What Is a Comet Tail? Two Tails Pointing the Same Wrong Way
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A comet's tail does not stream behind it like smoke from a train. It points away from the sun regardless of which way the comet is travelling, so on the outbound leg of an orbit the comet is chasing its own tail. That single fact told astronomers something was flowing outward from the sun long before anyone measured it.
Why it points where it does
A comet nucleus is a small body of ice, dust and rock, typically a few kilometres across and extremely dark. Far from the sun it is inert. As it approaches, solar heating causes the ices to sublimate directly from solid to gas, releasing gas and carrying embedded dust with it, and that material forms a large diffuse atmosphere around the nucleus called the coma, which can be larger than a planet while containing almost nothing. Two forces then act on that released material, both directed away from the sun. Radiation pressure, the momentum carried by sunlight, pushes on dust grains. The solar wind, a stream of charged particles flowing continuously outward, interacts with ionised gas. Neither has anything to do with the comet's direction of travel, so both tails extend anti-sunward at all times. The tail is also extraordinarily tenuous, with densities far below any laboratory vacuum, and the Earth has passed through a comet tail without detectable effect despite public alarm at the time.
The two tails and the rest of the structure
What looks like one tail is generally two, formed by different mechanisms and distinguishable in photographs:
- •The ion tail, made of gas ionised by ultraviolet light and swept directly outward by the solar wind, which appears blue from carbon monoxide ions, is straight and narrow, and shows knots and disconnection events when the solar wind changes
- •The dust tail, made of grains pushed more gently by radiation pressure, which appears yellowish from reflected sunlight and curves because grains released earlier have moved into slightly different orbits
- •The coma, the diffuse envelope around the nucleus, which is what gives a comet its fuzzy appearance in a telescope
- •A hydrogen envelope far larger still, invisible to the eye and detectable in ultraviolet
- •An antitail, an apparent spike pointing towards the sun, which is a perspective effect produced when the Earth crosses the comet's orbital plane and sees part of the curved dust tail edge on
- •Meteor showers, produced when the Earth passes through a stream of dust left along a comet's orbit, which is why several showers recur on the same dates annually
Where comets come from
Short-period comets, returning within about two hundred years, originate mostly in the Kuiper belt beyond Neptune and in the scattered disc, and their orbits lie near the plane of the planets. Long-period comets arrive on extremely elongated orbits from all directions, which implies a roughly spherical reservoir at enormous distance, the Oort cloud, proposed on exactly that orbital evidence and never directly observed. Objects are nudged inward by passing stars, galactic tides and planetary perturbations. Each passage close to the sun costs material, so a comet has a finite active lifetime and eventually exhausts its volatiles, becoming a dark inert object indistinguishable from an asteroid, or disintegrates entirely, which has been observed. The distinction between comet and asteroid has consequently blurred, with active asteroids and dormant comets occupying the middle ground. Comets are of particular scientific interest because their material has been kept cold since the solar system formed, so they preserve the original composition in a way that planetary material does not.
What the missions found
Spacecraft have transformed the subject. A fleet of probes met Halley's comet in 1986 and returned the first images of a nucleus, revealing a very dark surface with jets of material issuing from discrete active regions rather than uniformly. A later mission deliberately fired an impactor into a nucleus to expose subsurface material. Another collected dust samples in aerogel and returned them to Earth, finding minerals that form at high temperature, which implied substantial mixing in the early solar system between the hot inner region and the cold outer one. The Rosetta mission orbited a comet for two years and landed a probe on it, watching activity increase and decline through the approach to the sun, and its measurements found the ratio of hydrogen isotopes in the comet's water differing from Earth's ocean water, which weakened the case that comets delivered most of the Earth's water and strengthened the case for asteroids. The comet also proved to be two lobes joined at a neck, apparently a gentle merger of two bodies, which turned out to be a common shape.
The takeaway
Radiation pressure and the solar wind push released material directly away from the sun, so the tail ignores the direction of travel and leads the comet on the outbound leg. The straight blue ion tail is swept by the solar wind and the curved yellowish dust tail is nudged by sunlight. The material is thinner than any laboratory vacuum. Rosetta found the comet's water isotopes differ from Earth's oceans, weakening the comet delivery theory.