Rock comets, unlike other comets, which outgas primarily ice, have a nucleus made of rock. As a result, their outgassing can be unpredictable.. The reason this happens is thought to be similar to how mud cracks at the bottom of a dry lake bed, leaving small chunks loose on the surface of the body, which later get pushed off it due to radiation pressure sweeping.
Comet Humason, formally designated C/1961 R1 (a.k.a. 1962 VIII and 1961e), was a non-periodic comet discovered by Milton L. Humason on September 1, 1961. Its perihelion was well beyond the orbit of Mars, at 2.133 AU. The outbound orbital period is about 2516 years. The diameter of its comet nucleus is estimated at around 30−41 km.