Juno development, launch, and cruise, Including Earth flyby imaging Oct 9 2013 Options

[nprev]

Wow. I had no idea that perapsis was that low!

Makes me think that possibly no useful imagery will be possible then; the relative velocity's going to be quite high, I assume. Are they considering adopting the Cassini 'skeet-shoot' tactic, perhaps in the XM?

[Hungry4info]

It's not likely Juno will survive for a XM, and imaging science isn't a high priority as far as I know. (I get the strong impression JunoCam was installed kind of as an afterthought)

[volcanopele]

Even if they plan on having an XM, the JunoCAM won't be around for it. It should die due to radiation damage around halfway through the nominal mission.

[nprev]

Really! That is actually very interesting. The cam's progressive deterioration will in itself provide potentially useful engineering data.

[ZLD]

Wow, that is extremely low!

For reference:
Pioneer 10: 132,086 km
Pioneer 11: 42,838 km
Voyager 1: 277,492 km
Voyager 2: 650,272 km
Ulysses: 379,123 km
Galileo: 402,327 km (rough avg)
-219,194 km (closest at arrival),
-71,398 km (closest at final pass of Amalthea),
-and of course entered the atmosphere with a final 0.9 RJ / -7140 km
Cassini: 9.7 million km
New Horizons: 2.2 million km
(Source:NSSDC, PDS)

[Explorer1]

If nothing else, the unique perspective on the inner moons (Amalthea, Metis and such) will be something to look forward to (if they're planning to look of course).

[djellison]

I just had a quick look using Eyes on the Solar System (it has a baseline Juno trajectory)

CA's to Metis, Amalthea et al are on the order of 30,000 miles. Metis might be 7 pixels across. Amalthea maybe 20-30 pixels. We're talking on the order of 10km/pixel at those distances - and it would have to be a very very fortunate coincidence of geometry for them to even cross Junocams FOV.

Juno's orbit is near polar, remember - and at this time it'll be taking it's principle science data of Jupiter itself.

[machi]

So close?
Then imaging spectroscopy is theoretically possible (by JIRAM). We haven't usable data from NIMS (Galileo). But these aren't objectives of the Juno mission.

[Frank Crary]

CA's to Metis, Amalthea et al are on the order of 30,000 miles. Metis might be 7 pixels across. Amalthea maybe 20-30 pixels. We're talking on the order of 10km/pixel at those distances - and it would have to be a very very fortunate coincidence of geometry for them to even cross Junocams FOV.

The odds are much better than that: Juno is a spinning spacecraft. Every 30 seconds, JunoCam's field of view will scan out the entire plane perpendicular to the spin axis. It could image anything in that plane. Over the course of a periapsis, that plane slides over most of the inner jovian system. Depending on the orientation of the orbit and the location of the satellites, I'd guess there is about a 50% chance to image any given satellite on a given periapsis. They just need to put in commands to image at the right time and spin phase.

That does not, however, say what the range to the satellite will be. It could be quite distant. In fact, the range to the Galilean satellites is always large on purpose. The observations of Jupiter call for a pretty well-controlled orbit. For example, each periapsis is 192 deg. of longitude from the last (to give an even sampling grid for the magnetic field/internal core measurement.) To do that without using too much fuel, the orbit was designed to avoid the Galilean satellites: Even a distant (e.g. 100,000 km) encounter would perturb the orbit and require additional corrections.

Juno's orbit is near polar, remember - and at this time it'll be taking it's principle science data of Jupiter itself.

That's true of the mission's science goals, but I'm not sure about JunoCam. It is on the spacecraft for public outreach and education. It isn't tied to the formal mission science requirements. It does get the best view and range of Jupiter, I think its filters reflect that, and I suspect most of the images will be of Jupiter. But if satellite images support JunoCam's outreach and education goals, I'd think they would be taken.

[Frank Crary]

Wow. I had no idea that perapsis was that low!

Makes me think that possibly no useful imagery will be possible then; the relative velocity's going to be quite high, I assume. Are they considering adopting the Cassini 'skeet-shoot' tactic, perhaps in the XM?

JunoCam and Juno don't do any pointing as such. The spin axis will point either at Earth or perpendicular to the orbital plane (so nadir on Jupiter is in the spin plane.) JunoCam just takes images at a commanded time and a commanded spin phase. So I don't think they can do anything like a skeet shoot. But it also means the smear from the relative velocity is less of a problem than you might think. It's designed to image from a platform spinning at 2 rpm. That's 12 deg. per second. At periapsis, the clouds are 4300 km away and the spacecraft is moving at about 50 km/s. That's about 0.7 deg. per second of smear, not much compared to the smear from spacecraft spin the instrument's built to deal with.

[djellison]

I was told that the spin axis doesn't change. It's basically earth pointed throughout. Thus, it doesn't slide that axis across the system as you suggest. Yes - at some point, every moons orbit results in every moon passing through that plane at some point. However, you need a very close approach to get a worthwhile image, for those inner minor moons, it's at perihelion - so the moon will have to be on the EXACT opposite side of the spaceraft from Jupiter. And smear is a moot point - JunoCam is pushbroom.

Maybe I'm missing something regarding the orientation of the spacecraft and it's geometry - but it's only going to be at brief and fortunate times like the attached when a minor moon's going to both in the FOV, and close enough to be worth imaging. Basically - of the 360 degrees of it's orbit, those minor moons have to be in an 18 degree wide slab of it (less, actually ) to enter the JunoCam FOV, and do so fairly close to periapsis - a 1 in 20 event.

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[vjkane]

My recollection is that Doug is basically correct. Near perijove, the spacecraft will have either a nadir orientation for remote sensing (and imaging) of the atmosphere or an Earth orientation for gravity measurements so the antenna can point to Earth for tracking. Magnetic measurements presumably work equally well in either orientation. As I recall, the early orbits switch off between those orientations, and the latter orbits are only Earth orientation. In fact, as I recall, the remote sensing instruments will be turned off after the end of the nadir orientation orbits.

The difference in sun angle is small enough that the solar influx on the panels is enough to power the spacecraft with either orientation.

Anyone have better information?

[Frank Crary]

My recollection is that Doug is basically correct. Near perijove, the spacecraft will have either a nadir orientation for remote sensing (and imaging) of the atmosphere or an Earth orientation for gravity measurements so the antenna can point to Earth for tracking. Magnetic measurements presumably work equally well in either orientation. As I recall, the early orbits switch off between those orientations, and the latter orbits are only Earth orientation.

That's pretty much true, but maybe I can say this more clearly. On perijove for the MWR orbits, with Jupiter in the spin plane (spin axis perpendicular to orbital plane), the JunoCam or JIRAM having a chance to see a satellite are pretty small. They would have be crossing Juno's orbital plane at the time. That's also (more or less) true of the gravity orbits at the beginning of the mission. At first, the orbital plane is almost perpendicular to the Earth line (polar orbit, periapsis close to dusk local time.) But over the course of the mission, the orbital plane precesses and perijove moves towards noon. I think it moves about a degree to two per orbit, and it's at 30 or 40 closer to noon by the end of the mission. So, when Juno is 20 R_J from Jupiter, it will also be about 10 R_J behind Jupiter (as seen from Earth or the Sun.) The remote sensing fields of view would be scanning out a plane perpendicular to the Earth line and 10 R_J behind Jupiter. As Juno moves goes through periapsis, that plane would move sunward, get just a hair sunward of Jupiter itself at periapsis, and then move back again. That could greatly increase the odds of catching a satellite.

I think it's a moot point: JunoCam and JIRAM were really designed for atmospheric science, not satellite geology, so I'm not sure if this is more than a theoretical exercise. I think they could do it, but the results might be unimpressive.

In fact, as I recall, the remote sensing instruments will be turned off after the end of the nadir orientation orbits.

I don't think that's quite true. UVS (and WAVES, since radio astronomy might be called remote sensing) is designed to operate for the whole mission. The others are designed to accomplish all of their official goals in the first half of the mission. Because of that, they could designed to survive a lower radiation dose. But that just means no one is officially promising they will survive to the second half of the mission. They could. (For example, Galileo survived a far higher radiation does than it was designed for.) I strongly suspect the project will keep operating them for as long as they last; if they get lucky, that could be through the whole mission.

The difference in sun angle is small enough that the solar influx on the panels is enough to power the spacecraft with either orientation.

Towards the end of the MWR/nadir pointed perijove periods (orbit 10 or so, I think), there is a significant difference, and the power off the solar arrays is down. But that's what batteries are for. I think these perijoves were one of the things that determined the battery capacity.

[vjkane]

That's pretty much true, but maybe I can say this more clearly.

Thanks for the additional information. Are you part of the mission?

[ZLD]

New update about Juno: Link

The spacecraft assembly has been completed and it is undergoing testing now.