21 April 2009

Posted by Ryan Anderson

I got a bunch of questions about the BigPicture feature on the Cassini extended mission from an “enthusiastic” commenter, with whom I happen to be related (Hi mom!), and I thought I would dedicate a post to answering them.

1. How does a Jovian equinox work? Start by reviewing how one on earth works.

Well, the pictures are of Saturn, not Jupiter, but that doesn’t really matter since equinoxes work the same on all planets. An equinox occurs when, like the word implies, the night and day are of equal length. On Earth, this occurs on ~March 21 and ~September 22. During an equinox, the tilt of a planet on its axis is oriented parallel to the planet’s direction of motion. It’s a lot easier to show with a picture:

2. #5 Photo What are we seeing here? Rhea in front of Titan in front of the Sun?

Yeah, pretty much. Except that Titan is probably not exactly in front of the sun. I suspect the sun is off-screen, but the geometry is still essentially that Titan is between the sun and the spacecraft. Small particles like the ones in Titan’s hazy atmosphere tend to scatter light forward a lot more effectively than they scatter it back toward the light source, so that’s why Titan’s atmosphere looks so bright (plus the camera exposure, of course).

3. How many moons does this planet have, anyway?

Saturn has 61 moons. But that’s sort of an artificial number, because where do you draw the line between a moon and a large ring particle? Anyway, you can read all about Saturn’s satellites on Wikipedia.

4. How long does it take for info from Casini to come to Earth? Is it just a matter of a few minutes? A week?

Saturn’s orbit is 9.6 times as far from the sun as the earth, so that means that Saturn ranges from 8.6 to 10.6 astronomical units away from Earth (an astronomical unit is the distance from the earth to the sun). It takes light about 8 minutes to travel one AU. So, a radio signal from Cassini takes 1.2-1.5 hours to get back to earth.

5. What is a “shepard moon”? Why is Prometheus one?

A shepherd moon is a moon that orbits near a ring and whose gravity causes the ring to maintain a nice, sharp edge. Normally, rings want to spread out as the individual particles bump into each other and change their orbits slightly. If they were drifting toward the planet, that would mean that they were losing angular momentum. But then along comes the shepherd moon, on an orbit slightly closer to the planet. That means it is orbiting faster than the ring particles, so its gravity tends to try to drag them along with it. But by doing so, it gives them more angular momentum, so their orbit expands and ends up back where it began. The opposite happens for particles whose orbits are expanding when a more distant shepherd moon comes by. You can read more about this here, but sadly I’m not finding many good explanations of the phenomenon. Apparently there is an Enya album called “Shepherd Moons” though…

6. What point of view makes and eclipse? See Photo #10 and explain this to us.

An eclipse occurs when the shadow of one object falls on another object. So, a solar eclipse on Earth happens when the moon’s shadow falls on the Earth. A lunar eclipse occurs when the Earth’s shadow falls on the moon. In photo #10, the caption neglects to explain how Enceladus can be visible if it is in Saturn’s shadow. But remember what I said earlier about small particles scattering light forward? That applies to Saturn’s rings too. So, in the image, Enceladus is illuminated by Saturn’s glowing rings. Here’s an absolutely spectacular view of Saturn and its rings taken by Cassini a couple of years ago: