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Emily LakdawallaAugust 9, 2010

How to Recognize Titan from Quite a Long Way Away

You know, I could fill this blog almost entirely with the amazing images that Gordan Ugarkovic locates, processes into prettiness, and uploads to his Flickr account. Here's an awesome Hubble shot he found of Saturn, taken during the period last year when the geometry of Earth and Saturn lined up to permit us Earthlings to see (through telescopes of course) Saturn's moons passing across the face of the planet. Four moons are visible here: great big Titan, and then little white dots closer to the rings: Enceladus, Dione, and Mimas. Mimas is almost invisible at the right-hand edge of the disk, but you'll see it if you click to enlarge.

Saturn quadruple transit

NASA / STScI / Gordan Ugarkovic

Saturn quadruple transit
The Hubble Space Telescope caught four moons or their shadows crossing Saturn's disk in this image captured on February 24, 2009 at 14:22 UTC. Titan is the large, dark moon near the top; the other three moons are Enceladus (off the disk to the left), Dione (on the disk at left), and Mimas (at the edge of the disk at right).

Four moons with one shot! Cool.

But wait, there's more. Because Gordan spends a lot of time working with these images, he recalled, while working through the latest release of Cassini data, that Hubble had been imaging at a similar time. Gordan compared the times of Hubble and Cassini shots and found that the two spacecraft photographed the giant moon nearly simultaneously. Hubble's photo above was taken at 14:22 UTC, while Cassini shot a view of Titan at 13:06 UTC. That's 76 minutes' difference. But at this time, it took 70 minutes for light to traverse the more than 1.2 billion kilometers separating Saturn and Earth. So Hubble saw a view of the planets in the alignment that they were at 13:12 UTC, according to Cassini's internal clock -- which is to say (in Gordan's words) "the light hitting Hubble's detector above got its start just six minutes after what Cassini saw."

But Cassini had a very different view on Titan from Hubble's. Here are the two spacecraft views, juxtaposed:

Same place from two very different perspectives

NASA / STScI / JPL / SSI / Gordan Ugarkovic

Same place from two very different perspectives
In a remarkable coincidence (or maybe not?), at the time Hubble was busy snapping a quadruple moon transit across Saturn's disc - including Titan, Cassini also targeted the hazy moon. Even though they are separated in time by some 76 minutes, at the moment of taking this image it took light exactly 70 minutes to reach Earth from Saturn. Thus the light hitting Hubble's detector above got its start just six minutes after what Cassini saw - practically a simultaneous event (it usually takes several minutes to get 3 color exposures in the first place, anyway). Nevertheless, due to very different vantage points they each saw a completely different scene, practically seeing completely opposite hemispheres of Titan.

Hubble's view was from the direction of the Sun; Cassini was on the opposite side of Titan from the Sun, a position impossible to achieve for those of us stuck to Earth. Moreover, as Gordan wrote, "Cassini was at that moment 700 times closer to Titan than Hubble." The images display the different resolving powers of the two cameras, convolved with their different distances from their targets; both images are enlarged 1.5x from their original pixels.

We are all over the solar system, examining the planets, moons, and asteroids from every side!

(And for those of you who don't get the silly title of this post, go here.)

Read more: Enceladus, Dione, Cassini, Titan, global views, Saturn's moons, many worlds, Saturn, Mimas, Saturn's rings, Hubble Space Telescope, pretty pictures, amateur image processing

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Emily Lakdawalla

Senior Editor and Planetary Evangelist for The Planetary Society
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