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By Emily Lakdawalla

Update to my S/2008 S1 post: Cassini can fly past it!

Mar. 4, 2009 | 09:57 PST | 17:57 UTC

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UPDATE: This new moon is now named Aegaeon.

So it turns out I was totally wrong yesterday when I claimed that Cassini wouldn't be allowed to get close enough to the G ring to get a decent image of the newly discovered tiny moonlet S/2008 S1. And actually the clue was sitting right in front of me in the form of that "Cassini Mission Overview" slide I posted last week. Check out the "other icy satellites" row for year 6 and year 12 of the tour:

Click to enlarge >

Overview of Cassini's mission to Saturn
A summary of the completed and planned close flybys of Saturn's moons through Cassini's prime, extended ("Equinox"), and proposed extended-extended ("Solstice") missions. Credit: NASA / JPL

There are two "G arc" flybys planned for early 2010 and late 2015. Cassini mission planner Dave Seal gave me the goods on how close Cassini can get to the G arc and S/2008 S1.

Yes, the G ring is "dusty and extended", but the core of the ring, where the arc and this parent body lives, is only about 500 km wide radially and 500 km thick vertically. (The full ring is more like 11,000 km radial extent by 1,000 km vertical extent.)

So we could get quite close to it. Flying over or under it in an orbit with a small inclination would allow us to image S/2008 S1 quite safely in theory. And don't forget, both Pioneer 11 and Voyager 2 flew directly through the extended G ring (Voyager 2 through the "core"). Cassini has clipped the edges of the extended G ring region a number of times already, and will several more times in the coming years (it is, after all, one of the objects of interest to scientists). But you are correct in that we very specifically avoid flying through or very near to the core region, especially near this bright arc and parent body.

Cassini's two closest future flybys to S/2008 S1 will be on January 27, 2010, when it will pass at 13,305 kilometers, and December 19, 2015, when it will pass at a mere 1,900 kilometers. If they plan to take images, S/2008 S1 would be 6 pixels across in the first case and a whole 43 pixels across in the second case. Dave says "There has definitely been intensive discussion about imaging this coming January, and we'll most certainly take a shot at it in 2015 –- that is, if our extended-extended mission is approved in its current form."

Why wouldn't they image if they were flying that close? Well, these close flybys would be to understand the makeup of the G ring, and the best tools with which to study the G ring would be the dust counter and mass spectrometer. I'm sure imaging will be attempted if it doesn't interfere with observations that are more likely to provide data that really helps in figuring out how the G ring is put together. But I can't imagine they'd do a 1,900-kilometer flyby without shooting some images.

It turns out Cassini has, fortuitously, already had three close encounters with S/2008 S1, on September 5, 2007 (at 8,500 kilometers), on June 27, 2007 (at 15,400 kilometers), and on July 21, 2008 (at 11,600 kilometers). But planners didn't know that S/2008 S1 was there, so Cassini didn't see it -- the two just passed like ships in the night.

Thanks to Dave (and a few others) for the correction.

And, while I'm admitting mistakes, I've corrected a three-order-of-magnitude error I made in reporting the density of Pluto's atmosphere the other day (oops, confused microbars for millibars), and posted a correction about the Chandrayaan-1 miniSAR images from January. (In brief, Chandrayaan-1 miniSAR images look different from the Venus and Titan SAR images that I'm used to because they're shot from a very different incidence angle, much closer to vertical, which helps them search for water ice near the poles.) Thanks to Nigel Gunn and Paul Spudis for the corrections.

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