Most asteroids are irregularly shaped, unlike the spherically shaped major planets. Only Vesta can be seen with the naked eye. The orbits of most asteroids lie partially between the orbits of Mars and Jupiter. In 1990, a similar asteroid was found in the orbit of Mars.
Centaurs are asteroids with orbits in the outer solar system.
Asteroids are also classified by composition and albedo , most being one of three types. The M-type are composed of nickel iron and are bright. Toward the end of the 18th cent. By 1890 more than 300 asteroids had been discovered by visual means. Brucia was the first asteroid discovered by this method.