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The Sciences

A History of Pluto Exploration

As New Horizons' journey culminates in the July 14 Pluto flyby, explore how a search for Planet X over a century ago led scientists on a wild ride of dwarf planet discovery.

By Discover StaffJuly 13, 2015 5:00 PM
Pluto New Horizons 2015
(Credit: NASA/APL/SwRI)


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1905: Beyond Neptune

Percival Lowell launches a search for "Planet X." His calculations are based on inaccurate contemporary estimates of Uranus' mass, but his quest will eventually turn up a different world.

Percival Lowell Memoir
(Credit: Lowell Observatory)

February 18, 1930: Blink discovery

Clyde Tombaugh discovers Pluto from Lowell Observatory in Arizona by seeing it move on photographic plates over a six-day period.

Pluto discovery plates
(Credit: Lowell Observatory)

1950: More to the story

Gerard Kuiper and others propose the existence of a large belt of icy objects beyond Neptune.

Gerard Kuiper
(Credit: Lunar and Planetary Laboratory)

November 19, 1976: Methane measurement

Dale Cruikshank, Carl Pilcher, and David Morrison discover methane ice on Pluto, providing the first indication that the planet's surface is icy rather than rocky.

June 22, 1978: A Pluto companion

James Christy and Robert Harrington discover Pluto's largest moon, Charon. It appears as a slight elongation in these images of Pluto.

Charon discovery
(Credit: U.S. Navel Observatory)

1985: Size matters

A series of mutual occultations between Pluto and Charon begins, allowing scientists to measure the objects' diameters and much more.

1987: Icy out there

Marc Buie and Robert Marcialis lead teams that discover water ice on Charon.

Charon water ice Buie
(Credit: Mark Buie)

1988: Thin air

James Elliot and colleagues discover Pluto's thin atmosphere.

May 1, 1989: A call for exploration

About a dozen planetary scientists gather for dinner at a small Italian restaurant in downtown Baltimore. They call themselves the Pluto Underground, and the group decides to force the issue to get NASA on board with a mission to Pluto.

(Credit: NASA)

May 27, 1992: More and more icy

Toby Owen and colleagues discover nitrogen and carbon monoxide ices on Pluto.

August 30, 1992: No longer alone

David Jewitt and Jane Luu discover 1992 QB1, the first sighted partner to Pluto orbiting beyond Neptune, opening up a whole new realm of the solar system, that of the Kuiper Belt objects. Today, scientists know of more than 1,200 such trans-Neptunian objects.

1992 QB1 ESO
(Credit: ESO)

March 8, 1996: Speckled surface

Using the Hubble Space Telescope, Alan Stern and colleagues create the first "world maps" of Pluto, showing color and brightness changes on the planet's surface.

Hubble Pluto Map 1996
(Credit: A. Stern (SwRI)/Marc Buie (Lowell)/NASA/ESA)

November 29, 2001: Mission set

NASA selects the space probe New Horizons for a mission to Pluto.

643px-New Horizons spacecraft model 1
(Credit: NASA)

May 15, 2005: More companions

Hal Weaver and Alan Stern lead a team that uses the Hubble Space Telescope to discover Pluto's moons Nix and Hydra.

pluto moons
(Credit: NASA/ESA/H. Weaver (JHU/APL)/A. Stern (SwRI)/HST Pluto Companion Search Team)

January 19, 2006: Launch day

New Horizons launches from Cape Canaveral, Florida, to begin its nine-year, 3-billion-mile journey to Pluto.

(Credit: NASA/Ken Thornsley)

August 24, 2006: Dwarf demotion

The International Astronomical Union redefines the term "planet," demoting Pluto to dwarf planet status and lumping it with Eris, a Kuiper Belt object similar in size to Pluto discovered in 2005.

Dwarf Planets
(Credit: NASA/ESA/A. Feild (STScI))

February 28, 2007: Passing through

New Horizons flies past Jupiter, receiving a gravity assist that allows it to reach Pluto more quickly.

Jupiter New Horizons 2007

December 29, 2009: 1.5 billion miles down

New Horizons reaches its halfway point (in distance) to Pluto.

June 28, 2011: Party of five

Using the Hubble Space Telescope, Mark Showalter and colleagues discover Pluto's moon Kerberos.

July 7, 2012: Family of six

Using the Hubble Space Telescope, Mark Showalter and colleagues discover Pluto's moon Styx.

(Credit: NASA/ESA/M. Showalter (SETI Institute))

December 4, 2014: Go time

New Horizons awakens from hibernation and prepares for its Pluto flyby.

new horizons
(Credit: NASA)

July 14, 2015: Hello Pluto!

New Horizons makes its closest approach to Pluto, flying some 7,700 miles from the planet's surface.

Pluto New Horizons 2015
(Credit: NASA/APL/SwRI)

Read our latest coverage of the New Horizons mission here »

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