15. Comets

Discovery
Comets are common visitors to our night skies. A typical comet might appear as a fuzzy ball low in the evening sky. Some comets may even sport a tail and occasionally a spectacular comet can be seen in the daytime. Accounts of the rarer spectacular occurrences have been recorded by virtually all ancient civilizations. These rare apparitions caused panic and were thought by some to be poisonous vapours in the atmosphere.
Tycho Brahe observed the brilliant comet of 1577 and detected no parallax over a baseline of several kilometres which put the phenomenon well out of the atmosphere and observed no diurnal parallax putting the comet well beyond the moon.

Halley's Comet
Edmond Halley first suggested that comets are regular visitors of our solar system. He believed that several of the recorded bright comets might really be the same comet approaching the Sun at periodic intervals. He realized that one particularly bright comet was being sighted once every 76 years or so. There are Chinese records of comet Halley going back to at least 240 BC. The Bayeux Tapestry, which commemorates the Norman Conquest of England in 1066, depicts an apparition of Comet Halley. It was recorded in 1531, also in 1607, and again in 1682. Halley predicted the comet's next appearance to be in late 1758. When the comet appeared as predicted, Halley's theory was proven correct and the comet was named in his honor. This was a rare occasion when a comet was named for the person who predicted its future appearance. In most cases, comets are named for their discoverers. Comet Halley last approached the Sun in 1986 and will be back again in 2061.

Naming Comets
If you discover a comet, you can name it after yourself or a loved one! Often a new comet will have two or more discoverers and so have names such as Hale-Bopp, Swift-Tuttle etc. The new comet will also be given a temporary designation consisting of the year of discovery followed by a letter in alphabetical order of discovery that year, vis: 1999a, 1999b, etc. Later, the comets are given a permanent designation in order of perihelion passage: the year of first observed passage followed by a Roman numeral e.g. 1997-I, 1997-II, etc.

Orbits
Most comets arrive on our doorstep like a bolt out of the blue. Large ones have their brief moment of glory before sailing past the sun (or sometimes actually hitting it) then sailing back into the depths of space. The orbits of most of these interlopers are always very close to parabolic which led Oort to propose a source of comets in what we now call the Oort cloud about 100,000 astronomical units (AU) from the Sun. Trillions of comets may reside in this cloud, orbiting the Sun near the edge of the Sun's gravitational tug.

In addition to the comets in the Oort Cloud, billions more orbit the Sun beyond the orbit of Pluto. This belt of comets is called the Kuiper Belt.

Around a dozen "new" comets are discovered each year. Most of them are long-period comets, meaning that they have orbits that can take as much as 30 million years to complete one trip around the Sun.

The comets are perturbed by planets and can be ejected or captured into tighter orbits such as Halley's comet. These are the short-period comets, most of which take less than 200 years to complete one orbit.

Physical Nature
When comets (which are basically 20km diameter iceballs) approach the Sun they are heated and become active, "fuming iceballs". This gives rise to several distinct features:

nucleus: relatively solid, stable(?), mostly ices (H₂O, NH₃, CO₂ etc. with a small amount of dust and other solids.
coma: dense cloud of water, carbon dioxide and other gases sublimed off the nucleus. The coma fluoresces and this is what you see when a comet gets inside the orbit of Jupiter, it becomes much brighter than the tiny nucleus.
hydrogen cloud: huge (millions of km in diameter) but very sparse envelope of neutral hydrogen. (Not visible.)
dust tail: up to 10 million km long composed of smoke-sized dust particles driven off the nucleus by escaping gases; this is the most prominent part of a comet to the naked eye. The dust is slowly accelerated by solar radiation pressure and forms a long, smooth tail.
ion tail: up to 100 million km long composed of gas which is driven out of the coma by collisions with the solar wind and ionized (so fluorescing) in the solar ultraviolet radiation. The interactions with the solar wind is quite rapid and bright knots and streamers form as the nucleus "puffs away" under the heat of the sun. So the ion tail points in a straight line away from the sun while the "solar sailing" dust tail makes a gentle curve.

Origin
Each comet has a tiny solid part, the nucleus, often no bigger than a few kilometers across. The nucleus contains icy frozen gases with imbedded rock(?) and dust. At its center, the nucleus may have a small, rocky core. These bodies are thought to be a sample of the primordial material from which the solar system evolved. They are among the least changed objects in our solar system, and, as such, may yield important clues about the formation of our solar system.

Composition
Frozen water, carbon dioxide, ammonia and methane.

Appearance
As a comet begins its inward pass toward the Sun, it begins to warm up and turns from a dim cold object into one so bright that we can see it on Earth. This transformation occurs when the comet comes inside Jupiter's orbit and heat from the Sun vaporizes ice on the comet's surface. The evolved gases fluoresce (glow) in the solar radiation. "Vents" on the sun-warmed side may squirt fountains of dust and gas for several thousand kilometers. All this escaping material forms a large, tenuous atmosphere called the coma, typically a few hundred thousand kilometers in diameter. Solar energy and the flow of electrically charged particles, called the solar wind, blow the coma materials away from the Sun, forming the comet's long, glowing tail, which is often split into a straight tail of electrically charged ions and an arching tail of dust. The ion tails(s) of a comet always point away from the Sun and are caused by the collision of molecules in the coma with the solar wind. Changes in the solar wind are often "seen" since they immediately affect the ion tail. The dust tail is slowly accelerated by light pressure (solar sailing) and may form a broad slightly curved tail.

Almost all comet orbits keep them safely away from the Sun itself. Comet Halley comes no closer than 89 million kilometers from the Sun, just inside Venus's orbit. However, some comets, called sungrazers, may crash straight into the Sun or get so close they are tidally disrupted and may completely burn up.