This is planetary radio. Hello again everyone. I'm Matt Kaplan of the Planetary Society. This week on Planetary Radio, a conversation among friends about such

inconsequential topics as travel to the stars and life in the universe. Bruce Betts will also be here with another installment of What's Up

and even provide a bit of advice about amateur astronomy on a very tight budget.

We'll get started right after this.

Hi, I'm Emily Lochuwala, science and technology coordinator for the Planetary Society with random space facts.

Most people now believe that an asteroid colliding with the earth near what is now the

Yucatan Peninsula helped cause the mass extinction that ended the dinosaurs rule over the

Earth 65 million years ago.

But this is not the only impact in Earth's history.

So far, about 165 impact structures have been discovered on Earth

from the tiny 15-meter Haviland crater in a field in

Kiowa County, Kansas, to the enormous 300 kilometer Reedfort impact structure in South

Africa.

Photos of the Earth's Moon reveal thousands of craters.

So why aren't there thousands of craters visible on the surface of the Earth?

I'll tell you the answer to that question when I return in a few minutes.

Now, back to planetary radio.

Last September, three old friends got together for a long speculative conversation about subjects each has taken very seriously for decades.

Not that the conversation was all that serious.

Bruce Murray headed the Jet Propulsion Laboratory and now serves as chair of the Planetary Society's Board of Directors.

Lou Friedman once worked for, Bruce at JPL, and has been executive director of the society for more than two decades.

And then there's renowned physicist Freeman Dyson, the author of Disturbing the Universe and Infinite in All directions was in town for a conference at

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