Pioneer 10NASA’s Pioneer 10 unmanned spacecraft becomes the first human-made spacecraft to pass beyond the orbit of Pluto, the outermost known planet in the solar system. (Pioneer 10 merely crosses the planet’s orbital path; Pluto itself is in a different part of its orbit, nowhere near Pioneer at the time.) Launched in 1972, Pioneer 10 was the first spacecraft to cross the asteroid belt and observe the giant planet Jupiter at close range, blazing a trial for other outer solar system robotic exploration missions.