Arrokoth, trans-Neptunian object in the Kuiper Belt (see comet). First observed by the Hubble Space Telescope in 2014 and later nicknamed Ultima Thule (until its official naming in late 2019), it became the most distant object ever visited by a spacecraft when NASA's New Horizons made a flyby on Jan. 1, 2019, passing as close as 2,200 miles (3,500 km) to it, which was some 4.1 billion mi (6.6 billion km) from Earth. When New Horizons was launched in 2006, its mission was to explore Pluto; Arrokoth was not yet discovered, and the space probe was directed toward it after its Pluto flyby.

Although initial images showed what appeared to be two round spheres stuck together like a snowman, subsequent study has shown the object to consist of two sections or lobes, each a flattened sphere in shape. Formed some 4.5 billion years ago, Arrokoth is approximately 22 mi (36 km) at its longest point and reddish in color, with some brighter areas, especially where the two sections are joined. The highly reflective surface is lightly cratered and pitted, and may show signs of smaller objects that agglomerated to form it, especially on the larger section. Arrokoth appears to be composed of frozen methane and rock.

The Columbia Electronic Encyclopedia, 6th ed. Copyright 2012, Columbia University Press. All rights reserved.

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