Scientists study the light reflected by different parts of a comet. Gases contain different elements. Each element such as hydrogen , molecule such as water , or ion an electrically charged element or molecule has a distinct pattern of emission or absorption that can be determined in the laboratory; this pattern is known as its spectrum. By matching patterns between laboratory measurements and comet observations, scientists can determine the composition of the comet.
Every comet is made of the same basic ingredients — ice and dust. However, comets probably vary in how much of the ice is water ice and how much is ice made of other substances, such as methane, ammonia, and carbon dioxide. Comets also vary in the different types of trace elements and hydrocarbons are present. Several space missions, such as the European Space Agency's Giotto mission, have explored comets and provided detailed imagery of comet surfaces.
A few missions are intended to sample comets. NASA's Deep Impact mission will encounter Comet Tempel 1 in July , and will release a projectile into the comet surface to excavate a hole and expose a fresh surface on the nucleus.
The spacecraft will collect data on comet emissions and will relay the data to scientists on Earth. While the data from these missions will be from only a few comets and might not be representative, the data will greatly improve our understanding of comet compositions.
What do the orbital paths of comets look like? Based on observations of how comets move through the sky, scientists have determined that comets travel around our Sun in highly elliptical oval-shaped orbits. The time it takes to make a complete orbit is called a comet's period. Comet periods typically range from a few years to millions of years.
Where do comets come from? Comets are divided into short-period comets and long-period comets. Short period comets — such as Comet Halley — revolve around our Sun in orbits that take less than years.
Their orbital paths are close to the same plane of orbit as Earth and the other planets, and they orbit our Sun in the same direction as the planets.
Based on these orbital characteristics, short-period comets are believed to originate in the Kuiper belt , a disk-shaped region extending beyond Neptune.
The Kuiper belt contains small, icy planetary bodies, only a few of which have been imaged. In Comet Tempel 1, almost all the ice is water ice, but some is carbon dioxide ice—or "dry ice. That is why you might use "dry ice" to make "smoke" for a model volcano or "fog" for a stage play. As the comet gets close to the Sun, the carbon dioxide ice will vaporize before the water ice.
So, after thousands of years, even though the two kinds of ice were initially mixed together near the surface, only the water ice remains. The carbon dioxide ice a meter or so beneath the surface is more protected from the Sun's heat, so may survive, with water ice above it.
Tempel 1 contains materials from the outer, middle, and inner parts of the solar system. We are not sure. Comets probably formed in the outer solar system.
The inner solar system type of dust particles found in them could have traveled to the outer solar system where the comets formed. Or, not as likely, these dust particles could have arrived from other solar systems.
Water and carbon dioxide ices are both found in the outer solar system, so comets could pick up both ices while forming. This movie shows Comet Tempel 1's coma getting brighter after the Deep Impact collision. Deep Impact blasted lots of material from beneath the surface into the comet's coma. Remember, the coma is the cloud of dust and gas that boils off the nucleus as the comet's orbit takes it closer and closer to the Sun.
The coma contains material from near the surface of the nucleus. This material is what the Sun heats up most and what boils off first. Scientists saw what was in the coma right after the impact, and compared that with what was there before the impact. This way, they could get an idea what was added from the material blasted out of the hole in the nucleus.
But, whether before or after the blast, how do the scientists know what the coma is made of? After all, the comet and its coma are millions of miles away!
Comet Tempel 1's coma before impact. Colored dots stand for different materials that have boiled off the surface of comet's nucleus. It usually has an irregular shape because it is distorted by the solar wind. The hydrogen envelope gets bigger as the comet approaches the sun. The comet's dust tail always faces away from the sun. The tail is made of small one micron dust particles that have evaporated from the nucleus and are pushed away from the comet by the pressure of sunlight. The dust tail is the easiest part of the comet to see because it reflects sunlight and because it is long, several million kilometers several degrees of the sky.
The dust tail is often curved because the comet is moving in its orbit at the same speed that the dust is moving away, much as water curves away from the nozzle of a moving hose. Comets often have a second tail called an ion tail also called the plasma or gas tail. The ion tail is made of electrically charged gas molecules carbon dioxide, nitrogen, water that are pushed away from the nucleus by the solar wind. Sometimes, the gas tail disappears and later reappears when the comet crosses a boundary where direction of the sun's magnetic field is reversed.
Unfortunately, a parachute failure caused the capsule to crash into the desert floor and much, though not all of the data was lost.
Check out the official website for the latest science results. The main spacecraft flew by, studying the impactor and the crater it produced when it crashed into the comet about 24 hours later. Scientists hoped to better understand comets by looking at how craters form and the changes the impact had on other material that is naturally out-gassed.
They also measured the size of the crater, the composition of the interior of the crater and the ejected material. Previous: Dwarf Planets Next: Asteroids. Comet Hyakutake Photographed by Peter Ceravolo. Comet Diagram click to enlarge. Back to top. Evolution of a Comet in its Orbital Path click to enlarge. To see a comet in action, view the Orbit Simulator.
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