Ok, many thanks everyone for your replies, I'll try to answer to all of them,
but most suggestions don't apply to my problem or have already been tried... so
I hope to also clarify what I actually need.

> Stephen: Have you tried to use the transmit value to control the transparency of the
atmosphere?
This is not applicable because, for any non-zero transmit value, there will be a
sudden change when the camera goes from outside to inside the media container.
Also, the object inside the media should not become visible as soon as we enter
the atmosphere, only at lower altitudes, as in the video I linked. So even with
a transmit set by a function of the distance to the atmosphere boundary, this
would still not be what I need.

> Bald Eagle: Have a look at:
http://news.povray.org/povray.binaries.animations/thread/%3Cweb.5b32f0c22078c5ce458c7afe0
%40news.povray.org%3E/
This is interesting, especially because I don't use POV 3.8 so I had a glimpse
at new features ;-)
I downloaded alpha9893777 and proceeded to render the (50 frames) animation: at
no moment is the grey plane invisible behind or inside the media. I understand
this is not the purpose of that scene, and the hidden sphere is actually hidden,
but my need is to have an actually visible object, only hidden because of the
density of the media.
Basically, what I want is exactly what happens in nature with fog: the objects
are always there, but as you increase the distance to them, you can't see them
anymore because the droplets absorb their light and diffuse that of the sun
instead. But I can't simply use POV's "fog" feature because I also need to go
from outside to inside the foggy area, and when I'm outside, the foggy area is
in fact a sphere of non-uniform density.

> Alain: Try adding some absorbing media.
That's what I did, including in the scene I posted in my original message. It
does no good: no matter what combinations of absorption and scattering, you can
always see the object through the media. Emission is obviously not an option,
because the unlit hemisphere must stay black.

> Thomas de Groot: The following is about the best I can get
Thank you for your time and efforts :-) but unless you used some special
rendering options, when I rendered it the atmosphere is way too bright and the
sphere can still be distinctly seen through the media.

> clipka: You could use three independent media blocks, each one affecting only a
single colour channel
Maybe I do not understand what you mean, but this does not change the fact that
a constant density of 1 still allows seeing the sphere through the media. It is
not a matter of color itself, I can pretty much set any color I want, including
a color_map for the density, the object inside is always visible.

What I would probably need are densities higher than 1, or something that mimics
it. But since densities are looped in the 0-1 range, this is not possible with
the current implementation of density, unless I missed something...

More ideas welcome :-)

PS: this is a professional need, so unfortunately I cannot let go...

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