Cassini Looks at Saturn’s Rings, Gets a Bonus Planet

The entire universe in blog form
May 5 2014 12:16 PM

Cassini Spies Another Pale Blue Dot

Uranus and Saturn
The long view: Cassini looks past Saturn's rings to see Uranus, billions of kilometers away.

Photo by NASA/JPL-Caltech/SSI

Speaking of Saturn’s rings, on April 11, 2014, Cassini was taking a snapshot of Saturn’s thin outer F ring when it got a bonus: Uranus!

The picture above shows the seventh planet from the Sun as a small, pale blue dot (hmmmm, that sounds familiar). Uranus is actually a big planet, four times wider than Earth, but it was a staggering 4.3 billion kilometers (2.7 billion miles) from Saturn when this shot was taken, nearly on the opposite side of the Sun. It’s amazing Cassini saw anything at all (though, to be honest, in the image Uranus was brightened by a factor of 4.5 to make it more obvious, and the A ring—the broad ring at the lower right—was also brightened for clarity). Uranus was just barely more than a dot, even to Cassini’s narrow-angle camera.

Zooming in on Uranus.

Photo by NASA/JPL-Caltech/SSI


Uranus is generally visible from Earth using binoculars; it’s just on the edge of naked-eye visibility. Right now it’s an early-morning object near Venus in the predawn sky. Seeing it through even a small telescope is a bit shocking; it’s clearly a disk, not a point-like dot like a star, and a very distinctive blue-green. The upper atmosphere of the planet has a lot of methane which is excellent at absorbing red light. Sunlight that hits Uranus gets its red removed, leaving the blues and greens to reflect off and travel their way back to us.

With bigger telescopes, Uranus has another surprise: rings! You can’t see them in the Cassini shot—they’re far too faint—but they’re there. So we’re looking through the rings of one planet to spy on another that has rings as well. The solar system is a weird place.

Another funny thing that struck me in this image is how well-behaved the F ring looks! Usually it’s a mess, but it depends on what part you’re looking at. The moons Prometheus and Pandora can stir things up, for example, and sometimes an impact with a meteoroid can create havoc as well. It’s actually rather odd to see the F ring so well-groomed here (although, if you look carefully, you can see some twists and changes in it as your eye seeps around it).

It’s fun to see images like this, analyze them, and see what surprises lie therein. You might see an entirely other world … or you might have to just be satisfied with seeing the splendiferous glory of Saturn’s rings dancing to the tune of Newtonian gravity. I’ll take it.

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  


Sports Nut

Grandmaster Clash

One of the most amazing feats in chess history just happened, and no one noticed.

The Extraordinary Amicus Brief That Attempts to Explain the Wu-Tang Clan to the Supreme Court Justices

Amazon Is Officially a Gadget Company. Here Are Its Six New Devices.

Do the Celebrities Whose Nude Photos Were Stolen Have a Case Against Apple?

The NFL Explains How It Sees “the Role of the Female”

Future Tense

Amazon Is Now a Gadget Company


How to Order Chinese Food

First, stop thinking of it as “Chinese food.”

Scotland Is Inspiring Secessionists Across America

The Country Where Women Aren’t Allowed to Work Once They’re 36 Weeks’ Pregnant

The XX Factor
Sept. 18 2014 11:40 AM The Country Where Women Aren’t Allowed to Work Once They’re 36 Weeks’ Pregnant
Sept. 17 2014 5:10 PM The Most Awkward Scenario in Which a Man Can Hold a Door for a Woman
  News & Politics
Sept. 18 2014 3:19 PM In Defense of Congress Leaving Town Without a New War Vote
Business Insider
Sept. 18 2014 3:31 PM What Europe Would Look Like If All the Separatist Movements Got Their Way
Sept. 18 2014 3:24 PM Symantec Removes Its “Sexual Orientation” Filter
  Double X
The XX Factor
Sept. 18 2014 3:30 PM How Crisis Pregnancy Centers Trick Women
  Slate Plus
Behind the Scenes
Sept. 18 2014 1:23 PM “It’s Not Every Day That You Can Beat the World Champion” An exclusive interview with chess grandmaster Fabiano Caruana.
Sept. 18 2014 4:00 PM When The Cosby Show Got “Very Special” Why were The Cosby Show’s Very Special Episodes so much better than every other ’80s sitcom’s?
Future Tense
Sept. 18 2014 2:39 PM Here's How to Keep Apple From Sharing Your iPhone Data With the Police
  Health & Science
Sept. 18 2014 3:35 PM Do People Still Die of Rabies? And how do you know if an animal is rabid?
Sports Nut
Sept. 18 2014 11:42 AM Grandmaster Clash One of the most amazing feats in chess history just happened, and no one noticed.