Pluto moons may pose threat to Nasa spacecraft

Newly discovered moons around Pluto may pose a hazard to Nasa's New Horizons spacecraft, which is en route to the dwarf planet, according to scientists.

The spacecraft is now almost seven years into its nine-and-a-half-year journey across the Solar System to explore Pluto.

But since the probe was launched, astronomers have found two more moons around the icy world.

This raises the risk that dangerous debris could be orbiting Pluto.

"We've found more and more moons orbiting near Pluto - the count is now up to five," said Dr Alan Stern, chief scientist on the New Horizons mission.

"And we've come to appreciate that those moons, as well as those not yet discovered, act as debris generators populating the Pluto system with shards from collisions between those moons and small Kuiper Belt objects."

Pluto's moons are named Charon, Hydra, Nix, S/2011 P 1 and S/2012 P 1. Charon was discovered in 1978, Hydra and Nix in 2005 - as the mission was being prepared for launch. S/2011 P 1 and S/2012 P were spotted in 2011 by the Hubble Space Telescope.

Project scientist Hal Weaver, from the Johns Hopkins University Applied Physics Laboratory in Maryland commented: "Because our spacecraft is traveling so fast - more than 30,000 miles per hour - a collision with a single pebble, or even a millimeter-sized grain, could cripple or destroy New Horizons."

He added: "We need to steer clear of any debris zones around Pluto."

Mission scientists are using computer simulations, ground-based telescopes and the Hubble Space Telescope - to search for debris in orbit.

At the same time, the team is plotting alternative, more distant courses through the Pluto system that would preserve most of the science mission but avert deadly collisions if the current flyby plan is found to be too hazardous.

"We're worried that Pluto and its system of moons, the object of our scientific affection, may actually be a bit of a black widow," said Dr Stern, from the Southwest Research Institute in San Antonio, Texas.

Deputy project scientist Leslie Young explained: "From what we have determined, we can still accomplish our main objectives if we have to fly a 'bail-out trajectory' to a safer distance from Pluto.

"Although we'd prefer to go closer, going further from Pluto is certainly preferable to running through a dangerous gauntlet of debris, and possibly even rings, that may orbit close to Pluto among its complex system of moons."

New Horizons was launched on 19 January 2006 atop an Atlas rocket from Cape Canaveral Air Force Station in Florida. The mission will culminate in a close approach to Pluto on 14 July 2015, and the first-ever exploration of a planet in the Kuiper Belt - a region of space beyond Neptune containing many icy relics from the Solar System's formation.