Goodnight, moons! These are the best images we’re going to see of Nix and Hydra, the largest of the little moons swooping around Pluto and Charon.


Pluto’s largest moon Charon is massive enough to yank the dwarf planet around their mutual center of gravity, the barycenter. The two create a miniature binary system orbited by a handful of four tiny moons. Each moon—Styx, Nix, Kerberos, and Hydra—is diminutive by comparison, lumpy worlds so small their own gravity can’t even shape them into spheres. All are under 50 kilometers (30 miles) across their longest dimension, with Nix and Hydra as the two largest.

Pluto, Charon, Nix, and Hydra to scale in size, but not in orbital distance. Image credit:NASA/JPL/JHUAPL/SwRI/Emily Lakdawalla


Nix and Hydra were discovered shortly before the launch of the New Horizons mission, back in the days when Pluto was still considered a planet. Their initials are a subtle tribute to the spacecraft that would shortly send home the only decent images of the tiny worlds. Styx and Kerberos were discovered while the spacecraft was en route to the historic flyby. This explains a bias in the photos we’re receiving from the spacecraft: we hadn’t discovered the tiniest moons until the spacecraft’s primary observing targets were already set, so the only devoted photographs were filling in the handful of TBD slots.

Nix from angles during the flyby. Image credit: NASA/JHUAPL/SwRI

Nix is the largest of the small moons, and was closer to the New Horizons spacecraft during the flyby. Both of these mean we’re getting better images of Nix than of the other little moons.


Images taken at three different angles on the tiny world show its asymmetrical, potatoish shape. Our first detailed look was almost round, which was perplexing until mission scientist (and discoverer of Styx and Kerberos) Mark Showalter suggested we were looking up the long axis. His theory was vindicated hours later with the next shot from a new angle.

When New Horizons was barreling out of the system, it looked back at the crescent moon to capture details of the surface along the long shadows of the terminator.


Nix in high resolution black and white, and in lower-resolution enhanced colour near-infrared, red, and blue. Image credit: NASA/JHUAPL/SwRI

The oddest feature of Nix is a massive crater on its broadside. This crater is so large that it’s borderline between scarring the little world, or actually splitting it into pieces. Either Nix got lucky, or it’s a fragment of an older, larger moon shattered by some previous destruction.

From the enhanced colour view, the crater punches through a surficial blanket of white to reveal a darker reddish-brown interior, with some of that ejecta splaying out from the crater. We don’t yet know what this material is, but will know more when spectrometer data is downlinked.


Composite image of Hydra. Image credit: NASA/JHUAPL/SwRI

Hydra is the next largest moon. As both smaller and on the far side of its orbit when New Horizons zipped through Pluto. It has a weirder, more complicated shape than Nix, a double-lobe somewhat akin to the strange shape of comet 67P/Churyumov-Gerasimenko. It, too, could possibly be the result of a low-speed collision of a pair of smaller moons. Hydra in colour is less surprising, without any disruption in the dull monochrome surface.


We’ve yet to see the photographs of the diminutive Styx and Kerberos, although they’ll be making their way down the 4 kB per second downlink over the next fifteen months. As the downlink continues, New Horizons is burning to adjust its trajectory to place it in position for a flyby of a second Kuiper Belt Object, MU69, in 2019.


Top image: Nix [left] and Hydra [right] at their best. Credit: NASA/JHUAPL/SwRI

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