Pluto was discovered in 1930, but for most of the 20th century it remained little more than a distant point of light. Even the best telescopes could reveal relatively little about its surface, atmosphere or five known moons.
That changed dramatically with NASA’s New Horizons mission. After traveling across the solar system for more than nine years, the spacecraft flew past Pluto in July 2015 and transformed our understanding of this distant world. New Horizons then continued deeper into the Kuiper Belt, where its exploration continues today.
1. New Horizons Had the Highest Launch Speed of Any Spacecraft Before It
New Horizons launched from Cape Canaveral on January 19, 2006 aboard an Atlas V rocket.
The spacecraft left Earth at about 16.3 kilometers per second (36,000 mph), giving it the highest launch speed of any spacecraft at the time. Unlike many interplanetary missions, it did not first enter Earth orbit before beginning its journey into the outer solar system.
Despite this extraordinary departure speed, New Horizons is not the fastest spacecraft relative to the Sun. Its achievement refers specifically to the speed at which it departed Earth.
The rapid launch was important because Pluto was billions of miles away. Even so, the journey would take more than nine years.
2. Jupiter Gave New Horizons a Gravity Assist
New Horizons passed Jupiter in February 2007, approaching within about 2.3 million kilometers (1.4 million miles) of the giant planet.
Jupiter’s gravity increased the spacecraft’s speed and shortened its journey to Pluto by several years. But the encounter was much more than a navigational maneuver.
It provided the mission team with an opportunity to test New Horizons’ cameras and scientific instruments during a real planetary encounter. The spacecraft observed Jupiter’s atmosphere, rings and moons, including spectacular volcanic eruptions on Io.
The encounter therefore served as both a gravity assist and a full-scale rehearsal for Pluto.
3. New Horizons Made Humanity’s First Exploration of Pluto
On July 14, 2015, New Horizons became the first spacecraft to explore Pluto at close range.
At closest approach, it passed about 12,500 kilometers (7,800 miles) above Pluto’s surface while traveling at roughly 14 kilometers (9 miles) per second.
The encounter happened quickly. Instead of entering orbit, New Horizons raced through the Pluto system while its instruments mapped the dwarf planet, examined its atmosphere and studied its moons.
Because the spacecraft was so distant from Earth and could transmit data only slowly, returning the complete encounter dataset took more than a year.
4. Pluto Turned Out to Be a Geologically Complex World
Before New Horizons arrived, scientists expected Pluto to be interesting, but its remarkably varied geology came as a surprise.
The spacecraft discovered towering mountains made largely of water ice, enormous plains, glaciers, faults and other terrain showing evidence of extensive geological change.
Some regions contain very few impact craters. A surface with few craters must have been renewed relatively recently in geological terms, demonstrating that Pluto has not simply remained frozen and unchanged since the formation of the solar system.
Among the most dramatic mountains are Tenzing Montes, whose peaks rise several kilometers above the surrounding terrain.
5. Sputnik Planitia Is an Enormous Nitrogen-Ice Glacier
The most recognizable feature on Pluto is the vast heart-shaped region called Tombaugh Regio, named after Pluto’s discoverer, Clyde Tombaugh.
Its western lobe contains Sputnik Planitia, an enormous basin filled mainly with frozen nitrogen along with carbon monoxide and methane ices.
New Horizons revealed a surface divided into large polygonal cells. These are thought to result from slow convection within the nitrogen ice, with relatively warmer material rising and cooler material sinking.
The discovery provided striking evidence that Pluto’s surface is dynamic even though the dwarf planet receives only a tiny fraction of the sunlight that reaches Earth.
6. Pluto Has a Thin Nitrogen Atmosphere and Layers of Blue Haze
New Horizons confirmed and characterized Pluto’s extremely thin atmosphere, which consists mainly of nitrogen with smaller amounts of methane and carbon monoxide.
One of the mission’s most beautiful discoveries appeared when New Horizons looked back toward Pluto after closest approach. Sunlight passing through the atmosphere revealed numerous layers of haze extending high above the surface.
The haze appears blue when viewed in color. The color is associated with tiny atmospheric particles known as tholins, which form through chemical reactions involving methane and other molecules exposed to solar ultraviolet radiation.
Some of these particles eventually settle onto Pluto and contribute to the reddish-brown coloration seen across parts of its surface.
7. Charon Has Mountains, Canyons and a Dark Polar Cap
New Horizons also transformed our view of Charon, Pluto’s largest moon.
Charon has a diameter a little more than half that of Pluto, making the pair unusually close in size compared with most planet-moon systems. The two worlds orbit a common center of mass located outside Pluto itself.
Images revealed an unexpectedly complex landscape containing mountains, plains and an enormous system of fractures and canyons stretching across much of Charon.
Its north polar region is covered by dark reddish material. Research indicates that methane escaping from Pluto can become trapped at Charon’s extremely cold pole and undergo chemical reactions that ultimately produce heavier reddish compounds.
Rather than being an inert ball of ice, Charon preserves evidence of major geological changes in its past.
8. New Horizons Carries Some of Clyde Tombaugh’s Ashes
New Horizons carries an unusual connection between Pluto’s discovery and its first exploration.
A small quantity of the ashes of Clyde Tombaugh, who discovered Pluto in 1930, is attached to the spacecraft. Tombaugh died in 1997, nine years before New Horizons launched.
The spacecraft also carries several other commemorative items, including two US flags, a piece of SpaceShipOne and a 1991 US postage stamp bearing an image of Pluto and the words “Not Yet Explored.”
That description became obsolete on July 14, 2015 when New Horizons finally reached Pluto.
9. New Horizons Flew Past Arrokoth in 2019
Pluto was not the end of the mission. After the 2015 encounter, New Horizons continued deeper into the Kuiper Belt toward another target then known as 2014 MU69.
The spacecraft flew past the object on January 1, 2019. It was later officially named Arrokoth.
Arrokoth lies more than 6 billion kilometers (4 billion miles) from the Sun and consists of two flattened lobes joined together. Its unusual shape and relatively undisturbed condition provide clues to how small objects assembled during the early history of the solar system.
The encounter also established a remarkable record: Arrokoth is the most distant object ever explored by a spacecraft at close range.
10. New Horizons Is Still Exploring Beyond Pluto
More than two decades after launch and more than a decade after its Pluto encounter, New Horizons remains an active spacecraft.
The probe woke from a 321-day period of hibernation on June 23, 2026 in good health. At the time, it was roughly 5.9 billion miles (9.5 billion kilometers) from Earth and continuing its journey through the distant outer solar system.
New Horizons is now far beyond Pluto and continues to make observations of the Kuiper Belt and the environment at enormous distances from the Sun. Its instruments can study distant Kuiper Belt objects that cannot be visited directly while also measuring dust, charged particles and other properties of the outer heliosphere.
The spacecraft’s nuclear power supply gradually weakens with time, so New Horizons cannot operate indefinitely. But for now, the mission that gave humanity its first close view of Pluto continues to explore a region no previous planetary spacecraft has studied in such detail.