Astronomy Picture of the Day 07-02-04

Explanation: Yesterday, Cassini became the first spacecraft to enter orbit around gas giant Saturn, rocketing through a 25,000 kilometer wide gap in the distant planet's magnificent system of icy rings at about 15 kilometers per second. Turning to snap pictures, Cassini's narrow angle camera recorded this stunning close-up of a much smaller gap in the rings, the Encke Gap. A mere 300 kilometers wide, the Encke Gap is flanked by amazing structures within the rings -- scalloped edges and patterns of density waves are clear in the sharp image. While the rings of Saturn are likely debris from the breakup of a fair-sized icy moon, the Encke Gap itself is created by the repeated passage of a tiny moon. Only 20 kilometers wide that tiny moon, Pan, was also detected by Cassini's camera as the spacecraft approached the Saturnian system.

There are lots of new images of Saturn's rings, and more to come. Here is the LINK to them. Enjoy!

First pictures from Saturn orbit show rich ring detail
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: July 1, 2004

The first batch of photographs snapped by the Cassini Saturn orbiter earlier today reached the Jet Propulsion Laboratory around 8:30 a.m., zoomed-in shots of the planet's myriad rings showing a ghostly tapestry of icy, back-lit particles arrayed in sharply defined bands.

One of the images taken by Cassini from orbit of Saturn shows a close-up view of the planet's rings. Credit: NASA/JPL

Much brighter shots showing the rings from the sunlit side were expected to reach Earth later this morning, but scientists were elated at the initial results.

"Look at that structure, it's so regular!" marveled imaging team leader Carolyn Porco as a picture came in showing well-defined bands of brightness and darkness. "I'm wondering if we're looking at a density wave. This looks like it might be a density wave, but I'm not quite sure."

Density waves, caused by gravitational interactions with nearby moons, are thought to be "kissing cousins" of the waves that produce the spiral structure seen in galaxies like Earth's Milky Way.

"These are regions where the rings are communicating gravitationally with the moons exterior to them," Porco explained.

One of the objectives of Cassini's ring research is to study density waves in unprecedented detail and based on the first set of images, scientists will not be disappointed.

"With these kinds of images and with the data we're going to return from Cassini with stellar occulation observations, radio occulation observations, we are going to nail density waves, we are going to understand these critters," Porco said. "This is really a new era in the study of outer planet systems."

A few moments later: "There goes another one, which is mind blowing, absolutely mind blowing," Porco exclaimed. "Look at thatOoh... It's almost everywhere you look here, you can't miss one. They're just all over the place."

A few moments later: "Oh my God, look at that! ... These density waves are like books, just waiting to be read."

But this morning, as raw, unprocessed images flowed in, science wasn't the immediate objective. It was enough just to know Cassini's camera and other systems had worked as planned during close approach to Saturn.

The photo sequence began around 12:30 a.m., 18 minutes or so after Cassini finished a 96-minute rocket firing to brake into orbit around Saturn. Streaking just above the rings at speeds greater than 50,000 mph, Cassini's narrow-angle camera took a series of snapshots, opening its shutter for just five milliseconds per picture to avoid blurring.

One of the images taken by Cassini from orbit of Saturn shows a close-up view of the planet's rings. Credit: NASA/JPL

Each picture was separated from those on either side by about 600 miles because of Cassini's extreme velocity.

"We couldn't take a contiguous ring scan with images overlapping other images because we are speeding across the rings very fast," Porco said. "It takes us about a minute to take a picture and so in the time we shutter the exposure, read out the camera and get ready to take a picture again, we have crossed a thousand kilometers. Our field of view is only about, let's say, 100 to 200 kilometers. So never do we have overlapping images. Never will we be able to put this all together in a nice mosaic."

One of the world's leading ring experts, even Porco was surprised by the level of detail apparent in the first unprocessed pictures.

"I shouldn't be, I suppose, but I am surprised," she reflected. "You can think about this like we have done for 14 years and you know, well, we'll get density waves there and we'll take pictures. But it's remarkable to me at how startling it is to see these images for the first time. ... They're just beautiful, they're very sharp."

One picture that came in about an hour after the first image was received was especially intriguing, showing a density wave on the left with narrower and narrower bands of light and dark and a so-called bending wave on the right.

"Oh my God! This is, oh, this is really exciting!" Porco exclaimed. "If you look, the pattern now is decreasing to the left and that is the mark of a bending wave. ... And a bending wave, it's not the density of the particles that is being moderated or modified as you move across the rings, but it is the height of the ring plane. The darkness is created by shadows. Look at that! It's just a beautiful pair."

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