for dark carbonaceous objects (75% of known asteroids), S-types for stony (silicaceous) objects (17% of known asteroids) and U for those that did not fit into either C or S. This classification has since been expanded to include many other asteroid types. The number of types continues to grow as more asteroids are studied. The two most widely used taxonomies now used are the Tholen classification and SMASS classification. The former was proposed in 1984 by David J. Tholen, and was based on data collected from an eight-color asteroid survey performed in the 1980s. This resulted in 14 asteroid categories. In 2002, the Small Main-Belt Asteroid Spectroscopic Survey resulted in a modified version of the Tholen taxonomy with 24 different types. Both systems have three broad categories of C, S, and X asteroids, where X consists of mostly metallic asteroids, such as the M-type. There are also several smaller classes. The proportion of known asteroids falling into the various spectral types does not necessarily reflect the proportion of all asteroids that are of that type; some types are easier to detect than others, biasing the totals. Problems Originally, spectral designations were based on inferences of an asteroid's composition. However, the correspondence between spectral class and composition is not always very good, and a variety of classifications are in use. This has led to significant confusion. Although asteroids of different spectral classifications are likely to be composed of different materials, there are no assurances that asteroids within the same taxonomic class are composed of the same (or similar) materials. Active asteroids Active asteroids are objects that have asteroid-like orbits but show comet-like visual characteristics. That is, they show comae, tails, or other visual evidence of mass-loss (like a comet), but their orbit remains within Jupiter's orbit (like an asteroid). These bodies were originally designated main-belt comets (MBCs) in 2006 by astronomers David Jewitt and Henry Hsieh, but this name implies they are necessarily icy in composition like a comet and that they only exist within the main-belt, whereas the growing population of active asteroids shows that this is not always the case. The first active asteroid discovered is 7968 Elst–Pizarro. It was discovered (as an asteroid) in 1979 but then was found to have a tail by Eric Elst and Guido Pizarro in 1996 and given the cometary designation 133P/Elst-Pizarro. Another notable object is 311P/PanSTARRS: observations made by the Hubble Space