How big is Nix?

Nix is a small, icy moon of Pluto, about 36 kilometers (22 miles) across on average. It is not round but stretched out, about 48 kilometers (30 miles) long. NASA's New Horizons spacecraft photographed it when it flew past Pluto in July 2015.

Size

Because Nix is stretched out, its size depends on the direction it is measured in. A 2018 review of the New Horizons results gives its three widths, measured along lines at right angles to each other, as about 48 by 33 by 30 kilometers (30 by 21 by 19 miles). NASA's Pluto fact sheet gives nearly the same: 48.4 by 33.8 by 31.4 kilometers. The first measurements, published in 2016, were 50 by 35 by 33 kilometers, each uncertain by about 3 kilometers.

A ball with the same volume as Nix would be about 36 kilometers across. NASA's Jet Propulsion Laboratory gives its average radius as 18 kilometers, give or take 1. That puts its average diameter between about 34 and 38 kilometers.

Before New Horizons, astronomers could judge the size and shape of Nix mainly from its brightness, measured with the Hubble Space Telescope. From how that brightness changed over time, they had argued that it was long and thin. The flyby gave the first direct measurements of its size and shape. Its longest axis turned out to be about 1.6 times its shortest.

Size comparisons

The average width of Nix is about four times the height of Mount Everest, 8,849 meters (29,032 feet).
A marathon race of 42.195 kilometers (26.2 miles) is a little shorter than Nix is long.
Nix is about one and a half times as wide as a neutron star, which is about 24–26 kilometers across. A neutron star is the collapsed core of a massive star.
Pluto is about 2,380 kilometers (1,480 miles) across, 66 times the average width of Nix. Charon, Pluto's largest moon, is about 1,210 kilometers (750 miles) across, 34 times as wide as Nix.
If Pluto were shrunk to the size of a basketball, Charon would be a ball about 12 centimeters (5 inches) across. Nix would be a grain about 4 millimeters across, circling nearly 5 meters (16 feet) away.

Orbit and spin

Nix circles Pluto and Charon once every 24.9 days, about 48,700 kilometers (30,300 miles) from the center of the pair. That is about two and a half times as far out as Charon, and its orbit takes nearly four times as long.

Unlike Charon, which always keeps the same face toward Pluto, Nix spins much faster than it orbits. It turns once every 1.83 days, about 44 hours. Its spin axis is tipped over by about 123 degrees compared with the axis of Pluto and Charon, so it spins in the opposite direction.

An ancient, icy surface

Hal Weaver and a team of astronomers discovered Nix with the Hubble Space Telescope in June 2005. It is named for the Greek goddess of darkness and night, the mother of Charon. New Horizons found that its surface is mostly water ice, bright and gray.

The spacecraft's images show craters on Nix. There are enough of them to suggest that the surface is at least 4 billion years old. The largest crater is redder than the rest of the surface, perhaps because the impact uncovered different material from below. Scientists think Nix and Pluto's other small moons formed from debris after a giant collision created Pluto and Charon.