This Week’s Where In The Universe Challenge


It’s time once again for another Where In The Universe Challenge. Test your visual knowledge of the cosmos by naming where in the Universe this image was taken and give yourself extra points if you can name the spacecraft responsible for this picture. Post your guesses in the comments section, and check back later at this same post to find the answer. To make this challenge fun for everyone, please don’t include links or extensive explanations with your answer. Good luck!

UPDATE: The answer has now been posted below — don’t peek if you haven’t guessed yet!

This is Saturn’s small moon Janus, taken by the Cassini spacecraft. Here, Janus is illuminated by light from both the sun and Saturn.

This view looks toward the south pole of Janus (179 kilometers, or 111 miles across) which lies on the terminator just below the center of the image. Brightly lit terrain seen on the right is on the leading hemisphere of Janus. Light reflected off Saturn dimly lights the Saturn-facing side of Janus on the top left of the image.

Check out the Cassini website for more information about this image.

Check back next week for another WITU challenge!

32 Replies to “This Week’s Where In The Universe Challenge”

  1. Hm… small, irregular body, planetshine…

    It can be a few different moons of Jupiter or Saturn. I’m guessing a moon of Saturn here, though, which would mean Cassini, and the moon… Prometheus?

  2. An asteroid? A minor moon of Jupiter or Saturn (or, possibly, Uranus)? Perhaps even Deimos or Phobos??

  3. I’m working with some HiRISE images of Phobos & Deimos for my summer internship, and my first instinct was that it was Phobos. It’s pretty low resolution, so throw out HiRISE, and probably the Viking Orbiters as well. I doubt that it’s MGS…
    I’ll go along with Bill928 – Voyager 🙂

  4. Quite sure raulm is right, and beat me to it. 99.99% sure it’s Janus, as taken by Cassini.

  5. Lots of different opinions here. Dactyl looks very different to me, and Phobos looks more “lumpy” – for me, it’s a toss-up between Janus and Wild 2… and I go with … Janus, by Cassini, which is one of my “favourite spacecraft”.
    I didn’t actually toss – I seem to see light spread over much more than 180 degrees, with a darker zone in the generally lit area, i.e. probably two sources, possibly Saturn and the Sun.

  6. Seriously, though…

    The moon is lit from both sides. It is getting a lot of light from the right (presumably the sun), and about 10% of the light from the left and about 10 degrees up, assuming the camera gamma is somthing like my display. This leaves us with two options…

    If the light from the left is sunlight, bouncing off a planet, then the planet has got to look huge as seen from the moon. To intercept 10% of the light, the planet would have to take up a third of the sky as seen from the moon. I don’t think it is quite that close because you can see a distinct penumbra at about 7 o-clock, which means the illuminating body can’t be that big. Okay – maybe someone has tweaked the image gamma to make it look more arty, and it is in fact getting about 1% – it is still going to be pretty close in.

    The other option is that the body on the left is self-luminous. This is not absurd if you are talking about Jupiter and the picture is taken in the deep IR.

    This one’s fun. If we knew the tone curve of the camera, we could probably figure it out just form the shadows. Nice!

  7. It’s that moon of Saturn that’s half black and half white. Can’t remember what it’s called. (Memory’s the 2nd thing to go….). Triton?

  8. Saturn’s moon Janus, as taken by Cassini.

    Iapetus – no, Iapetus is almost round.

    It absolutely can’t be a Mars moon as taken by Voyager because Voyager didn’t fly by Mars.

  9. For Richard Kirk:

    Two Light Sources would indicate to me the sun and a large nearby planet (via reflected Sun light), Jupiter or Saturn, as been the light sources.

  10. I am now convinced that it is a picture of Dactyl orbiting about 90 kilometers above Ida and the spacecraft is Galileo. The proximity to both the sun and Ida explains both light sources and Dactyl is unusually round for a small object with a large crater.

  11. @ toseek

    Right, I had forgotten that NASA, in their infinite wisdom cancelled the Voyager Mars program, renaming it Viking several years later. I guess this is all of little consequence since I had the moon wrong anyway!

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