Their recorded video message, combined with a one-on-one interview with NASA's ISS program manager Kirk Shireman, make up a new video released by NASA to mark the anniversary.
"It is also one of the most valuable and unlikely achievements of humankind," Alexander Gerst, a European Space Agency astronaut and the current commander of the space station, said in the video. "If you're younger than 30 years of age, for almost every moment of your life, there was a human being in space, on the space stations Mir and ISS." [International Space Station at 20: A Photo Tour]
Even if spacefarers make up a tiny proportion of Earth's human population, he continued, having that presence on the space station is vital.
"Seven billion humans live on planet Earth; three humans live in space right now," Gerst said. "We are from three different continents, we are friends. We are here for you. We are your eyes looking down on this beautiful planet. We bring home a rare perspective onto ourselves. Our eyes see things down there that otherwise remain unseen — some good, some bad, some alarming."
He also touched on the debate that surrounded the program's birth. "There was a time when some seemingly crazy optimists dreamt about this ISS project. Other people, calling themselves realists, said that this was not possible," Gerst said. "Here we are, in the most complex machine that humanity has ever built."
But for Gerst, it shouldn't end with the space station. "ISS is a ship that teaches us how to fly beyond the horizon," Gerst said. "We are literally the first generation of fish that left Earth's homely ocean. The next generation will set their foot on Mars, using the knowledge and technologies developed on the International Space Station."
His crewmates, Russian cosmonaut Sergey Prokopyev and NASA astronaut Serena Auñón-Chancellor, also reflected on the anniversary.
"The ISS currently represents the brightest example of the victory of human thought combined with an outstanding example of international cooperation," Prokopyev said, congratulating the ground crews that have made the mission possible. "We wish prosperity to you in everything, we wish you kindness, peace and further success in achieving our common goals, both on Earth and in space."
Auñón-Chancellor chose to highlight the science that has been conducted on board the orbiting laboratory — more than 2,500 experiments involving researchers from more than 100 countries. "What's important to remember is that many of these experiments don't just benefit our ability to live in space, but also directly impact our lives on Earth," she said. "Space station science is truly off the Earth, for the Earth."
During his interview, Shireman focused on the international collaboration necessary to make the space station a reality. "The United States, of course, had their own spacefaring history and Russia had their own spacefaring history, and it was really neat to see it joined up," he said.
The International Space Station was fed by Russia's Soyuz rockets and Mir space station, by the U.S. shuttle program and by smaller hardware components. "All these pieces together had existed before; assembling them in a really unique spacecraft has really led to this successful program that we've had," Shireman said.
And for him, that's changed how humanity thinks about Earth and space. "People now 18 years old have never known a time when people didn't live and work in space," Shireman added. "They take it for granted that humans live and work in space… [the space station] has changed the mentality, the thought process of everyone on this planet."