The NY Times article Cassini’s Final Mission: Obliteration says that the Cassini spacecraft will end it's more than a decade long exploration of the Saturn system by raising the inclination of its orbit around the planet to about 64°, then orbiting between the rings and the planet 22 times before entering Saturn's atmosphere.

To protect Titan and other potentially life-bearing places, Cassini’s final mission calls for its obliteration. Researchers have dubbed the spacecraft’s swan song the “Cassini Grand Finale” because it includes an ambitious maneuver: 22 loops through the gap between Saturn’s surface and its innermost rings.

Before launching Cassini, NASA drew up plans to dispose of it in Saturn’s upper atmosphere, but they only developed its elaborate end-of-mission maneuver in the past couple of years. Once they found that some of Saturn’s moons may be potentially habitable to life, keeping the probe clear of them as it ran out of fuel became crucial.

Before Cassini can thread the needle through the planet’s gap, it must first reach the proper angle in its orbit. That means climbing to an inclination of approximately 64 degrees. The problem is that Cassini doesn’t have the power to propel itself to that position.

enter image description here An artist’s rendering of Cassini’s final mission, a series of 22 loops over a few months. Credit NASA

“Each of the six (Titan flybys) will raise the tilt of the orbit a little bit,” said Dr. Spilker, “until we get the right orbit to go in between the rings and the planet.”

...Linda Spilker (is) the Cassini project scientist at NASA’s Jet Propulsion Laboratory in Pasadena, Calif.

(my emphasis)

After reading the NASA news item Cassini Heads for 'Higher Ground' at Saturn, I still don't understand:

Why must Cassini reach an orbital inclination of about 64° before "threading the needle" between Saturn and its rings? (why not 40°, 50°, 60°...)

  • $\begingroup$ @TomSpilker I tried to get you to post another answer here before. Since you are also at NASA you may have some contact with the project scientist I've quoted here ;-) $\endgroup$ – uhoh Jul 28 '18 at 16:40

This is purely a speculation, as I have not found any official sources to confirm it, but I think they want to be able to view Saturn's pole during the fly by. The trigonometry works out at least:

threading the needle

If you want to get a close-up of the polar regions with a lower inclination, like 40, 50 or 60 degrees, the altitude required is larger than the gap between Saturn and the innermost ring, and therefore not possible.

  • $\begingroup$ That certainly makes sense to me. Whenever I read a science story in a major newspaper of record such as the NY Times where I'm somewhat familiar with the subject, they tend to nail it. If the 64° is necessary for polar observations and not just for "threading the needle", I'd expect that they'd get that information in there somewhere. I'm pretty sure the NASA Press Office and the Science Desk of the New York Times have established a good line of communication over the last half-century. $\endgroup$ – uhoh Jun 26 '16 at 11:06
  • $\begingroup$ I think this is almost for sure the right answer, so I won't wait any longer. The NYTimes hasn't amended their article, so let's just assume it's because of the crisis that all newspapers are in after the internet appeared and everyone reads for (virtually) free. Thanks! $\endgroup$ – uhoh Jul 19 '16 at 11:57
  • $\begingroup$ Considering how many images we just got of the polar regions... I think this must have been correct! $\endgroup$ – CommaToast May 3 '17 at 5:45
  • $\begingroup$ just fyi I've come across an "threading the needle" instance in Spaceflight 101 "Cassini... hit the brakes for orbital insertion while smaller thrusters were used for extensive orbital maneuvering with some 360 burns performed during the mission to set up over 160 flybys of Saturn’s moons, also using their gravity to control its orbit in a very complex mission design circumnavigating the gaseous planet, avoiding the rings and threading the needle when flying closer than 50 Kilometers to the moons." $\endgroup$ – uhoh Nov 26 '18 at 3:53

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