Nasa New Horizons Spacecraft Spots Small Moons Orbiting Pluto

The moons, Nix and Hydra, are visible in a series of images taken by the New Horizons spacecraft at distances ranging from about 201 million kilometres to 186 million kilometres.

The long-exposure images offer New Horizons' best view yet of these two small moons circling Pluto - the dwarf planet that professor Clyde Tombaugh discovered at Lowell Observatory in Flagstaff, Arizona February 18, 1930.

"Tombaugh's discovery of Pluto heralded the discovery of the Kuiper Belt and a new class of planet. The New Horizons team salutes his historic accomplishment," said Alan Stern, New Horizons principal investigator from the Southwest Research Institute, Boulder, Colorado.

Assembled into a seven-frame movie, the new images provide the spacecraft's first extended look at Hydra (identified by a yellow diamond) and its first-ever view of Nix (orange diamond).

"It's thrilling to watch the details of the Pluto system emerge as we close the distance to the spacecraft's July 14 encounter," added New Horizons science team member John Spencer, also from the Southwest Research Institute.

The first good view of Nix and Hydra marks another major milestone and a perfect way to celebrate the anniversary of Pluto's discovery, he noted.

Nix and Hydra were discovered by New Horizons team members in Hubble Space Telescope images taken in 2005.

Hydra, Pluto's outermost known moon, orbits Pluto every 38 days at a distance of approximately 64,700km while Nix orbits every 25 days at a distance of 48,700km.

Each moon is probably between approximately 40km and 150km in diameter but scientists will not know their sizes more precisely until New Horizons obtains close-up pictures of both of them in July.

Pluto's two other small moons, Styx and Kerberos, are still smaller and too faint to be seen by New Horizons at its current range to Pluto.