Comets
What are comets made of? Why do comets have tails? Why do comets get two tails? Why are there long-period and short period comets? What makes a meteor shower?
Comet Hale-Bopp comet is a classic example of a comet streaking across the sky with its twin tails as it flies past Earth. This comet was discovered in 1995 by Alan Hale and Thomas Bopp. It was one of the brightest comets to appear in the sky in many decades. We believe it is a very long-period comet, and will not approach Earth again for another 2,534 years.
A comet is a chunk of dust and ice that spends most of its time in the outer reaches of the solar system. Some comets have very elliptical orbits which periodically bring them close to the sun. When they get within about 5 AU from the sun, the solar heat creates a coma, an atmosphere forms around the comet, made of dust and ice that sublimes (changes directly from solid to gas).
At about 1 AU, the escaping dust and gas form two tails. The bluish ion tail (also called a plasma tail) is made of gas that is ionized by ultraviolet photons in the solar wind. The ion tail always points directly away from the sun, because it is blown by the solar wind. The white dust tail is made of dust particles that have escaped from the coma. The dust particles are electrically neutral, and not affected by the solar wind. They trail out behind the comet in its orbital arc.
This ESA simulation of a comet orbiting the sun shows how the tails of the comet appear and behave as the comet nears the sun.
This NASA simulation shows the encounter between the Deep Impact spacecraft and comet Tempel 1, where a probe was released that impacted the nucleus of the comet, punching a crater into the surface of the comet's nucleus in 2005. The information from this impact helps us to understand the details of the makeup of a comet, like how dense it is and what material it contains.
Please access the Solar Wind Tunnel software from the University of Colorado at Boulder to interactively investigate how the two tails of a comet arise in the Solar wind.
Ion tail of Comet Lovejoy shows quite a bit of structure, due to the fact that the solar wind varies over time. The solar wind is made of photons and charged particles like protons and electrons. The Ultraviolet light of the solar wind causes atoms and molecules in the comet to become ionized, or lose electrons. The solar wind blows the ions outward, away from the Sun. The ion tail can extend millions of kilometers into space. The tail dissipates when the comet gets far enough away from the Sun, and nucleus freezes into a solid rocky-ice material.
This is what a comet looks like up-close.
This image of Comet Churyumov-Gerasimenko was taken by ESA's robotic Rosetta spacecraft shows jets of material being sublimated from the surface of the comet as it nears the sun. Comets are typically rotating. As the jets increase in strength, they can cause the trajectory of the comet to become more unpredictable.
Copyright 2005 Pearson Prentiss-Hall
When comets are far from the sun, they are very dark objects. Although they are made of rocky ice, they are typically covered with a black soot, which makes them very hard to see. As they get close to the Sun, the heat of the sun causes the material to sublimate. Sublimation is a phase change, like melting. When a substance sublimates, it changes directly from a solid state to a gas state without becoming a liquid in-between.
Some comets come very close to the Sun at their closest approach, or perihelion. These comets are called Sun grazing comets. Sun grazing comets often fall directly into the Sun.
The path of Halley's Comet is well documented over the course of one orbit. The plane of the comet's orbit lies at an eighteen degree angle from the plane of the solar system. Many comets have very elliptical orbits, which carry them from the outer part of the solar system to very close to the sun. Occasionally the orbit of a comet will bring it close enough to Earth for it to be very visible to the naked eye.
Many comets spend much of their time far from the Sun in a belt called the Kuiper belt. The Kuiper belt lies outside the orbit of Neptune, at a distance of 30 - 50 AU from the Sun. Kuiper belt objects are interesting to astrophysicists, in that they are largely untouched since the origin of the Solar system and may provide clues about its formation.
The Kuiper belt was theorized by the Dutch astronomer Gerard Kuiper. The first Kuiper belt object was discovered in 1992, and soon afterward many other objects were found in this region. Comets that come from the Kuiper belt are called short-period comets.
Long period comets, with orbital periods over 200 years, may come from a region even farther from the Sun, called the Oort cloud. Comets coming from this region can have orbital paths coming from any angle away from the orbital plane of the planets in the Solar system.
This time-lapse photograph shows many meteors streaking through the sky during the Perseid meteor shower. It is called the Perseid meteor shower because the meteors seem to come from the region of the sky where we can see the constellation Perseus. Of course, the stars in that constellation are much farther away than the meteors.
Meteor showers occur when Earth passes through a region of space in its orbit where a comet has passed, leaving bits of comet dust. When the bits of comet dust enter Earth's atmosphere, they get very hot and burn up, producing streaks of light. They seem to originate from a specific place in the sky, like the constellation Perseus in this case, because every year when Earth passes through this part of its orbit, Perseus lies in the direction that Earth is moving at the time.
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