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