Why science is so awesome

I love science. Check out this photo of NASA astronaut Alan Bean descending the ladder of lunar module Intrepid during the Apollo 12 mission in 1969 and about to set foot on the Oceanus Procellarum.

Alan Bean (1932-2018) descending the ladder of the Apollo 12 LM, Intrepid, November 1969. Photo by Pete Conrad, NASA, public domain.

Know what’s so awesome about it? Other than the fact that – hey – these guys are on the moon, I mean. Have a look at the lighting. Bean’s on the shadow side of the lunar module (LM). See where the light falls? It’s hitting Bean’s LEVA (lunar extra-vehicular visor assembly) and OPS (oxygen purge system, that box atop the PLSS backpack) both just poking up into the sunlight.

Now, there’s no air on the Moon, so there’s nothing to diffuse the light and refract it to fill out shadows. But as you can see, the shadow is fairly well lit. You can see it not just in the way lighter colours such as Bean’s A7L suit are highlighted, but in the reflections on the foil on the shadow side of the LM descent stage, along with the edges of the ascent stage hatch. Equally, the surface shadow of the LM is jet black.

Another picture of Buzz Aldrin descending on the shadow side of the Apollo 11 LM shows similar apparent ‘fill-lighting’ and has been upheld by ‘lunar hoax’ conspiracists as proof that the whole thing was a stage job in which NASA were too stupid to realise that shadows in vacuum will always be jet black. And this despite the Apollo programme ultimately drawing in some 400,000 Americans who were among the world’s best engineers and scientists of the day, mostly employed by commercial enterprises, many of whom had to then work under the glare of relentless (‘annoying’) and commercially independent media attention. That’s also quite apart from the monolithic quantity of documentation, or the personal witnesses to what happened (including a couple of readers of this blog, one whose Dad was the pad safety officer at the Apollo 11 launch, and another whose Dad knew Neil Armstrong personally.)

Buzz Aldrin descends to the lunar surface, 20 July 1969, photo by Neil Armstrong, with Aldrin illuminated by light reflecting from the regolith. It’s one of 550 colour photos taken during that mission. Note how horribly over-exposed the regolith is, in order to properly expose the shadow side. Photo:NASA.

What we’re actually seeing in the photos is way cooler than the ‘hoax conspiracists’ imagine, and is associated with the actual physics of both light and photographic film. It’s correct that shadows in vacuum are black – this is why a moonless night away from artificial lighting appears dark to us on Earth (we can disregard starlight, mostly, from the human perspective) and why other astronomical bodies, including the Moon, appear sometimes as crescents. But sometimes we can see ‘Earthlight’ on the ‘dark’ side of the lunar crescent, and that’s a clue as to what’s happening in the Apollo photos.

In fact, what’s happening in both pictures – and shown up really well in the Apollo 12 one – is a demonstration of the basic physics of light. These are true everywhere, but they’re especially obvious on the lunar surface. The principle is simple enough: the illuminated side of the Moon reflects light. If it didn’t, you couldn’t see it from Earth. As it happens, the lunar regolith isn’t particularly great on reflection, as such things go – it bounces between 7 and 8 percent. That’s still enough to make moonlight pretty bright on Earth, especially under a full moon.

Now imagine what it’s like when you’re up close – as in, standing on the surface. Untempered by atmosphere. Yah.

The Apollo missions were deliberately planned to land with low sun-angles so the astronauts could get a better view of the terrain they were descending into, which is also why the LM usually landed facing away from the sun. The photo at the top of this post was taken during Apollo 12’s first EVA, with a sun angle of 7.5 degrees. The result – infill lighting solely produced by reflections off the regolith. Of course it isn’t as bright as direct sunlight because (a) as I mentioned, the albedo is only about 7 or 8 percent; and (b) not all the reflected light is heading back to source.

In terms of the photography, the Apollo astronauts used custom-modified Hasselblad 500El 70mm cameras with electric winding and a Zeiss Biogon f 5.6 60 mm lens, polarising filter, and a Reseau plate above the film plane. This last produced registration crosses that enabled distances to be subsequently calculated in the photographs. This system was a long-established standard in prior photogrammetric work, such as aerial photography, but in the lunar photos the registration marks were sometimes invisible in shadow or blurred out in places by brighter light. The NASA specification included coping with lunar temperature extremes, and the cameras were accordingly silvered to reduce heat absorption. They also used specially manufactured film cartridges because they could not be reloaded during a lunar EVA. The base stocks used on the Apollo missions were Kodak Panatomic-X for black-and-white, and various Ektachrome standards for colour.

The astronauts had only limited control over photography, typically fixed to 1/250 second shutter speed with focal length (depth-of-field) of either 5.6 for shadow or 11 for direct sunlight. The focus ring had only three positions, although that was usually OK given the depth of field and distances involved. Greater precision wasn’t possible in space-suit gloves, nor did the cameras (which were chest-mounted) have viewfinders. The astronauts practised aiming and shooting during training, but still had their fair share of duff results. At the time, only the good ones were released, but NASA have since published the whole lot here: https://www.flickr.com/photos/projectapolloarchive/

If you have a look at the picture at the top of this post you can see how the parts of the LM in direct sunlight (variously, because of the faceted shape, but clearly seen on the left of the frame) are over-exposed, along with Bean’s helmet where it’s struck by direct light. What’s more, if you look at the shadows on the surface, you’ll see they are very black in the flatter area in the shadow of the LM, but have infill where the ground is contoured because of the light angle. Also check out the crater kicked up by the LM’s descent engine on landing – note the gold tinge at the bottom? That’s direct sunlight being reflected off the LM’s gold-foil wrapping.The same’s true of the Apollo 11 photo by Neil Armstrong, where the regolith itself is clearly over-exposed, the shadows below the LM very black – but Aldrin is well lit by the reflected sunlight.

So there you have it. No conspiracy here – just basic optical physics, including the way cameras work. How cool is that? Thoughts?

Copyright © Matthew Wright 2018