A dim star shining feebly at magnitude +15.2 was first seen by Clyde Tombaugh at the Lowell Observatory in early 1930. He was looking for a planet beyond Neptune and blinked thousand of plates of images he took of star fields hoping to find it. Finally, on February 18, 1930 he blinked a pair of plates where a dim star moved just the right amount to lie beyond Neptune. On March 13, 1930 it was announced to the world that a planet had been found on the edge of the Solar System. On May 1, 1930 the dim little star became known as the planet Pluto on the recommendation of a young English girl, Venetia Burney. The world would forever fall in love with this remote, tiny, mysterious world.
Pluto was always the simplest planet to remember facts about since for decades very little was known about it. It was over 3 billion miles from the Sun, was 3600 miles in diameter, and had no moon, plain and simple. Pluto is so far away that the prying eyes of the largest telescopes barely resolved a tiny disc with a pale yellow hue. Pluto is an oddball out beyond the giant gas planets Jupiter, Saturn, Uranus, and Neptune. It is tiny and inclined 17.1º to the orbital plane of the rest of the planets. Even more bizarre is the fact that Pluto crosses Neptune’s orbit and is closer to the Sun than Neptune for 20 years. Nobody knew why and nobody questioned its planetary status. The limited knowledge of what we knew about Pluto remained valid until 1976 when methane ice was discovered on its surface. A rather large moon, Charon, was discovered in 1978 followed by an atmosphere in 1988. Pluto proved it was a planet like its terrestrial counterparts: Mercury, Venus, Earth, and Mars. Starting in 1992, small, icy worlds were discovered in a region beyond Pluto, which eventually became known as the Kuiper Belt and these members became known as Kuiperoids. They shared similar characteristics with Pluto, which eventually led to Pluto being reclassified in 2006 as a Kuiper Belt Object, and a dwarf planet because it is round.
Pluto’s pivotal year was indeed 2006 because besides it being reclassified, a spacecraft was finally heading its way. Pluto has Triton and Voyager 2 to thank for its mission since Voyager 2 flew past Triton in August 1989 and discovered geysers. Triton and Pluto were thought to be similar and scientists wanted to get a close-up look at Pluto. After a tough decade of politics and design issues, a mission was finally approved in November 2001 - the New Horizons spacecraft was born. Everything came together for the launch on January 19, 2006 while everyone marked July 14, 2015 on their calendars as that would be the day the New Horizons spacecraft would fly closest to Pluto and Charon; everyone would finally know what they really looked like.
What would Pluto look like? Would it be a dead, cratered, icy world or an active world with geysers and weather? Pluto has always been the imagination media for dreamers and space artists. The Hubble Space Telescope added to the fascination when it revealed contrasting light and dark features on Pluto along with a pinkish tan hue while Charon was grayish with less contrast. Charon was known to be composed mostly of water ice, but Pluto was more dynamic with its color due to surface methane ice being altered by the ultraviolet radiation from the Sun. Exactly what Pluto and Charon were going to look like made for exciting times in early 2015 as New Horizons grew near. Scientists wanted to time the flyby so that New Horizons would fly closest to Pluto along the region where the largest and brightest surface feature met a sprawling dark region. It was hoped that this contrasting region would yield the most science during the quick and brief flyby. It exceeded the wildest imagination!
As New Horizons neared Pluto the large bright area came into focus as a huge, heart-shaped region of smooth nitrogen ice covered with numbingly cold -391ºF methane and carbon monoxide ices. This feature is actually an impact basin filled by a giant glacier that is slowly flowing into nearby valleys and craters. There are cracks and pits in this giant glacial field, which shows that it is active. There are also small hills of water ice, which is as hard as rock in the intense cold. The glacier is warmed from underneath and softened just enough even in this intense cold to flow due to nitrogen’s low melting point. Nearby are solid water ice mountains as lofty as the Rockies that tower up to 11,000 feet, and jumbled lowlands of blocks of water ice that look like a coastline against the giant glacier plain. As small as Pluto is, it has some of the loftiest mountains in the Solar System with the most impressive being a pair of possible cryovolcanoes. One towers an impressive 18,000 feet and may be one of the tallest features on Pluto! The tops of these mountains have deep hollows, probably calderas, where an icy mixture of nitrogen and methane ice erupted and flowed down the sides. There are thick eruptive features and frozen flows, which is convincing evidence that these were once active volcanoes. Dunes are also seen on Pluto, which means the atmosphere had to be thicker at one time to create a strong enough wind to create them. New Horizons confirmed the reddish tint of Pluto and is obvious enough to be called “the other Red Planet.” Methane on the surface and in the atmosphere has been proven to be altered by the ultraviolet radiation of the Sun and converted into a class of complex organic compounds called tholins. This fine, sooty material slowly settles out of Pluto’s thin atmosphere and coats the surface giving it the reddish-brown tint. Pluto’s atmosphere was revealed in all its beauty when New Horizons flew behind it and saw it backlit by the Sun. Dozens of layers of blue haze hug the surface of Pluto and extend outward about 100 miles. The haze is thick enough for the mountains to cast shadows into it along the terminator.
Charon also proved to be more than a frozen, dead world. It has had an active and violent history as it is riddled with cracks up to 4-6 miles deep and hundreds of miles long. There are vast smooth areas where it appears that something flowed up against a mountain giving the effect that the mountain sits in a moat. The strangest feature is a dark reddish polar cap of tholins but it is a mystery why it appears there and nowhere else. The only predictions that were correct are that Charon is grayish due to mainly water ice and that it does not have an atmosphere. Pluto’s smaller moons, Hydra, Nix, Kerberos, and Styx, also proved to be fascinating and bewildering in spite of their small sizes, less than 40 miles across. Hydra and Kerberos each may actually be made of two smaller moons that merged together. Nix has an unusually large crater that splattered bright reddish material across its surface but was not shattered by the impact. These four moons all orbit Pluto chaotically, tumbling and swaying bad enough that it would be difficult to know where the Sun would rise the next day and even what time it would rise. Some of the moons may actually flip over as they orbit Pluto!
Who could ever imagine such a wild and fascinating place as Pluto? Pluto may never again be classified as a full-fledged planet, but it has proven to be a world with amazing geological diversification that is active to this day with flowing glaciers, floating hills, towering mountains, deep canyons, dune fields, and dynamic moons. Pluto is a world with an atmosphere that interacts with its surface, where ice sublimates into the atmosphere and snows back to the surface as organic soot. Who could ever imagine all of this in such a small package? The excitement and discovery will continue well into November as New Horizons continues to transmit the rest of the images and data back to Earth. Who knows what other awesome sights and discoveries will be revealed!