ISS Marks Two Decades of Continuous Human Presence, Built on International Cooperation

On Saturday, the International Space Station reaches a milestone that few of the people who built it expected to take for granted: twenty years of humans living aboard without a break. Astronauts and officials involved in the program are using the anniversary to point to the international cooperation behind the station and the research that, they say, benefits people on the ground.

Michael López-Alegría, a former astronaut who flew to the station three times, was the last person to visit before permanent crews arrived in 2000. He says those who helped assemble it find the two-decade mark hard to believe. In his words, the station is still in very good shape, and he calls it the most audacious and complex construction project ever attempted in space. He adds that it is remarkable how well everything fit together and still works. Across his missions he logged 67 hours and 40 minutes on 10 spacewalks, a NASA record surpassed only by Russian cosmonaut Anatoly Solovyev, who spent 82 hours and 22 minutes outside across 16 spacewalks.

The partnership rests on earlier experience. The United States ran Skylab, occupied for only 24 weeks with gaps between its three missions, while Russia operated Mir, which was inhabited for 12 and a half years with two short gaps. Robyn Gatens, NASA's acting director for the space station, says hundreds of people have supported the program over time, including mission controllers in both Houston and Moscow. She calls the continuous presence on so complex an international platform an amazing accomplishment, and says making it look easy, when it is not, is a tremendous feat.

The numbers give a sense of scale. The complex spans the length of a football field and is comparable in living space to a five-bedroom house with a gym, two bathrooms and a 360-degree bay window, the cupola, which offers views of Earth. Large solar arrays power it, and liquid-propellant rocket engines keep it from losing altitude. It flies more than 250 miles up at over 17,000 mph. Building it cost more than $150 billion, and it costs NASA over $3 billion a year to operate. More than 240 people from 19 nations have visited, and over 100 nations have sent research or educational projects.

Gatens says the biggest research payoffs have come in health, medicine and materials. Studies of bone loss and immune system depression in astronauts have fed into a better understanding of osteoporosis on Earth and of how cancer cells behave. A 2004 mission showed that astronauts could operate sophisticated ultrasound equipment in orbit. Its commander, retired astronaut Leroy Chiao, became the first person to vote in a U.S. presidential election from space; astronaut Kate Rubins cast her ballot from the station on Oct. 22. NASA also cites work on Earth's geology, oceans, farms and glaciers, improved methods of growing vegetables in space, confirmation of beliefs about cosmic rays, and a better grasp of how fire behaves in space. More recent work includes building human tissue in microgravity, with the aim of eventually producing entire organs.

The station has also supplied memorable moments. The longest spacewalk, 8 hours and 56 minutes, was performed by Jim Voss and Susan Helms in 2001 during assembly. Christina Koch and Jessica Meir carried out the first all-female spacewalk in October 2019. Scott Kelly and cosmonaut Mikhail Kornienko launched March 27, 2015, and returned March 1, 2016, a year-long stay meant to let researchers study Kelly's health during and after the flight in preparation for deep-space missions. By contrast, Bob Behnken and Doug Hurley's stay this summer lasted 64 days, among the shortest visits to the platform.

López-Alegría argues the station remains integral to exploration. He notes that reliable life-support systems for a long Mars mission, such as carbon dioxide scrubbers that keep air breathable for long periods without replacement, have not yet been built, and says the station is the best place to test them. That role, as a proving ground for the moon and Mars, is where officials expect its work to continue.