I hope the following very non-specialist post will interest some readers - and maybe even winkle out an expert reply or two.

Cassini is about to pull off a special trick. The next two Titan flybys are only 8 days apart. That single fact alone tells a great deal about what Cassini is up to. For Titan, 8 days is just half a month, meaning that the encounters happen on opposite sides of Titan’s orbit. Now it’s not that special for the orbits of two bodies to meet at two widely distant points. But for the two bodies to take exactly the same time to get from one point to the other is very special: it implies that their orbits must be twins. They must not only be the same size but the same shape too, in this case a fair approximation to a circle. The only way in which the orbits may (and do) differ greatly is in inclination. The double act will not last long, however, as Cassini’s orbit continues to be reshaped to meet it’s destined targets.

Over the coming weeks I hope this unusual event, and its implications for Cassini and the geometry of future Titan encounters, will be remarked upon at greater length by the Cassini team, or maybe by others here and elsewhere.

Watch out for the mission descriptions appearing on the Cassini website. Will we get a double-header for both flybys, as we did with Enceladus last year?

That also means that two different parts of Titan will be illuminated.
So we'll get to see one side, then the other. (Assuming Cassini gets a good view of the lit sides on both approaches.)

This has to do with shaping the orbit. Others can correct me if I'm wrong but I think this 8-day-apart pair of Titan orbits accomplishes a "180-degree transfer" -- it switches the periapsis of Cassini's orbit from one side of Saturn to the exact opposite side. Beginning April 4, there will be 10 (count them, 10) flybys of Titan on consecutive orbits, spaced 16 days (one Titan period) apart. The last of the 10 is on August 25.

--Emily

The 180 degree transfer maneuver, in which Cassini't orbit is first circularised to match Titan's before being elongated in the opposite direction, was also performed in January 2007. However on that occasion there was not an 8-day pair of targeted flybys. The geometry demands that there must have been a fairly close non-targeted flyby somewhere around that date, 8 days before or after one of the targeted ones. I went to Ciclops to look it up, but unfortunately this was just before 'looking ahead' started. On the Cassini site I find that although it's possible to view 'tour highlights' for every year, the full set of 'tour dates' is now only available for 2009 (an oversight?). The mission descriptions for the targeted flybys don't help either.

Was there a non-targeted flyby and what use was made of it? Any information welcome.

Planetary Society archive to the rescue! T24 and T25 were one and a half orbits apart with two non-targeted flybys in between. Equally effective no doubt, but not quite so neat as this month's left-right punch.

QUOTE form here: http://www.planetary.org/explore/topics/sp....html#180degree

Mon, Jan 29, 2007
2007-029T07:16 TARGETED FLYBY OF TITAN (38TI [T24])

Inbound 2631 km flyby, speed = 5.8 km/s, phase = 72°
Mon, Jan 29, 2007
2007-029T07:38 Descending ring plane crossing

r = 20.708 Saturn radii (1249323 km)
Thu, Feb 1, 2007
2007-032T09:57 Periapsis Rev 38

r = 15.6 Saturn radii, lat = -58°, phase = 58°
Sun, Feb 4, 2007
2007-035T01:08 Ascending ring plane crossing

r = 19.612 Saturn radii (1183213 km)
Mon, Feb 5, 2007
2007-036T07:42 Nontargeted flyby of TITAN (38TI)

Outbound 910072 km flyby, speed = 5.9 km/s, phase = 114°
Sat, Feb 10, 2007
2007-041T10:53 Apoapsis Rev 39

period = 18.1 days, inclination = 58.8°, r = 28.5 Saturn radii, phase = 121°
Thu, Feb 15, 2007
2007-046T03:09 Nontargeted flyby of TITAN (39TI)

Inbound 867510 km flyby, speed = 5.5 km/s, phase = 128°
Fri, Feb 16, 2007
2007-047T09:46 Descending ring plane crossing

r = 20.677 Saturn radii (1247446 km)
Mon, Feb 19, 2007
2007-050T11:50 Periapsis Rev 39

r = 15.6 Saturn radii, lat = -58°, phase = 59°
Thu, Feb 22, 2007
2007-053T03:12 TARGETED FLYBY OF TITAN (39TI [T25])

Outbound 1000 km flyby, speed = 6.2 km/s, phase = 161°

Just had lunch with Dave Seal, who confirmed this is a 180-degree transfer (known as a "pi transfer" to the navigators). The upcoming frequent Titan flybys are to bring the inclination of the orbit down to produce the desired geometry for viewing the equinox in August.

--Emily

Emily's post stimulated the the following random OT thought:

A possible question for joining UMSF. A spice kernel is? 1) Something you might want to eat in a novel by Frank Herbert. 2) Useful for seeing something interesting in a program by David Seal. 3) etc. etc.

Nevermind

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Next!

The "Nevermind" meant that I could see Stu coming with his gong Now we take the thread back to the topic of the pi transfer...or maybe an image of Casini doing a pie transfer.

And with that maneuver executed safely I have a question for the Cassini folks. Is this way of doing a pi transfer - an 8-day pair of targeted encounters - trickier than the way it was done last time in 2007? Is there an element of caution then and confidence now, or is it just a case of the way the maths pans out with no special challenges involved this time?

A separate question too: are there any particular advantages in terms of imaging opportunities or science potential in having two flybys unusually closely spaced?

The latest 'looking ahead' takes us up to the first of the two flybys:
http://ciclops.org/view/5550/Rev106

As a companion to that, I uploaded a video to youtube showing an animation of the flyby:

http://www.youtube.com/watch?v=Dcx-qXMxdwo

T51 Mission Description:
http://saturn.jpl.nasa.gov/files/20090327_...description.pdf

From the mission description:
RSS observes bistatic scattering from Titan's surface on the outbound side. The observation probes the midnorthern latitude regions of Titan's surface (30-40 degs; ~190-200 degs west longitude). Same- and crosspolarized components of the quasi-specular surface echo, if detectable, provide information about the dielectric constant and physical state of the surface region probed.

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This is in the mid-latitude region to the NW of the Dancing Monkey feature of Dilmun. (Or way to the N of Selk Crater).
This region may have also been imaged by SAR RADAR during the T43 pass (IIRC this will hit the PDS this April.)

Dielectric constant gives great information about the material properties of the surface stuff. Low dielectric constant stuff is more non-polar (like hydrocarbons) while higher dielectric constant stuff is more polar (like water).
But there's a catch: fluffy porous stuff lowers the apparent dielectric constant. So it's hard to tell if its solid wax (ca. 2.0-2.4), or fluffy snow (solid ice is 3.1, fluffy snow as low as 1.4). (Really rough angled surfaces can mess up this experiment too, but that doesn't seem to be a major problem on Titan - if it was, the SAR RADAR returns would be be really really bright).

Most of the blandlands have a pretty uniform dielectric constant of around 1.7. This is a smidgen higher than the global average of 1.6.
But based on a recently publication (Janssen et al., 2009), there is an much lower dielectric constant area near the bondary of the soon-to-be-observed region.
The low dielectic constant is about 0.9 - this is way to low to be any solid material. It has to be something likely organic AND fluffy (or porous).
Even more exciting, a region with the same dielectric properties is W Xanadu and Tui Regio.

I can't wait to see the T43 SAR Swath....

-Mike

BTW, my latest favorite article to carry around in my briefcase is:
Janssen et al., Icarus 200 (2009) 222-239. "Titan's surface at 2.2 cm wavelength imaged by the Cassini RADAR radiometer: Calibration and first results."
doi: 10.1016/j.icarus.2008.10.017

Is this Ontario Lacus?
http://saturn.jpl.nasa.gov/multimedia/imag...9/W00055206.jpg

Yes.

C'mon Gordan, you've had plenty of time for lunch. Ontario from less that 10,000 km must be worth a pretty job.

Heh, sorry - this is all I could pull from 3 smallish WAC frames and it's very contrast-enhanced. Someone else might do better.

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That's great, thanks! It's at lowish illumination angle and that's only going to get worse with the season, so this will likely be the best shot from ISS.

Just took another look at that. Is that a dark feeder valley curving up from below and into leftmost corner of the lake?

We'll tell you in the summer after we get SAR of it in T58. Radar don't need no stinkin' sunshine!

Hard to say, it could be a flatfield imaging artifact, just as the dark patch to the right of the lake is. The image is so low in contrast pretty much no conclusions should be made. It does appear to me that there's a channel feeding into the lower right part of the lake, but nothing conclusive. One of the frames is suggestive, the other two not so much.

I would be very careful about reading too much into that image. The effective resolution seems to be about the same as our Rev09 (June 2005) observation of Ontario Lacus, and we didn't see any evidence for drainage channels in that data either.

Trust me, I've been basically staring at this thing for the last hour and half... after a bit you start seeing shapes in that noise.

T52 mission description:
http://saturn.jpl.nasa.gov/files/20090403_...description.pdf

Rev108 Looking Ahead:

http://ciclops.org/view/5575/Rev108

Southern clouds from T-52 flyby grace the skies just north of the Mezzoramia Basin in this ISS NAC image (N00133269) taken April 4th. Click image to reveal Celestia grid location.

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That Rev108 thing talks about T52, and then says that Rev109 will include "Cassini's 54th flyby of Titan". So when is T53? Is it a nontargeted flyby or something?

T52 was the 53rd flyby of Titan. The first three flybys were Ta, Tb, and Tc with T3 being the fourth flyby. So the 54th flyby of Titan is T53.

Cassini raw images continue to come in! Two beauties (N00133397 and N00133398) taken on April 9th, look back at Titan post T-52 flyby and are layered together on a Celestia grid below. Hotei arcus is highlighted in these images.


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