It's hard to imagine a world without the International Space Station orbiting it. I was 8 when the ISS welcomed its first human inhabitants. On Monday, NASA celebrated the 15th anniversary of human space occupation. For those 15 years, humankind has had a continuous presence in space, with more than 200 people from 15 countries visiting orbit.

But all good things must come to an end: The space station, which wasn't technically completed until 2011, is slated to last only until 2024. And that's if the United States gets its way: Several countries contributed to the construction of the space station, and Russia's space program is vital for its upkeep — not to mention the fact that Russia provides the only current method of transport to the station. It's likely that the space station could physically survive for several years past the projected U.S. end date, upon which time it would crash down to Earth in a controlled de-orbit, but the space agencies that would have to be involved have yet to reach any public agreement.

Even in the best-case scenario, however, it's unlikely that we will get to celebrate the space station's 25th year of human occupation.

For now, we can enjoy the incredible science of the International Space Station while it lasts. Astronauts aboard do research that would be impossible on Earth, casting off the limits of gravity and subjecting test specimens to the vacuum and radiation of space. The problems they have to solve on board are helping us learn how spaceflight works, too — providing a training ground for future missions to cast us farther away from home. Right now, astronaut Scott Kelly is completing a year-long mission, the longest ever on ISS, while his identical twin — who retired from spaceflight in 2011 — stays home. The research conducted on the pair will help scientists understand the physical effects of long-term spaceflight.

The ISS is our kiddie pool, and the hope is that by the time we pack it up we'll be ready to dive into the deep end.

But there's something sad about saying goodbye to the place where we first dipped our toes in. Here's a brief passage from a 2013 feature on the ISS by The Post's Joel Achenbach, quoting the (now retired) Canadian astronaut Chris Hadfield:

“Station” — Hadfield often refers to it that way, as if it’s a proper name — “is so much more than some remote laboratory where some small number of people and robots are doing something that no one knows about. Station is so much more than that. It is our first great human outpost in space. It is our way of seeing our world that’s unprecedented in the history of the human species. It’s an amazing platform for human self-discovery.”

On this anniversary, just take a second to think about how amazing it is that humans have been living in space for 15 years. It's hard to be cynical about that. And what's even more amazing is the fact that this is just one of our baby steps. There's a lot of space out there waiting for us.

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