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The extremely cool NASA JPL video Triumph at Saturn (Part I) is really worth a watch and/or listen.

At about 45:45 it discusses the period after SOI (Saturn Orbit Insertion, July 1, 2004, 02:48 UTC) when the first images started coming in of Saturn's rings. In a JPL press conference later the same day, Carolyn Porco, Cassini Imaging Team Lead says:

I don't think you have to be a ring scientist to imagine what last night was for us. It was beyond description really. It was mind-blowing, it was every adjective you could think of.

I'm surprised at how surprised I am at the beauty and the clarity of these images. They are shocking to me.

This spacecraft allows us a very steady platform. This machine, you turn it, you point it, and it stays there. It's like a tripod in space.

At 9 to 10 AU Saturn gets about 1% of the sunlight that cis-lunar space gets, so all else being equal photographic exposures need to be 100 times longer.

Cassini's narrow angle camera has 12 micron pixels behind a 2 meter focal length f/10.5 reflector, so each pixel is only 1.2 arc seconds, and for extended objects in dim sunlight, f/10 is not a fast system.

So I would venture a guess that exposure times were perhaps seconds long rather than milliseconds.

Wikipedia's Cassini–Huygens says:

Smaller monopropellant rockets provided attitude control.

and notes that the spacecraft support the following acronyms:

  • AACS: Attitude and Articulation Control Subsystem
  • AFC: AACS Flight Computer
  • ARWM: Articulated Reaction Wheel Mechanism

And while I'm a strong advocate for articulated permanent magnet attitude control and momentum unloading in LEO for cubesats I haven't heard of articulated reaction wheels in deep space spacecraft before. But since I'm chronically uninformed that may be my fault.

Question: How exactly did Cassini provide rock-solid attitude control to enable high resolution low light imaging? (1.2 arc seconds/pixel for narrow angle camera)

I also wonder if Cassini ever had to slew; slowly rotate in a specific direction to de-blur an image of a moon during a close flyby.

screenshot from the extremely cool NASA JPL video "Triumph at Saturn (Part I)"

screenshot from the extremely cool NASA JPL video "Triumph at Saturn (Part I)"

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    $\begingroup$ I guess you want to read this: trs.jpl.nasa.gov/handle/2014/11604 $\endgroup$
    – asdfex
    Oct 26, 2021 at 12:01
  • $\begingroup$ @asdfex I don't see anything really helpful there. It's just an overview of the feedback algorithm presented as a talk for other feedback algorithm writers, and therefore over my head. I don't see anything obvious about an "articulated reaction wheel mechanism", I wonder if Cassini in fact had one? Hopefully there will be another document about the physical system as well. But thanks! $\endgroup$
    – uhoh
    Oct 26, 2021 at 12:17
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    $\begingroup$ It had "four Reaction Wheel Assemblies (RWA, three prime and a backup RWA mounted on an articulate-able platform)", also shown in the image on the first page, with a better image here: trs.jpl.nasa.gov/bitstream/handle/2014/46224/… $\endgroup$
    – asdfex
    Oct 26, 2021 at 12:54
  • $\begingroup$ @asdfex holy granola, that's an amazing paper, about a really amazing system! Indeed the reaction wheels were articulated, but that may have been used only when the "four Z-facing thrusters (executed) small ∆V burns" and not during imaging, but I'll need to read the whole thing first. $\endgroup$
    – uhoh
    Oct 31, 2021 at 21:33

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Cassini was the first spacecraft to use a Litton Hemispherical Resonator Gyro (HRG). The ultimate reference for the spacecrafts attitude was the Stellar Reference Unit and that could be used for control purposes most of the time, but the HRG was essential for maneuvers and for precision altitude control stabilization during data acquisition. Unfortunately I do not have the exact details, but see the first reference for the use of the HGR on Cassini and the second for a general reference on the HGR

https://www.spiedigitallibrary.org/conference-proceedings-of-spie/2803/1/Hemispherical-Resonator-Gyro-an-IRU-for-Cassini/10.1117/12.253429.short?SSO=1

https://en.wikipedia.org/wiki/Hemispherical_resonator_gyroscope

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