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NASA recently published this photo taken by Juno, at a distance of about 700,000kms from Jupiter:

enter image description here

The overall quality (very grainy around the darker regions, a bit washed-out in the brighter areas, and not the greatest dynamic range even in areas that are properly exposed) and resolution of this photo seems to compare poorly against images taken from ground-based telescopes:

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...and also from the images that we can get using Hubble:

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...both of which would have been shot at distances of closer to 600 million kms from Jupiter.

I understand Juno will get even closer to Jupiter than it currently is, but at the moment it's nearly 1000x closer to Jupiter than Hubble/Earth-based observatories and returning images that, at best, appear to be on par with what we can accomplish using terrestrial telescopes.

Is there a reason for this (like maybe the images released were shot in a low-resolution/low-bandwidth mode that doesn't actually use the camera's full capabilities), and what sort of improvements in the image quality can be expected as Juno reaches its closest orbital altitude? If the image quality can't be expected to drastically improve, was there a reason for not equipping the mission with better imaging technology that can at least clearly eclipse what we can attain by using Earth-local resources?

aroth
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    What is the blue thing on the north pole in the third image? – user985366 Aug 29 '16 at 19:02
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    Auroras in UV light http://www.nasa.gov/feature/goddard/2016/hubble-captures-vivid-auroras-in-jupiter-s-atmosphere – user985366 Aug 29 '16 at 19:08
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    To add to the Aurora comment/question: the image in question is a composite image combining a far-UV photo from June 2016 with a visible light image take in a previous year http://hubblesite.org/newscenter/archive/releases/2016/24 [Incidentally, I really wish composite images like this would be prominently labelled as such when released by Hubble/Nasa/Esa etc] – user2705196 Aug 31 '16 at 18:21

1 Answers1

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Short answer: JunoCam is not a scientific instrument; It was put onboard solely to get some neat pictures. It is not necessary for the scientific mission, and is mostly there just for public interest. You can interact with JunoCam by voting on what it takes pictures of.

Long answer: There are several reasons which combine to result in Juno only being able to take lower-quality pictures.

  1. Juno itself is spin-stabilized. It is constantly rotating, at about 2 rpm, so it cannot focus on any spot for long.
  2. According to Wikipedia, Junocam is "capable of color imaging at 1600 x 1200 pixels." This results in a resolution of 15 km/pixel. Compared to the Mars Reconnaisance Orbiter's HiRISE camera, which is able to take pictures at 0.3 m/pixel, this is not a very high resolution.
  3. The environment around Jupiter is filled with radiation. JunoCam is not inside the radiation vault, because it would be unable to take pictures. This means that JunoCam is only expected to survive about 8 orbits. The longer it orbits, the more its pictures will likely deteriorate. I recall reading an article somewhere about what this deterioration will look like, but I cannot find it anymore.
  4. At Jupiter's orbit, data rates are limited. JunoCam will only be able to return about 40 megabytes of data per orbit. This means that some quality may have to be sacrificed in order to get more pictures back.
Phiteros
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    Also, both ground-based telescopes and Hubble have huge apertures, and precise control over exposition etc. Even when Earth and Jupiter are at their closest, the signal will need more than an hour to get here and back again - that doesn't give you very precise control over the rather simple camera on Juno. – Luaan Aug 29 '16 at 07:49
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    Nice answer. "Juno itself is spin-stabilized. It is constantly rotating, at about 2 rpm, so it cannot focus on any spot for long." That would be a problem common to everyone looking at Juno. One could expose the camera multiple times for a short time and make several pictures, then combine (3D reconstruct) them with a better quality. – NoDataDumpNoContribution Aug 29 '16 at 12:46
  • https://www.missionjuno.swri.edu/pub/e/downloads/JunoCam_Junos_Outreach_Camera.pdf claims "a spatial scale of 3 km per pixel at perijove" and provides additional information on how still images are taken from a spinning platform – Jacob Krall Aug 29 '16 at 15:16
  • I also disagree with the statement "JunoCam is not a scientific instrument" - section 2 of the linked article shows the expected science return. – Jacob Krall Aug 29 '16 at 15:19
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    @JacobKrall I suppose what I meant by that is that it is not part of the primary scientific mission. Juno would have flown with or without the camera, and it was added almost as an afterthought. – Phiteros Aug 29 '16 at 15:21
  • Yes, I agree. I think that improves your answer: because it's not the primary science package, so it has to be cheap and small. The images @aroth showed are single frames taken from 700Mm, and PR images to show "hey, look, we're at Jupiter!" The actual high-quality images are taken from ~4.2Mm and integrated over longer exposure time, so much more detail will be visible -- but not of the entire planet. – Jacob Krall Aug 29 '16 at 15:26
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    Why would they only use a 2MP camera? In 2011 they could have gotten a 20+ MP sensor for nearly the same price (w/ respect to the price of a space probe). I know sensors require shielding in space, but couldn't they have used the same shielding they're using now? – BlueRaja - Danny Pflughoeft Aug 29 '16 at 16:30
  • Indeed... seems like a serious case of laziness that in 2011 they couldn't fit a better camera than that on there, even as an afterthought. – TylerH Aug 29 '16 at 17:13
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    Because the data rate for a 20MP camera simply isn't available and that camera is radiation hardened. – Joshua Aug 29 '16 at 17:36
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    Just adding more megapixels would only make the signal to noise ratio worse. Using a physically larger sensor would require significantly larger and more massive optics. And that is completely ignoring that the amount of download bandwidth is very limited; more megapixels would directly correspond to fewer images taken. – Dan Is Fiddling By Firelight Aug 29 '16 at 17:36
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    @TylerH - You have to understand "space cameras" (radiation and temperature hardened) are 1000x more expensive than what's in your phone. They are made by companies you've never heard of, and they are ridiculously expensive. Yes, the technology advances and prices come down, but cameras for spacecraft don't resemble consumer cameras at all. Most of them are hand-built from multiple off-the-shelf parts, all of which are wicked expensive. – Jasmine Aug 29 '16 at 18:29
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    @Jasmine: on this mission, "the CH electronics are designed around the Kodak KAI-2020 Charge-Coupled Device (CCD) image sensor" (per document I linked above) – Jacob Krall Aug 29 '16 at 19:00
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    @JacobKrall - That's just a sensor though. To configure that sensor into an actual camera, at the lowest cost I could find is $4000, and that does not include support equipment like power supplies, gimbals, heaters, and software and hardware to capture the images. So, $10K easily, for one of those only suitable for use on Earth. – Jasmine Aug 29 '16 at 19:30
  • JPGs could fix the 40mb limit... – noɥʇʎԀʎzɐɹƆ Aug 29 '16 at 19:42
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    @Jasmine I'm aware of "space cameras" actually and they still could have gotten a better resolution than 1600x1200 for not much difference in price. But yes, the bandwidth is the real concern. – TylerH Aug 29 '16 at 20:05
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    No, what I'm saying is I don't think so. If the difference between consumer cameras of different resolution might be 10x the price, that same difference in capability for professional hardware is likely 100x increase. I have on-the-job experience with these products, and the pricing is so high, I never got used to it. – Jasmine Aug 29 '16 at 21:08
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    @luaan: English correction: I think you mean exposure. "exposition" means something else in English. Sorry I can't private-message this. :/ – Peter Cordes Aug 30 '16 at 08:42
  • What image compression do they actually use? I doubt they would actually use JPEG (@uoɥʇʎPʎzɐɹC), since if you're going to pick a lossy image codec, you can do a lot better than that. (e.g. h.264 I frames can take advantage of redundancy between macroblocks, unlike JPEG where each DCT block is compressed separately). Lossless 10-bit 4:4:4 or RGB h.264 might actually be a reasonable choice if Juno has the CPU time... JPEG does have good error-recovery, but I think they always transmit with enough forward error correction that having images with errors in some of the blocks isn't an issue. – Peter Cordes Aug 30 '16 at 08:52
  • I should have checked first. https://www.missionjuno.swri.edu/pub/e/downloads/JunoCam_Junos_Outreach_Camera.pdf does say they can select between DCT-based lossy and lossless compression, and also that they have 4 color channels (RGB and methane?). They don't go into detail about the DCT-based lossy compression, so IDK if it's actually like JPEG or not. They need to store multiple images for processing, so they might actually use something like JPEG that allows for random access, and not something complex like h.264 I frames. (esp. not when so much radiation damage is expected.) – Peter Cordes Aug 30 '16 at 09:03
  • I'm not aware of a spacecraft that uses Jpeg compression, it can create artifacts that render the images unsuitable for scientific purposes. What they often do is to use some kind of a lookup table to do an 8 bit compression, and then use a lossless compression technique, which allows for quite a bit of data. Still, as has been mentioned, JunoCam isn't really intended for scientific purposes. – PearsonArtPhoto Aug 30 '16 at 12:11
  • I suspect the sensor alone for JunoCam costs at least $5000, probably quite a bit more. The optics surrounding it are more than just a standard set, they require special coatings for use in high radiation enviroments, and the survive vibration of launch. In addition, it has to work very well at low light, because Jupiter is much darker than Earth. Junocam was an aferthought, not originally intended to be a part of the mission, so I imagine the budget, mass, power, and money, for it was all quite low. – PearsonArtPhoto Aug 30 '16 at 12:14
  • Costs aside... what made people think the public would be particularly interested in lower-resolution photos of Jupiter than what we can presently get from a ground-based telescope? It's not to insult those who sent the probe or all their hard work, but it seems like, if you're going to go to all that trouble, why not put the best possible sensor on the camera? With modern compression techniques, you can get quite a few very high quality images into a 40MB transfer. – CommaToast Feb 08 '17 at 02:05
  • One major reason to have a camera is because it allows us to take pictures of places we can't see from the Earth - Jupiter's poles. Juno is on a polar orbit, so it crosses over the north and south pole of Jupiter. We can't get images of these places from the Earth, so it's nice to have some from Juno. – Phiteros Feb 10 '17 at 18:37