From the Start, Pluto was a Puzzle: Timeline

The path to Pluto is riddled with false starts and mistaken identities, but Mother Nature has been kind to its explorers. Here’s a look at some of the key moments leading up to humanity’s first close-up look at Pluto.

The path to Pluto is riddled with false starts and mistaken identities, but Mother Nature has been kind to its explorers. Here's a look at some of the key moments leading up to humanity's first close-up look at Pluto.

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Percival Lowell, an American businessman, mathematician and astronomy buff, spent the last decade of his life searching for what became known as "Planet X," a ghost world in the solar system's nether regions that had enough heft to gravitationally elbow the orbits of the two outermost giant planets, Uranus and Neptune.

He never found it, but working at the Flagstaff, Ariz., observatory that Lowell founded, astronomer Clyde Tombaugh in 1930 spotted a planet near where Lowell's calculations a quarter of a century earlier predicted Planet X should be. Though the newly found planet's mass could not be accurately measured for another 50 years, scientists came to suspect that it was too small to account for quirks in the giant planets' orbits.

The Planet X theory was officially retired when astronomers figured out that their estimates of Neptune's mass were off. With more precise data from the 1989 Voyager 2 flyby, the orbital discrepancies disappeared.

Planet hunting was a laborious process. Tombaugh would photograph a part of the sky, wait several days so that Earth would be at a slightly different viewing angle, and take another picture of the same region of the sky. He would then compare the two images, looking for a speck of light that appeared to move, relative to the fixed pattern of the background stars.

On Feb. 18, 1930, looking at photographic plates taken on the 23rd and 29th of January, Tombaugh spotted his elusive prey. He had naming rights to the newly found world and decided on "Pluto" -- the suggestion of an 11-year-old girl named Venetia Burney.

The English schoolgirl turned out to be well connected. Her grandfather passed along the suggestion to a University of Oxford astronomy professor, who cabled it to the Lowell Observatory. Pluto, the Roman god of the underworld, was in keeping with the practice of naming planets after Roman or Greek gods and goddess. It didn't hurt that the first two letter of Pluto were the initials of Percival Lowell.

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For the next 50 years, astronomers had a tough time ferreting out details of the solar system's ninth planet. By looking at changes in Pluto's position, they figured out that the planet orbits the sun every 248.5 Earth years at a distance ranging from 2.7 billion miles to 4.7 billion miles. That put Pluto roughly 40 times farther away from the sun than Earth. Scientists could estimate Pluto's brightness and make a guess as to how reflective its surface might be. And by studying the cycles of brightening and dimming, they determined that a single Pluto day, during which the planet completes one rotation, took the equivalent of 6.4 Earth days.

A breakthrough came in 1978 when U.S. Naval Observatory astronomer James Christy figured out that what looked like distortions in some pictures of Pluto -- written off by others as defective photographic plates -- was actually something moving from one side of Pluto to the other. Too big to be a mountain, too enduring to be an erupting volcano, Christy realized it was a moon. Confirmation took a few years. Christy named the moon Charon, the ferryman in Greek mythology whose boat transported dead souls across the River Styx to Pluto's underworld.

With the understanding that Pluto had a moon, new calculations revealed how shockingly small the planet to be. Watching the duo dance through a six-year cycle of occultations, astronomers realized that Charon had about one-seventh the mass of Pluto and a diameter that was about half as big as Pluto's. They also realized the two had different surface colors, with Pluto taking on a yellowish hue and Charon appearing gray. Charon orbits Pluto 20 times closer than the moon circles Earth.

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Pluto was always odd man out in the solar system's family of planets. Small enough to fit between Washington DC and Denver -- smaller even than Earth's moon -- Pluto seemed to be in a class by itself, unlike the inner terrestrial worlds Mercury, Venus, Earth and Mars, and a whole different beast than the gas giants Jupiter, Saturn, Uranus and Neptune that occupy what was believed to be the solar system's outer region.

Turns out that Pluto is far from alone. In 1992, astronomers Dave Jewitt and Jane Luu at the University of Hawaii find a faint speck of light coming from an object circling the sun farther away than Neptune. Since then, more than 1,000 similar objects have been discovered, with estimates that there are several hundred thousand more bodies bigger than 20 miles across still to be cataloged in the Kuiper Belt. The region is named for the Dutch-American astronomer Gerard Kuiper, who speculated about the existence of small bodies beyond Neptune in the 1950s.

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Spurred by the discovery of the Kuiper Belt, NASA decides to send a probe to Pluto, the best known and possibly largest resident of this unexplored region of the solar system. A few months before the spacecraft, named New Horizons, is due to launch, astronomers discover Charon isn't the only moon circling Pluto. Analysis of Hubble Space Telescope images reveal two small moons, later named Nix and Hydra (the letters "N" and "H" are a nod to the New Horizons mission), which are roughly 5,000 times fainter than Pluto itself. The moons are two to three times farther away from Pluto than Charon.

Five years later during a Hubble search to see if Pluto has rings, astronomers find a fourth moon, Kerberos. In 2012, a fifth moon, Styx, was found during a Hubble search for potential hazards for the New Horizons mission, still three years away from its encounter with Pluto.

Pluto is the only known Kuiper Belt Object with multiple moons.

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For a planetary probe, New Horizons is diminutive, about the size of a baby grand piano and weighing just over 1,000 pounds. But it launched aboard the biggest rocket in the U.S. fleet, outfitted with an extra solid-fuel kick motor to make it the fastest spacecraft to leave Earth orbit.

Launching on Jan. 19, 2006, from Cape Canaveral Air Force Station in Florida, New Horizons soared past the orbit of the moon in nine hours and reached Jupiter in 13 months. New Horizons used the giant planet's gravity to slingshot itself even faster and shave five years off the journey to Pluto. Traveling at more than 51,000 mph, the 3-billion-mile trip to Pluto would take more than nine years.

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Compact, lightweight and relatively inexpensive -- the mission cost $700 million, including the pricy Atlas 5 rocket ride -- New Horizons has no propellant for the massive braking burn that would be needed to shed its speed and drop into orbit around Pluto. Instead, the spacecraft will conduct its science on the fly, a campaign that began in January 2015 and culminates on July 14 when it passes within 7,800 miles of Pluto and 17,900 miles of Charon. So much data will be collected during the flyby that it will take about 16 months for everything to be transmitted back to Earth.

"You have to really be into delayed gratification if you want to be on this mission," New Horizons lead scientist Alan Stern told Discovery News.

The spacecraft is outfitted with miniature cameras, a radio science experiment, ultraviolet and infrared spectrometers and space plasma experiments to study the global geology and geomorphology of Pluto and Charon, map their surface compositions and temperatures, and examine Pluto's atmosphere.

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