Comets
Icy bodies that grow tails as they near the Sun.
Overview
A comet is a small icy body that releases gas and dust when it approaches the Sun, forming a glowing coma and often two distinct tails. Comets originate in the cold outer Solar System and preserve some of its least altered material.
- A comet's nucleus is typically only a few kilometres across; the coma it produces can exceed the Sun in diameter.
- Comets have two tails with different physics — one of ions, one of dust — and they point in different directions.
- Tails always point away from the Sun, so an outbound comet travels tail-first.
- Rosetta's two-year escort of comet 67P is the most detailed study of a comet ever made.
Anatomy of a comet
The nucleus is a solid body of ice, dust and rock, typically a few kilometres across and among the darkest surfaces in the Solar System — the 'dirty snowball' picture is closer to a very dark, porous, dusty ice. As the comet approaches the Sun, surface ices sublimate directly to gas, lifting dust with them.
That released material forms the coma, a diffuse envelope around the nucleus that can grow to more than a million kilometres across, larger than the Sun. Beyond it, a vast hydrogen cloud is detectable in ultraviolet. Nearly everything visible about a comet is this temporary escaped atmosphere, not the tiny solid body producing it.
Two tails, two mechanisms
The ion tail forms when solar ultraviolet ionises cometary gas, and the solar wind's magnetic field carries those ions directly away from the Sun. It is bluish, straight, and can show sudden structural changes when the solar wind shifts.
The dust tail forms from grains pushed outward by radiation pressure. Because dust grains retain their orbital motion and respond less sharply than ions, the dust tail curves along the comet's path and appears yellowish-white, reflecting sunlight rather than emitting.
Both tails point broadly away from the Sun regardless of the comet's direction of travel, so a comet moving outward after perihelion leads with its tail rather than its nucleus — it travels tail-first.
Where comets come from
- Short-period comets, with orbital periods under about 200 years, mostly originate in the Kuiper Belt and scattered disc beyond Neptune. Their orbits tend to lie near the plane of the Solar System.
- Long-period comets come from the Oort Cloud, a hypothesised roughly spherical reservoir extending tens of thousands of astronomical units out. Their orbits arrive from all directions, which is the principal evidence for the cloud's spherical geometry.
- Comets are not permanent. Each perihelion passage removes material, and they eventually exhaust their volatiles, break apart, or are ejected or captured. Some become effectively dormant and are hard to distinguish from asteroids.
- Meteor showers occur when Earth crosses a stream of debris left along a comet's orbit — the Perseids come from comet Swift–Tuttle, the Orionids from Halley's Comet.
What Rosetta found
ESA's Rosetta mission escorted comet 67P/Churyumov–Gerasimenko for over two years from 2014 and deployed the Philae lander to its surface. It observed the nucleus's activity change through perihelion, mapped a bilobed shape indicating a low-speed merger of two bodies, and measured a bulk density so low that the interior must be highly porous.
Its measurement of the deuterium-to-hydrogen ratio in 67P's water was notably different from Earth's ocean value, weakening the case that Jupiter-family comets were the main source of Earth's water and shifting attention toward asteroidal delivery. It also detected molecular oxygen in the coma and glycine, an amino acid — results relevant to how prebiotic material is distributed.
Continue in the data
Catalogues, hubs, and reference pages that hold the underlying records for this topic.
Frequently asked
- Why do comets have tails?
- Because solar heating sublimates ices on the nucleus, releasing gas and dust. Solar ultraviolet ionises the gas and the solar wind sweeps those ions straight away from the Sun, forming a narrow blue ion tail; radiation pressure pushes dust grains outward more gently, forming a curved white dust tail. Both point broadly away from the Sun rather than backwards along the comet's motion.
- How often does Halley's Comet return?
- About every 75 to 76 years. It last reached perihelion in 1986 and is next expected in 2061. The interval varies slightly because gravitational interactions with the planets perturb its orbit between passages. Chinese records allow its apparitions to be traced back to 240 BCE.
- Are comets dangerous?
- A large impact would be, but the population is far smaller than the near-Earth asteroid population and long-period comets are the harder case because they arrive from the outer Solar System with little warning. No known comet is on an Earth-impact trajectory. Earth passes through cometary debris streams routinely — that is what a meteor shower is — and those particles are harmless.
- Did comets bring water to Earth?
- Some, probably, but the evidence has shifted. Measuring the deuterium-to-hydrogen ratio in cometary water tests the idea, and Rosetta found 67P's ratio significantly different from Earth's oceans. Combined with results from other comets and from carbonaceous meteorites, current evidence favours asteroids as the dominant source, with comets contributing less than once assumed.