On July 14, we are to clear the last of the big hills. After a journey of nine and a half years and three billion miles, the New Horizons spacecraft is to go past Pluto, once the ninth and outermost planet, the last of the known worlds to be explored. This is the beginning of the end of a phase of human exploration. The crawling-out-of-our-cradle-and-looking-around part is over.

Beyond the hills are always more hills, and beyond the worlds are more worlds. So New Horizons will go on, if all goes well, to pass by one or more of the cosmic icebergs of the Kuiper belt, where leftovers from the dawn of the solar system have been preserved in a deep freeze extending five billion miles from the sun. If all goes well, spacecraft like Dawn, now orbiting the asteroid Ceres, and the Rosetta spacecraft orbiting Comet 67P/Churyumov-Gerasimenko, will keep picking away at the supporting cast of solar system characters, who, as in many a well-crafted movie, could turn out to be the most interesting characters.

But the inventory of major planets — whether you count Pluto as one of those or not — is about to be done. None of us alive today will see a new planet up close for the first time again. In some sense, this is, as Alan Stern, the leader of the New Horizons mission, says, “the last picture show.”

It’s hard to write these words and know what they might feel like 50 years from now. I never dreamed, when Apollo astronauts left the moon in 1972, that there might come a day when there was nobody still alive who had been to the moon. But now it seems that could come to pass. How heartbreaking is that?

You could say that we have reached the sea, the very icy and black sea between us and the stars. Whether we will ever cross that sea nobody can say.