The bringer of fire, hiding in the rings

By Phil Plait | June 23, 2010 12:04 pm

After yesterday’s depressing picture, how about one that will make you smile?

The ever-amazing Cassini spacecraft sent back this pretty nifty shot of Saturn’s icy moon Rhea playing peekaboo in the rings:

cassini_rhea_prometheus

Beautiful, isn’t it? You can see that Rhea was on the other side of the rings from Cassini when this image was taken, and that the spacecraft was almost, but not quite, in the plane of the rings, too.

But there’s more to this shot… Take a closer look. What’s that, hiding in a gap in the rings, apparently hovering over Rhea’s terminator (the line dividing day and night)?

cassini_rhea_prometheus2Surprise! It’s Prometheus, a tiny potato orbiting the planet much closer in. It’s far smaller than Rhea, only about 120 km (75 miles) long versus Rhea’s 1530 km (950 miles) diameter. Rhea is Saturn’s second largest moon — only Titan is bigger — and one of the ten biggest moons in the entire solar system. Prometheus, on the other hand, is so small it wasn’t even discovered until the Voyager 1 probe spotted it in 1980.

Nice. And I’m sure there’s science galore to be extracted from this image, but sometimes I think pictures like this will have a more lasting impact because they are simply so amazingly cool.

Tip o’ the F Ring to CICLOPS imaging team leader Carolyn Porco. Credit: NASA/JPL/Space Science Institute.


Related posts:

- Cassini: 10 years and counting
- Dr. Tongue’s 3D House of Prometheus
- The real Pandora, and two mooning brothers


CATEGORIZED UNDER: Astronomy, Pretty pictures

Comments (16)

  1. Pi-needles

    I’d never have spotted the smaller moon without that arrow I must admit. Unless you look closely it looks like its part of Rhea or the rings. ;-)

    Cool photo – as always from Cassini – surely the mission that has returned the most & best photographs ever! 8)

  2. I spotted the irregularity that is Prometheus in the first picture, but at first I thought you were going to tell us that there was some lensing going on in the “empty” part of the rings that was distorting or magnifying Rhea’s terminator.

    But anyway, I like potatoes.

  3. Can we move there? Its starting to look cleaner and more inviting than here.

  4. XPT

    I’d be happy to leave on a one-way mission to the Saturn system!

  5. TKoz

    That’s no moon. That’s a space station!

  6. ByJove

    I trust the space science community has gotten the message “Pictures are good! People like pictures! Pictures get us funding!” And will therefore have at least a modest camera on virtually everything launched from now on, even if the primary mission doesn’t really need it.

  7. Where’s the giant eagle ready to rip out that moon’s liver?

    Uh-oh. Being greco-geeky again.

  8. Mapnut

    Are we sure Cassini didn’t stop somewhere along the way and pick up Ansel Adams?

  9. Must be fake! I mean where are the stars eh? :P

  10. Radwaste

    “Can we move there? Its starting to look cleaner and more inviting than here.”

    Silly! Don’t you remember the sea of hydrocarbons in the Gulf of Mex– I mean, Titan?

  11. andy

    Talking of Rhea, have there been any more results about its possible ring system? IIRC, apart from the magnetometer measurements that suggested the existence of a ring system in the first place, there is evidence in the form a band of ultraviolet-bright material along Rhea’s equator that may be the result of ring material colliding with the moon. Anything else?

  12. I'd rather be fishin'

    Bloody typical government conspiracy! As soon as I finish teaching my course on physics and astronomy, NASA comes out with all the FREAKIN’ NEAT photos. TANJ!!

  13. Pi-needles

    @3. Non-Believer Says:

    Can we move there? Its starting to look cleaner and more inviting than here.

    Yes but it’s a lot colder and there’s just no atmosphere! ;-)

    @ 5. TKoz Says:

    That’s no moon. That’s a space station!

    No, right first time, it *is* just a moon. We think! If not its very well disguised. ;-)

  14. Messier Tidy Upper

    @ 11. andy Says:

    Talking of Rhea, have there been any more results about its possible ring system? IIRC, apart from the magnetometer measurements that suggested the existence of a ring system in the first place, there is evidence in the form a band of ultraviolet-bright material along Rhea’s equator that may be the result of ring material colliding with the moon. Anything else?

    Good question – but one I’m afraid I can’t really answer other than saying I haven’t heard any more about it.

    I second you asking it though & hope to find out the answer myself! Have any follow up studies – or better still images – of Rhea’s rings being taken or are any planned to be?

    @ 6. ByJove Says:

    I trust the space science community has gotten the message “Pictures are good! People like pictures! Pictures get us funding!” And will therefore have at least a modest camera on virtually everything launched from now on, even if the primary mission doesn’t really need it.

    Yes indeed. I think there’ve been a few missed opportunities in this regard with probes that should’ve had cameras on them but didn’t. For example, the long running and remarkable if unheralded Ulysses mission which got a very different angle on the solar system and our Sun especially its polar regions and also the capsule Galileo dropped into Jupiter. I’d have loved to seen what a plunge into that largest of our systems planets really looks like & we could’ve seen that back in the 1990′s. What could’ve been. Sigh. :-(

  15. Messier Tidy Upper

    For more on the Ulysses mission which was launched 20 years ago – on the 6th Oct. 1990 from the Shuttle Discovery – and explored the Sun’s poles looping around them three times & also flew through the tails of comets Hyakutake & McNaught-Hartley see :

    http://en.wikipedia.org/wiki/Ulysses_(spacecraft)

    & for more on the Jupiter atmospheric entry capsule which accompanied and formed part of the Galileo spacecraft which entered the Jovian atmosphere in December 1995 and was :

    “…by far the most difficult atmospheric entry ever attempted; the probe had to withstand 230 G and the probe’s 152 kg heat shield [which] made up almost half of the probe’s total mass, … lost 80 kg during the entry.”

    See :

    http://en.wikipedia.org/wiki/Galileo_(spacecraft)#Galileo.27s_atmospheric_entry_probe

    which is also my source for the above quote.

  16. I’ve long been interested in Saturn and his moons. I propose to discuss the anomalies of Rhea’s surface. Here is some facts on the site:
    http://aleksey-galan.blogspot.com/2010/06/anomalies-in-surface-of-rhea-saturn.html

    I’ll be glad to new ideas and explanations, thank you!

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