Comets

Comets 

                                                                                                                                                                    Comets have been seen in the sky since antiquity. Comets Halley, for example, is shown in the Bayern Tapestry, which depicts the Norman Invasion of the British Isles in the year 1066. It was seen by Ancient Chinese and Greek too. In general, and like everything else in the sky, comets were consider omen or Harbingers of human events.

Sometimes they were good omens. William the Conqueror liked his chances in 1066 after seeing one and sometimes bad- that some comets didn't do so well for king Harold 2nd.

A comet is bright enough to see with the naked eye shows up in the heavens every few years or so and some can get spectacularity bright.

When we think of a comet, we probably picture a fuzzy blob and a long tail stretching away from it. Comets are in many ways similar to asteroids. They're roughly hewn chunks of stuff left owe from the formation of the solar system. Unlike Asteroid's, which mostly rocks with a dash of ice and may be metal. Comets are a more balanced mixture of ice and rock, and by ice we mean frozen water, but also frozen Carbon dioxide, Carbon monoxide, Ammonia - things we normally think of as gases on Earth, And by rocks we mean rocks also with gravel and dust.

Astronomer's sometimes called comets  "Dirty Snowballs" which isn't a half bad term. It's that ice that makes comets well comets.

When they're way out in deep space they're so cold that they're basically inert lumps of ice and rock, But many are on elliptical orbits that take them from those sub-freezing depths into our neck of woods where the sun can warm them.

As they heat up the ice turns directly into a gas, the gas then flows away from the comets, creating a cloud around it. This makes the comets looks fuzzy and actually, in the past they've been called Hairy Stars.

The solid part of the comet is called Nucleus and the gaseous cloud around it is called the Coma. Latin for Hair, In fact that's why we called them Comets.

As the ice sublimates, the bits of rock and gravel nucleus as well. This material is what forms the comet tails, but that happens to depend on which material we're talking about. The gas and dust from comets form two different tails. Gas molecules emitted by the comet get ionized by the sun's ultraviolet light. That means they lose electrons, becoming charged, and charged particles are highly susceptible to a magnetic field.

The Solar Wind is a stream of charged particles blown out by the Sun and carries a magnetic field with it. As the wind hits the ionized gas from the comet, it picks up those particles and carries them downstream away from the sun.

The solar wind is usually moving far, faster than the comet, so this "ion tail" winds up pointing directly away from the sun. The dust, on the other hand, is influenced more by sunlight. Light from the sun exerts a small but inexorable pressure, and this pushes on the dust particles.

The dust streams away, but because the pressure isn't as intense as the solar wind is on the gas tail, the dust tail blows away more lazily and tends to lag behind the comet in its orbit, That means the two tails usually points in two different directions!.

In some comets, like 1977 incredibly bright and gorgeous comet Hale- Bopp. The dust tails look white or a teeny big yellowish, due to reflected sunlight, while the ion tail glows blue or green, depending on the primary constituents of the gas.

Carbon monoxide tends to emit blue light, while carbon molecules glow a ghostly green. A comet tail can stretch for tens of millions of kilometer but despite their length, tails are incredibly low density as low as a few hundred atoms per cubic centimeter. 

The air we breathe is a million billions times denser!. In 1910, Earth passed through the tail of comet Halley deadly gas, had been detected in the tail! but nothing happened. 

Comets are classified by their orbits, If they have an orbital period less than 200 years then they're called short-period comets. These tend to orbit the sun in the same plane as the planets and go around the sun in the same direction as well. From Earth, we see them sticking near the elliptical, the line across the sky that marks the annual path of the sun. Comets that takes longer than 2 centuries to go around the sun are called Long term Comets and have orbits that are tilted, that means they can appear anywhere in the sky.

Every time they get near the sun and start outgoing they lose mass over time they get smaller, eventually, they should evaporate. Some do this all at once because of they dire into the sun, skimming our star's surface and those we called Sundivers.

Many of those may actually be pieces from a bigger comet that broke up in space nearly a thousand years ago but besides those, we know of some comets with a few years. Even a century is like a single flap of a mosquito's wing compared to the lifetime of the solar system!.

They could have orbits that last for millennia or more but then something tweaks them, makes them fall towards the sun.

In fact, there may be 2 much regions, since we have both short period and long period term comets. And studying from Earth is Hard. The Coma obscures the nucleus, making it nearly impossible to see it directly.

Soviet Mission Vega 1 was the first to successfully get pictures of the nucleus, these images were used to better determine the position of the nucleus. It was dark reflecting only 4%  of the light that falls on it. Most of the Halley nucleus is covered with dust laced with darker molecules, with only a few spots emitting gas. Most likely there are deposits of ice under the surface, and only some of them receive enough heat from the sun to sublimate and blow at gas, This has been seen with the other comets as well.

The surface of the comets must be inhomogeneous different in different places that fact was brought home magnificently in 2014 by another European Mission Rosetta.

Because of Rosetta, the first time in human history we have a chance to study the comet so closely. 

In 2004 Nasa Stardust space probe physically passed through the Coma of the Comet Wild 2, collecting samples that were returned to Earth. Carefully analysis of the material found the presence of an organic, carbon-based molecule in them. And not just any random molecules, but complex ones, including Amino Acids! which are the building blocks of life on Earth, Amino acids are what we used to create proteins in our bodies.

It's possible that ingredient of the life didn't start here, but instead was brought by a comet to our planet from a comet impact. This means all life on Earth is a part of Alien race. 

When we look into our sky, we're looking at our own origin. Comets are like Time Machine allowing us to investigate our past, 4 billion years back hinting at the secrets of the origin of life itself.