From humble beginnings as a single cargo module in 1998, the International Space Station (ISS) has gone on to become an expansive and truly one-of-a-kind research facility. Today marks its 20th birthday, an impressive milestone for any scientific laboratory let alone one hurtling through space at around 17,000 mph (27,300 km/h).

The Zarya Functional Cargo Block formed the very first piece of the International Space Station, and after lifting off from the steppes of Kazakhstan exactly 20 years ago it was ready for duty in low-Earth orbit just nine minutes later.

Today, 16 pressurized modules make up the ISS, where an international crew of six people live and ply their trade. Some 230 astronauts from 18 different countries have come and gone through the station's airlocks in its history, all working to further our understanding of space through pioneering experiments in the microgravity environment.

One particularly fascinating demonstration of this is a landmark study involving identical twins Scott and Mark Kelly. Scott spent 340 days living aboard the ISS across 2015 and 2016, while his brother stayed right here on planet Earth. Because the pair share almost identical genomes and many life experiences, the hope is that by closely examining differences in gene expression between the two, scientists can start to unravel some of the mysteries around the physical effects of spending time in space. That work is ongoing, though scientists are already reporting some interesting early results.

But life aboard the ISS isn't all rainbows and intermittent sunshine (its astronauts see 16 sunrises and sunsets everyday). In August, astronauts caught wind of an air leak. This turned out to be a two-millimeter hole and though the crew was quick to find and seal it, it serves as a very real reminder of the dangers of living and working in space.

The ISS is expected to continue operations until at least 2024, and possibly longer depending on how its aging hardware holds up, so we can expect the stream of scientific discovery to continue for a little while yet. In celebration of its birthday, be sure to take a look at our collection of the best photos ever taken from the orbiting laboratory.

Source: NASA

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