My collage of all the asteroids and comets visited by spacecraft is probably the single most popular image I have ever posted on this blog. I've now updated it to be in color and to include Toutatis, which Chang'E 2 photographed last week. When I first started working on a color version, it didn't seem to hang together very well as a composition, because the colors were all over the map. But when I hit upon the idea of trying to represent the bodies with correct relative brightness, it seemed to come together nicely. Vesta is not included because it dwarfs the others. (I'd have to increase the height and width of the montage by factors of three.)
Now I have to go into my various caveats. Although the montage is in color, the colors are approximate. Color data is only available for 11 of the 16 objects shown here, and those color images were taken through spacecraft cameras equipped with different sets of filters. No attempts were made to calibrate the color of one image to another. For the five objects for which there is no color data, I just fudged a brown color that seemed consistent with others. I would welcome comment from astronomy types on the correctness of the relative redness or blueness of different bodies shown here.
I did try to be more precise with albedo. In the version above, the relative albedo is correct: the darkest thing really is the darkest thing, and so on. Albedo varies by a factor of 20 from the brightest object, Itokawa (albedo of 0.54, which puts it in the realm of Io and Ganymede), down to the darkest object, Hartley 2 (albedo of 0.028, blacker than coal). Below is an alternate version in which I've made the brightest pixels on Steins and Annefrank (albedo 0.34) to be white, and the rest linearly scaled. This pushes Itokawa beyond the realm of reality but it's only a few pixels anyway. And the comets and Mathilde are nearly invisible. Comets are dark! Even this isn't totally precise because there's phase angle differences and other confounding issues. Don't do science on this montage. It's an illustration.
As with the previous version, anyone is welcome to use this for not-for-profit purposes. In particular, I want teachers and presenters to use this in classrooms and in talks; my original purpose in putting this together was to improve bad presentations. I do love getting emails from people who use it and like it! (I also love hearing from people who see it "in the wild" in other people's presentations.)
If you wish to publish it in a book/magazine/etc., please contact me for permission. I'm also happy to produce alternate versions as needed -- without text, without comets, comets only, with Phobos and Deimos, etc. Just ask!
Want to buy one? I've made it into a poster for sale at our store.