An Atlas V rocket that is to carry the New Horizons spacecraft on a mission to the planet Pluto lifts off from launch pad 41 on Jan. 19, 2006, at the Cape Canaveral Air Force Station in Cape Canaveral, Fla.

When New Horizons, the unmanned NASA spacecraft en route to Pluto, was originally green lit in 2001, astronomers thought Pluto only had one moon. It was also still considered a planet.

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A lot has changed since then, and new discoveries could put the spacecraft's mission in jeopardy. Pluto, demoted to "dwarf planet" status, now has five known moons and may even have rings similar to Saturn. Those moons act as "debris generators" and could make New Horizons' approach dangerous, scientists from the project said Tuesday.

"Because our spacecraft is traveling so fastmore than 30,000 miles per houra collision with a single pebble, or even a millimeter-sized grain, could cripple or destroy New Horizons," project scientist Hal Weaver said in a statement.

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Launched in 2006, New Horizons passed Mars in April of that year; Jupiter in 2007; Saturn in 2008; and Uranus last year. It is set to reach Pluto in July, 2015 and will monitor it from nearby before heading out of the solar system.

"We're going into some unknown hazards," Alan Stern, principal investigator of the project, says. "The concerns we have are a lot higher than they were a few years ago."

Stern says the team is planning a "backup trajectory" in case New Horizons' projected path seems littered with debris. Currently, it takes about six and a half hours for a signal from Earth to reach New Horizons; by the time it reaches Pluto, it'll take nine hours.

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NASA's Pluto-Observing Spacecraft Faces Rough Future