Humankind reached an important milestone Monday. There has been human presence in space for 15 continuous years, thanks to the International Space Station.

ABC News reports that NASA and its partners worldwide celebrated the milestone. The astronauts currently aboard, from the United States, Russia, and Japan celebrated with a special dinner.

Commander Scott Kelly, who is seven months into his yearlong mission, thinks the biggest benefit of the space station is the potential for further long-term exploration, deeper into space.

Astronaut Kjell Lindgren referred to the station, during a news conference, as a “bridge,” and a place to test future technologies that would need to be developed and understood for a “successful trip to Mars.”

The first permanent crew arrived at the International Space Station November 2nd, 2000. Since then, 220 people have visited, from 17 different countries.

The United States leads in visits to the station, since space shuttle flights were needed to deliver important station pieces. Russia follows, with Canada and Japan tied for third. Representatives from America and Russia have been aboard the station at all times.

According to NASA, 26,500 meals have been served. The station has grown from three rooms to 13 over the past 15 years. The International Space Station weighs about 1 million pounds.

The most important element of the experiment, according to Commander Kelly, is the ability for humans to survive in space. Kelly’s mission, along with Russian representative Mikhail Kornienko, includes over 400, mostly medical, experiments. American astronauts have never spent this long in space, consecutively. Russians, however, did so decades ago on the Mir station.

NASA hopes to keep the station running until, at least, 2024, as maintenance will become required more and more. NASA administrator Charles Bolden called the milestone a “remarkable moment, 5,478 days in the making”