"Asteroids hit the earth all the time," said former astronaut Dr. Ed Lu, in a CNN interview that aired Sunday. "Really small ones are just the shooting stars you see when you look up in the sky." And this is basic Astronomy 101. But what you don't hear people talk about are the medium-sized rocks, like that kind that landed in Siberia in 1908. This was known as the Tunguska Event, or the Great Siberian Explosion.

When the meteor exploded above the Tunguska River in Siberia it detonated with an "estimated power 1,000 times greater than the atomic bomb dropped over Hiroshima [...] leveled trees over 40 kilometers away and shook the ground in a tremendous earthquake."

Lu said asteroids that size hit about every couple of hundred years. "So, there's about a 50 percent chance that in your lifetime, another explosion of that size is going to happen somewhere on Earth.

But don't start panicking just yet. According to Lu, "We actually have the technology to deflect an asteroid, if we have adequate warning, and by adequate warning, I mean decades in advance. And the problem now is that no one is doing a comprehensive map of all the threatening asteroids."

Lu said that less than one percent of asteroids larger than the one that leveled Tunguska (itself about 40 meters wide) have been tracked to date. The B612 Foundation, of which Lu is CEO, is an NGO dedicated to completing the the B612 Foundation Sentinel Space Telescope, which is expected to be launched in 2018 and meant to orbit the sun. Lu writes that the Sentinel will provide a comprehensive map ot the locations and trajectories of threatening asteroids. "By the end of its planned lifetime, Sentinel will have discovered well over 90 [percent] of the asteroids that could destroy entire regions of Earth on impact (those larger than 350ft in diameter) and more than 50 [percent] of the currently unknown DA14-like near-Earth asteroids."

That planned lifetime is 6.5 years. Here's Lu giving a TED talk about how to deflect an asteroid and further explanation on the space telescope.