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Achievement Four: International Space Station, Living in Space (1998-present)

The Expedition One crew hosts a group of visitors in the International Space Station.

NASA/Newsmakers/Getty Images

Missi­ons to space lasting several weeks can­ a­chieve some amazing results. We can reach the moon, service satellites and telescopes and test all sorts of equipment. But here's the thing: Exploring other planets, or what's beyond our solar system, is going to require serious time -- months, even years. The human body isn't designed to live in space. So getting us to the point of exploring farther than ever before requires long-term tests on the effects of space life on the human body. That's where a permanent space station comes in.

The International Space Station isn't the first space station, but it's by far the most impressive. Russia launched Salyut 1 in 1971, which orbited Earth for less than a year due to a series of equipment failures. The United States sent up Skylab in 1973, which failed as well, lasting less than two years. Russia launched its second station in 1986. That was Mir, and it was in operation until 2001, when it was purposefully decommissioned. Mir measured 107 feet (33 meters) long and 90 feet (27 meters) wide and weighed more than 100 tons, and it hosted astronauts almost continuously throughout its time in space [source: NASA]. But it was nowhere near the ISS in accommodations. The ISS is an orbiting, top-of-the-line laboratory. When it's done in 2010, the ISS will measure 356 feet (108.5 meters) long and 238.8 feet (72.8 meters) wide and weigh 450 tons [source: JAXA].

It's a feat of engineering unmatched by any other permanent home in space. The first two modules of the station arrived in orbit in 1998, where they were attached to form the initial structure. In 2000, the first crew arrived to stay awhile. Since then, the U.S., Russia and 13 other countries have sent additional modules, equipment and crews to the ISS, and it's now manned continuously. Several astronauts have spent hundreds of days onboard.

­During the last eight years, occupants of the station have studied human bone loss during extended time in microgravity, radiation levels in space and how to protect against them, different techniques for doing in-space soldering to repair equipment, and countless other experiments, repairs, space walks and robotics innovations. They've also studied the effects of space on several "space tourists" who pay tens of millions of dollars to experience life in orbit.

Everything we're learning is getting us closer to a manned mission to Mars. Without the ISS, we'd be stuck close to home, unsure whether astronauts could survive a trip to the outer limits, let alone function well enough after months in space to perform tests within the harsh environment of Mars. And speaking of Mars...

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