JUNO to Jupiter
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Last week, NASA software engineer Kevin Kyle uploaded this picture of the shadow of Io on Jupiter, from Juno's flyby of perijove 22:
The crispness of the umbra is due to the Sun appearing five times smaller from Jupiter than from Earth, so it casts sharper shadows. But the shadow looks very big compared to Io transits pictured from Earth or like this one from the Hubble Space Telescope:
The lunatic fringe who believe all NASA photos are faked sometimes cite pictures of Earth as evidence, in which the the continents appear different and inconsistent sizes. I thought the Juno picture of Io's sharp shadow would give a way of demonstrating what's going on numerically. First we figure out how big the shadow would look from very far away where we can see an entire hemisphere of the planet. This is simple geometry:
By filling in the radii of the Sun, Io, and Jupiter, and the distances separating them, we can calculate the slope of the line, and thus the size of the umbra that Io casts on Jupiter. I find the ratio of Jupiter's diameter to umbra size to be 46:1. It seems reasonable when compared against the Hubble image for a sanity check.
Then we consider how things look for an observer who is a height h above a planet:
d is the distance to the observer's horizon, and r is the radius of the visible disc. This is obviously smaller than the full hemisphere radius R, so objects in images taken from far away will be smaller by a factor.
The triangle with sides r, d, and h, and the one with sides R, d and R+h are similar. Therefore:
By Pythagoras theorem we also have:
Thus:
... and this can be rearranged to:
So then I just measured the Juno image in a crappy drawing program:
and found the ratio of Jupiter's diameter to Io's shadow to be 6.6. This meant that:
... and:
Given the rough and ready nature, this compares well to Juno's height of approximately 8,000 km given in the NASA credit for the image. All's well with the world ... and Jupiter (but not with the lunatic fringe :pac: ).-1 -
Amazing to think that the Juno Jupiter mission thread hasn’t been updated in over 2 years...
Juno has now gone into its extended mission after a highly successful primary mission, which clearly demonstrated that a solar powered probe could operate in the outer solar system. Juno is now exploring Jupiter’s large Galilean moons in addition to taking close up images of Jupiter.
Ganymede, largest moon of Jupiter and the largest moon in the solar system (larger than the innermost planet, Mercury) - imaged by the NASA Juno Jupiter orbiter mission on June 7th 2021. Tectonic stresses as well as craters on the icy crust are clearly visible and a layer of saltwater is believed to exist below the surface.
Details as small as 600 metres are visible in this close up image, the first close up image of Ganymede since the Galileo mission in 2000.
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Montage of images taken by Juno of Io, the highly volcanic inner Galilean moon of Jupiter on March 1st. Resolution is about 35 km per pixel.
Since 2021 Juno has imaged Jovian moons Ganymede and Europa in close up.
Juno will make two very close flybys of Io later this year and in early 2024, with images expected to be 1km resolution per pixel or better.
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The first of the up close Io images from Juno’s latest flyby have been released, and they are superb. The resolution is excellent.
There were some concerns in relation to the camera electronics due to radiation exposure but Juno really pulled it off spectacularly! 👍👍
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More absolutely stunning close up images of the tortured volcanic surface of Jovian moon Io, taken by the NASA Juno orbiter on February 3rd.
The surface of volcanic lava lake Loki, visible in the left image at upper right, is reflecting in the sunlight as never seen before by previous missions.
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