Pluto And Charon: Their True Colors Are Shining Through

We are a curious species; we like to explore alien places that lie far beyond the comfortable world that we know. And so we sail into the mysterious and dangerous darkness of distant space to see that which no one has ever before seen, kissed by the irresistible desire to understand what exists beyond the familiar.

New Horizons is an interplanetary space probe that was launched as part of NASA’s New Frontiers program, with the mission of performing a flyby study of the distant dwarf planet Pluto and its quintet of icy moons–especially its large moon Charon.

The distant, frozen twilight region of our Solar System, where the Pluto system is situated, was unexplored until New Horizons wandered its way into this faraway fringe of our Sun’s region of influence called the Kuiper Belt. New Horizons then dispatched back to the watchful eyes of curious astronomers on Earth a treasure trove of previously long-hidden wonders from the unknown. Now, marking the anniversary of New Horizons’ historic flight through the Pluto system on July 14, 2015, mission scientists have released the most accurate natural color images of Pluto and its large moon Charon to date.

New Horizons was engineered by the Johns Hopkins University Applied Physics Laboratory (API) and the Southwest Research Institute (SwRI). A team led by Dr. Alan S. Stern was responsible for seeing to it that the spacecraft was launched in 2006 with the primary mission of performing a flyby study of the Pluto system in 2015, and a secondary mission of visiting and studying one or more other denizens of the distant Kuiper Belt in our Solar System’s outer limits.

The Kuiper Belt is the frigid home of a multitude of sparkling, icy comet nuclei and other frozen objects, both large and small. It is located beyond the orbit of Neptune, the outermost major planet of our Sun’s family. New Horizons’ secondary mission, to perform a flyby of a Kuiper Belt Object (KBO), is scheduled to occur in the decade following the Pluto flyby. New Horizons is the fifth artificial object to reach the escape velocity necessary to free itself from the gravitational grip of our Solar System.

On January 19, 2006, New Horizons was launched from Cape Canaveral Air Force Station by an Atlas V rocket directly into an Earth-and-solar escape trajectory with a breathtaking speed of about 36,400 miles per hour. After a transient encounter with asteroid 132524 APL, New Horizons soared off to Jupiter, making its closest approach to our Solar System’s banded behemoth on February 28, 2007, at a distance of 1.4 million miles. The Jupiter flyby gave New Horizons a gravity kick that increased its speed. The flyby also provided a general test of New Horizons’ scientific capabilities, sending back to Earth important data about the planet’s magnetosphere, atmosphere, and many moons.

Most of New Horizons’ voyage that followed the Jupiter flyby was spent in hibernation mode, except for brief annual check-outs. This was done in order to preserve on-board systems. On December 6, 2014, New Horizons was deliberately shaken out of its long slumber and was brought back online for its scheduled encounter with Pluto, and instrument checkout began. On January 15, 2015, the New Horizons spacecraft began to approach its quarry.

On July 14, 2015, New Horizons soared 7,800 miles above the fascinating, beautiful, and alien surface of Pluto that was unlike any other planetary surface ever before observed in our Solar System. This made New Horizons the first spacecraft to explore the very distant dwarf planet.

On October 25, 2016, the last of the recorded data from the historic Pluto flyby was obtained from New Horizons. Having finished its flyby of Pluto, New Horizons was maneuvered for a flyby of a second KBO (486958) 2014 MU 69. This encounter is scheduled to occur on January 1, 2019, when it will be 43.4 astronomical units (AU) from our Sun. One AU is the average distance between the Earth and Sun, which is about 93,000,000 miles.

The purpose of the New Horizons mission is to gain a new scientific understanding of the formation of the Pluto System, the Kuiper Belt, and the evolution of the ancient Solar System. The spacecraft gathered data about the surfaces, atmospheres, interiors, and environments of Pluto and its moons–especially Charon. It is also planned to study other frozen objects inhabiting the mysterious and remote Kuiper Belt.

This secondary stage of the New Horizons mission began in 2011 with a dedicated hunt for a suitable KBO as the target for the historic flyby of one of these very remote objects dancing around in our Solar System’s distant deep freeze.

In order to make this very difficult choice, ground telescopes were first used by mission scientists. Specifically, large ground telescopes with wide-field cameras, particularly the duo of twin 6.5-meter Magellan Telescopes in Chile, the 8.2-meter Subaru Observatory in Hawaii, and the Canada-France-Hawaii Telescope were used by astronomers in order to search for potential targets. Through a citizen-science project, the general public also helped with this search by combing through telescopic images for potentially suitable mission candidates in an endeavor named the Ice Hunters Project.

On October 15, 2014, astronomers revealed that HST’s search had come up with three potential targets. All three objects had estimated diameters of about 19 to 34 miles, making them much too small to be observed by ground-based telescopes. The trio of potential targets were also about 43 to 44 AU from our Sun, which placed the future encounters in the 2018 to 2019 period.

On August 28, 2015, 2014 MU69 was selected as the flyby target for the secondary New Horizons mission of exploration through the Kuiper Belt. The necessary adjustment for this future encounter was performed with four engine firings between October 22 and November 4, 2015.

In addition to the scheduled flyby of 2014 MU69, the extended secondary mission for New Horizons includes plans for the spacecraft to conduct observations of, and search for ring systems around, between 25 and 35 different KBOs. Also, New Horizons will continue to observe the gas, dust, and plasma composition of the Kuiper Belt before the mission extension will come to an end in 2021.

The science objectives of the flyby over 2014 MU69 include determining its morphology and geology, and mapping its surface composition, specifically looking for carbon monoxide, methane, ammonia, and water ice. This will help planetary scientists determine how 2014 MU69 formed and has since evolved. New Horizons is also planned to measure the tiny KBO’s surface temperature.

In Our Solar System’s Dark Deep Freeze

The Kuiper Belt was named after the Dutch-American astronomer Gerard Kuiper (1905-1975), who is usually given credit for being the first to predict its existence. The Kuiper Belt is a distant, twilight region of our Solar System, situated beyond the frigid realm of the quartet of majestic, giant, gaseous major planets of our Sun’s family–Jupiter, Saturn, Uranus, and Neptune. The Kuiper Belt extends from the orbit of the outermost planet Neptune (at 30 AU) to about 50 AU from our Star. In a number of ways, the Kuiper Belt shares certain characteristics with the Main Asteroid Belt that circles our Sun between the orbits of Mars and Jupiter. However, the Kuiper Belt is approximately 20 to 200 times more massive than the Main Asteroid Belt.

But, like the Main Asteroid Belt, the Kuiper Belt is populated by small bodies that are the lingering relics of the ancient era of planet formation in our Solar System. The asteroids are all that is left of an abundant ancient population of rocky planetesimals (the building blocks of planets) that bumped into one another and merged together to create ever larger and larger bodies. The asteroids are similar to the rocky and metallic planetesimals that ultimately created the four inner solid planets of our Solar System: Mercury, Venus, Earth, and Mars. In contrast, the Kuiper Belt is the remote home of myriad icy comet nuclei that are like the dusty and icy planetesimals that collided with one another and merged to create the gigantic quartet of outer planets.

Many asteroids are composed primarily of rock and metal. However, most KBOs are made up of volatiles (“ices”), such as ammonia, methane, and water. The Kuiper Belt is also the frigid domain of a trio of officially recognized dwarf planets: Pluto, Haumea, and Makemake. A small number of our Solar System’s moons, such as Phoebe of Saturn and Triton of Neptune, are believed to have been born in this distant deep freeze far from the melting heat and brilliant light flowing out from our roiling, fiery Star.

Ever since the Kuiper Belt was actually discovered back in 1992, the number of known KBOs has skyrocketed, and more than 100,000 KBOs are predicted to be more than 62 miles in diameter At first, astronomers thought that the Kuiper Belt was the primary domain of short-period comets, which are those that sport orbits that swing them around our Star less frequently than every 200 years. However, more recent studies that have been conducted since the mid-1990s, have revealed that the Kuiper Belt is actually dynamically stable, and that the true home of the short-period comets is really the scattered disc. The scattered disc is a dynamically active region of our Solar System, that was likely created by the outward migration of Neptune about 4.5 billion years ago, in our 4.56 billion year old Solar System’s infancy. Scattered disc objects, such as Eris, possess extremely eccentric (out-of-round) orbits that take them as far as 100 AU from our fiery Star.

The icy inhabitants of the Kuiper Belt, along with the frozen denizens of the scattered disc, are collectively designated trans-Neptunian objects. A third region of our Solar System, that is also believed to be the home of an abundant population of comet nuclei, is the very distant Oort Cloud. The Oort Cloud is a thousand times more remote than the Kuiper Belt, and not nearly as flat. It is also the home of long-period comets, which are those sporting orbits that take them more than 200 years to circle our Star. The still somewhat hypothetical Oort Cloud is an enormous shell of dancing, icy comet nuclei that encircles our entire Solar System–and stretches halfway to the nearest star beyond our Sun.

Poor Pluto is the largest known denizen of the Kuiper Belt, as well as the second-largest trans-Neptunian Object, after Eris, that is located in the scattered disc. Although it was originally classified as a major planet after its discovery in 1930 by the American astronomer Clyde Tombaugh, Pluto’s status as just another member of the heavily populated Kuiper Belt resulted in its unceremonious eviction from the pantheon of major planets. However, final attempts to classify this little world with a big heart have failed to reach a definite conclusion concerning its planetary status. Pluto was re-classified as a dwarf planet in 2006, and it is composionally similar to many, many other KBOs. Indeed, its orbital period is characteristic of a particular class of KBOs called plutinos. Plutinos share the same 2:3 resonance with Neptune.

Showing Their True Colors

Three years after New Horizons gave humanity our first close up and personal views of Pluto and Charon, planetary scientists are still uncovering the myriad wonders of those two frozen, fascinating small worlds in our Solar System’s dimly-lit deep freeze.

The new natural-color images are the result of improved calibrations of data collected by New Horizons’ Multispectral Visible Imaging Camera (MVIC). “That processing creates images that would approximate the colors that the human eye would perceive–bringing them closer to ‘true color’ than the images released near the encounter,” explained Dr. Alex Parker in a July 20, 2018 Johns Hopkins University Applied Physics Laboratory (JHUAPL) Press Release. Dr. Parker is a New Horizons science team co-investigator from Southwest Research Institute in Boulder, Colorado.

However, MVIC’s color filters don’t closely match the wavelengths that are picked up by our human eyes. Because of this mismatch, mission scientists applied special processing to alter the raw MVIC data into providing a good estimate of the colors that our eyes would see if they could. The colors revealed proved to be more subdued than those produced from the raw MVIC color data. This is because of the narrower range of wavelength that is sensed by human vision.

Both images of Pluto and Charon were obtained when New Horizons soared towards its closest approach to Pluto and its quintet of mysterious moon-worlds on July 14, 2015. The image of Charon was taken from a range of 46,091 miles and Pluto from 22,025 miles. Each is a single color MVIC scan, with no additional data from other New Horizons instruments or imagers added. The most fascinating features on both Pluto and Charon are clearly visible, from the bright expanse of Pluto’s nitrogen-and-methane-ice laden big “heart” (Sputnik Planitia), to Charon’s reddish north-polar region (Modor Macula).

Preparations are currently underway for New Horizons’ upcoming encounter with 2014 MU69, recently nicknamed Ultima Thule–meaning “beyond the known world”. This close encounter, with the most distant object as yet visited by a spacecraft, is scheduled for January 1, 2019. The historic meeting between New Horizons and the mysterious little KBO will take place a billion miles farther from our Sun than Pluto. Today, approximately 3.8 billion miles from Earth–over 40 times farther from our Star than Earth–the spacecraft is operating normally and will start obtaining long-distance observations and measurements of Ultima in late August 2018.

“Even as we celebrate the third anniversary of the historic exploration of the Pluto system–the most distant worlds ever explored–we’re looking forward to the far more distant and record-shattering exploration of Ultima Thule, just five months from now,” Dr. Stern commented to the press on July 20, 2018.

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