[X] My Assistant

[JTN]

A while back I gave myself the impression (by playing with JPL Horizons) that the next instance of the famous orbital "swap" was going to take place around Jan/Feb 2006. Is that right?
I'm not familiar with the precise orbital dynamics[*], but I got the impression that the moons would become close, but not spectacularly so. (But I don't have the data in front of me now.)
Does anyone know what sort of Cassini observations are planned (indeed, what if any interesting observations can be made, other than ultra-precise trajectory determination)? Any eye candy?

[*] I had a quick Google, but couldn't find any good animation to help visualise what really happens, with a rotating frame or something - just this, which I'm not convinced by. Anyone know of anything better?)

[elakdawalla]

I'm trying to write a description of this for a Web story but I'm not confident that I have the details exactly right. Can anybody critique this and tell me if there's any errors?

Here is how the dance works. Epimetheus and Janus are small, irregularly-shaped moons with diameters of about 120 and 180 kilometers (about 75 and 110 miles), respectively. Their orbits around Saturn are separated by only 50 kilometers (30 miles). Since Cassini arrived at Saturn, Epimetheus has been the inner of the two satellites. Because it is closer to Saturn, Epimetheus travels at a faster angular rate than Janus, so inner Epimetheus has slowly, inexorably been catching up to outer Janus. As the two have approached each other in their orbits, Epimetheus tugs on Janus from behind as Janus tugs on Epimetheus equally. The mutual tugging causes them to exchange angular momentum. Epimetheus gains momentum and rises in orbit as Janus loses an equivalent amount of momentum and falls. But this switch of orbital altitudes makes Janus -- still ahead of Epimetheus in its orbit -- the faster of the two. As a result, Janus creeps ahead. It will continue to creep slowly ahead of Epimetheus until it catches up from behind in four more years.

Thanks! --Emily

[pat]

I'm trying to write a description of this for a Web story but I'm not confident that I have the details exactly right.  Can anybody critique this and tell me if there's any errors?

Here is how the dance works.  Epimetheus and Janus are small, irregularly-shaped moons with diameters of about 120 and 180 kilometers (about 75 and 110 miles), respectively.  Their orbits around Saturn are separated by only 50 kilometers (30 miles).  Since Cassini arrived at Saturn, Epimetheus has been the inner of the two satellites.  Because it is closer to Saturn, Epimetheus travels at a faster angular rate than Janus, so inner Epimetheus has slowly, inexorably been catching up to outer Janus.  As the two have approached each other in their orbits, Epimetheus tugs on Janus from behind as Janus tugs on Epimetheus equally.  The mutual tugging causes them to exchange angular momentum.  Epimetheus gains momentum and rises in orbit as Janus loses an equivalent amount of momentum and falls.  But this switch of orbital altitudes makes Janus -- still ahead of Epimetheus in its orbit -- the faster of the two.  As a result, Janus creeps ahead.  It will continue to creep slowly ahead of Epimetheus until it catches up from behind in four more years.

Thanks!  --Emily

You might want to add that since Janus is more massive than Epimetheus its orbital altitude decreases by less than Epimetheus' orbital altitude increases (conservation of angular momentum etc). So Janus doesn't 'occupy' Epimetheus' old orbital position and vica versa. Janus moves in but not as far in as Epimetheus' pre-event orbital radius while Epimetheus moves out past Janus' pre-event orbital radius. Four years later the event happens the other way around with Janus catching up to Epimetheus.

[elakdawalla]

You might want to add that since Janus is more massive than Epimetheus its orbital altitude decreases by less than Epimetheus' orbital altitude increases (conservation of angular momentum etc). So Janus doesn't 'occupy' Epimetheus' old orbital position and vica versa. Janus moves in but not as far in as Epimetheus' pre-event orbital radius while Epimetheus moves out past Janus' pre-event orbital radius. Four years later the event happens the other way around with Janus catching up to Epimetheus.

Thanks, Pat, I'd forgotten about that detail. I just posted the story here.

--Emily

[gpurcell]

Thanks, Pat, I'd forgotten about that detail.  I just posted the story here.

--Emily

That's a nifty little write-up, Emily. Thanks!