Help Shape the Future of Space Exploration

Join The Planetary Society Now  arrow.png

Join our eNewsletter for updates & action alerts

    Please leave this field empty

Headshot of Emily Lakdawalla

Planetary Society Advent Calendar for December 5: Epimetheus

Posted by Emily Lakdawalla

05-12-2009 9:17 CST


Epimetheus is one of the many small moons of Saturn that are referred to by the Cassini mission team as "rocks" though they are probably mostly made of ice, not rock. I think that the name "rock" comes from their lumpy shape, like a rock you might pick up on the ground. That lumpy shape arises from their small size; they do not have strong enough self-gravity to overcome the strength of the stuff they're composed from, so when a chance encounter with another thing in orbit around Saturn bangs a huge hole into one of them, the hole stays. However, the "rocks" do have enough gravity that some dust does manage to stick to them, giving them an oddly smooth surface in places.


NASA / JPL / SSI / color composite by Gordan Ugarkovic

Cassini flew within 40,000 kilometers of Saturn's moon Epimetheus to capture the images necessary to compose this natural-color view on December 3, 2007. Epimetheus is 135 by 108 by 105 kilometers in diameter and shares an orbit with the slightly larger Janus.
At 135 by 108 by 105 kilometers in diameter, Epimetheus is the second biggest of the rocks. The biggest is Janus. Epimetheus and Janus orbit Saturn at pretty much, but not quite the same distance. The very slight difference -- just a few kilometers -- in their orbital distance from Saturn means that one of them orbits very slightly faster than the other. Every two years, they approach relatively close to each other in their orbits. But they never meet. They don't crash, they don't even do-si-do; in square dance terms, it's more of a "bow to your partner." Once they get within six degrees of each other, they start exchanging momentum, the inner (faster) moon slows and its orbit expands, while the outer (slower) moon speeds up and its orbit shrinks. They exchange positions relative to Saturn, and start moving apart, to approach again two years later. For the last two years, Epimetheus has been the farther of the two from Saturn; they're now starting their interaction period, and they'll exchange orbital positions in the middle of January.

Go to the blog homepage to open the most recent door in the planetary advent calendar!

See other posts from December 2009


Or read more blog entries about:


Leave a Comment:

You must be logged in to submit a comment. Log in now.
Facebook Twitter Email RSS AddThis

Blog Search


Advocate for Space!

Fifteen years ago, Society members and passionate space advocates like you helped save the Pluto mission. Now we can do the same for missions to Europa and Mars.

Join over 27,600 people who have completed their petition and consider a donation to support advocacy efforts.

Sign Our Petition

Featured Images

LightSail-B on the bench
Blue Origin New Shepard after first landing
New Shepard test flight and booster landing
Suni Williams and Doug Hurley in Crew Dragon
More Images

Featured Video

MISSIONS: Dawn In The Asteroid Belt With Marc Rayman

Watch Now

Space in Images

Pretty pictures and
awe-inspiring science.

See More

Selfies to Space!

Take flight with a selfie on LightSail™ in 2016!

Send a Selfie Now

Connect With Us

Facebook! Twitter! Google+ and more…
Continue the conversation with our online community!