Meet Styx, the Wee Moon of Pluto

We may earn a commission from links on this page.

Styx, the faintest moon of Pluto, is absolutely miniscule. This fuzzball of a rock is just a few kilometers across, and is making me squeal in unconstrained glee.

Styx is even smaller than we first estimated: the highly-elongated moon is just 7 kilometers (4.5 miles) long and 5 kilometers (3 miles) across. The smaller revised size means that Styx is relatively bright, probably with an icy surface akin to what we’ve already observed on the larger-yet-still-small moons Nix and Hydra.


Composite image taken by the New Horizons just 12.5 hours before closest approach during the flyby, from roughly 631,000 kilometers (391,000 miles) away. Image credit: NASA/JHUAPL/SwRI

Styx completes an orbit around the barycenter of the binary dwarf pair Pluto and Charon every 20.2 days. It’s the closest of the small moons to the barycenter at just 42,656 kilometers (26,500 miles). All four moons are in nearly circular and planar orbits around the dwarf pair, and are in orbital resonance with each other. These tiny shards of moons are sometimes used as evidence that the system was formed by a massive collision.


Nix, Hydra, Styx, and Kerberos during New Horizon’s approach to the Pluto-Charon system. Image credit: NASA/JHUAPL/SwRI


The new image is composed of photographs taken by the New Horizons spacecraft just 12.5 hours before the spacecraft’s closest approach. It was still 631,000 kilometers (391,000 miles) away from the faint moon when it captured the component images. The composite has a resolution of 3.1 kilometers per pixel; we anticipate eventually receiving images with up to 1.8 kilometers per pixel resolution as the data downlink continues.


Discovery of Styx on July 7, 2012. Image credit: NASA/ESA/M. Showalter

Styx is so small it was only discovered by the Hubble Space Telescope in 2012 when the spacecraft was already en route for its historic flyby. It’s just half as bright as Kerberos, the other tiny moon discovered in 2011.


Styx is named for the river dead cross to enter the underworld. If any notable features are identified on Styx, they will be named for river gods from around the world.

The New Horizons spacecraft is currently downlinking data from its flyby of the Pluto-Charon system, and is en route to a flyby of a second Kuiper Belt Object, MU69, in 2019.


Top image: Styx, the faintest of Pluto’s four smaller moons. Credit: NASA/JHUAPL/SwRI


Contact the author at or follow her at @MikaMcKinnon.