Photos: Delta 4-Heavy launch gallery

A frozen fragment left over from the formation of the solar system is set to become the most distant object ever explored by a human-built space probe, scientists working on the New Horizons mission said Friday.

Five years ago Tuesday, NASA’s New Horizons spacecraft barreled by Pluto for a high-speed encounter that gave humanity its first close-up look at a distant world that puzzled astronomers and planetary scientists for nearly a century. Scientists are now planning how to go back.

NASA managers formally approved the New Horizons mission another speedy encounter with an object at the frontier of the solar system, but denied a request from scientists to redirect the Dawn spacecraft to visit a third destination in the asteroid belt, opting to keep it in orbit around the dwarf planet Ceres, officials said Friday.