Pioneer 10 became the first probe beyond the planets in 1983

NASA’s Pioneer 10 became the first human-made object to travel beyond the planets on June 13, 1983, when it crossed the orbit of Neptune, 11 years after its launch on March 2, 1972. On the way it had been the first spacecraft to fly through the main asteroid belt and, in December 1973, the first to fly past Jupiter.

The milestone was Neptune, not Pluto, for a reason. Pluto, then counted as the ninth planet, follows a stretched orbit that carries it inside Neptune’s for about 20 years in every 248. It crossed inside on February 7, 1979, and did not cross back out until February 11, 1999, so in 1983 Neptune really was the outermost planet.

Nor did it leave the solar system in the sense scientists use today. Passing the planets is not the same as leaving the heliosphere, the Sun’s bubble of particles and magnetic fields. The first spacecraft known to have crossed that boundary into interstellar space was Voyager 1, in August 2012; it had already overtaken Pioneer 10 as the most distant human-made object on February 17, 1998, at 69.4 AU, or 69.4 times Earth’s distance from the Sun.

Pioneer 10 kept working long after its planned mission. NASA ended routine contact on March 31, 1997, for budget reasons, when the probe was 67 AU out, but engineers kept listening. The last telemetry arrived in April 2002, and the final faint signal on January 23, 2003, from 7.6 billion miles away.

It carries an aluminum plaque showing a man and a woman, the solar system, and Earth’s position relative to 14 pulsars. Now silent, it is heading roughly toward the red star Aldebaran, the eye of Taurus, which it should pass in about two million years.