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How can an object in the asteroid belt show cometlike activity?

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Illustration showing the eight planets of the solar system in order from the Sun-Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune-depicted to scale by size, with labeled diameters in miles and kilometers. The Sun is partially visible on the left for comparison. — Solar system The planets (in comparative size) in order of distance from the Sun.

An object in the asteroid belt that exhibits cometlike activity—such as a coma or a tail—is known as a main-belt comet or an active asteroid.1

While asteroids are typically composed of rock and metal and lack the volatile ices that cause comets to develop tails, main-belt comets represent a unique group that bridges these characteristics. The discovery of these objects, such as 133P/(7968) Elst-Pizarro, demonstrates that some bodies within the asteroid belt possess the ability to shed gas or dust when they approach the Sun.21

This activity suggests that these objects may contain subsurface volatile materials, similar to those found in comets, which become active under solar heating. Because these objects reside in the asteroid belt but display cometary features, they challenge the traditional, rigid classification between asteroids and comets, highlighting that some small bodies may have formed as icy remnants that have since lost their volatile materials or retained them beneath a rocky surface.1