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I'm trying to understand where Juno will pick up most of the slow, cumulative radiation damage within it's orbit.

Currently it is in the higher, 53 day orbit. The original plan was that tomorrow (2016-10-19) it would execute a roughly 395 m/s delta-v burn in order to drop down to a 14 day orbit.

If I understand correctly, most of the radiation that will damage Juno's instruments is associated with Jupiter's magnetic field and lies within a few Jovian diameters of the planet. The 54 day orbit spends slightly less time near jupiter than a 14 day orbit of the same perijove would, but it's a small difference.

If Juno is accumulating most of its radiation damage near the planet, then it should be able to last for 30 or more of these longer period orbits. However, I had just read that Juno would not last for the same number of flybys (over 30) if it remained in this higher orbit, due to accumulated radiation damage.

Here is one example. The lower, 14 day orbit is called the "Science Orbit" in several different articles I've read.

From the Spaceflight 101 article Rocket Burn postponed for NASA’s Juno, to remain in elongated Jupiter Orbit until December

Passing Jupiter on Wednesday, Juno will be outbound again with its next close pass of the planet on December 11 which will be the next opportunity for the Period Reduction Maneuver – pending evaluations of the suspect check valve signature.

The option of a third Capture Orbit has be prepared as part of numerous contingency scenarios worked out for this mission, however, there is some urgency in getting Juno down into the science orbit due to the limited lifetime of the spacecraft in the extreme radiation environment of Jupiter. (my emphasis)

Are my assumptions about the location of the highest damaging parts of the orbit incorrect, and a substantial amount of radiation damage happens far from Jupiter, and thus in each 54 day orbit the accumulated dose is much larger than in each 14 day lower, "Science" orbit?

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above: "Capture Orbit Design – Image: NASA/JPL/LASP". From Spaceflight 101

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above: sketch of a few of Juno's lower 14-day orbits and illustration of the strongest radiation bands near Jupiter. From the Lost Angeles Times

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above: "Animation of Jupiter's Synchrotron Radiation Torus. Credit: NASA/JPL - Caltech". From the Vatican Observatory Blog

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uhoh
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  • The design to drop down to 14 day orbits might not be related to radiation damages, but to achieve an optimal coverage of Jupiter all around. A careful plan which now already might be a bit out of sync, – LocalFluff Oct 18 '16 at 11:58
  • @LocalFluff I found an example of an article discussing why the shorter 14 day "Science" orbit was necessary to avoid accumulating radiation damage, or something to that effect. – uhoh Oct 18 '16 at 12:21
  • Maybe because the closer orbit would precess into higher inclination, and the worst radiation is equatorial, and significant even at greater distances? – LocalFluff Nov 24 '16 at 07:38
  • @LocalFluff that's a really interesting suggestion. The planned maneuver seems to keep the periapsis about the same, I'll try to check what would be different. The biggest precession effects would not be in inclination though, would they? – uhoh Nov 24 '16 at 08:54
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    @LocalFluff I've plotted the originally planned orbit from Horizons. It starts out with an inclination of nearly 90 degrees and maintains that inclination. The periapsis precesses, but I don't know if that has a big impact on the radiation. – uhoh Nov 27 '16 at 14:51
  • The radiation belts do not make it to the poles of the planet and the synchrotron emission in your animation is a light-of-sight view so it looks to get closer to the planet than the radiation belts actually do. The purpose is to reduce the accumulated radiation dosage and prolong the mission but still allow for unique science. There is also the advantage of looking at the auroral regions (i.e., poles) which has not been done before at Jupiter. – honeste_vivere Nov 29 '16 at 18:10
  • @honeste_vivere I don't quite understand everything you are saying. I've asked why the change would partially mitigate radiation dose, and you are saying reduce the accumulated radiation dose - is this just saying exactly the same thing, or are you explaining something new and I'm missing your point? – uhoh Nov 30 '16 at 01:13
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    @uhoh - I was trying to explain why lowering the periapsis would lower the accumulated radiation dose to the spacecraft, which is why they wanted to do this in the first place. The lower periapsis would allow the spacecraft to "slip inside" of the main radiation belts on inbound and sneak under again on outbound. In the current state, if my understanding is correct, the spacecraft is going through more of the meat of the Jovian radiation belts. – honeste_vivere Nov 30 '16 at 13:36
  • OK, as far as I can tell using JPL Horizons, the periapsis didn't change between the 53 day orbits and the 14 day orbits. You can see my plot in this question. I downloaded the data with 2 hour time steps, so the periJoves (periapses) look ragged because the flyby is really quick and that's not enough samples to get a smooth orbit. – uhoh Nov 30 '16 at 17:53
  • The lower orbit may have allowed them to get the measurements done more quickly because Juno would spend more time within instrument range of Jupiter. So radiation dose would be reduced by reducing mission time. – Hobbes Feb 21 '17 at 10:38
  • @Hobbes It's a small effect. Since both elliptical orbits have relatively high eccentricity, the velocities at periJove are similar if it's only the apoJove distance that's changed. The perijove velocities are different in the various orbit plans in the Horizons kernel archive (10-20%), but those are due to differences in perijove, and that distance probably has multiple constraints on instrumentation, slewing rate, data collection rate, radiation... I'll look in more detail tomorrow. – uhoh Feb 21 '17 at 10:52
  • let me rephrase: in a lower orbit, Juno spends less time out of instrument range. – Hobbes Feb 21 '17 at 10:57
  • @Hobbes oic! I'm so focused on the fly-by data at circa 4,000 km that I wasn't considering all the other observations. Thanks. – uhoh Feb 21 '17 at 11:19

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I am going to advance the answer "It wasn't." to the question:

Why would Juno's originally planned orbit lowering partially mitigate radiation damage?

According to Space Daily's recent article Juno to remain in current orbit at Jupiter.

Juno's larger 53-day orbit allows for "bonus science" that wasn't part of the original mission design. Juno will further explore the far reaches of the Jovian magnetosphere - the region of space dominated by Jupiter's magnetic field - including the far magnetotail, the southern magnetosphere, and the magnetospheric boundary region called the magnetopause. Understanding magnetospheres and how they interact with the solar wind are key science goals of NASA's Heliophysics Science Division.

The operative paragraph:

"Another key advantage of the longer orbit is that Juno will spend less time within the strong radiation belts on each orbit," said Scott Bolton, Juno principal investigator from Southwest Research Institute in San Antonio. "This is significant because radiation has been the main life-limiting factor for Juno."

uhoh
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