Love is easy to spell and notoriously difficult to define, but ever since its dramatic discovery in 1930, Pluto seems to have captured the love of many people the world over. Perhaps this is because Pluto is a small and very remote world, lost in the incredible, frigid darkness of the outermost region of our Solar System, where our Sun appears in its dark sky as just another bright star swimming in a mesmerizing sea of starlight. Mystery is captivating, and Pluto has remained an alluring and bewitching mystery for almost a century, located as it is so very far from the feeble fire of our distant Star. On July 14, 2015, after a decade-long, very treacherous, and difficult journey through our Solar System, NASA’s plucky New Horizons spacecraft successfully made its historic closest approach to Pluto, at about 7,750 miles above its long-hidden surface–approximately the same distance that it is from New York to Mumbai, India–making it the first-ever space mission to explore a world so far from our own planet.
Humanity has loved Pluto from a distance for a long time–but, now, for the very first time, it can be studied up close and personal, revealing its intriguing features to its many distant admirers far, far away on a little blue watery planet much closer to the Sun.
“I’m delighted at this latest accomplishment by NASA, another first that demonstrates once again how the United States leads the world in space. New Horizons is the latest in a long line of scientific accomplishments by NASA, including multiple missions orbiting and exploring the surface of Mars in advance of human visits yet to come; the remarkable Kepler mission to identify Earth-like planets around stars other than our own; and the DSCOVR satellite that soon will be beaming back images of the whole Earth in near real-time from a vantage point a million miles away. As New Horizons completes its flyby of Pluto and continues deeper into the Kuiper Belt, NASA’s multifaceted journey of discovery continues,” noted Dr. John Holdren in a July 14, 2015 Johns Hopkins University Applied Physics Laboratory (JHUAPL) Press Release. Dr. Holdren is assistant to the President for Science and Technology and director of the White House Office of Science and Technology Policy. The JHUAPL is in Laurel, Maryland.
The Kuiper Belt is a frigid, murky, and mysterious domain far beyond the ice-giant Neptune–an enormous and beautiful blue gaseous planet, that is the eighth major planet from our Sun. Astronomers are first beginning to explore the distant Kuiper Belt, where a sparkling multitude of alien, frozen worldlets do a jitter bug around our Sun. Pluto is a relatively large denizen of the Kuiper Belt, and it was–at first sight–classified as the ninth major planet from our Star after its discovery in 1930. However, the growing realization among astronomers that this beloved, frozen little “oddball” is only one of several large icy bodies inhabiting the Kuiper Belt prompted the International Astronomical Union (IAU) to formally define the term “planet” in 2006–and poor Pluto lost its lofty classification as the ninth major planet in our Sun’s family. Now, freshly re-classified as mere dwarf planet Pluto, nevertheless, remains a small world of fascination, affection, and debate among members of the planetary science community who are trying to determine precisely how a “planet” should be defined.
“The exploration of Pluto and its moons by New Horizons represents the capstone event to 50 years of planetary exploration by NASA and the United States. Once again we have achieved a historic first. The United States is the first nation to reach Pluto, and with this mission has completed the initial survey of our Solar System, a remarkable accomplishment that no other nation can match,” NASA Administrator Dr. Charles Bolden said to the press on July 14, 2015.
The Pluto Story
The Pluto story began less than a century ago when a youthful farmer’s son from Kansas, the astronomer Clyde Tombaugh (1906-1997), was assigned the difficult task of searching for the elusive, mysterious, and possibly non-existent Planet X. According to theory, Planet X is a secretive giant planet that lurks somewhere in the frigid darkness beyond the orbit of Neptune. Using a telescope in Flagstaff, Arizona, Tombaugh did, in fact, discover a dim pinpoint of light that ultimately proved to be the fascinating, icy “oddball” Pluto–instead of Planet X.
Like other Kuiper Belt Objects (KBOs), Pluto is believed to be mainly made up of a mixture of ice and rock. It is truly a little world, only approximately 1/6 the mass of Earth’s Moon and about 1/3 its volume. Pluto also shows a highly inclined and eccentric orbit that carries it from 20 to 49 Astronomical Units (AU) from our Sun. One AU is equal to the mean Earth-Sun separation of about 93,000,000 miles. Pluto periodically travels towards our Star at a closer distance than Neptune! Fortunately, an orbital resonance with Neptune effectively prevents the two worlds from crashing into each other.
The Kuiper Belt–sometimes termed the Edgeworth-Kuiper Belt–is a very distant region of our Solar System, circling our Sun beyond the realm of the majestic giant planets. The Belt extends from the orbit of Neptune to about 50 AU. Neptune’s average distance from our Star is about 30.1 AU–its perihelion (when it is closest to our Sun) is 29.8 AU, while its aphelion (when it is furthest from our Sun) is 30.4 AU.
Pluto itself has five known moons: Charon, Nix, Hydra, Kerberos, and Styx. Charon is by far the largest of the quintet of moons, and it sports a diameter of about half the size of Pluto. Some astronomers think that Pluto and Charon actually compose a binary system because the barycenter of their orbits does not reside within either of the two worlds.
The demoted, small world that is Pluto was named for the Roman god of the underworld. Charon was discovered in 1978 by the American astronomer James Christy. Some astronomers think that Charon is a big chunk blasted off Pluto as the result of a monumental collision between Pluto and some unidentified object that was rampaging through the Kuiper Belt close to Pluto.
For most of the 20th century, it was thought that Pluto was a lonely little world where it dwells in the deep freeze of the outer Solar System, far from the delightful light and comforting heat of our Star. However, in 1992, the very first KBO (other than Pluto and Charon) was discovered, and astronomers came to the realization that Pluto is not really as far from the madding crowd of other similar objects, inhabiting the Kuiper Belt, as originally thought. Since 1992, many other frigid, frozen little worlds akin to Pluto–also displaying eccentric orbits–have been detected. The most important of these is the scattered disc object named Eris that was discovered in 2005. Eris is a bit more massive than Pluto. The realization that Pluto is just one of many KBOs resulted in its demotion and reclassification. However, there are astronomers who oppose this demotion, contending that Pluto should have kept its original designation as the ninth major planet from our Sun, and that the other similar dwarf planets that have recently been discovered should also be added to the list of major planets along with Pluto.
The New Horizons mission will help astronomers understand those remote, mysterious worlds haunting the edge of our Solar System. New Horizons is making the first reconnaissance of Pluto and its moons by traveling deeper into the mysterious, remote Kuiper Belt–which is actually a relic of our Solar System’s ancient formation.
New Horizons was launched on January 19, 2006. It took a swing past Jupiter for a gravity kick and scientific studies in February 2007, and it is conducting a five-month-long reconnaissance flyby study of Pluto and its moons. As part of New Horizons’ extended mission, the spacecraft is expected to zip farther into the Kuiper Belt to observe one or two of the icy, ancient worldlets dwelling in the remote deep-freeze of that dark, frigid region.
The National Academy of Sciences has designated the exploration of the Kuiper Belt to be of the highest priority for Solar System exploration. In general, New Horizons seeks to gain insight into how Pluto and its moons should be categorized as denizens of our Sun’s family–one category of Solar System inhabitants is composed of the inner, rocky, terrestrial planets (Mercury, Venus, Earth, and Mars), and the second category is made up of the outer, giant, gaseous planets (Jupiter, Saturn, Uranus, and Neptune).
Pluto and Charon are now classified as members of yet a third category of objects dubbed ice dwarfs. They possess solid surfaces but, unlike the rocky terrestrial planets, a large portion of their mass is icy material.
An up close and personal peek at these icy, dwarf worlds from a spacecraft promises to reveal the incredible and very ancient story about the origins and outer limits of our Solar System. New Horizons will also explore–for the very first time–how ice dwarfs like Pluto and kindred objects have evolved over time.
Icy Mountains And Plains, A Mountain In A Moat, And A Mesmerizing Misshapen Moon
The first tantalizing peek at Pluto and its moons hints of marvels yet to be discovered within this distant, dark, and frigid wonderland of our Solar System’s outer limits. Mountains carved of ice on Pluto and a new and very revealing view of the youthful surface of its active companion world, Charon, are among the first of a treasure trove of discoveries announced by the New Horizons team on July 15, 2015–just one day after the spacecraft’s first Pluto flyby!
“Pluto New Horizons is a true mission of exploration showing us why basic scientific research is so important. The mission has had nine years to build expectations about what we would see during closest approach to Pluto and Charon. Today, we get the first sampling of the scientific treasure collected during those critical moments, and I can tell you it dramatically surpasses those high expectations,” commented Dr. John Grunsfeld in a July 15, 2015 NASA Press Release. Dr. Grunsfeld is associate administrator for NASA’s Science Mission Directorate in Washington D.C.
“Home run! New Horizons is returning amazing results already. The data look absolutely gorgeous, and Pluto and Charon are just mind-blowing,” declared Dr. Alan Stern in the same Press Release. Dr. Stern is principal investigator for New Horizons at the Southwest Research Institute (SwRI) in Boulder, Colorado.
One of the new close-up images of an equatorial region, near the base of a bright heart-shaped feature on Pluto’s surface, reveals a mountain range with icy peaks jutting as high as 11,000 feet above the surface of this frozen world.
The icy mountains on Pluto are thought to have formed no more than a “mere” 100 million years ago–making them comparatively youthful structures in our 4.56-billion-year-old Solar System. This hints that the close-up region, which covers approximately one percent of Pluto’s surface, may still be geologically active today.
In addition, another close-up picture of Pluto shows a vast, craterless plain that is possibly still being shaped by geologic processes. Like the mountains of Pluto this plain is probably no more than 100 million years old. This frozen region is located north of Pluto’s icy mountains, near the center of the heart-shaped feature. The intriguing heart has been informally named Tombaugh Regio in honor of its discoverer Clyde Tombaugh. The plains region, that resembles frozen mud cracks on Earth, has been named Sputnik Planum after Earth’s first artificial satellite. It displays a a fractured, cracked surface composed of irregularly-shaped segments, approximately 12 miles across. Planetary scientists have proposed two possible scenarios to explain how the segments were formed. The irregular shape may have been caused by the contraction of surface materials, similar to what happens when mud dries. Alternatively, they may have been formed by the process of convection, in a way that has been compared to how wax rises in a lava lamp.
“This is one of the youngest surfaces we’ve ever seen in the Solar System,” commented Dr. Jeff Moore in the July 15, 2015 NASA Press Release. Dr. Moore is of the New Horizons Geology, Geophysics, and Imaging Team (GGI) at NASA’s Ames Research Center in Moffett Field, California.
Pluto cannot be heated by a gravitational dance with a much larger planetary body, like the myriad of icy moons that circle the four gaseous giant planets of our Sun’s family. Therefore, some other process must generate the mountainous landscape.
“This may cause us to rethink what powers geological activity on many other icy worlds,” commented Dr. John Spencer to the press on July 15, 2015. Dr. Spencer is GGI deputy team leader at SwRI.
The new and revealing view of Charon shows a young and varied terrain, remarkable for its apparent lack of craters. Older surfaces show more cratering than younger surfaces, and Charon is almost completely devoid of craters. A swath of cliffs and troughs–extending approximately 600 miles–indicates widespread fracturing of Charon’s crust. This fracturing is probably the result of internal geological processes. The new picture also shows a canyon that may be 4 to 6 miles deep. In Charon’s north polar region, there are mysterious dark surface markings with a diffuse boundary, suggesting a shallow stain or deposit on its icy surface.
An image of Charon released on July 16, 2015, shows an additional bewitching feature–a depression with a peak in the middle, that has been dubbed the Mountain in a Moat. The image reveals an area approximately 240 miles from top to bottom, including few visible craters.
New Horizons also observed one of the smaller members of the Pluto system. A new image of the moon Hydra shows that it has an apparently irregular shape, and that it is only about 27 by 20 miles. Hydra’s irregular surface is probably covered with water ice.
Dr. Jim Green, director of Planetary Science at NASA Headquarters in Washington D.C. told the press on July 17, 2015 that “With the flyby in the rearview mirror, a decade-long journey to Pluto is over–but the science payoff is only beginning.”