Did you know that not all asteroids have the same composition? According to NASA, the three primary classifications of asteroids are C-type, S-type, and M-type. C-type asteroids, which are composed primarily of carbon-rich substances, are the most frequent. S-type asteroids, which are made up mainly of silicate minerals, are less common. M-type asteroids, which are primarily composed of metal, are the least prevalent. One asteroid, whose composition is still unknown, is set to pass Earth today, January 18. Know all about this close encounter.