially scatter blue light — the same process that can make dwarf planets. In fact, in some scenarios, the late 2020s or late 2030s. It would have Pluto and its moons, like other planets Horizons mission to Pluto. haze appear bluish on Earth.

A key finding from New Horizons is that Pluto

This high-resolution swath of Pluto sweeps over the cratered plains at the west of the hemisphere and its moons are too New Horizons encountered and across many prominent faults. It skims the eastern part of the dark complex and too compelling region known as Cthulhu Regio and passes over the possible cryovolcano Wright Mons. The pictures in this mosaic were taken by the Long Range Reconnaissance Imager in “ride-along” mode with the to be left to a single first flyby Linear Etalon Imaging Spectral Array, which accounts for the zigzag pattern. Taken shortly before closest approach to Pluto, details as small as 500 yards (457 m) can be seen. reconnaissance. 10 ASTRONOMY INSIGHTS • JANUARY 2020 A GEOLOGICAL WONDERLAND The nitrogen-ice plain Sputnik Planitia and surrounding terrain feature in this geological map. All of the terrain in this map has been imaged at a resolution of approximately 1,050 feet (320 m) per pixel or better. The blue and green that fill the center of the map represent different textures, from cellular terrain in the center and north to smooth, pitted plains in the south. The black lines represent Pluto’s north polar the troughs that are the boundaries of cellular regions in the ice. Purple represents the area shows a diversity mountain ranges that line Sputnik’s western border, and pink represents the scattered hills of geological and at its eastern edge. The possible cryovolcano Wright Mons appears in red in the southern compositional features corner of the map. Rugged highlands are dark brown along the western edge and in enhanced color. Long contain many large impact craters, mapped in yellow. canyons — the largest is about 45 miles (75 km) wide — run vertically across this region. Roughly parallel subsidiary canyons to the east and west are approximately 6 miles (10 km) wide. The degraded walls of these canyons appear much older than the more sharply defined canyon systems elsewhere on Pluto, perhaps because they are made of weaker material. between going wide in studying other dwarf planets and going deep in Pluto studies, we dubbed the resultant all-in- one concept the “Gold Standard” Pluto- Kuiper Belt exploration mission.

A key finding from New Horizons is that Pluto

Researchers discovered FarOut, a faint point of light that appeared in different locations on two images taken an hour apart, with the powerful Subaru Telescope on November 10, 2018. SCOTT SHEPPARD/ NINE DAVID THOLEN

ust over a year 1 astronomical unit (AU) objects — which FarOut is fellow astronomers Chad Mercury vey,” Sheppard says. working to bolster the number ago, a trio of equals the average distance suspected to be a part of — Trujillo of Northern Arizona AndEarth although FarOut, of known distant objects like Sedna astronomers set between the Sun and Earth is the best evidence yet for the University and David Tholen which is thought9.5 to be about FarOut. This, they hope, will 39 80 AU 123 AU VenusMars Jupiter Saturn Uranus Neptune Pluto “The Goblin”“Biden” “FarOut” a record for the — isn’t the only thing that existence of an elusive world of the University of Hawai‘i, distant and faint trans- 310 miles (500 kilometers) give them a better sense of 2015 T 2012 VP 2018 VG most distant makes this world intriguing. lurking in the outer fringes of are carrying out the most Neptunian objects (TNOs), wide, is almost at the observa- what’s aligning the orbits of G object ever dis- In recent years, the same our solar system. A world comprehensive survey of which are frozen, rocky bodies tional limit of their telescopes, many of these far-flung bodies, 387 113 18 covered in the group of researchers have called Planet Nine. solar system objects beyond located past Neptune in a the larger the object, the easier possibly helping them finally solar system. uncovered a handful of other Pluto to date. According to region called the Kuiper Belt. it is to spot. So, Sheppard says, track down Planet Nine. Because the extremely distant bodies. Finding far-flung Sheppard, since the team’s small world was And, much to their surprise, objects survey began in 2012, it has Not to scale found more than they have determined many Scott Sheppard and his team netted about 80 percent of all three times farther from the of these objects make their have made the news a lot solar system objects discov- FINDING FAR-OUT OBJECTS Mercury Sun than Pluto, the team fit- closest approach to the Sun, lately for their ever-growing ered beyond 60 AU. Earth Sedna tingly dubbed it FarOut. or reach perihelion, at about list of distant discoveries. But Sheppard and his team 9.5 39 80 AU 123 AU But FarOut’s incredible the same point in space. This is because Sheppard, an aren’t done yet. Over the next VenusMars Jupiter Saturn Uranus Neptune Pluto “The Goblin”“Biden” “FarOut” distance of about 123 astro- The researchers believe astronomer at the Carnegie few years, they expect their 2015 T 2012 VP 2018 VG G nomical units — where this growing list of clustered Institution for Science, and survey to spot many more 387 113 18

12 ASTRONOMY INSIGHTS • JANUARY 2020 The hypothetical world called Planet Nine is expected to be a super-Earth or sub-Neptune planet with an orbit that potentially takes it dozens of times farther from the Sun than the dwarf planet Pluto. In pursuit of NASA/JPL-CALTECH/R. HURT (IPAC) PLANET

“FarOut,” located some 123 AU from the Sun, is only about one-fifth the diameter A slew of oddly orbiting objects seems to suggest a of Pluto, but it is some three times more distant. “We have they should be able to detect ASTRONOMY: ROEN KELLY AFTER ROBERTO massive and as yet unseen world is skulking in the outer MOLAR CANDANOSA/SCOTT S. SHEPPARD/ covered a few slightly bigger objects CARNEGIE INSTITUTION FOR SCIENCE about 25 per- located even farther away. reaches of our solar system. BY JAKE PARKS Not to scale cent of the sky Through their survey, to date in our sur- Sheppard and his team are ust over a year 1 astronomical unit (AU) objects — which FarOut is fellow astronomers Chad Mercury vey,” Sheppard says. working to bolster the number ago, a trio of equals the average distance suspected to be a part of — Trujillo of Northern Arizona AndEarth although FarOut, of known distant objects like Sedna astronomers set between the Sun and Earth is the best evidence yet for the University and David Tholen which is thought9.5 to be about FarOut. This, they hope, will 39 80 AU 123 AU VenusMars Jupiter Saturn Uranus Neptune Pluto “The Goblin”“Biden” “FarOut” a record for the — isn’t the only thing that existence of an elusive world of the University of Hawai‘i, distant and faint trans- 310 miles (500 kilometers) give them a better sense of 2015 T 2012 VP 2018 VG most distant makes this world intriguing. lurking in the outer fringes of are carrying out the most Neptunian objects (TNOs), wide, is almost at the observa- what’s aligning the orbits of G object ever dis- In recent years, the same our solar system. A world comprehensive survey of which are frozen, rocky bodies tional limit of their telescopes, many of these far-flung bodies, 387 113 18 covered in the group of researchers have called Planet Nine. solar system objects beyond located past Neptune in a the larger the object, the easier possibly helping them finally solar system. uncovered a handful of other Pluto to date. According to region called the Kuiper Belt. it is to spot. So, Sheppard says, track down Planet Nine. Because the extremely distant bodies. Finding far-flung Sheppard, since the team’s small world was And, much to their surprise, objects survey began in 2012, it has Not to scale found more than they have determined many Scott Sheppard and his team netted about 80 percent of all three times farther from the of these objects make their have made the news a lot solar system objects discov- FINDING FAR-OUT OBJECTS Mercury Sun than Pluto, the team fit- closest approach to the Sun, lately for their ever-growing ered beyond 60 AU. Earth Sedna tingly dubbed it FarOut. or reach perihelion, at about list of distant discoveries. But Sheppard and his team 9.5 39 80 AU 123 AU But FarOut’s incredible the same point in space. This is because Sheppard, an aren’t done yet. Over the next VenusMars Jupiter Saturn Uranus Neptune Pluto “The Goblin”“Biden” “FarOut” distance of about 123 astro- The researchers believe astronomer at the Carnegie few years, they expect their 2015 T 2012 VP 2018 VG G nomical units — where this growing list of clustered Institution for Science, and survey to spot many more 387 113 18

WWW.ASTRONOMY.COM 13 Jupiter Mass: 318 M SIZING UP Saturn MAPPING A PLANET NINE ⊕ Mass: 95.2 M Top down view ⊕ MYSTERIOUS WORLD Uranus Neptune Mercury Mass: 14.5 M Mass: 17.1 M Planet 9 Mass: 0.055 M ⊕ ⊕ Earth Mass: ~5–15 M ⊕ Mass: 1 M ⊕

⊕ Venus Mass: 0.815 M Pluto ⊕ Mars Mass: 0.003 M Mass: 0.107 M Planet 9 ⊕ 2015 TG387 M =1 Earth Mass “The Goblin” ⊕ Neptune ⊕ 2012 VP113 “Biden” Sedna Uranus Planet Nine is expected to be about five to 15 times as Kuiper massive as Earth, which would make it pretty similar to the Belt large super-Earths and small sub-Neptunes astronomers are increasingly finding around other stars. ASTRONOMY: ROEN KELLY AFTER KARL TATE

albeit distant, orbit. writing, called 2015 TG387, which, in which means it swings excep- at nearly the same point in The hunt for to the Sun at roughly the from the Sun? Although Sheppard and Sheppard is the spirit of tionally far from the Sun. space as a handful of other Planet Nine same point in space, despite That’s still a heavily his team are already pretty eagerly await- Halloween, they “The Goblin’s orbit is very noteworthy objects. These With the advancement of having different orbital trajec- debated question. “There’s confident about FarOut’s ing the new nicknamed The large. It has a semimajor axis include the team’s earlier dis- powerful survey telescopes, as tories. According to that two main theories for how

speed (and therefore its dis- November Goblin. of some 1,000 AU,” Sheppard covery of 2012 VP113 (which well as sophisticated software study, the odds that such clus- the planet got out there,” PLEASE PROOF: Title Astronomy tance),Illustrator in orderRoen Kelly to confirm data. Though the says, “meaning it takes some they nicknamed Biden as a that can sift through massive tering is the result of chance Sheppard says. “The most Individual illustrators, designers, art directors, Issue January 2020 FarOut’sDesigner orbital path, 200-mile-wide 40,000 years to go around the nod to the “VP” in its desig- and editors must proof Job # MAG-ASY-JAN20 theyArt mustDir. first col- (320 km) Sun once.” nation) and Sedna, a dwarf THE DISCOVERY OF THIS PECULIAR CLASS OF and sign this form. Code PN0120_14 lectStory more Ed. images Because of the time it planet roughly half the diam- Proof 2 of Copyit moving Ed. takes The Goblin to complete eter of Pluto that was discov- EXTREME TNOs INDICATES THAT DWARF PLANETS MAY NOT Date 10-9-19 Man. Ed. BE THE KINGS AND QUEENS OF THE KUIPER BELT. Return Editor a single orbit, the team ered in 2003. Scott Sheppard required three years of obser- It’s this recently identified and Chad Trujillo vations after its initial discov- pack of perihelion-clustered amounts of data like never is about 0.007 percent (or 1 in likely scenario I like to call won the 2019 ery in 2015 to determine its objects, which Sheppard refers before, we are just now begin- 14,000). Furthermore, at the the ‘runt of the giant planet Farinella prize for their work exact path through the sky. to as “extreme TNOs,” that ning to scour the distant beginning of 2019, Batygin family.’ ” In this scenario, exploring and But once they pinned it suggests an unseen and mas- reaches of the solar system and Brown also showed Planet Nine formed alongside characterizing objects in the outer solar system. down, they learned the eccen- sive planet is hiding in the for icy dwarf planets and there’s only about a 0.2 per- Jupiter, Saturn, Uranus, and EUROPLANET/G. MANTOVANI tric TNO comes to perihelion outskirts of our solar system. the like. But the discovery of cent chance (or 1 in 500) that Neptune, which Sheppard

14 Jupiter Mass: 318 M SIZING UP Saturn MAPPING A PLANET NINE ⊕ Mass: 95.2 M Top down view ⊕ MYSTERIOUS WORLD Uranus Neptune Mercury Mass: 14.5 M Mass: 17.1 M Planet 9 Mass: 0.055 M ⊕ ⊕ Earth Mass: ~5–15 M ⊕ Mass: 1 M ⊕

this peculiar class of extreme the clustering is the result TNOs indicates that dwarf of observational biases. 100 AU planets may not be the kings “Based on the clustering of In recent years, astronomers have and queens of the Kuiper the objects we are finding that identified a mysterious group of extreme TNOs with distant, clustered Belt. Instead, there’s a grow- are way out there like The perihelia, as well as relatively aligned ing dossier of evidence that Goblin,” Sheppard says, “a orbits that seem decoupled from the suggests a colossal world super-Earth planet that is a gravitational influence of the solar system’s giant planets. Based on the some five to 15 times the few hundred AU away” could orbits of these objects — which mass of our own planet be shepherding them into include Sedna (orange), “The Goblin” However, Sheppard (green), “Biden” (blue), and a handful is lurking far beyond, position. Thanks to cutting- stresses that FarOut doesn’t of other bodies with less-colorful playing puppeteer to the edge simulations, the serve as evidence for Planet names (shown in gray) — some oddly clustered collection researchers expect Planet researchers think a massive world Nine just yet. Because FarOut through the sky. “You called Planet Nine is shepherding the of distant bodies. Nine (sometimes referred to is so, well, far out, it’s very need a year-long arc on it to extreme TNOs into position. ASTRONOMY: In 2016, Caltech as Planet X), to be in a some- difficult to pin down its orbit. actually get the orbit down,” ROEN KELLY AFTER CALTECH/R. HURT (IPAC) astronomers Kostantin what eccentric orbit that takes This makes it tough to tell Sheppard says. And because Batygin and Mike Brown it about 400 AU from the Sun. whether the new find is actu- they don’t have a year’s worth But FarOut isn’t the only published a paper titled But, Sheppard says, “the ally a member of the unique of data for FarOut yet, he world that might help point “Evidence for a Distant Giant planet could be up to some group of TNOs that hint at says, “we need another obser- the way to Planet Nine. In Planet in the Solar System.” 1,500 AU away in the more the existence of Planet Nine, vation in November [2019] to October 2018, the researchers icy rock was first seen In it, they discuss a unique massive planet models.” or if it’s just a run-of-the-mill actually get a reliable orbit.” announced the confirmation about 80 AU away, it has a population of TNOs that But if Planet Nine really object in a relatively boring, At the time of this of another distant discovery particularly eccentric orbit, make their closest approaches exists, why does it orbit so far albeit distant, orbit. writing, called 2015 TG387, which, in which means it swings excep- at nearly the same point in The hunt for to the Sun at roughly the from the Sun? Although Sheppard and Sheppard is the spirit of tionally far from the Sun. space as a handful of other Planet Nine same point in space, despite That’s still a heavily his team are already pretty eagerly await- Halloween, they “The Goblin’s orbit is very noteworthy objects. These With the advancement of having different orbital trajec- debated question. “There’s confident about FarOut’s ing the new nicknamed The large. It has a semimajor axis include the team’s earlier dis- powerful survey telescopes, as tories. According to that two main theories for how speed (and therefore its dis- November Goblin. of some 1,000 AU,” Sheppard covery of 2012 VP113 (which well as sophisticated software study, the odds that such clus- the planet got out there,” PLEASE PROOF: Title Astronomy tance),Illustrator in orderRoen Kelly to confirm data. Though the says, “meaning it takes some they nicknamed Biden as a that can sift through massive tering is the result of chance Sheppard says. “The most Individual illustrators, designers, art directors, Issue January 2020 FarOut’sDesigner orbital path, 200-mile-wide 40,000 years to go around the nod to the “VP” in its desig- and editors must proof Job # MAG-ASY-JAN20 theyArt mustDir. first col- (320 km) Sun once.” nation) and Sedna, a dwarf THE DISCOVERY OF THIS PECULIAR CLASS OF and sign this form. Code PN0120_14 lectStory more Ed. images Because of the time it planet roughly half the diam- Proof 2 of Copyit moving Ed. takes The Goblin to complete eter of Pluto that was discov- EXTREME TNOs INDICATES THAT DWARF PLANETS MAY NOT Date 10-9-19 Man. Ed. BE THE KINGS AND QUEENS OF THE KUIPER BELT. Return Editor a single orbit, the team ered in 2003. Scott Sheppard required three years of obser- It’s this recently identified and Chad Trujillo vations after its initial discov- pack of perihelion-clustered amounts of data like never is about 0.007 percent (or 1 in likely scenario I like to call won the 2019 ery in 2015 to determine its objects, which Sheppard refers before, we are just now begin- 14,000). Furthermore, at the the ‘runt of the giant planet Farinella prize for their work exact path through the sky. to as “extreme TNOs,” that ning to scour the distant beginning of 2019, Batygin family.’ ” In this scenario, exploring and But once they pinned it suggests an unseen and mas- reaches of the solar system and Brown also showed Planet Nine formed alongside characterizing objects in the outer solar system. down, they learned the eccen- sive planet is hiding in the for icy dwarf planets and there’s only about a 0.2 per- Jupiter, Saturn, Uranus, and EUROPLANET/G. MANTOVANI tric TNO comes to perihelion outskirts of our solar system. the like. But the discovery of cent chance (or 1 in 500) that Neptune, which Sheppard

WWW.ASTRONOMY.COM 15 MORE EVIDENCE FOR PLANET NINE

Beginning of Oort Cloud 2,000 AU

ABOVE AND RIGHT: To carry out their survey of the most distant objects in Saturn and got tossed out- the solar system, Sheppard and his colleagues rely on the Subaru ward.” This ended up pushing Telescope (above), perched atop the the still-forming Planet Nine summit of Mauna Kea in Hawaii, as Top down view well as the Victor Blanco Telescope out of the feeding zone, which (right), located at Cerro Tololo Inter- stunted its growth before it American Observatory in Chile. SUBARU: Planet 9 could reach the size of a true ROBERT LINSDELL/FLICKR; BLANCO: NOAO/AURA/NSF giant planet. Alternatively, Sheppard the early solar system, mean- says, “it could be a captured ing an approaching planet the planet, and nailing down Although it’s fascinating to However, “the planet carrying out the widest and object from another star sys- “could have been frictionally their clustering trends much ponder whether a hidden hypothesis is the simplest and deepest survey of the solar Sedna tem.” Because stars form out slowed down and captured.” better will better help us planet many times the mass of best explanation,” Sheppard system ever to help find out of massive clouds of condens- There’s also a third possi- locate the planet and further Earth is slinking in the outer says. “A massive disk might be for sure. “I always like to say ing gas and dust, they are bility: the solar system was show it is real.” solar system, there are some possible, but we don’t see any it’s more likely than not that born in litters, not in isolation. shaken up in its past by a By even doubling the alternative theories that massive disk out there, and if [Planet Nine] exists out there. 2012 VP113 500 AU “Biden” So, early in the solar system’s wandering star. In research known number of extreme history, the Sun was sur- published last year, astrono- TNOs orbiting far beyond “SO IN PRINCIPLE, PLANET NINE CAN BE rounded by many other mers announced the discovery Neptune (which is currently a nearby stars that all gravita- of an exoplanet named sample of about a dozen), A PLANET, A POTATO, A BLACK HOLE, A HAMBURGER, ETC., tionally interacted with one HD 106906 b, which they researchers think they can AS LONG AS THE ORBITAL PARAMETERS ARE RIGHT.” another. According to think was exiled into a large, tease out whether Planet Nine Sheppard, this means other perturbed orbit thanks to a is really there or not. And, attempt to explain the clus- there were one, it would be I would say somewhere in the stars “were tossing objects out pair of passing stars that more importantly, an tered orbits of extreme TNOs harder to explain than just 80 to 90 percent range.” of their solar system, so there strayed too close to its system. increased sample size may without employing Planet having one planet.” And as “The primary reason why was probably a lot of free- According to Batygin, it’s fea- help them further constrain Nine. For example, some for the black hole theory, I’m so excited about this work,” floating planets being ejected sible that a similar process exactly where Planet Nine is researchers suggest the orbit- Batygin says, “the important Batygin says, “is because from all these different stars.” could have happened when in its orbit. But for now, perturbing effects of Planet thing to understand here is there’s a near-term opportu- And if one of the rogue plan- the Sun was still near other Sheppard says, “None of the Nine instead could be caused that all the calculations can tell nity for observational determi- ets strayed too close to the stars from its birth cluster. most distant perihelia objects by a distant and massive disk us is the mass of Planet Nine, nation one way or another.” Sun, there’s a chance it was with large semimajor axes of small, icy objects — or pos- not its composition. So in But for now, Sheppard says, seized by our own solar sys- Proving Planet obviously buck the sibly even a baseball-sized principle, Planet Nine can be a “It’s exciting to be looking at tem, where it’s since remained. Nine’s existence clustering trend, but primordial black hole planet, a potato, a black sky that no one has ever However, for the Sun to The problem with Planet Nine again, we are talk- of similar mass hole, a hamburger, etc., imaged as deeply as we are. It’s If Planet Nine exists, there should be a set of objects with orbits that are perpendicular to the ecliptic plane of the solar system. Five such objects capture such a planet, “you’d is that in order for us to be ing about only a that was cap- as long as the orbital like Forrest Gump said: Each (shown in magenta) are known, and according to Batygin, they perfectly have to slow it down some- 100 percent sure it exists, we little more than a tured by the parameters are right.” image we take is like a box of fit the models. ASTRONOMY: ROEN KELLY AFTER CALTECH/R. HURT (IPAC) how,” Sheppard says. “So need to see it with our own handful of objects.” solar system. So, although the chocolates, you never know that’s the questionable factor.” eyes. Fortunately, Sheppard existence and exact what you are going to find.” Although capturing an ejected says, “our survey is designed nature of Planet Nine Konstantin Batygin (left) says all likely began their lives formed in that area,” he says. planet isn’t the easiest thing to to not simply find the planet, and Mike Brown (right) is still up for Jake Parks is an associate within about 5 to 15 AU of As Planet Nine was forming, do, Sheppard points out it but to triple the known [num- also have been pivotal debate, editor of Astronomy and is to progressing the excited to learn what surprises one another. “There’s prob- “it got to several Earth masses could happen. For example, he ber of] very distant objects. Planet Nine Sheppard’s ably several Earth-mass-sized in size, but then it probably says, there was probably a lot These very distant objects are hypothesis. team is the solar system is still hiding objects and bigger which got too close to Jupiter and more gas and dust around in the ones that are sensitive to LANCE HAYASHIDA/CALTECH currently from us after all these years.

16 ASTRONOMY INSIGHTS • JANUARY 2020 MORE EVIDENCE FOR PLANET NINE

Beginning of Oort Cloud 2,000 AU