On June 13, 1983, a small spacecraft no bigger than a car crossed an invisible line and became the first human-made object ever to travel beyond the farthest known planet. Pioneer 10 had launched from Cape Canaveral eleven years earlier, in March 1972, on a tiny rocket. Now it was sailing past the orbit of Neptune, almost three billion miles from Earth. (Pluto was inside Neptune's orbit at the time, which is why Neptune counted as the outer edge.)
Pioneer's mission had already been a string of firsts. It was the first spacecraft to cross the asteroid belt, the wide ring of rocks between Mars and Jupiter that scientists worried might smash any ship into pieces. It went through without a scratch. Then in December 1973, it sailed past Jupiter, sending back the first close-up photos ever taken of the giant planet's swirling red storm. Bolted to its side was a famous gold plaque designed by astronomers Carl Sagan and Frank Drake - a kind of cosmic postcard showing a man, a woman, and a map of where Earth is in the galaxy, just in case any aliens happened to find it.
Pioneer 10 kept whispering radio signals home for thirty more years. NASA caught its last faint signal on January 23, 2003, when it was about 7.6 billion miles away. The probe is still out there, drifting silently toward the red star Aldebaran in the constellation Taurus. It will not arrive for about two million years - but when it does, the gold plaque will still be shining on its side.
