Comet
Icy bodies that develop comas and tails near the Sun.
A comet is an icy, small Solar System body or interstellar object that warms and releases gases when passing close to the Sun, a process called outgassing. This produces an extended, gravitationally unbound atmosphere or coma surrounding the nucleus, and sometimes a tail of gas and dust blown out from the coma. Comets have been observed and recorded since ancient times by many cultures and religions.
- type
- Small Solar System body or interstellar object
- composition
- Ice, dust, and small rocky particles
- nucleus_size
- From a few hundred meters to tens of kilometers across
- coma_size
- Up to 15 times Earth's diameter
- tail_length
- May stretch beyond one astronomical unit
- orbital_periods
- From several years to potentially millions of years
Lore & Background
Comets usually have highly eccentric elliptical orbits, with a wide range of orbital periods. Short-period comets originate in the Kuiper belt or its associated scattered disc, while long-period comets are thought to originate in the Oort cloud. Hyperbolic comets may pass once through the inner Solar System before being flung to interstellar space. The appearance of a comet is called an apparition. Comets differ from asteroids in that they contain volatile material that sublimate as they approach the Sun to produce the characteristic tail. However, the discovery of main-belt comets and active centaur minor planets has blurred the distinction between asteroids and comets. In the early 21st century, some minor bodies with long-period comet orbits but characteristics of inner solar system asteroids were called Manx comets.
Reader's Guide
Comets are significant as remnants from the early Solar System, preserving volatile materials and organic compounds. Their outgassing provides insights into solar radiation and solar wind interactions. The nucleus of a comet is composed of rock, dust, water ice, and frozen gases, often described as 'dirty snowballs' or 'icy dirtballs.' Research has found organic compounds including amino acids and components of DNA and RNA in comet dust, suggesting a possible role in the origin of life. Comets have been visited by uncrewed probes such as NASA's Deep Impact, which blasted a crater on Comet Tempel 1 to study its interior, and the European Space Agency's Rosetta, which became the first to land a robotic spacecraft on a comet. Roughly one comet per year is visible to the naked eye, with particularly bright examples called 'great comets.' The total potential comet population in the Oort cloud is estimated at about one trillion.
Did You Know?
- The coma of a comet may be up to 15 times Earth's diameter, while the tail may stretch beyond one astronomical unit.
- The word 'comet' derives from the Greek κομήτης, meaning 'long-haired star'.
- Comet nuclei have a very low albedo, reflecting as little as 3% of light, making them among the least reflective objects in the Solar System.
Frequently Asked Questions
What is a comet?
A comet is a small, icy body—either a native Solar System object or a visitor from interstellar space—that begins releasing gas and dust when it draws close to the Sun. This outgassing produces a luminous envelope called a coma and, in many cases, a visible tail streaming away from the Sun.
What is a comet made of?
The solid nucleus of a comet is a jumbled mixture of frozen water, other ices, fine dust, and tiny rocky fragments. Because of this mix, astronomers have long nicknamed comets "dirty snowballs."
What are a comet's most dramatic features?
As solar heating drives volatiles out of the nucleus, a coma can swell to roughly fifteen times the width of Earth. The resulting gas-and-dust tail can stretch farther than one astronomical unit, making the object visible to the naked eye from our planet.
How big is a comet's nucleus compared to the coma and tail?
The hard core of a comet typically spans from a few hundred meters to several tens of kilometers across. Despite that modest size, the surrounding coma and tail can make the whole object appear thousands of times larger than its actual solid body.
How long does a comet take to complete one orbit?
Return periods vary enormously: some comets swing by the Sun every few years, while others take millions of years to circle back. Short-period comets like Halley's recur on a schedule of a few decades, whereas long-period ones may not reappear for many human generations.
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