So, knowing that the result you want is of course quite unrealistic (and if you zoomed out to space it would look "wrong"), here's an idea for how you can (mostly) achieve it in Terragen without comping, etc. Copy/paste the following into the node network and it'll create an Atmosphere node you can just plug-in:
<terragen_clip>
<planet_atmosphere
name = "Planet atmosphere 01"
gui_use_node_pos = "1"
gui_node_pos = "260 360 0"
gui_group = ""
enable = "1"
input_node = ""
gui_use_preview_patch_size = "0"
gui_preview_patch_size = "1000 1000"
enable_primary = "1"
enable_secondary = "1"
centre = "0 -6378000 0"
radius = "6378000"
seed = "0"
haze_density = "0.01"
haze_horizon_colour = "0.25 0.25 0.25"
bluesky_density = "0.15"
bluesky_horizon_colour = "0.200000003 0.25 0.3000000119"
bluesky_additive = "0"
bluesky_additive_colour = "0.1438666731 0.3779166639 1"
redsky_decay = "0"
haze_exp_height = "60000"
bluesky_exp_height = "60000"
ceiling_adjust = "1.866666667"
ceiling = "112000"
floor = "-16000"
haze_glow_amount = "0.5"
haze_glow_power = "1"
bluesky_glow_amount = "0"
bluesky_glow_power = "0.75"
enviro_light = "1"
enviro_light_tint = "1 1 1"
anisotropic_enviro_light = "1"
shadow_function = ""
ambient = "0 0 0"
fake_dark_power = "0"
fake_dark_sharpness = "10"
bluesky_density_colour = "0.2158000022 0.4535000026 1"
redsky_decay_colour = "0.805896461 0.6354003549 0.3678794503"
ozone_factor = "0.5"
improved_glow_model = "1"
number_of_samples = "16"
adjust_to_distance = "1"
enable_ray_traced_shadows = "0"
>
</planet_atmosphere>
</terragen_clip>
The basic idea is to use very low density and very high Exp(onential) Height values. What this gives you is an atmosphere that extends much further up above you (the hazes, at least), but is so low density that it doesn't attenuate light nearly as much over long distances. So you get a general, diffuse haze, but features are clearly visible even 100s of kilometers into the distance. Compare it with the default atmosphere and you'll see the dramatic difference. I think the key insight here is the use of very large Exp Height values to offset the lower densities, basically.
Hope that helps.
- Oshyan