Objects · Solar System scale
Comet
An icy body orbiting the Sun. When it approaches the inner Solar System, solar heat sublimes its ices, producing a bright coma and one or two luminous tails stretching millions of kilometres. Leftover material from the outer Solar System's formation.
Notable example
Halley's Comet has been observed on every return since at least 240 BC. It last visited the inner Solar System in 1986 and returns next in 2061.
Source: NASA Solar System Exploration
Structure of a comet
A comet has four distinct parts. The nucleus — the solid core — is typically 1–50 km across and composed of ice, dust, and rocky material. When within roughly 5 AU of the Sun, solar radiation heats the surface, sublimating ices and releasing gas and dust. This activity creates the coma — a diffuse cloud of gas and dust surrounding the nucleus that can grow to thousands of kilometres across, sometimes larger than a planet.
From the coma extend the two tails: the straight, blue ion tail (plasma pushed by the solar wind) and the curved, pale dust tail (particles pushed by radiation pressure). Both always point away from the Sun, regardless of the comet's direction of travel. The Rosetta mission's close study of Comet 67P revealed a rubber-duck-shaped nucleus 4.1 km across, with jets of gas and dust erupting from active regions.
For the full story on how comets, asteroids, and meteors relate, see our guide to asteroids, comets and meteors.
Frequently asked questions
- The key difference is composition and origin. Asteroids are rocky or metallic bodies, mostly found in the asteroid belt between Mars and Jupiter — dry remnants of the inner solar system. Comets are icy bodies from the outer solar system (the Kuiper Belt or Oort Cloud) that contain frozen water, carbon dioxide, methane, and ammonia mixed with dust and rock. When a comet approaches the Sun, the ices sublimate (turn directly to gas), releasing dust and creating the characteristic bright coma and tail. See our asteroids, comets and meteors guide for the full taxonomy.
- A comet actually has two distinct tails, always pointing away from the Sun (not behind the direction of travel). The ion tail (plasma tail) is straight and blue-tinged — it is gas ionised by solar ultraviolet radiation, pushed directly away from the Sun by the solar wind at hundreds of km/s. The dust tail is curved and yellowish-white — fine dust particles released from the nucleus are pushed away by radiation pressure but more gently, curving along the comet's orbital path. Both tails can stretch for millions of kilometres. NASA's comet overview illustrates both tail types.
- Comets originate in two reservoirs at the solar system's edge. Short-period comets (orbital period under 200 years, like Halley's Comet at ~76 years) come from the Kuiper Belt — a disc of icy bodies beyond Neptune, 30–50 AU from the Sun. Long-period comets (periods of thousands to millions of years, arriving from any direction) come from the Oort Cloud — a vast spherical shell of icy bodies extending roughly 2,000–100,000 AU from the Sun. Gravitational disturbances — from passing stars, giant molecular clouds, or the galactic tide — nudge these bodies inward toward the Sun.
- Halley's Comet (1P/Halley) is the most famous — it returns every 75–76 years and has been recorded since at least 240 BC; its next perihelion is 2061. Comet Hale-Bopp (1997) was visible with the naked eye for 18 months — the longest naked-eye apparition in recorded history. Comet Shoemaker-Levy 9 (1994) fragmented and impacted Jupiter over six days, giving scientists the first direct observation of a solar system collision. The ESA Rosetta mission (2014–2016) was the first spacecraft to orbit a comet nucleus — 67P/Churyumov–Gerasimenko — and deploy a lander (Philae).
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