International Space Station

The most expensive structure ever built ($150 billion), the ISS has hosted over 270 astronauts from 21 countries with continuous habitation since November 2000—more than 25 years. Orbiting at 28,000 km/h, it has enabled 3,000+ experiments spanning biology, physics, and materials science, including zero-gravity crystals grown 10 times more perfectly than on Earth. The ISS demonstrates cooperation among 15 nations, costing less per scientific output than #5 (Cassini-Huygens) when amortized over its lifespan, proving world powers can collaborate in space despite terrestrial disagreements.