Twenty million passengers embark on cruises each year, vacationing in deluxe "floating cities" that offer everything from swimming pools to shopping malls to ice skating rinks. And the ships just keep getting bigger: The average cruise ship has doubled in size in just the last ten years. Some engineers fear that these towering behemoths are dangerously unstable, and the recent tragedy of the Costa Concordia has raised new questions about their safety. Now, NOVA brings together marine engineering and safety experts to reconstruct the events that led up to famous cruise disasters, including the ill-fated Concordia, the Sea Diamond, and the Oceans.

The asteroid that exploded over Siberia—injuring more than 1,000 and damaging buildings in six cities—was a shocking reminder that Earth is a target in a cosmic shooting range. From the width of a football field to the size of a small city, these space rocks have the potential to be killers. In a collision with Earth, they could set off deadly blast waves, raging fires and colossal tidal waves. But some audacious entrepreneurs look up at asteroids and see payday, not doomsday. Some asteroids are loaded with billions of dollars’ worth of elements like iron, nickel, and platinum. NASA is planning an ambitious mission to return samples from a potentially hazardous asteroid, and would-be asteroid miners are dreaming up their own program to scout for potentially profitable asteroids. Will asteroids turn out to be our economic salvation—or instruments of extinction?