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Juno spacecraft question



 
 
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  #1  
Old August 10th 11, 06:01 PM posted to sci.space.policy,sci.space.history
Pat Flannery
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Posts: 18,465
Default Juno spacecraft question

Since Juno has three very large solar arrays on it to give sufficient
electrical power via sunlight (rather than RTGs) at Jupiter's distance
from the Sun, why wasn't it equipped with an ion engine to cut its trip
time down some? The arrays would generate lots of power while it's in
the inner solar system, as it will be for a lot of time till the Earth
encounter in October of 2013 slingshots it out to Jupiter:
http://www.youtube.com/watch?v=sYp5p2oL51g
Also, what's the purpose of the fabric cover over the transceiver dish?
They expecting to encounter a lot of micrometeors on the way?

Pat
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  #2  
Old August 10th 11, 06:31 PM posted to sci.space.policy,sci.space.history
Rick Jones
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Default Juno spacecraft question

In sci.space.history Pat Flannery wrote:
Since Juno has three very large solar arrays on it to give
sufficient electrical power via sunlight (rather than RTGs) at
Jupiter's distance from the Sun, why wasn't it equipped with an ion
engine to cut its trip time down some? The arrays would generate
lots of power while it's in the inner solar system, as it will be
for a lot of time till the Earth encounter in October of 2013
slingshots it out to Jupiter:


Concerns about added complexity?

How close was Juno to an Atlas V 551 configuration's mass limits?

rick jones
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  #3  
Old August 12th 11, 02:06 AM posted to sci.space.policy
Quadibloc
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Posts: 7,018
Default Juno spacecraft question

On Aug 10, 11:01*am, Pat Flannery wrote:
Since Juno has three very large solar arrays on it to give sufficient
electrical power via sunlight (rather than RTGs) at Jupiter's distance
from the Sun, why wasn't it equipped with an ion engine to cut its trip
time down some?


One obvious answer would be that while, as you note, the spacecraft
had a lot of extra energy available for that, this would have required
extra reaction mass to be added to the craft.

Since the goal was to put a specific scientific payload into orbit
around Jupiter, but _when_ that payload arrived didn't really matter
much, what point would there be in cutting down the trip time at the
cost of increasing the size of the booster needed to launch the craft
in the first place?

Instead, they launched Juno into a 2-year orbit - as if it was just
going to Mars, but needed a fast free-return trajectory - thus
reducing the required delta-V - and when it gets back to Earth, it
will do a gravity-assist maneuver which will finally give its orbit
the aphelion required to actually reach Jupiter.

So that shows where the emphasis is he on getting the thing to
Jupiter with the smallest booster they can get away with. Not getting
it there quickly.

Mind you, I'm surprised they didn't save even more fuel, and launch it
into a 1.5 year orbit, so that it would do the gravity-assist maneuver
*three* years later. It's not like they have any astronauts on it that
need life-support. But perhaps the relative speed of the spaceship and
the Earth wouldn't have been high enough for the required gravity
assist while still staying outside the atmosphere.

And doing more than one gravity assist involved diminishing returns
and wasn't necessary.

John Savard
  #4  
Old August 14th 11, 11:29 PM posted to sci.space.policy,sci.space.history
Brian Thorn[_2_]
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Posts: 2,266
Default Juno spacecraft question

On Wed, 10 Aug 2011 09:01:36 -0800, Pat Flannery
wrote:

Since Juno has three very large solar arrays on it to give sufficient
electrical power via sunlight (rather than RTGs) at Jupiter's distance
from the Sun, why wasn't it equipped with an ion engine to cut its trip
time down some?


Cost. It had to fit within the New Frontiers program budget. The
ion-powered Dawn busted that budget and had to beg to be reinstated.
Lesson learned.

Brian
 




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