NASA is spending Monday celebrating 15 years of humans living on the International Space Station.

On Nov. 2, 2000, the Expedition 1 crew — NASA astronaut Bill Shepherd and Russian cosmonauts Yuri Gidzenko and Sergei K. Krikalev — began their 141-day stay on the ISS.

Since then, there have been 45 expeditions and at least two people at a time occupying the space station, conducting research studies.

The first research study was on Protein Chrystal growth in space, which today is used to help treat diseases.

Right now, the six members of Expedition 45 — American astronauts Scott Kelly and Kjell Lindgren; Russian cosmonauts Mikhail Kornienko, Oleg Kononenko and Sergey Volkov; and Kimiya Yui  from the Japan Aerospace Exploration Agency, are living on the International Space Station orbiting 250 miles above the Earth.

NASA has an ISS 15th anniversary website with interesting numbers and facts about the 15 years humans have lived in space, including all the badges from the 45 expeditions to the space station.

A total of 220 people from 17 countries have called the orbiting outpost home. There's now enough livable room to compare to a six bedroom house

But what's truly incredible is the amount of science that's been done in the unique microgravity environment.

It began on the first mission with a protein crystal growth study before people even began living on board.

Since then, researchers on Earth have benefitted from 1,760 investigations from 83 counties, from working on vaccines to studying bone and muscle loss.

Right now Commander Scott Kelly is part of the year in space project, to study the effects of weightlessness on the body. The goal is to use the information for future missions to deep space.

Social media also gives people on Earth a look into the lives of those living on the ISS. Kelly, who is in the middle of a yearlong stay on the space station, updates his Twitter account daily, so the world can get a better glimpse into what Earth looks like from his point of view.

How long will the ISS be in use?

The American portion is funded through 2024 -- although it's still unclear how long the outpost will orbit the Earth.

(L-R) Expedition 45 crew portrait: Commander Scott Kelly and Flight Engineers Sergey Volkov, Mikhail Kornienko, Kjell Lindgren, Oleg Kononenko and Kimiya Yui.