Far out in the dim depths of the Solar System is a tiny world that remained hidden from Earth's prying eyes for eons. The sun appears star-like, shining at magnitude -18, barely 1/1000th as bright as seen from Earth but bright enough to read by, but still too dangerous to stare at. This tiny world guarded itself well until early 1930 when Clyde Tombaugh discovered it on February 18, 1930 from Lowell Observatory in Flagstaff, Arizona. Pluto was finally exposed, and immediately became almost everyone's favorite planet, only rivaled by Mars.
Pluto is a difficult world to study because it orbits the Sun no closer than about three billion miles yielding a disc of hardly 0.1 arcseconds across and shines no brighter than magnitude +13.6. Advances in technology since its discovery have gradually teased out Pluto's secrets. Scientists studied the fluctuations of Pluto's light curve in 1955 and discovered it rotates once every 6.39 days. In 1970 Pluto was observed to have a reddish tint and in 1976 the reason for this unusual color was discovered to be methane ice on the surface damaged by the ultraviolet radiation from the Sun, which reddens it. This actually gives Pluto a pinkish-tan color if seen close up. The discovery of this ice also meant that Pluto was much smaller than expected since the brightness is partly due to the reflection of ice rather than disc size. The original estimated size was 3600 miles in diameter, but it quickly shrank with each new discovery; it was jokingly predicted that Pluto would disappear by 1984!
The big excitement came on June 22, 1978 when James Christy discovered a moon orbiting Pluto, which he named Charon. Scientists finally had an important tool to unlock more of Pluto's secrets. The first secret to be revealed by observing how Charon orbited Pluto was that Pluto is tipped 122 degrees on its side, even more than Uranus! A series of eclipses between Pluto and Charon between 1985 and 1990, which allowed the size of Pluto and Charon to be measured, revealed that Pluto was no larger than about 1426 miles in diameter and Charon was almost exactly half as large at approximately 744 miles in diameter. The eclipses also provided a knife-edge tool to measure the light reflected from the two tiny discs. The crude map produced from this albedo data revealed Pluto to be mottled with vast expanses of light and dark material, which was correctly thought at the time to be ice and rock. Pluto has the largest contrasting surface features of any world besides Earth, and are similar to Saturn's moon Iapetus. This contrast may create uneven heating with the dark areas absorbing more of the feeble heat of the Sun while the brighter areas reflect the heat resulting in cooler areas. This differential heating could create the winds that were recently discovered that howl up to 225 miles per hour near the poles! However, the atmosphere is thin and frail, so these winds are not very forceful. The wind-chill would still be serious as the average surface temperature always hovers around -400 degrees F! Charon was found to be a nearly uniform grayish world probably due to a higher abundance of water ice and very little methane ice. It orbits Pluto once every 6.39 days, exactly the same rate that Pluto rotates once on its axis, which means Charon can only be seen from one side of Pluto and always remains in the same part of the sky.
On June 9, 1988 Pluto revealed a big secret when it briefly covered a star. The star gradually faded instead of suddenly winking out which meant that Pluto has an atmosphere. The atmosphere can only exist while Pluto is closest to the Sun and quickly freezes onto the surface as frost as Pluto moves out to the remote part of its huge, 248-year-long orbit. The composition of the atmosphere is mainly methane and nitrogen with traces of carbon monoxide and argon. No atmosphere has been detected around Charon. The Hubble Space Telescope confirmed the mottled, contrasting appearance of Pluto and its pinkish-tan color in the mid-1990s, and produced the sharpest images ever in 2010, which are still our best, at least until New Horizons gets close enough next month. Careful analysis of Pluto revealed hazes and possible clouds in its atmosphere, which has sometimes made scientists wonder if these will obscure some of the surface features. There is no fear of Pluto being completely shrouded like Titan and Venus because atmospheric conditions are unfavorable; the atmosphere is too thin, too cold, and too windy.
The big debate came in 2006 when Pluto was reclassified, often incorrectly stated "demoted" from planethood. The situation began in 1992 when several small objects were discovered in orbit around the Sun beyond Pluto that exhibited the characteristics of Pluto. The steady discovery of more of these tiny worlds during the 1990s and 2000s began to cast doubt as to whether Pluto should continue to be classified as a planet or be reclassified as one of these icy objects. Pluto has a strange orbit that comes within 2.75 billion miles of the Sun, closer than Neptune, but wanders out to 4.58 billion miles and it does this in a plane that is tilted 17.1 degrees to that of the rest of the planets. It is less than half the size of the smallest planet, Mercury, and is out of place among the gas giants: Jupiter, Saturn, Uranus, and Neptune. The late, great planetary scientist, Gerard Kuiper, predicted the existence of these remote icy worlds beyond Neptune, which are called Kuiperoids in his honor. The Kuiperoids share the characteristics of Pluto: most are reddish, icy, and travel in elongated orbits tilted significantly from the orbital plane of the rest of the planets. Because of these similar traits, Pluto was reclassified as a Kuiperoid to the anger of many. Pluto is also classified as a dwarf planet because it is round and its gravity is strong enough to shape it into a sphere.
Pluto has another interesting secret that has been steadily revealed over the past decade. Four new moons have been discovered, all much smaller than Charon. The Hubble Space Telescope discovered Nix and Hydra in 2005 and in 2011 Kerberos was discovered and then Styx in 2012. All four moons orbit beyond Charon and are far smaller with estimates between 10 and 100 miles across. Unlike Charon, which keeps its same side facing Pluto, these tiny moons may rotate on their axis, but confirmation must wait until New Horizons flies by. There could even be more moons awaiting discovery or even rings which would make Pluto more planet-like considering it also has an active atmosphere and maybe even active geology. Pluto has always been compared to Neptune's moon, Triton, which has active nitrogen geysers. It is not known if Pluto has them, but at long last we will soon have an answer.
The urge to send a spacecraft past Pluto was strong when Voyager 2 flew past Neptune and Triton in late August 1989. Various missions were conceived, all too expensive. One was finally approved but it grew too expensive and was cancelled. NASA let the science community of independent teams design a new mission and capped it at $650 million. New Horizons, designed by the John Hopkins University/Applied Physics Laboratory, was built and successfully launched on January 19, 2006. The spacecraft is no bigger than a grand piano but packed with instruments that will image, probe, and study Pluto and its moons in a way never before possible. The camera aboard New Horizons is better than any previously flown and far sharper images will be transmitted than what Voyager 2 returned at Triton. At closest approach approximately 6200 miles above Pluto's surface at 7:50 A.M. this coming July 14, New Horizons will be capable of resolving large boulders on the surface and easily observe any geysers if they exist.
Pluto will no longer be a fuzzy smudge. No longer will artists paint what it "might" look like. Pluto will be revealed in exquisite detail no less exciting than any other world visited by a spacecraft for the first time. Pluto and Charon will be full of unimagined surprises. There is only one moment in time in all of history when something poorly known and mysterious becomes well known and understood. That time has come for Pluto; that time is now!