# Unravelling Cassini's “ball of yarn” orbit around Saturn, tabulation of propulsive maneuvers?

The cool GIF on the NASA JPL Cassini resources page All Orbits (2004-2017): The Ball of Yarn is quite beautiful to look at, but it also hints at some beautiful engineering and orbital mechanics as well. Take a moment to look at the real thing there.

According to the page Saturn tour dates 2016, in the next few days Cassini will execute "short burn number 468" after it starts "rev 254" of its orbit around Saturn.

In my recent question Is Cassini's 183rd burn - or the associated orbital mechanics - some kind of record? I've noted that a popular news article states:

Cassini’s first graze of the F ring is scheduled to occur on December 4th. During that pass, Cassini will also briefly ignite its main engine to help fine-tune its orbit. It will be the 183rd time the spacecraft turns on its engine, as well as its last time, if all goes according to plan.

I'm wondering how to reconcile the "short burn" count > 460 with the number of times the engine has been turned on < 190.

Question: Is there a place where all of Cassini's propulsive maneuvers post-insertion (big and small, or at least big) are tabulated? Perhaps as a list of time and delta-v?

I looked at the orbital elements available in JPL Horizons and I can clearly see when many (most?) of the major changes in orbit happen, but just looking for changes here, or in changes in velocity that can't be explained by Saturn's gravity is a sketchy way to do this considering the complexity of this mission. Also, that would not distinguish propulsive maneuvers from gravitational deflections by Cassini's many close flyby's of Saturn's many satellites.

above: Teaser GIF to get you to enjoy the real thing here (since even the low-res version is larger than the stackexchange imgur's limit of 2 MB oops! MiB)

below: Some of the orbital elements of the Cassini spacecraft around the planet Jupiter from JPL Horizons. Discrete changes (steps) in the orbital elements hint at propulsive maneuvers.

EC: Eccentricity e
IN: Inclination i (deg)
OM: Longitude of Ascending Node OMEGA (deg)
W:  Argument of Perifocus w (deg)
A:  Semi-major axis a (km)

*******************************************************************************
Ephemeris / WWW_USER Tue Dec 20 20:03:00 2016 Pasadena, USA      / Horizons
*******************************************************************************
Target body name: Cassini (spacecraft) (-82)      {source: cassini}
Center body name: Saturn (699)                    {source: cassini}
Center-site name: BODY CENTER
*******************************************************************************
Start time      : A.D. 2004-Jun-01 00:00:00.0000 TDB
Stop  time      : A.D. 2017-Sep-29 00:00:00.0000 TDB
Step-size       : 720 minutes
*******************************************************************************

• Wouldn't the rest of the burns be performed using RCS - 183 main engine burns, 285 RCS burns? If the burn is minimal enough, starting the main engine might be an overkill and a risk to run out of its restartability capacity. – SF. Dec 21 '16 at 11:04
• @SF. that makes sense to me. Also if they are small delta-v adjustments, the main engine may not be controllable finely enough to make them accurately. Some of these flyby maneuvers are pretty close in distance and quite carefully timed. – uhoh Dec 21 '16 at 16:25
• per "Introduction to the Design of the Cassini Spacecraft", maneuvers less than 5m/s are generally done on the RCS (hydrazine monopropellant, 1N thrusters), greater than 5m/s are generally done on the main engines (MMH/NTO, 490N). 132kg of hydrazine is loaded at launch, may as well use it. :) – Russell Borogove Sep 14 '17 at 14:58
• 490N accelerates a 3t spacecraft by 0.16 m/s^2 so that's a 30 second burn for 5 m/s. Assuming the thrusters are used 4 at a time they'd give a little more than 1mm/s^2, so very fine control is possible. researchgate.net/publication/… – Russell Borogove Sep 14 '17 at 15:04

I can't unravel the ball of yarn for you, but maneuvers less than about 1 m/s were done with the ACS thrusters, not one of the main engines. While the main engine is more efficient, it is not accurate enough for very small burns.

You might be able to extract information about Cassini's maneuvers from its Event Kernel (EK) files, publicly available as part of the Planetary Data System distributions.

• I had thought about doing a parallel integration of the state vector (including Saturn's higher gravitational moments and Titan), looking for sudden deviations between it and published ephemeris as indications of delta-v maneuvers, but it looks like this might save me some trouble; I'll take a look. At a very first glance with just a text editor see a lot of ISS_NAC_ShutterBladeAMoveISS_NAC_ShutterReset but it makes sense that propulsive maneuvers would be very infrequent compared to science events. Thanks! – uhoh Sep 15 '17 at 1:36
• There is a question Cassini: difference between trajectory and orbital trim maneuvers in this list? to which the first part might be an answer. – uhoh Sep 17 '17 at 16:48