Three astronauts link hands about 260 miles above Earth. They swing open the doors to the International Space Station and flick on the lights. What do they see? Only three puny, cramped rooms. The spacemen heat water for hot drinks. They activate the toilet that they will share. “Now we can live,” says astronaut Bill Shepherd. “We have lights, we have hot water, and we have a toilet.”

That was 20 years ago. Since that historic first mission, 241 people have lived on or visited the International Space Station (ISS).

What is the ISS? “It’s 500 tons of stuff zooming around in space,” says Mr. Shepherd. “And it’s all run for 20 years with almost no big problems.”

Mr. Shepherd and Russians Sergei Krikalev and Yuri Gidzenko blasted off from Kazakhstan on October 31, 2000. They arrived at the ISS two days later. The three astronauts spent most of their mission trying to get equipment to work. “Each day seemed to have its own set of challenges,” Mr. Shepherd says. He is retired now.

Since then, the station has grown—a lot. Now it measures almost the length of a football field. Eight miles of electrical wiring and an acre of solar panels keep it running. Today, the station has a lookout tower, three toilets, six sleeping compartments, and 12 rooms. Three of those rooms are high-tech labs. What a difference from what the first crew saw!

Three astronauts live on the ISS now. They celebrated the 20-year anniversary with dinner and by remembering the station’s early days. In 2000, astronauts could talk to people on the ground only occasionally. Communication blackouts could last hours. Astronauts now can have contact with flight controllers on Earth almost all the time. They can even use an internet phone for personal calls.

But the current crew does have to watch out for one thing: space junk! This year, the orbiting lab has had to dodge debris three times.