Two alien worlds, superposed

By Phil Plait | September 22, 2010 7:00 am

Every time I think Cassini can’t possibly send back a more dramatic image from Saturn, it takes my preconceived notions and crushes them to dust.


Oh. My.

That shows the moons Titan (in the background) and Rhea, the two largest of the gazillions of moons circling the ringed planet. Titan has an atmosphere, which is why it looks fuzzy.

This picture is crazy amazing! Rhea looks like it’s about to smash into Titan — if you’re curious, this is what my nightmares look like — but in fact they’re very far apart. Rhea was about 1.1 million kilometers (680,000 miles) away from Cassini when this was taken, and Titan was more than twice that. So they were actually separated by over a million kilometers, about three times the distance of the Moon from the Earth! Cassini used the narrow-angle camera (essentially a big telescope) to get this shot, so it looks foreshortened — Titan is actually more than three times wider than Rhea, but it looks smaller than that here because it’s twice as far away. The two moons are in no danger of collision.

What a stunning shot! And I love how these two worlds are so different. Icy Rhea is pitted and cratered, but Titan’s thick atmosphere smudges out all details in visible light. Together, they’re an excellent example of diversity in the Saturn system, and a reminder of just why we sent Cassini there in the first place.

Tip o’ the spacesuit visor to Carolyn Porco. Image credit: NASA/JPL/Space Science Institute

CATEGORIZED UNDER: Pretty pictures
MORE ABOUT: Cassini, Rhea, Saturn, Titan

Comments (38)

Links to this Post

  1. The Top 14 Astronomy Pictures of 2010 | December 18, 2010
  1. Murff

    When are we sending a lander to Titan?!?!

    Beautiful picture…

  2. Messier Tidy Upper

    @^ Murff : Already done – as far back as 2005 January 14th – as one part of the Cassini mission; the Huygens lander :

    Although I’d love to see another dedicated lander mission sent there preferably with a rover component. :-)

  3. Messier Tidy Upper

    Rhea looks like it’s about to smash into Titan — if you’re curious, this is what my nightmares look like

    Why is a Titan-Rhea collision a nightmare for you BA?

    All the way out in the Saturnian system I can’t see it doing much harm. What’s the worry? Unless you’re directly involved in studying it or living there.. 😉

    An Earth-Mars or Earth-Venus collision OTOH ..!!! :-O

    Yeah that would be a nightmare. It is even one that actually came true .. something like four and a half billion years ago and resulted in the formation of our Moon via one Big Splash.

    My mind’s eye is running an animation of the next sequences on from that image when Titan and Rhea meet in a gigantic flash as I type ..

    Great image. Thankyou Cassini esp. Carolyn Porco. (& the BA) once again for sharing these superluminous pics adding that extra touch of wonder and marvel into our lives. :-)

  4. Crap! Asimov had the wrong title… it should have been Lucky Starr and the Moons of Saturn, and this would be the best cover ever.

  5. Peter Eldergill

    Something is returning…something is returning..something is returning….

  6. Nigel Depledge

    Most excellent!

    Another fantastic pic from the CAssini team.

  7. Nigel Depledge

    Murff (1) said:

    When are we sending a lander to Titan?!?!

    Erm … dude, we already did that.

    Cassini carried the Huygens lander to Titan.

    Or did you mean a rover, in the style of Spirit and Opportunity?

  8. Michel

    Simply WOW !!!! again
    Go Cassini!
    (but I still love the mars Rovers a litle bit more)

  9. I’m willing to bet Titan’s blur is also due to the spacecraft locking on to Rhea as the primary target. Titan would be drifting slightly in the background throughout the exposure.

    Way cool pic. I’ll be waiting for it to show up in next year’s MARS WILL APPEAR BIGGER THAN THE MOON email.

  10. Appallingly! Looks like a game of the universe in the balls.

  11. Jeff

    I really think if you woke up one day on Titan, other than the cold and suffocation, I really think you’d think you were on earth, maybe in Greenland, it probably is that earth like from the surface.

    The longer I live, the more morning I wake up and observe earth, the more I realize this is just a planet , albeit a special one, with all the characteristics of a planet. It took me a very long time to see it this way, our human civilization tends to block this reality out.

    I think I should have been born a Pleistocene man. I would have liked that, being born and living in raw nature without hardly any intervening human facade. I’ve morphed into a luddite, and I am getting more and more impatient with human arrogance.

  12. Missy B

    Peter, lmao. Tap-tap-tap-tap, tap-tap-tap-tap…

  13. Messier Tidy Upper

    @9. Michel Says:

    Simply WOW !!!! again Go Cassini! (but I still love the mars Rovers a litle bit more)

    Ah, for me it will always be Voyager II – all the four gas giants and the outer two for the very first time. :-)

    @2. Mike J : great minds think alike? You beat me by a minute. The BA’s links filter strikes again. LOL (what else can you do?) :-)

    @5. Korpil Says:

    Crap! Asimov had the wrong title… it should have been Lucky Starr and the Moons of Saturn,

    Nice reference but then we’d be stuck with ‘Lucky Starr and the rings of Jupiter’ which .. hmm …? (Equivocal wibbly hand gesture.) Bands of Jupiter perhaps?

    … and this would be the best cover ever.

    I thought that was Spiderbait’s cover of ‘Black Betty’ song myself! 😉

    (Wracks brain for other good cover songs.)

  14. Rachel

    Just to let you know, it’s things like this and your obvious passion for your work that inspire/re-inspire people. I’m for botany, through-and-through, but seeing your posts reminds me of the wonder I had for rest of the universe before I had settled on my own passion…and you keep me looking up at the stars. Thanks for that.

  15. kevbo

    a) are there any 3D pics like these (could be generated by taking two pictures in sequence, giving Cassini time to move to a slightly different point of view


    b) when is Ted Turner going to colourize these?

    (yes, ‘colourize’. that’s the way we spel up hear in Canada, eh?)

  16. Messier Tidy Upper

    @ ^ Kevbo : Not just in Canada either – we spell it ‘colour’ with a ‘u’ here in Oz and I gather also in the UK and the civilised world generally! 😉 😛

    [me, 13 – unless the numbers have changed again.]

    I thought that was Spiderbait’s cover of the ‘Black Betty’ song myself! (Wracks brain for other good cover songs.)

    NB. Yeah, I changed that via edit in time from the Dixie Chicks cover of REM’s Landslide, what can I say, I’m half-asleep as usual. 😉

    To see – or hear what I mean :

    A number of other versions on Youtube too. :-)

    Like this one :

    Or if you want something mucho mellower :

    For the Dixie Chicks version of Landslide which looks like its been covered by a few people .. Smashing Pumpkins & I could’ve sworn REM did it (?) too. Seems it was a Stevie Nicks / Fleetwood Mac song originally.

  17. Jeff

    Yes, phil does inspire. I was a kid growing up being inspired into science by Carl Sagan. Phil seems to be the first scientist to replace Carl in this role. Why, oh why, do scientists have to be such downers in general???? Why do people who are complete airheads, like Lindsey Lohan, have to have all the charisma?? I’d rather watch Lindsey and be completely bored than most of the scientists they interview on , say, NOVA.

    Phil has the correct combination of scientific competence (it is easy to see from his analyses of various topics that he has a strong science background), with the free-wheeling spirit to hypothesize on new things, and the ability to put it out there with good blogs and interviews. Very few scientists can do that ..

  18. Russell


    I love it when I get into the office, pour my coffee, sit down and open up BA and BAHZINGA!
    A photo like this pops up! I quickly emailed this to my 11yr old daughter at home.

    Question: #2 Mike J posted a link to the Huygens probe wiki page. It says there that the probe sent back information for 90 minutes. I wonder why so short time? What do they think happened to it. I think the locals walked up to it and turned it off….they don’t want to be probed and poked.

  19. Messier Tidy Upper

    @ ^ Russell : Hey I posted that link too! 😉

    Assuming you wanted a serious answer – it froze. The on-board batteries ran out, it cooled down, lost all power and the Huygens became a frozen artefact .. Now it sits buried under methane snows, waiting to be uncovered by the next visitors to Titan’s icy surface, whoever they’ll be.

    Hopefully those next visitors will be human too & sooner rather than later.

  20. Mike J

    Haha, hello there, Messier Tidy Upper. You know, while I fumbled for the link, I thought: man, what am I doing, someone is absolutely bound to do that too. But then, if we all decided to go “meh, screw that”, the poor guy would’ve ended without his link.

    @Russell: I’m happy to notify you that no alien interaction was recorded, but it’s also possible the aliens got the probe from behind. Also, MTU is absolutely right. Huygens is now lying as frozen as robots can be below the soupy yellow atmosphere of Titan, a tantalizing metallic artifact apparently created by some mysterious spacefaring civilization.

    Here, let me add a bit more of that “Landing On Titan Is Just As Awesome As It Sounds” mood:

    -Mike J

  21. Ross

    Wow, that’s incredible! I remember watching a Horizon documentary broadcast while
    Cassini-Huygens was still on route to Saturn, It was the most exciting thing ever for me at the time :p

  22. Michel

    @Messier Tidy Upper
    My first love was the IRAS. The dutch/uk/us Infra Red Astronomical Sattelite. It gave totally new views.
    I´ve still got the original decal on my former camera and now eyepiece case.

  23. Tribeca Mike

    The solar system’s biggest clown nose?

  24. andy

    Just shows us what we missed at Jupiter thanks to Galileo’s faulty antenna. High time to send another dedicated orbiter mission there.

    Another planet fully deserving of a dedicated mission is Neptune, doubt I’ll see that in my lifetime though. :-(

  25. Kurt Larson

    Is the reason there are no stars visible in this shot that there is so much light coming off Titan that the camera’s exposure time has to be so short that the stars don’t have time to make an impression?

  26. Jamey

    It looks to me like there’s actually two cloud layers on Titan – the really thick bottom layer, and then a top layer that kinda looks like a shell around it.

  27. Jeff

    I just did a student problem inspired by this image of Rhea based on concepts of mass versus weight. In researching this, I just stumbled on this:

    RHEA WAS DISCOVERED BY GIOVANNI CASSINI in 1672. How appropriate that the Cassini spacecraft took this.

  28. @26. Yeah, that’s pretty much it. Exposing for the stars to be visible would have made the sunlight sides of both moons appear as featureless smears of white on the image. And that’s no fun.

    @27. There ARE two cloud layers on Titan! Basically, there’s the main atmospheric cloud cover as well as an upper-level haze made up of hydrocarbons, created by the interaction between the sun’s UV radiation and the molecules in Titan’s clouds. All this makes Titan’s atmosphere even thicker than ours.

    Cassini is teh kewlest.

  29. Messier Tidy Upper

    23. Michel Says:

    @Messier Tidy Upper : My first love was the IRAS. The dutch/uk/us Infra Red Astronomical Sattelite. It gave totally new views.

    IRAS was also significant for indicating exoplanets existed showing the stars Vega, Fomalhaut and Beta Pictoris had dusty disks of material forming planets around them. Nice choice. :-)

    I recall reading about those & other IRAS discoveries in one or two of Patrick Moore’s books but it was before my time. Today we know of and have actually directly imaged Beta Pictoris b and Fomalhaut b – plus we now know that Vega has its pole (which is hotter and bluer than its equatorial region) facing us.

    For me, the 1989 fly-past of Neptune by Voyager II was inspirational and one of the things that really got me interested in astronomy in a huge way. :-)

    @21. Mike J : Good point. :-)

    @25. andy Says:

    Another planet fully deserving of a dedicated mission is Neptune, doubt I’ll see that in my lifetime though.

    Seconded by me. :-)

    Voyager II only imaged 40% of Triton for instance and there’s still so much to discover there. We need a long term orbiter watching the Neptunean seasons change .. which, of course, takes place over many decades throughout its 165 year orbit.

    We can’t hope to see all of that without some hu-uuge medical breakthroughs – but to see a lot more from Neptune close-up. Oh yes please! 😀

    I wish we were sending more spacecraft to more planets. All the gas and ice giants deserve dedicated missions if you ask me.

  30. Amazing pic!…No man made object will ever be as beautiful and majestic as to what the universe has to offer.

  31. Jeff

    Messier: “I wish we were sending more spacecraft to more planets. All the gas and ice giants deserve dedicated missions if you ask me.”

    you are so correct. It is so sad with all the money out there that is being squandered, how much more could it cost to really reconnaissance with all the Jovian planets and moons. They’ve sent dozens to Mars, why not also the Jovians, which are far more fascinating I think with their flotilla of moons, they are absolutely mind-blowing. Every time I see these new images of the outer worlds, it wows me.

  32. Gary

    Only slightly off-topic, but I had to share this. This day, September 23 2010, marks the 164th anniversary of the discovery of Neptune in 1846, and as its orbital period is also 164 years, it is one Neptune birthday since we spotted it! Happy birthday, Neptune!

  33. Ron

    Phil, I am consistently amazed by how my fellow fans of your blog are so in tune with the science, the politics, and the wonder of Bad Astronomy.

    At the same time, I am deeply saddened that it is left to me to point out your failure to make bad puns of the phrases “big telescope” and “foreshortened”.

    I assume you’re keeping those in your back pocket for wOOtstock in Boulder (October 23, Boulder Theater, tickets still available!). Or at least you’d better be!

  34. Messier Tidy Upper

    @33. Gary : Well spotted! :-)

    Wish I’d seen that earlier.

  35. Brett Glass

    Nonetheless, Ron, I think you will agree that Phil has achieved his, er, objective without focusing everything up. I wouldn’t pan his posting.

  36. Ed H.

    It’s interesting, I know that the camera being used is such that ALL items in the view should be 100% in focus. I know that Titan only looks blurry because of its atmosphere, not because it’s out of focus.

    Yet it is a completely convincing “depth of field” effect.


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