Humans have been living aboard the International Space Station for 20 years. What comes next?

The space station is old. It leaks from time to time, requiring patches like the ones the astronauts installed last month. The toilet breaks. The batteries need to be replaced. It has to dodge micrometeorites — this year alone the station has had to maneuver three times to avoid getting hit. And sometimes it does get tagged, like the time in 2016 when a piece of space debris cracked a window.

But despite the inherent dangers of space, the airless void, the radiation, the bits of debris shooting around in orbit several times faster than a speeding bullet, astronauts have somehow managed to live aboard the outpost continuously for 20 years.

On Nov. 2, 2000, NASA astronaut Bill Shepherd and his Russian counterparts Yuri Gidzenko and Sergei Krikalev became the first crew to live and work on the station for an extended period, starting a streak that continues today. This month NASA is celebrating the anniversary and the work that comes from the orbiting laboratory, science experiments that include beginning to 3-D print human organs, growing protein crystals and studying the effects of space on the human body.

For years, the station has been not just one of humanity’s greatest engineering feats — atop the architectural pantheon with the pyramids — but also a way for nations to forge unlikely alliances while astronauts learned to live and work in space, and prepare for extended missions to the moon and Mars.

But as the station continues to show its age, there is concern about what comes next and whether the United States will find itself in a position similar to 2011, when it retired its fleet of space shuttles without a backup ready. That left the space agency dependent on Russia to fly its astronauts to space until SpaceX ended an ignominious chapter earlier this year with the launch of its Crew Dragon spacecraft as part of NASA’s Commercial Crew Program.

Now the concern is that the station will one day need to come down — in what would be a carefully coordinated but spectacular crash into the ocean — before its successor is ready.

NASA Administrator Jim Bridenstine in recent weeks has been sounding the alarm, telling Congress it needed to better fund the efforts and plan for the future.

“We think about Apollo era, and as much as we loved it, it came to an end,” he said during a recent Senate hearing. “We had a gap of about eight years before Space Shuttle. And then after Space Shuttle retired, we had another gap of about eight years before Commercial Crew. We want to make sure that there is no gap in low Earth orbit for the United States of America.”

The next station used by U.S. astronauts likely won’t be owned and operated by NASA but, rather, by a company like Axiom, which is building a commercial space station that it says would build on ISS’s legacy but cost less to assemble and be easier to maintain.

On the outside, it would look a lot like the ISS, with habitation modules, solar arrays and docking ports. But the inside would be dramatically different, with the “largest window observatory ever constructed for space.”

“We want the customers to have this great, comfortable, luxurious feel,” said Mike Suffredini, Axiom’s co-founder, who led NASA’s ISS program for a decade. “We’re even looking at how we cook food on orbit … to make the food a little more tasty.”

The interior is being designed by Philippe Starck, the French architect and designer, known for working on everything from furniture to yachts to corporate headquarters. His vision for the Axiom station is to “create a nest, a comfortable and friendly egg, which would feature materials and colors stemmed from a fetal universe.”

In other words, a far cry from the ISS.

Even if it wasn’t designed by Starck, though, ISS is magnificent, maybe even something of a miracle, a giant erector set assembled in orbit.

“The space station is by far, I would say orders of magnitude, the most audacious construction project in space we’ve ever contemplated,” said former NASA astronaut Leroy Chiao. “And we actually pulled it off.”

As if the vacuum of space didn’t present enough challenges, “we were doing it with people who spoke different languages, had different national cultures and different political priorities,” said Pam Melroy, a former NASA astronaut.

“Anybody can go see a launch and be inspired by the rocket,” she said. “But I wish people could actually see the station for real in person because it’s just an amazing engineering feat. It looks like a work of modern art in space as you get close to it.”

From the ground, it’s not bad, either.

NASA has a service called “spot the station,” where sky watchers can enter their email and location to be notified when the lab will fly overhead. On a clear sky, it’s often the brightest spot in the sky, streaking like a spark on a long, flowing arc toward the horizon at traveling 17,500 mph and lapping Earth every 90 minutes.

Photographers with quick trigger fingers sometimes catch it in silhouette, its giant solar arrays, looking like the wings of some sci-fi spaceship.

But for all the romanticism of spaceflight, the reality is space exploration is a difficult and potentially deadly endeavor that requires resourcefulness and dedication by the astronauts and teams of experts in mission control monitoring the systems around-the-clock, every day.

“I mean, it may sound may sound crazy, but it turns out it’s really difficult to design and build a reliable life support system,” Chiao said. “You know, on both the Russian side and the American side, our life support systems are always breaking down, always needing work, always needing spare parts.”

If it’s not air or ammonia leaks, or dodging space debris, there are sometimes problems with the plumbing.

Like the time Chiao and his crew mates noticed “the most horrible smell you can imagine,” he recalled. “And we were like, ‘What could that be?’ And we’re lifting up panels and looking behind panels. And then we lifted up the panels near the toilet, and these horrible green globules started coming out of the panel. And: ‘Oh, my goodness.’ ”

In September, NASA announced that it was sending a new toilet to the station. “Boldly Go!” read the headline on the news release. The cost of the newly designed toilet was $23 million, and it’s smaller and lighter and better suited for women, while able to recycle more urine into drinking water. (Actual astronaut joke: “Today’s coffee is tomorrow’s coffee!”)

The ISS is a laboratory where researchers get the rare opportunity to remove the one variable always present on Earth: gravity.

“The ability to take away a variable is how science moves forward,” said Eugene Boland, the chief scientist at Techshot, an Indiana research company. “And so having a station for 20 years without the effect of continuous gravity is kind of one of those like mind-blowing experiences. As a scientist, that, you know, is that shiny new tool that most scientists will never see in their tool box.”

If you 3-D print an organ on the ground, gravity collapses it like a souffle, so scientists have to reinforce, say, the walls of a heart’s chamber. But then scientists started thinking about the weightless environment of space: “What if we didn’t have to add extra scaffolding to hold open cavities like you would have in an organ like the heart?” Boland said. What if things stood on their own “because there is no gravity”?

Some of the best research has been on the human body. Scott Kelly spent nearly a year in space, and researchers have been studying how he compares with his twin brother, Mark, for years. Scott Kelly had several physiological and chromosomal changes during his time in space, such as changes in his gene expression, and his telomeres, which protect the ends of chromosomes, lengthened in space.

But the station is also a grand experiment itself, a study in human dynamics: What happens when you bring together representatives of many different countries, shoot them up into orbit and study them like lab rats to see how they get along?

At first no one was sure how this was going to work out — Russians and French, Japanese and Germans, men, women, Black, White — a rotating cast of different languages and cultures stuffed together for months at a time on a spaceship no bigger than a football field.

Twenty years on, the results are encouraging, with the space station not just a laboratory but also a tool of diplomacy. It’s a revelation that “kind of surprised people, me included,” said former NASA astronaut Michael López-Alegría. “There are some significant tensions, especially between the U.S. and Russia right now. But in the space world, we’ve got to get along somehow.”

Astronauts learn each other’s languages and cultures. They often make sure to eat meals together, share music and stories. And so alongside science and exploration, international relations may be the legacy of the station — and some are pushing it for a Nobel Peace Prize.

“When you think about those 20 years of the people,” said Leland Melvin, a former NASA astronaut, “gay, straight, Muslim, Christian and Catholic, atheist, these different colors, these different lifestyles — all these people were able to come together and build something from one module to this international outpost the size of a football field without fighting, without warring. That is worthy of a peace prize.”

Not long after arriving on the space station 20 years ago, two members of the very first crew found themselves by one of the windows, taking a moment to watch Earth go by.

They were two highly trained military officers, Bill Shepherd, a U.S. Navy SEAL, and Yuri Gidzenko, a fighter pilot from the Soviet Union’s Air Force. In another life, they were trained to kill each other.

The world turned, and “half an orbit later,” it was Shepherd’s turn: “I was a Navy SEAL, and we were here, here and here.”

Now, though, they were astronauts, not military officers, up on the space station looking down at an Earth without borders that, from their perspective, seemed peaceful.