Bad Radiation Dosage

I’m prepping material for my “Universe Update” program at NightLife, and I wanted to talk about how the Mars Curiosity rover (a.k.a. Mars Science Laboratory or MSL) measured its radiation exposure while in transit to the Red Planet. And I came across the hideous diagram pictured above.

Oh, NASA. What what what what what are you thinking when you put together a diagram like this for public consumption?

The title of the bar graph is “Comparison of Some Radiation Exposures to Mars-Trip Level,” and evidently, the designers are trying to convey the sizable quantity of radiation to which a traveler to Mars would be exposed: according to the accompanying press release, “The findings, which are published in the May 31 edition of the journal Science, indicate radiation exposure for human explorers could exceed NASA’s career limit for astronauts if current propulsion systems are used.” But the diagram doesn’t send that message well at all.

For the uninitiated, the x-axis of the diagram above uses a logarithmic scale, so each bold horizontal line corresponds to a factor of ten change in the magnitude of radiation dosage. (To their credit, the authors of the caption describe this in the second paragraph.) Eyeballing the bar chart, the “MSL Six Month Transit to Mars” dosage looks about 100 times the “US Annual Average, All Doses.” But ratio of the areas (which our eye-brain vision system perceives more naturally) is probably closer to 2.33:1. That’s a vast disparity between what we perceive versus the message NASA is intending to send.

(I should note that the press release from the Southwest Research Institute includes links to some nice images and diagrams, but they all relate to the process by which the numbers in the bar graph were derived.)

Compare this diagram to absolutely brilliant xkcd “Radiation Dose Chart” from several months ago. It even uses the same units as the NASA bar chart: millisieverts (mSv). In his diagram, Randall Munroe has created a much more complicated but much more viscerally satisfying visualization of the variation in radiation doses—from “Sleeping next to someone (0.05 mSv)” to “Ten minutes next to the Chernobyl reactor core after explosion and meltdown (50 Sv)”! That corresponds to a factor of about a million, meaning that the Chernobyl accident would give you a million times the dosage of a one-night stand.

If you want to try understanding the NASA data using the xkcd chart, look for “Normal yearly background dose… (~4 mSv)” near the center of the chart, which corresponds to the “US Annual Average, All Doses,” and “Dose causing symptoms of radiation poisoning if received in a short time (400 mSv, but varies)” in the middle right, which is roughly equivalent to “MSL Six Month Transit to Mars” on the NASA bar graph. Now, do you find that the xkcd gives you a better feeling for the ratio? It’s not trivial to compare those two numbers, but the area-based approach still works better than the NASA image—and it could have easily been adapted to depict the Mars radiation data in an easily-understood manner.

Next time, NASA designers should take a close look at Noah Iliinsky’s “Properties and Best Uses of Visual Encodings” chart and think about how the visual elements they choose underscore—or undermine—the message they’re trying to send. (More on Noah’s spiffy chart in my next post.)

A Fine Aerosol Diagram

New results from the Cassini spacecraft reveal the chain of events (so to speak) that leads to the formation of complex aerosols in its atmosphere. Aside from the spiffy science, the NASA announcement includes the very nice diagram pictured above.

What I like about the graphic is that it tells the story very plainly and simply, yet with considerable detail and substantial visual interest: nice little PAHs and aerosols, decent image of Titan’s surface, Saturn in the background (tilted too much with respect to the ring plane, but that’s nothing new), and so on. It even includes altitude info on the right-hand side clearly indicating where specific processes take place. All in all, a lot of info packed into a single image.

And anther detail. I’m already on record as not being a fan of lens flares in the fulldome environment, and in general, I seem them as kind of cheesy. But this might be the first time I’ve seen a lens flare used as a didactic element, suggesting the flow of photons from the Sun. Nice touch!

The only thing that gives me pause is the depiction of “energetic particles” as little arrows pointing away from Saturn. The particles are trapped in Saturn’s magnetic field, so they aren’t really shooting out of the planet in straight lines, which makes that depiction a little deceptive. But then, the only real solution would be to depict Saturn’s magnetic field with particles streaming from it, and that might be a little cumbersome. So I suppose I can forgive the diagrammatic shorthand.

Another more mundane quibble. The NASA webpage for the diagram include links to smaller versions at 1600×1200, 1028×768, and 800×600, but those are all windowboxed versions of the (obviously portrait, not landscape) diagram. Thus, the only version of above image that you can download at its original aspect ratio is the full-resolution version: a whopping 2000×2776 pixels! Not the greatest for, say, linking to blog entries.

Anyway, nice work, Cassinifolk! I like the diagram. And the story it tells…