NASA's New Horizons mission may encounter tumultuous clouds of ice and rock when it flies by Pluto in 2015, and engineers are prepared move the spacecraft on an alternate course by the dwarf planet to spare the probe from fatal debris strikes, officials said Tuesday.

Artist's concept of New Horizons flying by Pluto in July 2015. Credit: JHUAPL/SWRI
 
"This is our first time to the Kuiper Belt, the very frontier of our planetary system," said Alan Stern, the New Horizon principal investigator from the Southwest Research Institute in Boulder, Colo.

The probe's first destination is Pluto, where it will make a close approach in July 2015, returning the first close-up pictures of the distant planet and gazing its scientific eyes on five nearby moons.

Since the 1,000-pound craft launched in January 2006, scientists have discovered a fourth and fifth moon circling Pluto, raising concerns the space around the planet might be fraught with dangerous bits of debris generated from pinball-like collisions between objects in the past.

"We've found more and more moons orbiting near Pluto — the count is now up to five," Stern said. "And we've come to appreciate that those moons, as well as others not yet discovered, act as debris generators populating the Pluto system with shards from collisions between those moons and small Kuiper Belt objects."

The Kuiper Belt is a disc of objects outside the orbit of Neptune. Pluto is considered one of the belt's larger worlds, and plans call for New Horizons to target at least one more Kuiper Belt object after its encounter of Pluto.

Unlike gas giant planets, which have rings of icy debris concentrated in rings, studies show Pluto's gravity could hold a more chaotic cloud of ice and rock.

"If there's a debris belt, it's very likely to be a three-dimensional tar ball," Stern said. "No matter how you approach the planet, you probably can expect to run through that gauntlet."

Wiring, cables, instrument apertures, and the propellant tank on New Horizons are susceptible to debris hits.

"So I'm a little bit worried," Stern said Tuesday during at a meeting of the American Astronomical Societ's Division for Planetary Sciences. "My spacecraft is going very fast at 14 kilometers per second (31,300 mph). Even a strike by something the size of a BB would be fatal."

New Horizon's navigators have plotted nine alternate paths by Pluto. Their acronym is SHBOT, or Safe Haven Bail Out Trajectory.

Up until 10 days before closest approach, controllers can command New Horizons to switch to one of the backup trajectories to avoid pockets of dense debris and keep the probe safe, according to Stern.

"We're making plans to be able to cope with what we may discover on final approach in the last weeks and months before we get there," Stern said.

Mission officials say any chance in course near Pluto will not risk scientific discoveries.

"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," said Leslie Young, the mission's deputy project scientist at the Southwest Research Institute. "Although we'd prefer to go closer, going farther 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."