Halley's Comet

Halley's Comet was the first comet recognized as returning periodically.

Edmond Halley compared historical sightings and inferred they were the same comet.

Halley's Comet last passed perihelion in 1986.

Halley's Comet is expected to return near perihelion in 2061.

Halley's Comet orbits retrograde, opposite to most planets.

Giotto photographed Halley's nucleus up close in 1986.

Halley's nucleus is irregular and very dark, not a bright ball of ice.

The Eta Aquariid meteor shower comes from debris left by Halley's Comet.

The Orionid meteor shower also comes from Halley's debris stream.

Halley's orbital period is roughly 76 years, varying with planetary gravity.

The nucleus is a comet's solid body, made mainly of ice, dust, and rock mixed together.

Near the Sun, cometary ice can sublimate directly to gas and carry dust with it.

The diffuse envelope of gas and dust around the nucleus is the coma.

The solar wind influences the ion tail, directing it roughly away from the Sun.

Dust-tail particles have their own orbits, often producing a curved tail.

A comet's tail can point ahead as it moves away from the Sun; it is not exhaust trailing its motion.

A coma can be many times larger than its nucleus; a photo's bright cloud is not the solid body's size.

Meteor showers can occur when Earth crosses debris left by a comet.

A meteor is light produced when a small particle enters the atmosphere, not a falling star.

Most returning comets still require a telescope to observe.

Comet brightness depends on activity, not just distance from Earth.

Short-period comets generally have orbital periods under 200 years.

Many long-period comet orbits suggest origins in the distant Oort Cloud.

Comets preserve early Solar System material and help us study planetary formation.

Comets generally move faster at perihelion and slower at aphelion.

Planetary gravity, especially Jupiter's, can change comet orbits and sometimes disrupt their nuclei.

Cometary gas jets exert small reaction forces that precise predictions must include.

Orbital elements refer to an epoch; neglected perturbations can accumulate over time.

P in a comet designation means periodic; C is commonly used for comets that are not short-period.

Dark dust often coats comet nuclei; containing ice does not make their surfaces snow-white.