When astronaut Bill Shepherd and cosmonauts Yuri Gidzenko and Sergei Krikalev boarded the first pieces of the International Space Station on November 2, 2000, they clasped hands in unison.

"At that moment, we were thinking of the present," Krikalev told NASA in October 2010. But, he added, "subconsciously, we understood that was a certain threshold we were supposed to cross."

Since Expedition 1, the facility's first long-term mission, humans have lived aboard the ISS every second of the next 7,300 days, all while the now-football field-size laboratory has careened around Earth at 17,500 mph.

The station has far outlasted expectations engineers gave it a 15-year life expectancy though its future remains uncertain. NASA thinks the ISS can remain in orbit until 2028 or so, but that depends on whether or not the US and other countries keep funding the project, to the tune of billions per year.

An early version of the Space Station, back when it consisted of just three primary modules, on September 1, 2000. NASA

Private space companies are hoping to build new stations with the help of NASA; some may be way fancier than the current station, or have a lot more room.

But no matter what comes next, engineers and commercial operators will look to the space station as the foundation for all of our future space travel endeavors.

They'll learn from crew members' solutions to common engineering problems, like when one cosmonaut used tea leaves to find a leak in one of the station's main modules. And they could build on ISS research by digging deeper into how to grow fresh produce off-planet, or how the human body reacts to months of microgravity.

Expedition One commander William (Bill) Shepherd in the Zvezda Service module of the International Space Station (ISS) Alpha, May 24, 2011. NASA

Shepherd, a NASA astronaut, has said Expedition-1's first day was "hectic" as the crew prepared to broadcast leaving their spaceship and entering the space station.

"There was kind of a very busy scramble to do the initial things that we had to do and, particularly, to find the TV hook-up and the TV cable," he told AmericaSpace in October 2015. "We were really close to the wire getting all that rigged and happy, and we almost missed it."

The team eventually succeeded in hooking up the equipment, though things never really quieted down: The crew spent more than four months conducting 22 science experiments between piecing the station together. Shepherd likened their mission to "trying to build a house and live in it at the same time."

On that mission, Shepherd, Krikalev and Gidzenko installed the station's solar arrays, which boosted the station's power, plus the $1.4 billion US "Destiny" research module. They also exercised multiple hours a day to stave off bone and muscle loss that can occur in microgravity.

On March 18, 2001, the crew packed up and left after their replacements arrived: Expedition 2 aboard NASA's space shuttle Discovery. But Expedition 1's legacy lives on in the sections they installed, and in hidden surprises, like an aluminum cover for a ventilation duct with a message scrawled on it.

"I wrote on the back of it that this was something that was manufactured by the first expedition, figuring that no one would ever see that," Shepherd said. But in 2014, one of the crews finally found his message.

"There are a few other things hidden away in various parts of the original modules on Space Station that I don't think people have found yet... But some day," he said.

The International Space Station (ISS). NASA

Thanks to nearly 100 crewed missions over its first 20 years, the ISS now boasts dozens of modules, or sections, installed by astronauts and cosmonauts. In all, the flying laboratory now has more livable room than a six-bedroom house.

As it's gotten bigger, ambitions for the station's use have also grown. Most crews now conduct upwards of 200 experiments during a six-month mission a 10-fold increase from two decades ago.

All told, the station has hosted 3,000 science experiments since 2000. Crew members' work has ranged from growing fresh produce in space to studying how living in space affects the human body, an area vital to understand before long-term missions to the moon and Mars begin.

Cosmonaut Maxim Suraev, Expedition 22 flight engineer, holds Mizun lettuce plants from the BIO-5 Rasteniya-2 (Plants-2) experiment in the Service Module during Expedition 20. NASA

In one prominent example, known as the Twin Study, NASA astronaut Scott Kelly spent 340 days aboard the Space Station. His identical twin brother and fellow astronaut, Mark, remained on Earth.

The study found that Scott Kelly's time on the station likely changed his body mass, his bone density, and even the way his genes expressed themselves.

In addition to scientific research, ISS crew members have helped shoot feature films, like the IMAX movie "Space Station." And recently, NASA has ratcheted up commercial activity at the facility. In October, it sent up bottles of Este Lauder skincare product and instructed astronauts to photograph them for a commercial. The agency is also in talks with a production company to make a space-based reality show, called "Space Hero," that would allow one contest-winner to travel to the station and live there for 10 days.

Astronaut Robert L. Curbeam Jr. (left) and European Space Agency (ESA) astronaut Christer Fuglesang, both STS-116 mission specialists, participate in a spacewalk on December 12, 2006. NASA

The ISS has cost NASA and other space agencies approximately $150 billion to build and operate. And like any large and elaborate home, it needs constant upkeep and repairs; the agency still spends about $3 billion to $4 billion per year so crew members can keep the station in working order.

Sometimes the work takes them outside. To date, crew members have performed at least 231 spacewalks, many to conduct repair work.

Such operations, also called "extravehicular activities," usually require intense concentration and stamina, since they more nakedly expose crew members to the dangers of space. They're also grueling, since the spacesuits, called extravehicular mobility units, weigh about 280 pounds and limit normal human motions.

But they're not all risky tedium: They can also dazzle in rare moments, as retired astronaut Peggy Whitson previously told Business Insider while recounting a moment outside the ISS.

"I could see myself in a space suit, I could see the Earth behind me in the solar arrays, and I was like, 'holy cow, I really am an astronaut!' Because you forget. You're in this moment. You're getting a job done," Whitson said.

"Mixed feelings about International Women's Day: why do we still have 364 mens days a year? Proud to have Peggy Whitson by my side: future 2-time Space Station commander." Thomas Pesquet courtesy of ESA/NASA

Astronaut Terry Virts also experienced his own zen moment during a maintenance procedure. When he had a few moments to rest, he paused outside the station as it orbited 240 miles above the Earth.

"The only sound I heard was the faint, high-pitched whine of the spacesuit fan, and my own breathing, and for a few glorious seconds it was just me and the universe," the retired astronaut wrote in his book "HOW TO ASTRONAUT: An Insider's Guide to Leaving Planet Earth." (Astronauts sometimes describe a powerful "overview effect" from such experiences.)

Russian cosmonaut Sergey Ryazanskiy takes a break during a six-hour spacewalk to help with assembly and maintenance on the International Space Station, August 22, 2013. Johnson Space Center

But, he added, such moments were rare.

"Ninety-nine percent of my time was spent repairing equipment and storing gear and putting grease on bolts and running on a treadmill. And 1% of it was spent hearing from God and seeing creation from a perspective that I'd never thought possible," he wrote.

And even as crew members work around the clock to keep the station in normal working order, unexpected problems arise.

Since 2000, crews onboard the ISS have dealt with more than a dozen moderate to serious maintenance issues, including oxygen generator failures, air leaks, and torn solar panels.

Such problems have become more frequent in recent years, particularly on the station's Russian side where some of the oldest modules reside. The segment has seen a toilet go bust, an oxygen-supply system break down, and an air leak grow larger.

NASA astronaut Serena Aun-Chancellor works on the current US space station toilet. NASA

Crew members found the leak by watching tea leaves float around in microgravity, then patched it with Kapton tape until a more permanent fix can be made.

Such issues are only likely to grow more common as the ISS hurtles into its twilight years.

"All modules of the Russian segment are exhausted," Gennady Padalka, a cosmonaut, told RIANovostion October 15, 2020.

The space station is projected to remain in orbit until at least 2024, though possibly through 2030.

But all good things must come to an end. And when the ISS does, other habitable stations are expected to take its place. Some companies, like Blue Origin, are already working on their own "orbital habitats," which would apparently lay the foundations for a space-based economy of millions of workers.

Others, like Axiom, hope to build new modules that add on to the space station initially, yet later detach to become part of an independent orbital outpost before the world's space agencies retire the ISS into the Pacific Ocean.

An artist's rendering of Axiom's Earth Observatory, a section within its planned International Space Station module. Axiom Space

The Axiom plan is likely to be a welcome development, at least for the current crew aboard the station.

Asked what they think a fitting 20th birthday gift for the ISS would be, all three crew members of Expedition 64, the crew currently living there Kate Rubins, Sergey Ryzhikov, and Sergey Kud-Sverchkov agreed on what they'd want.

"We welcome any and all modules," Rubins said on a recent conference call. "Particularly if they have some stowage space."

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