How big is Hydra?

Hydra is the outermost of the five moons known to orbit Pluto as of 2026. It is not round: it is an elongated, icy body about 50 kilometers (31 miles) long, measured along its longest side. Across its other two sides it measures about 36 and 32 kilometers (22 and 20 miles).

Size

Hydra is too small for its own gravity to have pulled it into a ball. Scientists describe its shape with a stretched ball that has three different widths, called a triaxial ellipsoid. The best fit to images from NASA's New Horizons spacecraft is about 50 × 36 × 32 kilometers.

A sphere with the same volume would be about 37 kilometers (23 miles) across. That is the mean size that NASA's Jet Propulsion Laboratory lists for Hydra.

Before New Horizons flew past Pluto in July 2015, Hydra was only a point of light in telescope images, and its size had to be estimated from its brightness. NASA's estimates for Nix and Hydra ranged from about 32 to 113 kilometers. The first analysis of the flyby images, published in 2016, gave about 65 × 45 × 25 kilometers, uncertain by about 10 kilometers. A 2018 review of the mission's results gives the refined figures used here.

Size comparisons

Hydra's longest side is a little longer than a marathon, which is 42.195 kilometers (26.2 miles).
Pluto's moon Nix is almost the same size and shape, about 48 × 33 × 30 kilometers.
Pluto is about 2,380 kilometers (1,480 miles) across, about 48 times the length of Hydra.
If Pluto were shrunk to the size of a basketball, Hydra would be about 5 millimeters long, circling about 6.5 meters (21 feet) away.
The Grand Canyon, about 450 kilometers long, is about nine times as long as Hydra.

Discovery and name

Hydra was discovered in June 2005 by Hal Weaver and a team of astronomers using the Hubble Space Telescope. Pluto's moon Nix was discovered in the same year. Hydra is named after the nine-headed serpent that the hero Hercules fought in Greek and Roman mythology.

A fast-spinning, bright moon

Hydra circles the balance point of Pluto and its largest moon, Charon, at a distance of about 64,700 kilometers (40,200 miles), taking about 38 days to go around once. It spins much faster than it orbits, turning once every 10.3 hours. Its spin axis is tipped far over compared with those of Pluto and Charon.

Its surface is mostly water ice and very bright. Hydra's geometric albedo, a measure of how strongly a surface reflects light, is about 0.83. Most objects in the Kuiper belt, the zone of icy bodies beyond Neptune, have albedos below 0.2, typically about 0.1.

Craters on Hydra suggest that its surface is at least 4 billion years old. Scientists think Hydra and Pluto's other small moons formed from debris left over from a giant collision that also produced Pluto and Charon as a pair.