13.3 The "Long Haired" Comets
Comets differ from asteroids primarily in their icy composition, which causes them to brighten dramatically as they approach the Sun, forming a temporary atmosphere. Historically, they were often seen as omens of disaster.
General Physical Appearance of Comets
A comet is a relatively small chunk of icy material, typically a few kilometers across.
It develops an atmosphere, known as the head or coma, as it approaches the Sun.
A very faint, nebulous tail, extending several million kilometers, may also form.
From Earth, a typical comet appears as a faint, diffuse spot of light, smaller than the Moon and less brilliant than other celestial objects due to artificial lighting.
Comets wander among the stars, slowly shifting their positions nightly, but unlike planets, most appear at unpredictable times.
They typically remain visible for varying periods, from a couple of weeks to several months.
When observed in the real sky, comets are distinctly different from meteors (shooting stars); meteors burn up in seconds, while comets are visible for weeks in nearly the same part of the sky.
Range of Cometary Orbits
The study of cometary orbits began with Isaac Newton, who suggested they orbit the Sun on extremely elongated ellipses.
Edmund Halley developed these ideas, publishing calculations of 24 comet orbits in 1705. He notably recognized Comet Halley as a periodic comet, returning at average intervals of 76 years.
Comet Halley's return period varies from 74 to 79 years due to gravitational perturbations from giant planets.
Its aphelion (furthest point from the Sun) is beyond Neptune's orbit.
It was last seen in 1986 and is predicted to return in 2061.
Comet orbits can be categorized by their periods:
Short-period comets: Return in less than one or two centuries, like Comet Halley. Many of these, known as Jupiter-family comets, have had their orbits altered by close approaches to giant planets, most often Jupiter.
Long-period comets: Take thousands of years to return, if at all.
Notable comets and their periods include Swift-Tuttle (133 years), Churyumov-Gerasimenko (6.5 years), and Encke (3.3 years, the shortest known period).
Size and Composition of a Typical Comet's Nucleus
The nucleus is the real comet—a small, solid core of ancient icy material, typically only a few kilometers across.
It is usually hidden by the much larger, glowing atmosphere (coma) and is responsible for producing this atmosphere and the tail.
The modern theory, known as the “dirty snowball” model, was proposed by Fred Whipple in 1950.
According to this model, the nucleus is composed substantially of water ice, mixed with other ices (like carbon monoxide and carbon dioxide), silicate grains, and dust.
The “dirt” in the dirty snowball consists of dark, primitive hydrocarbons and silicates, similar to material found on dark asteroids.
Nuclei are difficult to study from Earth due to their small size and dark nature.
Spacecraft missions, such as the Giotto flyby of Comet Halley in 1986, provided the first direct measurements, revealing an irregularly shaped, black nucleus about 10 kilometers long with jets of escaping material.
The Deep Impact mission in 2005 impacted Comet Tempel 1.
The Rosetta mission provided the most extensive study, revealing Comet Churyumov-Gerasimenko's (67P) strangely misshapen, double-lobed nucleus, approximately 5 kilometers across. It has a dark surface rich in organic carbon compounds, sulfides, and iron-nickel grains, and a low average density of 0.5 g/cm^3, indicating high porosity.
Atmospheres of Comets
The spectacular activity that makes comets visible is caused by the evaporation of their ices when heated by sunlight.
Beyond the asteroid belt, cometary ices are solidly frozen. As a comet approaches the Sun, it warms up, and if water is the dominant ice, significant quantities vaporize above 200 K.
This evaporation releases dust mixed within the ice.
Due to the comet's weak gravity, both gas and dust flow away into space at speeds of about 1 kilometer per second.
Evaporation is often sporadic, occurring in sudden spurts or confined to a few active areas on the surface.
The expanding atmosphere, or head (coma), can reach enormous sizes, often as large as Jupiter, sometimes approaching a million kilometers in diameter.
Most comets also develop tails as they near the Sun. Tails are extensions of the atmosphere, composed of gas and dust.
Comet tails always point away from the Sun, a phenomenon first theorized by Newton as a repulsive force of sunlight.
There are two main components to a comet's tail:
Dust tail: The brighter part, composed of smoke-sized dust particles. It curves slightly as these particles spread out along the comet's orbit and can be up to 10 million kilometers long.
Ion tail (or gas tail): A fainter, straight tail made of ionized gas. It is pushed directly outward from the Sun by the solar wind of charged particles and can extend for hundreds of millions of kilometers.
Discoveries of the Rosetta Mission
The Rosetta mission, launched in 2004, was designed to orbit Comet Churyumov-Gerasimenko (67P) and deploy a lander, Philae.
Key discoveries and observations from Rosetta, which began approaching 67P in August 2014, include:
Nucleus Morphology: The nucleus of 67P is strangely misshapen and double-lobed, approximately 5 kilometers across. This shape is tentatively attributed to the collision and merger of two independent comet nuclei.
Surface Features: The surface is surprisingly rough, with sharp angles, deep pits, and overhangs, differing from the smoother appearance seen in images of Halley's nucleus. Imaging resolution was nearly 100 times greater than previous missions.
Composition: The dark surface is rich in organic carbon compounds, mixed with sulfides and iron-nickel grains.
Density and Porosity: 67P has a low average density of only 0.5 g/cm^3, indicating a highly porous structure with a large amount of empty space.
Activity Patterns: Less than 1\% of the surface is active at any given time. Gas and dust are released in narrow jets, typically a few meters across, persisting for only a few minutes. Activity levels are strongly dependent on solar heating.
Water Isotope Analysis: Isotopic analysis of deuterium in the water ejected by 67P revealed that its water composition is different from Earth's water. This suggests that comets like 67P might not have been the primary source of Earth's oceans.
Philae Lander's Fate: The Philae lander successfully touched down on November 12, 2014, but bounced and came to rest under an overhang, where insufficient sunlight prevented sustained battery charging, limiting its operational time.