# If Juno's long orbit means a lower rate of radiation damage, why the planned short orbit?

The Juno Spacecraft (also Wikipedia, launch 05-Aug-2011, Jupiter orbit insertion 05-Jul-2016) was scheduled for a burn to reduce it's period from about 54 days to 14 days, while maintaining a similarly challenging low altitude periJove of only 4000 km above the planet's equator. Among the technical challenges are the intense radiation near Jupiter due to particles trapped in the planet's strong magnetic field. This lead to some extreme measures, including the 1 cm thick Tantalum Juno Radiation Vault to protect roughy 200 kg of electronic equipment.

It completed it's first periJove on 27-Aug-2016, and on the second periJove it was scheduled to execute the orbit reducing burn on 19-Oct-2016. Due to potential issues with the propulsion system discovered in telemetry prior to the scheduled burn, it was postponed. The burn was not executed during any subsequent burn, and now it seems the decision has been made to leave Juno in the longer orbit for the duration of the mission, according to Space Daily's Juno to remain in current orbit at Jupiter.

The article says:

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.

"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."

Question: What was the purpose of the planned change from 53 days to 14 days? Considering the advantages of staying in the longer orbit, what were the even more compelling advantages of the short orbit?

below: Plot of Juno's originally planned orbit from historical kernels (source and plot described here)in the JPL Horizions database. Juno remains in a single plane but with a reduced apoJove, The in-plane precession of the apses is due to Jupiter's substantial oblateness and large $J_2$. Left: projection into J2000 Ecliptic YZ plane. Right: XZ plane, coincidentally it is edge-on in this view due to mission schedule relative to Jupiter's orbit.

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

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

• I think the bonus science thing is a euphemism for making the best out of what they have, not that it's a happy accident. – GdD Feb 21 '17 at 9:47
• @GdD you don't think that the longer orbit had any advantages at all? Of course one keeps things positive in press releases partly because news headline-writers are always looking for the bad side of any news, but could the orbit planning have involved a balance of potential advantages and disadvantages for both longish and shortish orbits? – uhoh Feb 21 '17 at 9:57
• I think if the potential benefit of a higher orbit was that great they would have planned the mission with a few of them before the descent burn @uhoh. The engineering challenges and cost of a close orbit design means that the must have had good reasons. – GdD Feb 21 '17 at 10:35
• @GdD you seem to want to force a uni-axial paradigm You can have simultaneously a great reason for doing something and a great reason for not doing it. There could be several considerations. I'm not saying your conclusion is wrong, just saying that there could be multiple important factors beyond what's mentioned in a given article. – uhoh Feb 21 '17 at 10:40
• This is why you asked the question @uhoh, and I think it's a good one. I'm not answering this because I don't have hard data. – GdD Feb 21 '17 at 10:41

Money. The mission was proposed to a cost cap, which includes operations costs. The science objectives could be achieved with a shorter primary mission and therefore lower cost using tighter orbits.

• I had a hunch :) On the manpower side, can personnel multiplex? With 53 days between flybys will people spend more time on other things than they would have at 14 days? – uhoh Feb 21 '17 at 16:44
• The science team should be able to ramp down something close to proportionally, but the engineering team would only be able to ramp down a little. Activity planning would be reduced, but spacecraft monitoring and housekeeping is more a function of time than activities. I suspect that the Juno team is considering exactly those trades as we speak, to see how long they can stretch their already approved funds. – Mark Adler Feb 21 '17 at 16:49
• I'm not disagreeing with you, it's an interesting answer. Do you have any links to evidence on it? – GdD Feb 22 '17 at 9:13
• @GdD, Mark Adler works at JPL. I also recently attended a talk by Steve Levin, Juno Project Scientist, and he gave the same explanation. – pericynthion Feb 22 '17 at 23:37
• I'm not questioning the provenance @pericynthion, I was hoping to learn more if there were docs available to read. – GdD Feb 23 '17 at 8:54

I can see several reasons for those shorter orbits.

• first of all, you have to notice that the probe is in a polar orbit around Jupiter. One of the goals of the mission is to make several "maps" of Jupiter (jupiter's gravitational and magnetic fields, variation in atmospheric composition, etc.), so the shorter the orbit the more data you get for a fixed amount of time (eg the lifespan of the probe).

• if you want to make the same amount of orbits, but with the longer one, it would take 34 times 53.5 days = 1819 days (~5 years) instead of 34 x 14 days = 476 (~15 months) for the shorter one. The wikipedia article cited in OP mentioned that the orbits configuration allows the probe's solar panel to be always facing the sun. With a mission time around 5 years, and knowing that Jupiter has an orbital period of 11.86 years, you can deduce that at least a few orbit would have got the probe into Jupiter shadow, and so without the possibility to energize itself through its solar panels.

• last but not least, the said polar orbits let the probe navigate with a minimum contact with Jupiter's dense radiation belts

These orbits minimize contact with Jupiter's dense radiation belts, which can damage spacecraft electronics and solar panels, by exploiting a gap in the radiation envelope near the planet, passing through a region of minimal radiation

• Thanks! On point 1, the lifetime is not necessarily measured in clock time. For example accumulating radiation dose and damage depends on the particular path, and may be better represented by number and duration of the close passes, and rotation of the apses. Point 2; That's a good point! if the periJove is on the far side, there isn't enough fuel to avoid the inevitable solar eclipse. If you can find somewhere how soon it will be and add the info and link to your answer that would be great! – uhoh Feb 21 '17 at 13:26
• Point 3 I don't understand - it looks to me like the longer orbit will actually pass farther from the radiation bands than the closer one. – uhoh Feb 21 '17 at 13:27
• You're welcome. I do agree with your comment on point 1. But for a certain amount of radiation allowed by the design, you decrease any other failure risk shortering the mission. I will make some research on point 2. – le_daim Feb 22 '17 at 8:30
• About point 3, I don't know what to think to be honest. You can have two orbits with similar periods (let's say 30days), but one very elliptical with a very low PeriJove (and radiations+++) and one more circular with a ~constant radiation level. But here, I talked about where around Jupiter the probe fly-by, and you can see on the picture showing "strongest radiation bands near Jupiter" that there is almost no radiation at the poles. – le_daim Feb 22 '17 at 8:30