On July 14, 2015, the New Horizons spacecraft will zip past Pluto and its five known moons. Nobody really knows what it will find.  When I was a kid, before I wanted to be Einstein and before I wanted to play second base for the Yankees, I wanted to be a cowboy. Specifically, I wanted to ride a horse over the hills and mountains of the West that we always passed by on car trips and see what was on the other side.  I imagined that over every hill, there would be a new hill, new mysteries and frontiers. New territory and new possibilities.  This hardly makes me unique. Humans have been seeking out new territory ever since they wandered out of Africa. You can imagine many sound evolutionary reasons for such wanderlust: encouraging diversity, finding new resources, reducing the effect of a single-point catastrophe like drought or a plague. Maybe even the dream of reinventing yourself in a place where nobody knows you used to be a dog.  For most of my life, the hills that have beckoned some of us are worlds, the lands under the sun but beyond the sky. And our horses have been robots with names like Mariner, Viking, Voyager.  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.  This is not to say that marvelous or even the most fundamental discoveries of all aren’t still in store for us here in our own backyard. The first reconnaissance of the solar system has yielded mysteries that will take more than a human lifetime to resolve, especially given the anemic pace of NASA’s explorations and the increasingly long time it takes to get a space mission approved and actually out there.  As long as scientists can keep prying nickels from the clenched fists of legislators, probes will keep popping off to Mars every couple of years. NASA now has plans to send a spacecraft to Jupiter’s moon Europa, which has a salty ocean underneath its ice, and may get around to flying a probe through the geysers spurting from the Saturnian moon Enceladus someday to sniff for organic materials. My personal favorite is the idea to send a boat to float around on the methane seas and lakes of Titan, Saturn’s biggest moon.  The discovery of one microbe anywhere out there that was not our own would be the greatest scientific event of the age.  It all began when Mariner 2 went past Venus in 1962.  Myths and hopes have been punctured along the way, starting with the dream of life on Mars. Mariner 4 went past Mars three years later and sent back pictures of a cratered, dead-looking world. Goodbye, dreams of finding canals and ancient apocalyptically advanced and dying civilizations. Hello, sifting the red sands for microbes or anything organic.  I came into this game when, as a late-blooming writer for Discover magazine, I climbed onto the back of a pair of horses named Voyager in 1980 for the first of two passes by Saturn. I could hardly believe my luck.  In the press room at the Jet Propulsion Laboratory, we reporters could watch the latest Voyager images coming down from space at the same time as the scientists in their own building a few yards away. If somebody or something was holding up a “Yankee Go Home” sign in some distant crater, or in fact just waving, we would all see it at the same time. Every morning, befuddled scientists, unused to doing instant science in public, would report for a news conference and confess that they didn’t know what this or that really was.  We were all seeing new territory, things nobody had seen before, all at the same time. It was like riding on the nose of the spacecraft.  For the next decade, every couple of years or so, the tribes would gather in Pasadena, Calif., for weeklong planetary encounters that were half celebrations of humanity and curiosity, half parties. At some point, some evening, Jonathan Eberhart, the Science News correspondent and folk singer, would sing space ballads in his whiskey voice to a roomful of mellowed-out space fans.  While particle physicists pursuing string theory were locking into what they thought was an equation suitable for a T-shirt that would explain creation, Voyager was wending outward past Saturn, Uranus and Neptune, showing just how diverse and unpredictable the arrangements of the material universe could be.  Every planetfall was a surprise.  What mostly received attention were volcanoes. Sulfur on the Jovian moon Io, smog and hydrocarbon lakes on Titan, cracks in the fresh ice of Europa, plumes of nitrogen and water on Triton, dark spokes on the rings of Saturn.  The idea of something, anything — even water — moving so far from the light and warmth of the sun, out there on the shoreline of the solar system, seemed miraculous to me.  A version of this article appears in print on July 7, 2015, on page D5 of the New York edition with the headline: A Great Ride While It Lasted. Order Reprints| Today's Paper|Subscribe
When the New Horizons craft passes Pluto as expected July 14, we will have explored the known worlds. There are other finds ahead, but a personal journey will end.