It was a strange mistake. But it revealed the greater truth about the nature of our universe. Physicist Brian Greene explains how the prevailing assumption about the fabric of space has changed dramatically in the last century — twice.

What's six miles wide and can end civilization in an instant? An asteroid — and there are lots of them out there. Blogger Phil Plait explains all the ways asteroids can cause great destruction, and what we must do to avoid them.

Astronomer Jill Tarter wants to know if we're alone in the universe. She's spent her career hunting for signs of sentient beings elsewhere in space. In this talk, she explains why searching for cosmic company is important to the entire human race.

Is there more than one universe? Physicist Brian Greene shows how the unanswered questions of physics have led to the theory that our own universe is just one of many in the "multiverse."