It’s because it is designed to work on a spacecraft that is spinning constantly. The reason Juno spins is to save on weight.
Most spacecraft have gyroscopes to control their attitude. But Juno needed to save weight, and more importantly also, power because it’s a solar powered satellite and to be able to continue under solar power at the distance of Jupiter it needs massive solar panels. And even so it has a total of only 500 watts available for everything. I think it’s the first spacecraft to go as far as Jupiter relying only on solar power.
Others use radio thermal generation - radioisotopes that are so radioactive they remain hot to the touch and the heat is used to generate power for the duration of the mission (not nuclear power plants, they don’t use chain reactions, just the heat, so for instance they use Plutonium 238, a form of plutonium that’s useless for bombs or for fission reactors).
So their ingenious solution is to rotate the entire spacecraft which helps to keep it stable in its orientation. It needs to point towards the sun for solar power. But the orbit is designed in such a way that it’s also got Jupiter to one side as it orbits it, so this means its instruments scan across the surface of Jupiter. Each of its instruments gets a shot at imaging Jupiter, one scan line at a time, as it spins around.
Other spacecraft have used this technique of a scan line where you just take repeatedly photograph a line of pixels and then combine them together to make an image which is endless in length, finite in width. This works well for photographs of scenes that don’t change much from one minute to the next.
This is a rarely used technique, but it has been used before, notably on the Pioneer missions, for instance Pioneer 10 and 11:
“Pioneers 6-9 demonstrated the practicality of spinning a spacecraft to stabilize it and to simplify control of its orientation. Measurements made by these spacecraft greatly increased our knowledge of the interplanetary environment and the effects of solar activity on Earth. New information was gathered about the solar wind, solar cosmic rays, the structure of the Sun's plasma and magnetic fields, the physics of particles in space, and the nature of storms on the Sun which produce solar flares.”
The Pioneer Missions, see also Pioneer 10 which describes the instruments including a camera, which it used to take photos such as these:
One advantage is that it lets all its instruments operate simultaneously. JunoCan will be able to take many images, but the main bottleneck is transmission back to Earth, as so often the case with deep space images - remember how long it took New Horizons to return images to Earth? Juno is not so far away, but still will only return about 40 megabytes of data for each 14 day orbit. JunoCam
Still it should be able to return some spectacular images. Jupiter is huge, the Red Spot alone is comparable in size to Earth even now that it’s shrunk a bit.
It's 1600 by 1200 pixels and will achieve 15 km / pixel - which is nearly 10 times better than Hubble's best resolution of 119 km/pixel and at any time after that.
By comparison, Galileo's CCD was 800 by 800
It will be able to take photos that are better resolution than this photo by Galileo which it says is 10s of kms per pixel:
So it’s not too shabby :).
As another example, in this photo from voyager
Programs Will Share Excitement of Voyager Discoveries
the small white spot to bottom right of Jupiter’s red spot has a diameter same as Earth, so about 12,742 km. So at 15 km / pixel, it would be 849 pixels wide. And the Red Spot itself, about twice its diameter, would fill an entire image of JunoCam at its closest. So you can expect images like this one
to high enough resolution so that if you put it on a screen 1600 by 1200 pixels, this image would fill the entire screen. Indeed if you click through to that image here: APOD: August 27, 1996 click on the image on that page to see it full resolution, then if I’ve got it right, that’s very approximately the resolution for the very highest resolution images from JunoCam taken when it is closest to Jupiter.
It starts its first science orbit at the end of August so we’ll see those first images some time around then hopefully.
It's now in a 53 day orbit, which gives them lots of time to test out the instruments before the first science mission flyby. They then have another 53 days to get the data back from that and work on what happened, before finally doing another burn when closest to Jupiter (most fuel efficient time to do it) before going into the final 14 day science orbit. So we get first science at the end of August. And it's another two-three months before they do regular science missions - I think its that 53 + 14 so 67 days after that if I understand it right, as I don't think they'd be able to do science during the second burn to reduce it to 14 days - that's interpretation, they didn't actually say that but don't see how they could.
So, that second burn is mid October according to Juno probe enters into orbit around Jupiter - BBC News
So If I got this right, expect some science return at the end of August. Probably nothing during that mid October because of the burn. So then the next science images and data would be end of October / early November and then from then on get data every two weeks until the instruments give out due to the high levels of radiation there.
It takes a long time to download data, 40 megabytes of photos for instance takes 14 days for the JunoCam to download them. So I think that's partly why they need as much as 53 days just to check out the instruments etc before the first science and then again want that 53 days again, to have a lot more than 14 days to download the science data and check things over for the first orbit after the first science.
Then after they have done all that they should be ready to do it more quickly, only 14 days to download the data and set up the next science run for each close flyby.
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