When the word “comet” comes to mind, you may envision a dazzling streak traversing the sky. Perhaps a relative of yours witnessed a comet prior to your birth, or maybe you had the opportunity to see one yourself when comet Nishimura made its appearance near Earth in September 2023. However, have you ever wondered what these extraordinary celestial objects are composed of, where they originate from, and why they possess such remarkable tails?
In my role as a planetarium director, my primary focus is to ignite excitement and interest in space among individuals. Nothing captivates people’s curiosity about our planet’s position in the universe quite like comets. These astronomical phenomena are incredibly unpredictable and often remain undetected until they approach the Sun. Even after all these years, I still cannot contain my enthusiasm when a comet enters our field of view.
So, what exactly is a comet? Comets are remnants left over from the formation of our solar system around 4.5 billion years ago. As the solar system took shape, most of the gas, dust, rocks, and metals gravitated towards the Sun and the planets. However, what remained uncaptured eventually evolved into comets and asteroids.
Renowned as “dirty snowballs” or “icy dirtballs” among astronomers, comets consist of agglomerations of rocks, dust, ice, and frozen forms of various gases and molecules. The core of a comet, known as the nucleus, is composed of these ice and dirt clumps. Around the nucleus lies a porous and almost fluffy layer of ice, resembling a snow cone. This façade is encased by a dense crystalline crust, formed when the comet approaches the Sun and its outer layers undergo heating. Thus, with an outer shell exhibiting a crispy texture and an inner core possessing a fluffy consistency, comets have been likened to deep-fried ice cream by astronomers.
Most comets span a few miles in width, with the largest known comet measuring approximately 85 miles in width. Due to their relatively small size and dark appearance compared to other celestial bodies within the solar system, comets usually go unnoticed unless they draw near to the Sun.
As a comet approaches the Sun, it gradually heats up. This increase in temperature leads to the direct conversion of the frozen gases and molecules constituting the comet from solid ice to gas through a process called sublimation. Consequently, this sublimation process releases trapped dust particles from beneath the comet’s surface.
The liberated gas and dust form a visible cloud surrounding the comet, referred to as a coma. Upon interaction with the Sun, this cloud gives rise to two distinct tails. The first tail, known as the ion tail, primarily comprises gas. The Sun’s radiation strips electrons from the gases present in the coma, rendering them positively charged—it is these charged gases that astronomers term ions. Subsequently, the solar wind propels these ionized gas particles directly away from the Sun, resulting in the formation of a tail that appears blue in color. The blue hue arises from the presence of vast quantities of carbon monoxide ions within the tail.
Conversely, the dust tail emerges from the expulsion of dust particles during sublimation. These particles are driven away from the Sun due to the pressure exerted by the Sun’s light. The tail reflects sunlight and trails behind the comet as it progresses, thereby impacting the tail with a curved shape.
As a comet nears the Sun, its tail becomes longer and brighter. In fact, the tail can extend significantly beyond the nucleus, reaching lengths of up to half a million miles.
The question now arises—where do comets originate? All comets possess highly eccentric orbits, traversing elongated ovals with extensive trajectories that bring them in close proximity to the Sun while also taking them far away from it.
The speed at which an object orbits the Sun is directly proportional to its proximity to the Sun, in accordance with the principle of conservation of angular momentum. Analogously, an ice skater spins at a faster rate when they draw their arms closer to their body. Similarly, comets accelerate as they approach the Sun. The majority of their journey entails relatively slow movement within the outer regions of the solar system.
Many comets are believed to originate from a distant region within our solar system known as the Oort cloud. The Oort cloud is projected to be a spherical shell comprising numerous small bodies surrounding our solar system, with its inner boundary located approximately 2,000 times farther from the Sun than Earth. For comparison, Pluto lies just about 40 times farther away.
Comets stemming from the Oort cloud require over 200 years to complete a single orbit, a measure known as the orbital period. Due to their extended orbital periods, these comets are referred to as long-period comets. Astronomers often struggle to gather substantial information about these comets until they approach the inner region of the solar system.
On the other hand, short-period comets possess orbital periods lasting less than 200 years. One famous example is Halley’s comet, which comes close to the Sun every 75 years.
Although such a timeframe may appear extensive to humans, it is abbreviated for a comet. Short-period comets predominantly emerge from the Kuiper Belt—a region beyond Neptune containing asteroids and notably serving as the former abode of Pluto.
Within the realm of short-period comets, a subset only reaches the farthest point from the Sun approximately at Jupiter’s orbit. These comets possess orbital periods of fewer than 20 years and are known as Jupiter-family comets.
Comets remain within the inner solar system for a relatively brief period, typically ranging from weeks to months. As they approach the Sun, their tails elongate, and their luminosity intensifies before waning as they depart for the outer reaches of the solar system.
Nonetheless, even short-period comets visit infrequently, and their porous interiors occasion intermittent disintegration. Consequently, predicting their behavior becomes an intricate task. Astronomers track comets as they approach the inner solar system and utilize observations to formulate projections. Yet, they can never ascertain with certainty whether a comet will achieve sufficient brightness to be seen with the naked eye while passing by Earth or if it will disintegrate and fizzle out upon entering the inner solar system.
Regardless, comets will continue to tantalize and captivate humans, compelling them to gaze skyward for generations to come.
This article, originally published on The Conversation, was authored by Shannon Schmoll from Michigan State University.
