The sun transforms comets from dark, frozen objects into the glowing, tailed spectacles we see in the night sky. As a comet falls inward on its orbit, solar heat vaporizes its ices, solar radiation pushes dust away from its surface, and the solar wind sculpts streams of charged gas into tails that can stretch for millions of kilometers. At extreme close range, the sun can tear a comet apart entirely.
How Solar Heat Wakes a Comet Up
A comet spends most of its life as a quiet, dark chunk of ice and rock, sometimes only a few kilometers across. The transformation begins when its orbit carries it within about 2 to 3 astronomical units of the sun (roughly the distance of the asteroid belt). At that range, solar heating warms the surface enough to trigger sublimation: ices skip the liquid phase and convert directly into gas, which escapes from the surface and carries fine dust particles along with it.
Not all ices sublimate at the same temperature. The most volatile compounds, carbon monoxide and nitrogen, begin turning to gas at extremely low temperatures, around 20 to 30 degrees above absolute zero. Carbon dioxide follows at roughly 85 K. Water ice, the most abundant frozen material in a comet, requires temperatures above 140 K to sublimate. Because of this layered response, a comet’s activity changes as it approaches the sun. The most volatile gases escape first, even at great distances, while water-driven activity dominates closer in. Together, the escaping gas and dust form the coma, a fuzzy envelope surrounding the solid nucleus that can swell to tens of thousands of kilometers wide.
Two Tails, Two Forces
Once a coma develops, the sun shapes it into the comet’s most recognizable feature: its tails. Most comets grow two distinct tails, each created by a different solar force.
The dust tail forms when sunlight itself pushes on tiny particles released from the nucleus. Photons carry momentum, and when they strike dust grains, they nudge them away from the sun. Because dust is relatively heavy and slow to accelerate, it tends to lag behind the comet’s motion, curving gently into a broad, yellowish-white arc. You’re seeing reflected sunlight when you look at a dust tail.
The ion tail (also called the plasma tail) is a product of the solar wind, a stream of charged particles constantly flowing outward from the sun. Ultraviolet radiation from the sun ionizes gas molecules in the coma, stripping away electrons and creating electrically charged particles. The solar wind then catches these ions and accelerates them almost directly away from the sun. Meanwhile, the magnetic field lines embedded in the solar wind drape around the comet’s nucleus, funneling the ions into a narrow, straight structure. Ion tails glow blue because ionized molecules emit their own light rather than simply reflecting sunlight. This tail always points almost exactly away from the sun, regardless of which direction the comet is traveling.
Solar UV Breaks Molecules Apart
The sun doesn’t just heat and push cometary material. Its ultraviolet radiation actively breaks molecules apart through a process called photodissociation. Water molecules, for example, get split into hydrogen and hydroxyl fragments. Carbon monoxide can be ionized or broken down as well. These reactions change the chemical makeup of the coma as gas moves farther from the nucleus, creating shells of different chemical species that astronomers can identify through the light they emit.
The intensity of this process fluctuates with the sun’s 11-year activity cycle. During solar maximum, UV output increases enough to boost the rates of ionization, dissociation, and fluorescence by factors of up to four. That means the same comet passing through the inner solar system during a solar maximum can look noticeably different in brightness and emission characteristics than it would during solar minimum.
The Rocket Effect on a Comet’s Orbit
Every time gas jets off the surface of a comet, it acts like a small thruster. The escaping material pushes back against the nucleus, creating what scientists call non-gravitational acceleration. This effect is subtle but measurable, and it means a comet’s orbit is never determined by gravity alone.
Because a comet’s surface isn’t uniform, outgassing tends to be stronger on the sunlit side and can be concentrated in specific active regions. The resulting thrust doesn’t always point straight away from the sun. It can have sideways and out-of-plane components that slowly shift the comet’s path over time. For comet 67P/Churyumov-Gerasimenko, studied up close by the Rosetta spacecraft, scientists modeled this acceleration by combining the total gas production rate, the speed of escaping gas, and the comet’s mass. These non-gravitational forces have been used to explain the orbital behavior of many comets, and were even invoked to account for the unexpected acceleration of the interstellar object ‘Oumuamua as it passed through our solar system.
Over many orbits, this rocket effect gradually changes a comet’s trajectory, potentially shifting its closest approach to the sun or altering when it arrives at perihelion (its nearest point to the sun). The effect also means that predicting a comet’s return date requires accounting for how much material it loses on each pass.
Thermal Stress and Fragmentation
Solar heating doesn’t just vaporize surface ice. It can crack a comet apart. When a nucleus approaches the sun, its surface heats rapidly while its interior stays cold. This temperature difference creates mechanical stress in the material, much like pouring hot water into a cold glass. For comets with nuclei between roughly 30 meters and 5 kilometers across, these thermal stresses can exceed the tensile strength of the material and trigger fragmentation.
Smaller fragments, under about 30 meters, heat evenly throughout and don’t build up dangerous temperature gradients. Larger nuclei, over a kilometer, develop the highest stress in their near-surface layers, which is consistent with observations of comets developing visible activity (coma formation) as they approach the sun. The breakup of Comet Shoemaker-Levy 9, famously torn apart before crashing into Jupiter in 1994, may have been initiated by thermal fracturing before tidal forces from Jupiter finished the job. Larger objects tend to survive longer but fragment closer to the sun, often triggering a cascade where each piece breaks further into smaller chunks.
Sungrazing Comets and Total Destruction
The most extreme example of the sun’s effect on comets is what happens to sungrazers, comets that pass within a few solar radii of the sun’s surface. The Kreutz family of sungrazers, a group sharing similar orbits with perihelion distances of just 1 to 2 solar radii, have been observed in large numbers by the SOHO solar observatory. Over 20 years, SOHO detected more than 3,000 of these objects.
Almost none survive. The typical Kreutz comet brightens as it approaches, peaking at around 10 to 15 solar radii from the sun’s center, then fading and disappearing well before it reaches its closest point. This pattern indicates complete vaporization and destruction. Based on how quickly they fade, most SOHO-observed Kreutz comets are estimated to have been less than 100 meters across before they began losing material. The smallest ones detected were probably only 5 to 10 meters in diameter, and even smaller ones likely exist but are too faint to spot.
Out of all the Kreutz comets SOHO has watched, only one survived perihelion: Comet Lovejoy (C/2011 W3), which emerged from behind the sun to the surprise of many astronomers. Its survival was attributed to a larger-than-average nucleus, though it did fragment significantly during the encounter. For the vast majority of sungrazers, a close pass means total annihilation.
What Happens Over Many Orbits
Each time a comet rounds the sun, it loses material permanently. A typical short-period comet might shed a thin layer of ice and dust on every pass, gradually building up a dark, insulating crust of non-volatile material on its surface. Over hundreds or thousands of orbits, this process can shut down surface activity almost entirely, leaving behind what’s sometimes called an extinct comet: an object that looks and behaves more like an asteroid than a comet.
The sun, in other words, is both what makes comets spectacular and what eventually destroys them. The same solar energy that creates glowing tails visible across the solar system also steadily erodes the nucleus, alters the orbit, and fractures the structure until, over time, nothing is left but a trail of dust and gas spread along the comet’s former path. When Earth passes through one of these debris trails, the particles burn up in our atmosphere as meteor showers.