His art and engineering are taking us to Saturn.

Charlie Colhays is on planetary radio. called Cassini is closing in on Saturn after more than six years in space.

Today we'll talk with the man who was the mission's designer.

Bruce Betts will give us a solar system's special event update in What's Up, along with his latest trivia

contest.

First though, let's go to Emily, who reminds us that, like people, some planets are

denser than others.

I'll be right back with Charlie Colhays.

Hi, I'm Emily Lachto-walla with questions and answers. A listener asked, I read that Jupiter has an earth-sized core of ice and rock, but what

evidence do we have for thinking that Jupiter is mostly atmosphere? How do we know it's not really

a giant planet of rock?

It's true that the interior of Jupiter, and indeed all of the rest of the planets and moons in the solar system,

is not directly visible to our science instruments,

but just a few measurements can yield a ton of clues to the mysterious interiors of the planets.

The first important fact scientists learned about planetary interiors was their masses.

Isaac Newton was able to estimate the masses of the known planets by combining his insights

into forces in motion with Johannes Kepler's laws about the motions of the planets. He proved that the gravitational forces

acting between the Sun and the planets are directly proportional to the masses of

the planets. From calculations like these you can learn that Venus has just about the same mass as Earth,

but Jupiter is over 300 times as big, but this technique doesn't tell us what the mass is made of.

To learn more, stay tuned to planetary radio.

Jolly Colhays joined the Cassini team 25 years ago and was soon balancing

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