NASA's Juno To Get a Close Look At Jupiter's Moon Ganymede (phys.org) 17
On Monday, NASA's Juno spacecraft will come within 645 miles of the surface of Jupiter's largest moon, Ganymede. "The flyby will be the closest a spacecraft has come to the solar system's largest natural satellite since NASA's Galileo spacecraft made its penultimate close approach back on May 20, 2000," reports Phys.Org. From the report: Along with striking imagery, the solar-powered spacecraft's flyby will yield insights into the moon's composition, ionosphere, magnetosphere, and ice shell. Juno's measurements of the radiation environment near the moon will also benefit future missions to the Jovian system. Ganymede is bigger than the planet Mercury and is the only moon in the solar system with its own magnetosphere -- a bubble-shaped region of charged particles surrounding the celestial body.
Juno's science instruments will begin collecting data about three hours before the spacecraft's closest approach. Along with the Ultraviolet Spectrograph (UVS) and Jovian Infrared Auroral Mapper (JIRAM) instruments, Juno's Microwave Radiometer's (MWR) will peer into Ganymede's water-ice crust, obtaining data on its composition and temperature. Signals from Juno's X-band and Ka-band radio wavelengths will be used to perform a radio occultation experiment to probe the moon's tenuous ionosphere (the outer layer of an atmosphere where gases are excited by solar radiation to form ions, which have an electrical charge).
Juno's science instruments will begin collecting data about three hours before the spacecraft's closest approach. Along with the Ultraviolet Spectrograph (UVS) and Jovian Infrared Auroral Mapper (JIRAM) instruments, Juno's Microwave Radiometer's (MWR) will peer into Ganymede's water-ice crust, obtaining data on its composition and temperature. Signals from Juno's X-band and Ka-band radio wavelengths will be used to perform a radio occultation experiment to probe the moon's tenuous ionosphere (the outer layer of an atmosphere where gases are excited by solar radiation to form ions, which have an electrical charge).
Re: (Score:1, Troll)
This is a win for the good guys!
Who says something so stupid; what are you in, HR??
or 1038 km (Score:1)
nuff said
Largest (Score:1)
the solar system's largest natural satellite
Why the "natural" qualifier? Is there an enormous Death Star orbiting somewhere, that we have not been told about?
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Maybe a Death Star gets built some day. They're just future-proofing the statement, although who knows, maybe Planet 9 has a larger satellite and the statement still turns out to be false.
Re: Largest (Score:3, Informative)
Why not any closer ? (Score:3)
All fly-bys seem to pass quite a long way away. Surely 1/2 the distance would mean better pictures, etc, and we could learn more about Ganymede. So: why not ? Are they afraid of being hit by debris orbiting Ganymede ? What am I missing ?
Re:Why not any closer ? (Score:4, Informative)
They don't have the fuel to keep providing thrust or constant course adjustments, especially at roughly one light-hour distance from Earth. Simply measuring that moon well enough to plan the trajectory, especially that that closely with Jupiter nearby affecting orbits, is extraordinary piloting.
Re:Why not any closer ? (Score:5, Informative)
Not all flybys are far away. In October 2008 the Cassini spacecraft flew by Enceladus at 25 km! Considering it was passing by at about 18km/sec. whatever features it could see on the surface would be zooming right by it. I mean the angle subtended by the relative motion would be huge; you'd really have to turn your head (or camera) FAST to keep something in view. (Actually I don't think Cassini HAD an independently steerable camera platform like the Voyager spacecraft, so it had to turn the entire spacecraft in order to aim the camera. In that case I doubt it would have been possible to safely spin the spacecraft that quickly!). So no close up photos during the flyby :(. (But maybe it was on the night side anyway).
The only other cases where a spacecraft came so close to an astronomical body was when it was going to "hover" (like the asteroid probes) in a very close (and slow) orbit, or land, or impact (on purpose like the Deep Impact comet probe or by accident like the Mars Climate Orbiter). Maybe a few lunar probes came very close in their (relatively) fast orbits but I think the moon's lumpy gravitational field kept the mission planners from getting them too close.
In any case, I wonder if the Cassini mission planners had known then about the "plumes" of ice crystals coming up from Enceladus' South Pole. If they had, would they have flown Cassini at such a low altitude? Or maybe they weren't passing anywhere near the South Pole. I know they deliberately went through them later but presumably at the much higher altitude they would be much less dense. Hitting (a lot of) snowflakes at 18km/sec could have been damaging (especially if they didn't know to protect the cameras!).
Re:Why not any closer ? (Score:4, Interesting)
By the way, skimming the surface of an (active!) moon at only 25km while zipping by at 18km/sec. while controlling it from more than a billion km away with an hour time lag is not easy! At the time, I actually knew the (senior?) flight(?) programmer on the Cassini project at JPL so I guess it was his job to make it happen.
Talking with him I also came up with an idea for a movie. Some nefarious person (at the time it was Saddam Hussein) should offer him $5M (back when that was real money) to do a very modest course correction ($5M more when the job was done). The course correction? To alter the trajectory of Cassini when it did its gravity slingshot past earth so that instead of harmlessly flying by, "accidentally" crashes into the desert near Baghdad. Why? Because it carries 70(?) kg of highly enriched plutonium in containers that are designed to withstand catastrophic accidents including re-entry. This would occur and then in the movie some James Bond like guy would have to retrieve it (and liberate some beautiful scientist from Saddam's harem).
My friend humored me by saying to me "don't tell this to anyone". Of course, I later figured out the plutonium they use can't be used to make a bomb; it's the wrong isotope. (That's in addition to all the other safeguards). I presume the same is of all the other plutonium they've used in all the other nuclear powered American spacecraft, including the lunar lander from Apollo 13 that they had to ditch in the Pacific after it kept Tom Hanks alive. On the other hand, it still would be useful for a dirty bomb...
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Little things like that have never stood in the way of Hollywood, from Bullshot Drummond through Bond to Austin Powers. The SFX department have a death ray for that.
I doubt they have any interaction at all in these flybys. They'll have a go-nogo decision meeting several hours out and execute either an abort or the mission trajectory. Before you add the error correction laye
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Juno may not be designed target instruments that close because the target would be moving too fast relative to the craft. This flyby is a bonus, and wasn't part of the mission's original goals. (The craft's current orbit is an accident of a thruster glitch.) Thus, being closer may not be feasible and/or helpful. It may also not have enough fuel for certain flight paths. It kind of has to "go with the flow" to save fuel.
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Some of it I'm sure is just timing. Different teams on the mission want different flight paths so their instruments get the best readings and the actual trajectory has to balance all of that along with fuel constraints together.
Even if they wanted to just drop orbit in closer for a bit then raise back up after the flyby, where they're right back on the same path, that could screw up later timings for other encounters.
It can be a bit of a brain-melter when you put all the pieces together. Fascinating field,