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.
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 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," project scientist Hal Weaver said in a statement.
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.
He says it was too late to change New Horizons' flight path as new discoveries about Pluto were being made.
"When the first new moon, Hydra, was discovered in 2005, New Horizons was already finished—it was literally in testing," he says.
If New Horizons survives its dangerous approach, it will continue past Pluto and further into the Kuiper Belt—the ring of debris, asteroids, and other celestial bodies that surround the solar system. The team expects to keep receiving data from New Horizons into the 2030s.
"When we wanted to fly by Pluto for reconnaissance, we had no idea there'd be four moons," Stern says. "But when we get there, we may discover a bunch more moons."
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Jason Koebler is a science and technology reporter for U.S. News & World Report. You can follow him on Twitter or reach him at firstname.lastname@example.org.