While writing about asteroids earlier this week, I didn’t get a chance to mention the different methods of classifying asteroid types. One is called the Tholen system, created by David Tholen, and uses a combination of spectral analysis and measurements of the asteroid’s ability to reflect light. The other method is the Small Main-belt Asteroid Spectrographic Survey (a.k.a. SMASS), and it only uses spectral analysis. SMASS also does not use the same wavelengths for its spectral analysis as the Tholen system.
The important thing to know is that there is not currently one standardized system to classify asteroid types; at least two are in use, and there is no guarantee that either of them will still be in use in the future. Which means that, as a science fiction writer, I have some flexibility whenever I write about asteroids.