In the ongoing observations of distant Pluto by NASA's Hubble Space Telescope, yet another moon has been discovered orbiting the icy dwarf planet, scientists announced Wednesday.

It brings to five the number of moons found around Pluto, four of which were spotted in recent years by Hubble as mission operators for the New Horizons spacecraft plot navigation plans for the robotic probe's close encounter in 2015.

Credit: NASA, ESA, and M. Showalter (SETI Institute) See larger image

"The inventory of the Pluto system we're taking now with Hubble will help the New Horizons team design a safer trajectory for the spacecraft," said Alan Stern of the Southwest Research Institute in Boulder, Colo., the mission's principal investigator.

The newest moon is estimated to be irregular in shape and 6 to 15 miles across and circling in a 58,000-mile-diameter orbit around Pluto that is assumed to be co-planar with the other satellites in the system.

"The moons form a series of neatly nested orbits, a bit like Russian dolls," said team lead Mark Showalter of the SETI Institute in Mountain View, Calif.

Scientists say they are intrigued that small Pluto could have such a complex collection of satellites and gives additional clues for the formation and evolution of the Pluto system. The favored theory is that all the moons are relics of a collision between Pluto and another large Kuiper belt object billions of years ago.

"The discovery of so many small moons indirectly tells us that there must be lots of small particles lurking unseen in the Pluto system," said Harold Weaver of the Johns Hopkins University Applied Physics Laboratory in Laurel, Md.

Hubble observations in 2006 uncovered two small moons, Nix and Hydra. In 2011 another moon, P4, was found in Hubble data.

Pluto's largest moon, Charon, was discovered in 1978 in observations made at the United States Naval Observatory in Washington, D.C.

New Horizons, launched from Cape Canaveral in 2006, is a nuclear-powered probe headed for the frozen fringes of the solar system. It will fly by Pluto at a speed of 30,000 miles per hour, using seven instruments, including cameras, spectrometers and in-situ particle measurement devices to reveal the landscape and chemical makeup of the planet and moons, provide stereoscopic imaging for 3-D visualizations of the topography, calculate the composition and density of material coming off of Pluto's rapidly escaping atmosphere and search for rings and more moons.

The new moon announced Wednesday is designated S/2012 (134340) 1. It was discovered in nine separate sets of images taken by Hubble's Wide Field Camera 3 on June 26, 27 and 29, 2012 and July 7 and 9, 2012.