back of the envelope: daylighting on Mars

Sometimes I need to work out a rough calculation to check whether my idea of something science-fictiony has any basis in reality. It doesn’t need to be super-rigorous*, but close enough to tell if my conception is way off the mark.

In this case, I’ve been thinking about how it might feel to walk around a city on Mars. It’s likely to be mostly underground to help shield against radiation, but there should be as much daylight as possible to save energy. On Earth, that kind of daylighting comes from skylights, windows, and (my personal favorite) light tubes.

But what about on Mars? Mars is farther from the Sun than Earth is, so it gets less light on the surface, but how much less? Is walking down a Martian street destined to feel like a gloomy overcast day?

First I had to get a grip on how to measure daylight. An obvious comparison is solar radiation, measured in Watts per square meter. Depending on time of year, Mars gets between 1/3 and 1/2 as much solar radiation as Earth, because it’s about 50% farther from the Sun. That’s handy for figuring solar power output, but the human eye isn’t so linear.

Another way is illuminance, measured in lux. Though the exact conversion factor between solar radiation and lux is a bit tricky due to the eye’s reaction to different wavelengths, I gather that the relationship is linear. Thus, using some standard Earth values and scaling them:

on Earth on Mars (min) on Mars (max)
direct sunlight 110,000 lux 38,000 lux 55,000 lux
indirect daylight 20,000 lux 6,800? lux 10,000? lux
clear sunrise/sunset 400 lux 130 lux 200 lux

(I’m assuming that indirect daylight is scattered as well in Mars’s pink sky as it is in Earth’s blue. Something to check later.)

Filling in a few other Earthly values for comparison:

bright overcast 25,000 lux
dark overcast 10,000 lux
studio lighting 1000 lux
office lighting 500 lux
cloudy sunrise/sunset 40 lux

So it looks like daylight on Mars wouldn’t look too different from daylight on Earth. It’s orders of magnitude more light than during “golden hour” on Earth, which is plenty to get around by. It would probably feel like a partly-cloudy day, since there would be more light than even the brightest overcast day, with sharply-defined shadows.

For daylighting, this probably means that Martian interiors would need twice as many Solatubes to get the same level of illumination, but we’re still talking about a fraction of the available daylight. In other words, using Earth-style lighting techniques should keep a Martian city street from feeling gloomy.

*Note the use of Wikipedia sources. Kids, don’t use Wikipedia as a source if you want anyone to take you seriously.

the documentation lied: publish_stream vs. publish_actions

It may surprise you to discover I still work with Facebook on a daily basis. I did leave Facebook as a user over three years ago, but wrangling the Graph API is still a core part of my job. (For the web developers among you: It’s like your relationship with Internet Explorer.)

Last week I was updating Measured Voice to match changes in the Facebook permissions dialog, and I noticed that the documentation said one of my permissions was now out of date:

“Facebook used to have a permission called publish_streampublish_actions replaces it.”

– from the Facebook API’s Extended Permissions documentation

In a fit of eagerness, I broke the first rule of API usage* and switched out publish_stream for publish_actions. However, it soon became obvious that the two weren’t equal. The auth tokens produced before and after my update were markedly different:

with publish_stream

Requested Granted
manage_pages ? manage_pages
read_insights ? read_insights
user_about_me ? user_about_me
user_status ? user_status
publish_stream ? publish_stream
? publish_actions
? video_upload
? create_note
? photo_upload
? share_item
? status_update

with publish_actions

Requested Granted
manage_pages ? manage_pages
read_insights ? read_insights
user_about_me ? user_about_me
user_status ? user_status
publish_actions ? publish_actions

Requesting publish_stream gave me publish_actions anyway, which makes sense if the two are being treated as equals, but it also gave me a whole passel of other permissions I hadn’t asked for. As it turns out, at least one of those permissions is still necessary to do what I need: post status updates and photos to a Facebook Page.

But which one? I checked the documentation again, and… well, none of them are documented at all. Not listed anywhere, not mentioned as deprecated, not anything. Huh. Some of them do sound like permissions I’d need (photo_upload and status_update, for example), but without documentation it’s just a guess.

It sounds like the documentation is ahead of the actual API development, and reflects some design goal instead. Or maybe this is (yet another) API bug. Either way, I’m going back to requesting publish_stream until they get their facts straight. It still works. (For now.)

* “If it ain’t broke, don’t upgrade to the new revision.”

three rules to put more science in your fiction

I won’t start an argument about the definition of “hard SF” or the state of scientific accuracy in fiction, but here are a few handy rules for science-fiction writers who want a quick test of real-science groundedness.

I call them Joi’s Laws, because SF writer Joi Weaver put them so well. (The headlines are her laws; the rambling context is mine.) Each one is a challenge: avoid this common crutch when starting your own story. Think of them like the Bechdel Test for solid science.

No FTL Travel

Space is big, and ships are slow. That’s the reality for at least another century, so embrace it. Faster-than-light travel lets your characters hop from star system to star system, but what are you really gaining? How is a rock around Epsilon Eridani inherently more interesting than a rock in the Main Belt?

Even if your story is set in the far future around a far star, FTL travel is dispensable. Joss Whedon didn’t need it for Firefly, and his (solar-system-sized) universe was packed full of interesting locations.

No Aliens

Everyone knows it: aliens in most stories are just humans with funny foreheads. Even the most unusual aliens in the most mind-bending stories turn out to have mostly human attributes, because a) it’s hard to imagine anything truly alien, and b) it’s harder to relate to truly alien aliens. So stop trying. Humans and animals have bizarre enough variations to fill a century of stories.

My own corollary: No Monsters. Monster stories are great and all, but 99% of new sci-fi is already cluttered with zombies, mutant viruses, and killer robots. Next time you need something terrifying, how about the interplanetary DMV instead?

No Artificial Gravity

This one is tough even for me. I’m obsessed with gravity, and I honestly believe we’ll be a second-rate spacefaring species until we learn to control it. Still, no one is close to controlling gravity even a little bit, so spaceships with a solid one-gee field working at all times are still pure fantasy.

Besides, fifty years of astronaut hijinks teach us that weightlessness is one of the best things about space travel, and we haven’t yet explored the spectacle of low-gravity sports. The only reason your characters would actually choose to be in a one-gee field (occasionally) is due to health concerns, and a treatment for bone loss is much more believable than gravity control.

“It’s more a guideline than a code.”

Good stories can still be told if they violate Joi’s Laws. (I’m going to see the next Star Trek film just like everyone else.) They’re not a guarantee of a good story, either. If you want to tell a *new* story, though, keep these in mind to give yourself a bit of a real-science challenge.


my SXSW space tweeps slides

On March 11th, 2012 in Austin, TX at the SXSW Interactive conference, I had the privilege of being on a panel of space enthusiasts called “How to Win Friends and Influence Space Exploration“. You can listen to audio of the panel at the SXSW site.

The panel was fun, and I put together a slide show to serve as spacy background for it. Those slides are big and unwieldy because they have huge images and video, but you’re welcome to have them if you’d like:

I might try to put the audio and slides together somewhere so you get the full effect, but that’s going to be a pain. If someone with better skills than me would like to do that, you have my blessing. Please send me the result if/when you post it so I can link it in here.

on life and its sources

Tonight I read the geeklet a story at bedtime, the kind of thing that’s designed to be restful with a hint of mind-broadening moral reassurance. As I finished, he looked thoughtful.

“We wouldn’t be here without this.” He tapped on the floor. “I don’t mean the floor, or the neighbors downstairs. I mean the ground underneath us.”

“That’s right.”

“We also wouldn’t be here without this.” He held up is palm, and this time I wasn’t sure what he meant. “We wouldn’t be here without the sky.”

“That’s also true.” I stood up and turned out the light. “Good night.”

He wasn’t done, though. “The sky wiped out the dinosaurs so we could take over.”

“Mm hmm,” I said, without even a pause. “Good night.”

“Good night, daddy.”