IntroductionThe small Kuiper Belt object officially known as Arrokoth — or by its scientific designation (486958) 2014 MU69 — is the most distant and the most primitive object ever explored by a spacecraft. It’s double-lobed, meaning it has two rounded pieces in contact with each other, and looks like a partially flattened snowman. Arrokoth is also very red—even redder than Pluto.
The Kuiper Belt, where Arrokoth is located, is a region of space beyond Neptune that is swarming with small, icy and ancient objects. Because they are so far from the Sun, Kuiper Belt objects have been only slightly heated since their formation, and are thought to be well-preserved samples of what the outer solar system was like following its birth more than 4.5 billion years ago.
On Jan. 1, 2019, New Horizons made a high-speed flyby of Arrokoth, approaching as close as 2,198 miles (3,538 kilometers) of the object. During the encounter the spacecraft’s cameras revealed a world not quite like any explored before.
Arrokoth was discovered June 26, 2014, by New Horizons science team member Marc Buie using NASA’s Hubble Space Telescope. The team was searching for Kuiper Belt objects that their robotic spacecraft could reach with fuel left over after its Pluto flyby in July 2015. It was the faintest and smallest object, but it was the most reachable within the mission's timeline.
The object was designated (486958) 2014 MU69 and later given the formal name, Arrokoth.
Size and Distance
End to end, the overall shape of Arrokoth measures about 22 miles (35 kilometers) long. It’s about 12 miles (20 kilometers) wide, by 6 miles (10 kilometers) thick. The dimensions of the two lobes are given below (see Structure).
At an average distance of 4 billion miles (6.5 billion kilometers), the orbit of Arrokothlies 44.6 astronomical units from the Sun. One astronomical unit (abbreviated as "AU"), is the distance from the Sun to Earth.
Orbit and Rotation
Arrokoth's orbit is located about 4 billion miles (6.6 billion kilometers) from the Sun, or roughly 1 billion miles (1.6 million kilometers) beyond Pluto. Its orbit is close to circular, and is inclined by only about 2.5 degrees to the plane of the ecliptic (the plane in which most of the planets orbit). It takes about 293 Earth years for the small world to make a single trip around the Sun.
Arrokoth is a member of the “cold classical” Kuiper Belt, a group of objects that makes up about one-third of the Kuiper Belt. The term “cold” doesn’t refer to the surface temperature of the objects, but rather to their orbits. Cold classical objects have extremely circular orbits that indicate they haven’t been pushed around a lot by the gravity of the giant planets — and that they’ve probably stayed about the same distance from the Sun since the formation of the solar system. (“Hot classical" KBOs have non-circular, inclined orbits that cause them to stray much farther from their average position).
These objects are called "classical" KBOs because their orbits are similar to those discussed in the 1950s by Gerard Kuiper (1905–1973), the Dutch-American astronomer for whom the Kuiper Belt is named. The largest two Kuiper Belt objects are dwarf planets Pluto and Eris, with Pluto being slightly larger at a diameter of 1,477 miles (2,377 kilometers). Most KBOs are much smaller.
Arrokoth rotates on its axis every 15.92 hours. Like Pluto, it rotates on its side — its axis of rotation is extremely tilted with respect to the plane of its orbit, by 98 degrees. When New Horizons flew past, Arrokoth's south pole was pointed almost directly at the Sun.
Potential for Life
Arrokoth is much too cold to support life as we know it.
Moons and Rings
Arrokoth does not have moons or rings.
The shape of Arrokoth indicates that its two lobes most likely formed separately and gently merged within a cloud of particles early in the history of our solar system. This is more evidence that Arrokoth is primordial — existing since the beginning of the solar system. The two lobes are similar in color, adding evidence to this interpretation.
Images from New Horizons show that Arrokoth is a bi-lobed contact binary. This means it’s actually two objects that began orbiting each other until they somehow gently merged. The strange shape of Arrokoth, unlike any object previously visited, was the biggest surprise of the New Horizons spacecraft’s flyby.
A three-dimensional model created from images taken by the New Horizons spacecraft shows that Arrokoth is elongated and flattened. The larger lobe was found to be "lenticular," which means it's flattened and shaped like two lenses placed back to back. It has dimensions of approximately 14 × 12 × 4 miles (22 × 20 × 7 kilometers). The smaller lobe is more rounded and is approximately 9 × 9 × 6 miles (14 × 14 × 10 kilometers) in its dimensions.
The surface of Arrokoth doesn’t reflect much light, but there are bright regions, with the brightest being at the “neck” where the two lobes are joined. There are also two bright spots inside the largest crater on the surface.
In color and composition, Arrokoth resembles many other objects found in its area of the Kuiper Belt. It's very red — even redder than Pluto — and is, in fact, the reddest outer solar system object visited by spacecraft thus far.
The reddish hue is believed to result from changes in the surface materials caused by ultraviolet light and cosmic rays over billions of years. Scientists found evidence for methanol, water ice and organic molecules — a mixture very different from most icy objects explored previously by spacecraft.
No atmosphere has been detected at Arrokoth. It is too small for its gravity to hold onto an atmosphere.