Today is the fortieth anniversary of a very important day in human history. On June 13, 1983, Pioneer 10 passed beyond Neptune.
In that moment it became the first human-made object to leave our modern Solar System. Although the story is more complicated than that.
Because in 1983, Pluto was still considered a planet. Pluto didn’t stop being considered a planet for another quarter century — in 2006 in fact.
So Neptune did not yet mark the edge of our solar system in 1983. But in the intervening decades, the rules changed.
40 years ago our human reach got bigger. Since then, our solar system got smaller.
There is another quirk to this story. Pluto’s orbit is very eccentric – in the geometric sense of the word. So much so that it occasionally intersects with Neptune’s orbit, temporarily placing Pluto closer to the sun than Neptune. This happens once every 248 years, most recently from February 7, 1979 until February 11, 1999.
This means that back in 1983, it was actually reasonable to consider Neptune’s orbit as the boundary of the solar system, irrespective of Pluto’s planetary status.