Hundreds of thousands of stars circle slowly around the celestial North and South Poles of the night sky. Planets move slowly through the constellations. Satellites and the International Space Station move visibly across the night sky as they orbit the earth. But once in a while, something bright appears in the night sky that was not there before.
Meteoroids
Meteors are visible streaks of light which suddenly streak across the sky. The brightest of them are visible even in the daytime. They are only visible for a few seconds, and then they are gone.
This brilliant streak is caused by a particle no larger than a grain of sand. When it enters the earth’s atmosphere at a speed of up to 44 miles per second, the resulting friction creates so much heat that it vaporizes. The heat is so intense that it gives off a brilliant streak of light. Only this streak of light is properly referred to as a meteor.
Every solid particle which moves through the solar system is called a meteoroid. It can range in size from a speck of dust to a large boulder. According to the International Astronomical Union, a meteoroid is “considerably smaller than an asteroid and considerably larger than an atom.” Other proposed definitions limit the size of the object to something smaller than 32-1/2 feet in size. The Near Earth Object definition limits the size of meteoroids to under 163 feet.
The brightest meteors are caused by the largest meteoroids. Very large meteors, or fireballs, are sometimes heard as well as seen, due to the sonic boom when it enters the atmosphere. Officially, a fireball is a meteor which has an absolute magnitude of -3 when seen at zenith. Unofficially, a fireball makes everyone along its path look up, sparking many UFO reports along the way. A fireball is also sometimes called a bolide. If it hits magnitude -17, it is called a superbolide.
A meteor can also be accompanied by other sounds besides the sonic boom, such as hissing or crackling. These sounds are probably caused by the ionization trail left behind by the meteoroid. They probably work the same way the sounds of an aurora borealis work.
Like the aurora borealis, the color of a meteor is a clue about the composition of the meteoroid, because different elements give off different colors when ionized or heated. Common meteor colors are yellow (iron), nickel (green), and red (silicate). A meteor can change color as its outer layers vaporize, exposing the inner layers.
Meteorites
A meteorite is a meteoroid which was large enough that part of it survived all the way down to the surface of the earth. The outer part of the meteorite usually looks dark and melted.
A meteorite which is recovered after someone saw it falling is called a fall. Meteorites which are recovered separately from seeing them fall are called finds. Meteorites are officially named for the place where they are found. If several meteorites are found at the same location, a number is added.
Meteorites are divided into three categories according to their composition: those made of silicate minerals, those made primarily of nickel-iron, and those which have large portions of rocky materials. Some meteoroids are also made of large portions of ice, but the ice does not survive the fall.
Meteorites are also divided according to their structure. About 86% of all meteorites are chondrites, which are made of small round silicate particles called chondrules. Chondrules are believed to be some of the oldest parts of the solar system. Some chondrites have even been found to contain amino acids.
About 8% of meteorites are achondrites. They are made up of various igneous minerals. Although both chondrites and achondrites are believed to come from the asteroid belt, chondrites never formed into a larger solar system object, while achondrites did. Most achondrites are believed to be pieces of asteroid crust. The HED achondrites are thought to have come from asteroid 4 Vesta. One small group of achondrites may have originated on the moon, while another may have originated on Mars.
The rest are iron or stony-iron meteorites. About 5% are iron with iron-nickel alloys. These are thought to have once been part of an asteroid’s core. The remaining 1% are stony-iron meteorites made of iron-nickel and silicates.
Tektites are not meteorites. They are formed from sand which was melted and splashed up when the meteorite landed. They are sometimes called meteoric glass.
Because they are usually small enough to be slowed by the atmosphere, nearly all meteorites land at terminal velocity and make only as much of a hole as anything else falling out of an airplane. However, a very large meteoroid can’t be slowed down very much by the atmosphere.
What happens next depends on the type of meteoroid. A very large nickel-iron meteroid can leave a large hypervelocity impact crater while remaining intact. Most known meteor craters were caused by nickel-iron meteoroids. Meteorites are usually found at these craters.
Most icy and stony meteoroids, including silicate meteoroids, are usually disrupted in the atmosphere. If the meteoroid is large enough, this disruption can cause a massive atmospheric concussion. This is what is thought to have happened at Tunguska. These fragments usually don’t survive to become meteorites, although for very large events, such as Tunguska, a meteorite may lie at the bottom of Lake Cheko, which is thought to be an impact crater.
The largest stony meteoroids will reach the surface of the earth and cause huge craters. However, the meteoroid is usually destroyed in the process.
Comets
A comet is a collection of dust, rocks, ice, and frozen gases which is between half a mile and 100 miles in diameter. According to NASA, a comet must be composed of at least 85% ice, which can include ice consisting of carbon monoxide, carbon dioxide, methane, and ammonia.
The comet becomes visible when its orbit takes it close enough to the sun for part of the comet to evaporate and create a visible coma. This coma is what makes the comet bright and fuzzy-looking. At this point, a comet can be closer to the sun than the orbit of Mercury. A few sungrazing comets come within a few thousand miles of the sun’s surface.
As the solar wind interacts with the coma, it blows part of the coma away from the sun. This is the comet’s tail. The tail can be many times larger than the comet, up to 100 million miles long, or it can be almost non-existent. It depends on the angle of the comet’s orbit, the strength of the solar wind, and the composition of the comet. The interaction between the solar wind and the comet’s tail generates X-rays as well as visible light.
The large majority of comets are either short-period comets or long-period comets which orbit the sun in a narrow elliptical orbit which usually keeps them far from the sun. Long-period comets originate in the Oort Cloud, which is nearly a light year away from the sun, at the gravitational boundary of the sun. A few of these comets may only come close to the sun once in their lives before returning to interstellar space.
Short-period comets originate in the Kuiper belt and its associated scattered disc, which lies beyond the orbit of Neptune. They have an orbital period of less than 200 years. Halley’s Comet is one of the best-known short-period comets.
Whenever a comet comes close to the sun, the amount of material which evaporates can make the comet unstable. Sometimes the comet breaks up permanently. These comets sometimes become the source of future meteor showers.
Meteor showers
During a meteor shower, thousands of meteors are visible during a short period of time. An intense meteor shower can produce more than 1,000 meteors per hour.
Meteor showers are named for the bright star which is closest to the point in the sky from which the meteors seem to be radiating. For example, the Leonids meteor shower, which peaks near November 17, radiates from near the star Gamma Leonis.
The reason meteor showers appear to be radiating from a single point in the sky is because most meteor showers originate from the debris left behind by comets. The debris still follows the same orbit as the comet once did. The fixed point will move across the sky together with the stars as the earth slowly turns.
