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View All Targets -- Seventh Edition

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Cassini Scientist for a Day -- Spring 2009

Cassini Scientist for a Day is a national essay contest for U.S. students in grades 5-12. Students choose one of three images the Cassini spacecraft can take on a given date and time set aside for education, and write a 500-word essay as to why they think it will yield the best science results. Winners and their classes participate in teleconferences with Cassini scientists.

Target 1: Dione

Dione has many interesting features which are perfect to be studied with Cassini's cameras. The leading side of the moon, that is, the side that faces the direction it is traveling in, has a lot of craters on it.

How do you think all those craters ended up on the front of the moon?

Think of it like putting your hand out the window of a car that is driving in the rain. Most of the raindrops are hitting the front of your hand, while the back of your hand stays mostly dry. This is just like how rocks in space would mostly be hitting the front side of the moon.

One fascinating discovery scientists made was that the back side of the moon also has lots of craters, and since Dione only faces one direction as it orbits Saturn, they think that some intense meteor impacts actually changed the way Dione is oriented.

The first images of Dione taken nearly 30 years ago revealed what appeared to be bright, wispy terrain laced throughout the back side, or trailing hemisphere. More detailed pictures from the Cassini spacecraft revealed that these were actually ice cliffs created by fractures in the surface.

Meet the winners and read the essays

Target 2: Prometheus and Saturn's Rings

Although it is a small moon, Prometheus is very interesting for a number of reasons. First of all, it is not the round shape we would expect, but is instead about twice as long as it is wide! It has ridges and valleys from impact craters, and we think that it is a very porous icy body because it seems to have a low density.

Although this moon may look small, it is what it does to the rings that is super cool. Prometheus is called a "shepherd moon" because of its interaction with the inner edge of Saturn's F Ring. It turns out that the gravitational field of Prometheus steals material from the F Ring as the moon passes, and this creates kinks and dark channels that are visible in the ringlet.

The rings and Prometheus orbit at slightly different rates so the interactions between the two are seasonal, or vary with time. These are dynamic events, and they are exciting to watch. We are actually in a period of time where Prometheus is actively transforming the rings, so we can't wait to see what happens next.

Meet the winners and read the essays

Target 3: Epimetheus and Janus

Janus and Epimetheus have a special relationship. They actually share an orbit. Normally, this bizarre situation would result in a collision between them or cause one of them to go spiraling into Saturn or crashing into the rings. However, these moons manage to share an orbit by swapping positions roughly every four years. Currently Janus orbits closer to Saturn by 50 kilometers, or 30 miles. Next year, in 2010, they will change places.

This strange couple also appears to sculpt the outer edge of the main rings. Ring particles at the outer edge of the A ring orbit Saturn seven times for each six orbits of Janus and Epimetheus. Through this resonance, they help contain Saturn's main rings. However, scientists still have questions about exactly how this works. We'd also like to understand how this orbital switch affects the wakes that Janus creates in the A ring.

Meet the winners and read the essays

Janus (left) and Epimetheus