Space/Science — HabitableZone

Space/Science » in reply to re: Atmosphere as filter, and landing site predictions...

re: 2100m rim visible for ~100km?
DTB — (archived)

Nick,

Thanks for the reply!

Hesitant as I am to suggest a correction -- I got chased from astronomy to political science years ago by the fact that I'm better with words than math -- I think at 50km, a 2.1km high feature should be visible, though barely, and that a bit should show even at the center of a 160km crater.

The corresponding simplication for Mars should be d=2.45 * h^0.5 (the constant should change as sqrt(R), I think).

For 2100m, that gives 112km for the rim to just reach the horizon.

That seems consistent with a chart I found from the Pathfinder IMP team showing height above the horizon (in pixels) by distance, derived from the non-simplified formula. They predict a 1000m feature would be on the martian horizon at 85km.

http://www.lpi.usra.edu/meetings/lpsc97/pdf/1814.PDF

Of course, you have to be a *lot* closer for the feature to stick appreciably above the horizon -- a couple of pixels worth of distant rim wouldn't amount to much at all. Possible it'd be noticable at low sun angles if illuminated when the crater floor is already shadowed, but in general I imagine you'd miss it if not looking for it.

On the other hand, haze would be a real issue -- if I recall you could barely distinguish the "far knob" in the pathfinder images, and that was only 30 or 40km?

A small issue either way. Those were still very helpful predictions about landing site characteristics.

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