The International Space Station has housed people for 20 years, but the $150 billion experiment will soon come to an end

• Monday marks the 20th anniversary of astronauts' continuous presence on the International Space Station.
• Over those two decades, the station has hosted 240 crew members and 3,000 science experiments.
• But the ISS requires constant maintenance and is seeing more issues like broken toilets and air leaks as it ages. A cosmonaut described some parts as "exhausted."
• In the future, privately owned modules may be added to the ISS. But it will eventually get deorbited, perhaps by 2030.

When astronaut Bill Shepherd and cosmonauts Yuri Gidzenko and Sergei Krikalev boarded the first module 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 7,300 days, all while the now-football-field-size laboratory has been careening 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 the US and other countries keep funding the project, to the tune of billions per year.

Private space companies are hoping to build new stations with NASA's help, some of which might be far fancier or bigger than the current station.

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 a cosmonaut used tea leaves to find a leak in one of the station's modules. They will likely also build on ISS research by digging deeper into how to grow produce off-planet and how the human body reacts to months of microgravity.

Like 'trying to build a house and live in it at the same time'

Shepherd has described Expedition 1's first day as "hectic." The crew broadcast the process of 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, then subsequently spent more than four months conducting 22 science experiments while piecing the station together. Shepherd likened the mission to "trying to build a house and live in it at the same time."

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 per day avoid the bone and muscle loss that can occur in microgravity.

That first crew left on March 18, 2001, after their replacements arrived for 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, a crew 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.

As the space station has grown, so has its use

Thanks to nearly 100 crewed missions over 20 years, the ISS now boasts dozens of modules, or sections, installed by astronauts and cosmonauts. The flying laboratory, which orbits 240 miles above the Earth, now has more livable room than a six-bedroom house.

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.

In total, the station has hosted 3,000 science experiments since 2000. In one prominent example, known as the Twin Study, NASA astronaut Scott Kelly spent 340 days aboard the Space Station while his identical twin and fellow astronaut, Mark, remained on Earth. The study found that Scott Kelly's time on the station likely changed his body mass, bone density, and even the way his genes expressed themselves.

ISS crew members have also helped shoot a few 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 an Estée Lauder skincare product and had astronauts 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 winner to travel to the station and live there for 10 days.

Keeping a $150 billion home in operating order

The ISS has cost NASA and other space agencies approximately $150 billion to build and operate. It needs constant upkeep and repairs — the agency still spends about $3 billion to $4 billion per year to keep the station in working order.

Sometimes that work requires crew members to venture outside. To date, they've performed at least 231 spacewalks.

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

But floating in space is also dazzling, as retired astronaut Peggy Whitson previously told Business Insider.

"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.

Astronaut Terry Virts also described the awe of taking a look around outside the station.

"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."

Other astronauts, too, often describe a powerful "overview effect" from such experiences.

But Virts added that these moments of reflection 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.

Despite all that regular maintenance work, unexpected problems arise on the ISS. Since 2000, crews have dealt with more than a dozen moderate to serious 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, which is where some of the oldest modules reside. The segment has had a toilet go bust, an oxygen-supply system break down, and an air leak grow larger.

Crew members found that leak by watching tea leaves float in microgravity, then they patched it with Kapton tape as a temporary fix.

Such issues are 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 RIA Novosti in October.

What's next for the aging station

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.

When the ISS does, other stations are expected to take its place. Some companies, like Blue Origin, are already working on their own "orbital habitats" that could lay a foundation for a space-based economy of workers.

Others, like Axiom, hope to build new modules that could at first be added to the existing space station, then later detach to become part of an independent orbital outpost.

For now, though, the three crew members of Expedition 64 — astronaut Kate Rubins and cosmonauts Sergey Ryzhikov and Sergey Kud-Sverchkov — are continuing their daily duties on the ISS.

Asked what a fitting 20th birthday gift for the station might be, the three agreed: "We welcome any and all modules," Rubins said on a recent call. "Particularly if they have some stowage space."