Peeking past Rhea

By Phil Plait | February 28, 2011 10:00 am

If I had to pick a single word to describe the system of moons swarming around Saturn as seen by Cassini, it would be "bizarre", "amazing", "exquisite", "jaw-dropping", and "Holy Haleakala!"

See for yourself:

[Click to enchronosenate.]

Wow! I love these shots showing perspective! The moon at the top is Rhea, which is about 1500 km (950 miles) across. We’re looking past its south pole here. The moon farther away is Dione, which is 1100 km (700 miles) in size. And since Cassini was very nearly in the plane of Saturn’s equator, the rings are nearly edge-on. Note that Dione is on the other side of the rings as seen by Cassini, so the bottom of the moon is obscured by the rings. We can’t see Saturn itself, but it’s off to the left in this shot.

Rhea is only a little bigger than Dione, but is a lot closer in this shot: 61,000 km versus 924,000 for Dione! That’s why Dione looks so much smaller. As seen by Cassini in this shot, it’s actually more than twice as far as our Moon is from the Earth. Both moons are composed of mostly water ice, with some rock. Both have been heavily battered by impacts, as you can see.

What a gorgeous, spectacular picture. I never get tired of these.

Image credit: NASA/JPL/Space Science Institute

Tip o’ the rings to Carolyn Porco on Twitter.

Related posts:

The more distant moon
Dione and Rhea, sitting in a tree
Two alien worlds, superposed
Midnight on a ringed world

CATEGORIZED UNDER: Astronomy, Pretty pictures

Comments (35)

  1. DerKlempner

    Thanks, Phil. Pictures like this are the main reason I read your blog!

  2. Beau

    Wow, this is an absolutley amazing picture!

  3. Kyle

    OK I love that picture, heck I love most all of Cassini’s pictures.

  4. Joe R.

    Not just obscured by the rings, but it looks like there is even some ring shadow being cast on that moon. Really cool.

  5. fmobus

    THAT’S NO MOON! THAT’S A-ctually two moons and the rings of saturn!

  6. Cassini is truly an historic mission. These images and the science will provide joy and education for generations to come!

  7. Carey

    @4 Joe R.

    That’s probably not a shadow, since it doesn’t follow the curvature of the southern hemisphere of Dione, is too sharp given the distance to Dione from the camera, and is probably too narrow, given the current season on Saturn (not sure about that last part though).

    It’s pretty likely a ring of dark material that is only seen in silhouette against the bright background of Dione.

    Oh, just noticed: If you click on the image to get the enlarged version, you can see that dark band extending away on either side of Dione, so definitely not a shadow.

  8. Chris


    From the slopes of Haleakala….. many thanks for the plug!


    Chris from Maui

  9. Murff

    Wasn’t it Cassini that they gathered thousands of signatures and digitally put them on board? Just struck me that I couldn’t remember.

    Great photo though!

  10. Blizno

    I am sitting with mouth agape at the beauty of the scene.
    Wonderful. Wonderful.

  11. pete

    Just woke up to such a beautiful photo. Cassini mission is such a wonderful project. My word for this photo is breathtaking

  12. KaneHau

    Aloha Chris (#8)… from the observatories on the Big Island of Hawai’i :)

  13. WOMBAT

    There’s no stars visible in this picture.
    Thus, obviously faked on a Hollywood stage.
    Thus, the moon landings never happened.


    The hazy ring occultation is amazing. We need more pictures of things seen *rhymesigh* through rings.

  14. Messier Tidy Upper

    What a gorgeous, spectacular picture. I never get tired of these.

    Same here. Love these Cassini images. Superluminous. :-)

  15. Theron

    Wombat, you are on to something. Why, all those Hubble shots are just back lit pieces of cloth with pinholes in them. Get Bill O’Reilly on the case!

  16. Sawdust Sam

    Could that dark line be the F ring, dense enough at this angle to appear solid? It’s also visible – in the enlarged picture – in the foreground below the other rings.

    We need Someone Who Knows.

  17. Chief

    Reminds me of the charts for planetary density, ie lighter element body will float on “water” (assuming body of water found that is large enough). This is another reason why we need to get out there. No matter how familiar the planets seem, there is always some thing more interesting out there if viewed in a different way.

    I really need to replace the 10″ scope I had access to when growing up.

  18. Sam H

    Exactly what is Phil’s fascination with Haleakala, Maui? Great stargazing site? Fascination with Volcanoes? Personal attachment to Hawaii in general? Hmmm…

  19. I don’t intend to steal Phil’s thunder, but I bet a lot of readers would be interested in the sourceblog for these Cassini images:

    I have it in my google reader also. I actually really enjoy seeing the images from the Ciclops page and then seeing which ones Phil will comment on.

    Ad astra,
    Ben H.
    Houston, TX

  20. Grand Lunar

    Cassini will surely (or at least ought to be) remembered as one of the most photogenic planetary missions!

    Now we have to see if it can spot the Eye of Iapetus!*

    *kudos to those that know the reference. I’m sure Phil does!

  21. Correctamundo, Sawdust! That is the F ring!
    Now can you BA Blogees see the third moon here?
    No BS, there’s 3 moons here.

  22. mfumbesi

    [QUOTE] We can’t see Saturn itself, but….[/QUOTE]
    I take exception to that statement, it should be HER, as in “We can’t see the lovely lady Saturn”.
    As you were.
    Gorgeous pics by the way.

  23. Regner Trampedach

    The Astronomy Bum @ 23: I believe it must be Prometheus, the inner shepherd of the F ring, that can be seen on the near side of the rings (but just inside of the F ring) at about the 4:20 position on Dione. It could also be Pandora, but Prometheus is more elongated, so may bet stays on Prometheus. Pretty neat picture, to understate just a wee bit.
    Cheers, Regner

  24. Regner wins! (What does he win?) Bragging rights forever!

  25. Nigel Depledge

    Once again, Cassini delivers the Awesome!

    Well done (again) to the whole Cassini team!

  26. Nigel Depledge

    @ Grand Lunar (22) –
    My Google-fu tells me it is full of stars.

  27. James Pap

    Gentlemen,sorry if this is a dumb question,but could someone explain why Dione being smaller and further away(from Cassini’s POV) than our Moon is to us,looks so big in this picture?

    In other words,Luna is almost 3 times bigger and 2 times closer to Earth but doesn’t look this big.Why?

  28. Tom F.

    Because Cassini’s narrow-angle camera is essentially a telescope with a CCD camera hooked to it. It gives a magnified view compared to what your eye would see.

  29. The next moon out from Rhea really should have been named Ghonnor, what fun that would have been. What not funny? I bet you all laughed at Uranus.

  30. James Pap

    Thanks for the answer Tom F. ,even though after reading it , i feel dumb and ashamed.Cheers.

  31. Jenn

    I will never ever get tired of the beautiful photos Cassini keeps sending us. It’s sad knowing it’ll only last until 2017.


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