The real Pandora, and two mooning brothers

The entire universe in blog form
May 6 2010 11:45 AM

The real Pandora, and two mooning brothers

Cassini continues making loop-de-loops around Saturn, returning tens of thousands of way cool pictures. Like this one:


Phil Plait Phil Plait

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


From 1.3 million kilometers (800,000 miles) away -- 3 times as far as the Moon is from the Earth -- Cassini spied this pretty scene. It shows, obviously, Saturn's rings to the right. The very thin ring extending to the left is the F-ring; it's very faint and wasn't even discovered until 1979, when Pioneer 11 passed the planet.

The two moons are Pandora (the flying saucer-shaped one) on the left, and Epimetheus on the right. Usually, in pictures like this, perspective is a problem; one moon is much farther away than the other, so your sense of scale gets a bit bollixed. But in this case, both moons are about the same distance from Cassini! Pandora is about 114 x 84 x 63 km (68 x 50 x 38 miles) in size, and Epimetheus is a bit heftier at about 144 x 108 x 98 km (86 x 64 x 58 miles). In this shot, the rings are in the background relative to the moons, and Pandora is just a hair closer to Cassini than Epimetheus. I was surprised that they appeared so close together, so I did some checking. Pandora orbits Saturn at a distance of about 141,700 km (85,000 miles), and Epimetheus orbits at 151,400 km (91,000 miles). So really, they never get any closer than 10,000 km (6000 miles) to each other. Since they do look pretty cozy in this image, there really is a little perspective going on, since Epimetheus must be a few thousand kilometers farther away. That's only a trifle compared to the more than 1 million km distance Cassini was from the pair when it took this shot, though. What this means is that if you compare the sizes of the two moons you get a good idea of their relative diameters, but their positions relative to Saturn are a little messed up due to perspective. Got it?

Once I got the orbital distance of the moons, I was curious how long it takes them to orbit Saturn. Turns out, it only takes about 15 hours for Pandora and about 17 hours for Epimetheus! So both moons are screaming around the planet at speeds of roughly 60,000 kph (36,000 mph), far faster than even low Earth satellites move. That's because Saturn is a lot more massive than the Earth, and has far more gravity. It yanks much harder on those moons, whipping them around at greater speeds.

There's more, too. Pandora is one of the shepherd moons of the F-ring, helping it maintain its shape. It shares an orbit (more or less) with the moon Prometheus. As it happens, Epimetheus shares an orbit (more or less) with the moon Janus.

Now follow along here: in mythology, Epimetheus and Prometheus were very close brothers. Their names means hindsight and foresight, respectively. Prometheus gave us fire and civilization, and had his liver pecked out by birds every night for his sins against the gods. Epimetheus was supposed to give mankind positive traits, but was a bit of an absent-minded goofball, and he ran out of raw materials before he got to us. For this, the gods gave him the "gift" of Pandora, whom he married.

Well, that's not fair! Being smart and clever and helpful gets your organs ripped out of you, and being an idiot with no eye for the future gets you rewarded*.

Of course, in reality, Epimetheus's wife shares an orbit with his brother. That's gotta hurt.

Sigh. I think I prefer the confusion of the actual Prometheus and Epimetheus to the confusion of their mythical namesakes. Reality may not always be fair, but at least (to borrow a phrase from George Hrab) it's fair in its unfairness.

* Feel free to extrapolate this myth to science and politics, if you wish.

Related posts:



The Democrats’ War at Home

How can the president’s party defend itself from the president’s foreign policy blunders?

An Iranian Woman Was Sentenced to Death for Killing Her Alleged Rapist. Can Activists Save Her?

Piper Kerman on Why She Dressed Like a Hitchcock Heroine for Her Prison Sentencing

Windows 8 Was So Bad That Microsoft Will Skip Straight to Windows 10

Homeland Is Good Again! For Now.


Cringing. Ducking. Mumbling.

How GOP candidates react whenever someone brings up reproductive rights or gay marriage.

Building a Better Workplace

You Deserve a Pre-cation

The smartest job perk you’ve never heard of.

The U.S. Has a New Problem in Syria: The Moderate Rebels Feel Like We’ve Betrayed Them

The Ludicrous Claims Women Are Pitched at “Egg Freezing Parties”

Oct. 1 2014 11:48 AM An Up-Close Look at the U.S.–Mexico Border
  News & Politics
The World
Oct. 1 2014 12:20 PM Don’t Expect Hong Kong’s Protests to Spread to the Mainland
Oct. 1 2014 2:16 PM Wall Street Tackles Chat Services, Shies Away From Diversity Issues 
Oct. 1 2014 11:59 AM Ask a Homo: A Lesbian PDA FAQ
  Double X
The XX Factor
Oct. 1 2014 2:08 PM We Need to Talk: Terrible Name, Good Show
  Slate Plus
Political Gabfest
Oct. 1 2014 1:53 PM Slate Superfest East How to get your tickets before anyone else.
Brow Beat
Oct. 1 2014 2:24 PM The New Interstellar Trailer Is the Most Exciting Yet
Future Tense
Oct. 1 2014 2:26 PM The Apple Graveyard Leave a flower for a dead Apple product.
  Health & Science
Bad Astronomy
Oct. 1 2014 12:01 PM Rocky Snow
Sports Nut
Sept. 30 2014 5:54 PM Goodbye, Tough Guy It’s time for Michigan to fire its toughness-obsessed coach, Brady Hoke.