Craig Markwardt replied to Jeff Root:

Yes, I can see that. It is why I estimate that only 20% of
the RTG surface area is visible to the back side of the antenna
dish and other parts of the spacecraft sunward of the RTGs,
rather than the approximately 45% that would be visible if they
were plain cylinders.

I think the fractional (i.e. edge-on) area that actually faces
the antenna is much smaller than the outward-facing area. How
do you get 20%?

Just from looking at photos and guessing. Nothing better.

Note that the 20% figure is for the area of the RTG which is
visible to the back of the dish, etc., and is not the most
important thing to measure here, but was a response to Bruce
Woodcock's assertion that:

every RTG is not a spherical black body, but rather has fins
that are "edge on" to the antenna, which means only 2.5% of
the surface area of the RTG is actually facing the antenna.

My 20% figure may be high, but Bruce's 2.5% figure has *got*
to be *way* too low. Or else he mis-stated what the figure
represented. As I said, if the RTGs were plain cylinders,
approximately 45% of their surfaces would be visible to the
back of the dish, etc. (Depending on how it is measured-- it
could be closer to 60%.) The fins reduce that greatly, but
not to 2.5%! Don't you agree?

The more important statement by Bruce was:

the RTGs are located at the ends of the booms, and they only
see the antenna "edge on", subtending an angle of about 1.5%
of 4 pi steradians.

This figure has also got to be way too low. Look at photos
or a model. No way can it be only 1.5%. My estimate was 5%.
Don't you agree that 5% looks more like it?

-- Jeff, in Minneapolis

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