This week the New Horizons mission team celebrates the 30th anniversary of the discovery of Pluto's largest and first moon, Charon, by U.S. Naval Observatory astronomers James Christy and Robert Harrington.

Charon, whose discovery was officially announced on July 7, 1978, orbits nearly 11,390 miles (about 18,220 kilometers) from Pluto's surface and has a diameter of about 750 miles (1,210 kilometers). At half the diameter of Pluto, Charon is the largest moon relative to its planet in our solar system.

Charon's surface is covered in water ice, and its interior is known to be a nearly even combination of rock and water ice. Unlike Pluto, it has no substantial atmosphere.

"The historic discovery of Charon ushered in the modern understanding of Pluto as a double planet and the product of a giant collision that formed the system in much the same way as the Earth-moon system was formed," says New Horizons Principal Investigator Alan Stern.

The Pluto family grew just three years ago, when Stern and New Horizons Project Scientist Hal Weaver led a team that discovered two additional, much smaller moons, later named Nix and Hydra.

New Horizons is en route to fly by and reconnoiter the Pluto system seven years from now, in July 2015, turning these moons and their parent planet from points of light into well-mapped worlds.