Asteroids are somewhat arbitrarily differentiated from comets and meteoroids. Objects smaller than meteoroids are classified as micrometeoroids and interplanetary dust. Most meteoroids burn up as meteors before hitting a planet's surface. Near-Earth objects are classified as meteoroids, asteroids, or comets depending on size, composition, and orbit. By the 1950s, speculating about antimatter comets and meteoroids was a commonplace exercise for astrophysicists. These trails of meteoroids cause meteor showers when Earth encounters them. The study of meteorites also gives insights into the composition of non-ephemeral meteoroids. Some meteoroids ejected from the Moon get launched into orbits around the Sun. None of the meteoroids observed were associated with material from the comet's 1846 disintegration. Some stone meteoroids contain grain-like inclusions known as chondrules and are called chondrites. Most meteoroids disintegrate when entering the Earth's atmosphere. Some of the debris from collisions can form meteoroids that enter the Earth's atmosphere. Most meteoroids that strike the earth vaporize in the atmosphere. Particles which are affected primarily by radiation pressure are known as "beta meteoroids". After breakup of the parent body meteoroids are exposed to cosmic radiation.