Whether an asteroid can possess a tail like a comet touches upon the fundamental distinctions between the solar system’s small bodies. While most classic rocky asteroids do not develop tails, a growing number of exceptions have been observed. Asteroids are traditionally defined as rocky, metallic bodies, remnants from the early solar system that formed in the warmer inner regions. The spectacular, flowing tail is the defining visual characteristic of a comet, a different class of object entirely.
The Fundamental Difference Between Asteroids and Comets
The distinction between asteroids and comets lies in their composition and original location. Asteroids are composed mainly of non-volatile materials like rock and metal. They formed closer to the Sun, primarily residing in the Main Asteroid Belt between Mars and Jupiter, where temperatures were too high for water ice to remain solid.
Comets, in contrast, contain significant amounts of volatile ices, such as water, carbon dioxide, and ammonia, mixed with dust and rock. These icy bodies originated much farther out in the solar system, in the frigid Kuiper Belt or the distant Oort Cloud. Their far-flung origins allowed them to preserve these volatile ices, which are the source of their activity.
Asteroids typically have stable, circular orbits confined to the Main Belt. Comets often have highly eccentric orbits that bring them toward the Sun. The lack of surface ice on a typical asteroid means it remains solid and inert, explaining why asteroids ordinarily do not develop the gaseous atmosphere (coma) and tail associated with comets.
The Mechanism Behind Celestial Tails
The tail of a comet is created by sublimation, where ices transition directly from a solid to a gaseous state. As a comet approaches the Sun, solar radiation heats the surface, causing volatile ices to vaporize. This escaping gas carries dust particles away from the nucleus, forming a temporary atmosphere called the coma, and eventually the streaming tail.
The material forms two distinct tails due to different forces acting upon the gas and dust. The dust tail, composed of silicate particles, is pushed away by the pressure of sunlight (radiation pressure). Because these particles have mass, the dust tail often appears broad and curved, trailing slightly along the comet’s orbital path.
The ion tail, also called the gas or plasma tail, is made of ionized gases like carbon monoxide and nitrogen. This tail is affected not by light pressure, but by the solar wind, a fast-moving stream of charged particles from the Sun. The solar wind pushes the ionized gas directly away from the Sun, making the ion tail appear straight and often blue.
Active Asteroids and the Tail Exception
The observation of asteroids with tails led to the creation of a new classification: active asteroids. These objects maintain the orbital characteristics of an asteroid, but occasionally display a coma and tail. They were initially called Main-Belt Comets (MBCs) because their activity was linked to the sublimation of ice within the asteroid belt.
One trigger for activity is the sublimation of volatile ice trapped beneath the surface. Although the outer layers of Main Belt asteroids are too warm for ice, some bodies may have preserved subsurface ice that is periodically exposed by minor impacts or thermal fracturing. For instance, 133P/Elst-Pizarro displays a cometary dust tail around its closest approach to the Sun, suggesting recurring sublimation.
Not all active asteroids are icy, which is why the broader term is used. Activity can also be triggered by non-sublimation mechanisms, such as a recent, small-scale collision that generates a cloud of debris mimicking a tail. The asteroid P/2010 A2 is thought to have generated its tail-like structure from such an impact event.
Another mechanism involves rotational instability, where an asteroid spins so quickly that material is shed from its surface, creating a temporary dust tail. The asteroid (6478) Gault is an example of an active asteroid whose tail formation was likely caused by this rotational breakup. The existence of these active asteroids challenges the traditional separation between comets and asteroids, demonstrating that some rocky bodies can generate a tail through various processes of mass loss.