A class of asteroids included in older classification schemes, where the ‘U’ stood for unclassifiable, for asteroids which could not be allocated to one of the five compositional types C, S, M, E, and R. However, more recent schemes group the asteroids into 9, 11, or 14 different classes, depending on the technique used for analysis. In the 14-class scheme the original U class has been subdivided into R, A, M, P, Q, E, V, and T classes, in addition to the C, G, B, F, D, and S classes used. In the new scheme, the letter U is appended to the classification of objects that possess an unusual spectrum for their class (e.g. CU, MU).
Subjects: Astronomy and Astrophysics.