The head of NASA Science on this week's planetary radio. Hello again everyone. I'm Matt Kaplan with another chapter in our ongoing story about the

exploration of space.

Associate Administrator of NASA, Ed Weiler, is our special guest.

Later, Bruce Betts, who once worked for Ed, will be here with the latest what's up.

First up is Emily with this tail of a shrinking comet,

if you'll pardon the pun.

I'll be right back. Hi, I'm Emily Loch Duwala with questions and answers. A listener asked, with all the junk that's constantly streaming away from a comet, why doesn't the comet's nucleus eventually shrink down to nothing?

Comets are small icy bodies from the outer solar system whose orbits have been perturbed so that they fly into the inner part of the solar system.

Once there, solar heating raises their temperatures so that the ices that make them up start to boil.

The ice, which could be water, ammonia, carbon dioxide, or other materials, forms a

fuzzy transient atmosphere called a coma around the comet's center. When the comet gets close

enough to the sun, the solar radiation drives the gases away from the comet forming a tail.

The blowing gases can carry off dust particles as well, so with every trip around the sun, a significant amount of a comet's mass is lost.

It's certainly possible that some comets just evaporate completely.

Some have been observed to pass behind the sun and disappear, never to be seen again. But comets can meet their ends in a variety of other interesting ways.

Stay tuned to planetary radio to find out how. Ed Weiler was at the Jet Propulsion Lab near Pasadena, California, when the Cassini-Huygen

spacecraft arrived at Saturn.

He was more than an onlooker. As NASA's associate administrator for

Space Science Enterprise, Ed represented the agency's headquarters in Washington. But he is

also a scientist, with as much reason to be excited about the

mission as the team that has worked so hard to reach the ring planet.

Ed Weiler, thanks very much for joining us on planetary radio.

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