trans-Neptunian object in the Kuiper Belt (see cometcomet
[Gr.,=longhaired], a small celestial body consisting mostly of dust and gases that moves in an elongated elliptical or nearly parabolic orbit around the sun or another star. Comets visible from the earth can be seen for periods ranging from a few days to several months.
..... Click the link for more information.
). First observed by the Hubble Space TelescopeHubble Space Telescope
(HST), the first large optical orbiting observatory. Built from 1978 to 1990 at a cost of $1.5 billion, the HST (named for astronomer E. P. Hubble) was expected to provide the clearest view yet obtained of the universe from a position some 350 mi (560 km)
..... Click the link for more information.
 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™ Copyright © 2013, Columbia University Press. Licensed from Columbia University Press. All rights reserved.