Family portrait of Pluto's moons. It shows the size of the small moons and the edge of Charon. [NASA/JHUAPL/SwRI]

Pluto was the baby of the Solar System planets. Although only slightly bigger than Russia, Pluto is the biggest known dwarf planet. It has five moons, and that's more than there are in the inner Solar System. Earth has one moon, Mars has two, and Mercury and Venus have none.

Charon
Being small and far away, Pluto wasn't discovered until 1930. Then it took nearly fifty years before its large moon was found. The discoverer, James Christy, wanted to name the moon for his wife Char (short for Charlene). But the International Astronomical Union (IAU) said no. Pluto was the Roman god of the underworld, and Mrs Christy wasn't involved.

However, Charon was the ferryman who rowed the dead across the river Styx to Hades, Pluto's kingdom. This was a good name for Pluto's moon – and it also included Mrs Christy's name.

Charon is the only moon in the Solar System that's nearly half the size of its planet. It's also so close to Pluto that Pluto and Charon are a double dwarf-planet system. The New Horizons flyby in 2015 showed us that Charon was a fascinating object in its own right.

Rings and tiny moons?
When NASA created the New Horizons mission to Pluto, it was the only planet they hadn't visited. But there was a safety concern. Unknown small moons and rings could damage the spacecraft when it did its flyby. Because of the danger to the spacecraft, in 2005 astronomers used the Hubble Space Telescope to search for moons.

Nix and Hydra
The astronomers found two small moons. They wanted to name them to match the initials of New Horizons. So they chose Nyx and Hydra.

Hydra was the giant serpent with nine heads. It was finally destroyed by the hero Heracles. Hydra is Pluto's second largest moon.

But there was a problem with Nyx. In mythology, she is Charon's mother and the goddess of the night. And there was already an asteroid named Nyx. Fortunately, they were allowed to use the spelling Nix.

Hydra is somewhat bigger, but the two moons are a similar size. They're around 50 km (30 miles) at their longest lengths. In this image you can see where Nix and Hydra are. Their initials make NH, like New Horizons.

New Horizons on its way
As New Horizons was speeding on to Pluto, an object the size of Pluto was discovered. It was nearly twice as far away as Pluto, and was later named Eris. Instead of making Eris a 10th planet, the IAU made both Pluto and Eris dwarf planets.

The New Horizons team was still looking out for dangers to the spacecraft. Still no rings, but they found two more moons. The public was encouraged to vote on the names.

The winning names: Styx and Kerebros
Styx is the smallest moon and closest to Pluto-Charon. At its longest, it's about 12 km (7.5 miles). The name isn't only for the river that borders Hades. Styx is also the goddess of that river.

Cerberus is the three-headed dog that guards the entrance to Hades. That was another top name, but it was also another name belonging to an asteroid. The moon got the Greek version of the name: Kerebros. It's about 16 km (10 miles) in length. Its odd shape suggests it might have been formed by the collision of two smaller objects.