The researchers say this dust must be coming from collisions among small bodies similar to the comets or icy bodies that make up today's Kuiper Belt objects in our solar system.

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Mr. STERN: So a few weeks after the Pluto encounter when we've chosen our target, we'll fire the engines of the spacecraft, but we'll have to fly a distance something like from here to the sun or twice that far to reach these Kuiper Belt objects.

NPR: NASA Sends Spacecraft to Pluto

Unlike the larger Kuiper-belt objects, comet-sized ones would be too faint to show up in a single photograph even one taken by the mighty Hubble.

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But the forces exerted by objects in the Kuiper belt (a collection of small bodies orbiting the sun beyond the orbit of Pluto), or by the galaxy as a whole, were also far too weak to be responsible.

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Alan Stern expects that after the Pluto flyby in 2015 or so, there will be plenty of power left, both electricity from the plutonium generator and fuel, so New Horizons will keep on going to explore other Plutolike objects in this region of the solar system which is known as the Kuiper Belt.

NPR: NASA Sends Spacecraft to Pluto