New Horizons

The first spacecraft to fly past Pluto and the first to visit the Kuiper Belt — launched in 2006 when Pluto was still a planet, arriving nine years later to find a world of unexpected complexity. Then continued outward to the most distant object ever visited by a spacecraft. Principal Investigator: Alan Stern (Southwest Research Institute).

Key Facts

The Pluto Flyby — July 14, 2015

New Horizons spent nine years crossing the solar system, hibernating for much of the journey to conserve power. It could not orbit Pluto — travelling too fast, with no fuel for orbital insertion — so the entire science return came from a single pass lasting a matter of hours. Every instrument was pointed and firing simultaneously.

The signal confirming closest approach took 4.5 hours to reach Earth. When it arrived on the evening of July 14, mission control erupted.

What Pluto revealed:

• Tombaugh Regio — the heart-shaped feature seen even in approach images; its western lobe is Sputnik Planitia, a 1,000 km-wide nitrogen ice plain showing active convective cells
• Young surface — fewer craters than expected; Sputnik Planitia may be geologically active today, resurfaced within the last million years
• Mountains — water ice peaks up to 3,500 m, comparable to the Rockies; water ice is rigid enough at −230°C to form and hold mountain structures
• Atmosphere — multiple layers of blue photochemical haze; nitrogen atmosphere that partially freezes onto the surface near aphelion
• Colour — reddish-brown tholins (UV-irradiated nitrogen/methane organics) mixed with regions of pure nitrogen ice
• Charon — geologically complex: Mordor Macula (dark reddish north polar cap, likely tholins transported from Pluto), Virgil Fossae (deep canyon system suggesting ancient internal expansion)

Arrokoth — January 1, 2019

After Pluto, New Horizons was redirected toward 2014 MU69, renamed Arrokoth after the flyby. At ~6.6 billion km from the Sun, it is the most distant object ever visited by a spacecraft.

Arrokoth is a contact binary: two roughly spherical lobes (nicknamed "Ultima" and "Thule" during the mission, now Arrokoth's two lobes informally) that gently merged in the early solar system. The smooth, uncraterered contact neck suggests they came together slowly — a pristine remnant of solar system formation ~4.5 billion years ago, unchanged since the Sun's protoplanetary disk.

The name Arrokoth comes from the Powhatan/Algonquian language, meaning "sky" — chosen in consultation with Powhatan descendants.

Science Instruments

The Name

"New Horizons" carries multiple meanings: the mission crossed into the new horizon of the outer solar system; it gave humanity its first close look at a class of worlds (Kuiper Belt objects) that had been purely theoretical; and the acronym NH honours New Horizons' PI Alan Stern's prior Pluto mission proposals, and implicitly echoes the initials of the man who found Pluto: Noted Historically as Clyde Tombaugh.

Connection to Voyager

New Horizons follows a trajectory broadly similar to Voyager 1 — outward past Jupiter and into the outer solar system. But where Voyager's Grand Tour was a multi-planet mission, New Horizons was designed with one primary target: Pluto. It is the last in the line of first-reconnaissance missions — Mariner to Mars, Pioneer and Voyager to the outer planets, MESSENGER to Mercury, New Horizons to Pluto. Every planet (and dwarf planet) in the solar system has now been visited.