Let's continue on our journey to the Moon. Last week we talked about the physical characteristics of the Moon, its appearance in the sky and how it interacts with the Earth. This week we're going to take a look at how scientists have expanded our understanding of the Moon. From ancient astronomers using nothing more than their eyes and the first telescope observations of Galileo to the exploration by robotic spacecraft. And of course, the first tentative steps by the human explorers of the Apollo program.

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Wanted to point out an error in this episode, just after 19 minutes into the program, Pamela states that cratering rates are fairly constant throughout the solar system.

This is actually not true, and it's a major source of uncertainty when trying to create crater isochrons for dating surfaces on other rocky bodies. For example, the flux of impactors (number per unit area per unit time) on Mars is estimated to be approximately twice the flux of craters on the moon (see, for example, Ivanov, 2001, "Mars/Moon Cratering Rate Ratio Estimates" as well as Hartmann's latest Mars isochron paper from 2005).

This is also at least partially why, for at least a decade or so, the error bars on Venus' surface age from crater counts were a factor of two (between 500 and 1000 Myr).

The inner solar system is easier to extrapolate than the outer solar system, where we have reasons to believe that comets are a more significant impactor population than in the inner solar system. When I've talked with people about extrapolating isochrons to the outer solar system, they generally throw up their hands and say that it's still hard to get a good estimate when going to Mars.

Oh, and this happened to be the subject of my M.S. conditions last year, which is why I'm particularly attuned to it.

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