As of Monday morning humans have lived consecutively in space for 15 years. That's longer than the iPod has existed and before Americans first learned of hanging chads during the contentious election between Al Gore and George W. Bush. During that time more than 220 astronauts and cosmonauts from 17 countries have visited and lived on the $140 billion International Space Station.

On the crystal anniversary of the station, which should fly until 2028, the six inhabitants on board didn’t exchange gifts, but they did take a few moments to reflect on the accomplishment. “For me, the most important experiment is keeping humans alive for a long period of time,” said NASA astronaut Scott Kelly. “All of this is something we’re going to need to explore deeper into space for a longer period of time.”

Keeping the football-field sized laboratory, which has an annual budget of about $3 billion, flying hasn’t always been smooth sailing. In 2007, the three primary flight computers failed, leaving astronauts without control of its thrusters, oxygen generators, carbon dioxide scrubbers, and other environmental control systems. After several frantic hours they were able to reboot the system.

More recently, Italian astronaut Luca Parmitano’s helmet began to fill with water during a spacewalk, an incident that could have been fatal had he not been rushed back inside an airlock. “The closest we’ve ever come to hurting someone was the Luca incident,” then ISS program manager Mike Suffredini said. “It happened so fast, and it was over so fast, none of us had a chance to think about what was going on.”

During the course of the station’s lifetime, NASA has struggled with how to promote it. The ISS does many things, but there is no killer app to single-handedly justify its enormous cost.

The station also serves as a useful testbed for technology, such as its water recycler, needed for missions beyond low-Earth orbit. NASA recognizes the need for closed-loop systems for water and other consumables, but so far limited progress has been made on food, clothing, and other consumables that add considerable mass to long missions.

Finally, the agency has promoted the station as an important scientific laboratory for research in microgravity. The ISS does offer a unique platform, but despite some promising research, Earth-based labs with multi-billion-dollar budgets could unquestionably make more significant advances in various scientific fields.

Probably the most important contribution made by the station doesn’t involve any kind of technology at all.

Rather, the station has proven an unqualified success in terms of international diplomacy. As US-Russia tensions deepened in the spring of 2014 following Russia’s invasion of Crimea and undeclared war in the Ukraine, Russia’s chief space official, Dmitry Rogozin, created a firestorm by saying if Americans didn’t like his country’s actions they could get to space by jumping on a trampoline. But that bluster never really went anywhere. Instead, US astronauts kept training in Star City, just outside of Moscow, and kept launching from the Baikonur Cosmodrome in Kazakhstan. Crews on board the station continued to work hand-in-hand, as did US flight controllers, with their Russian counterparts.

In reality, both the United States and Russia need the station. America has no other piece of human-rated space hardware, and without the station Russia’s Progress and Soyuz vehicles have nowhere to go.