A shooting star is a meteor. It's usually caused by a tiny particle (possibly a bit of comet dust) that enters the Earth's atmosphere at a very high speed. The friction of the particle's collision with our atmosphere causes it to heat quite rapidly, and it burns brightly for just a moment or two before being vaporized by the intense heat.
Because of the way the orbit of these particles intersects with the Earth's rotation (and because it's dark!), the best time to see meteors is between midnight and about 6:00 a.m. It's estimated that on a clear, moonless night, you can see as many as 3 or 4 meteors per hour from anywhere in the world!
There are also several nights during the year when the likelihood of seeing meteors is even higher. These often occur when a trail of comet dust crosses the Earth's orbit, creating a meteor shower, and can be predicted fairly reliably. These yearly meteor showers are coming up: *
- The Quadrantids -- December 28-January 7 (peaks January 3-4 at about 40 meteors/hour)
- The Lyrids -- April 16-25 (peaks April 21-22 at about 20 meteors/hour)
- The Eta Aquarids -- April 21-May 12 (peaks May 3-6 at about 10 meteors/hour)
- The Southern Delta Aquarids -- July 14-August 18 (peaks July 28-29 at about 20 meteors/hour)
- The Perseids -- July 23-August 22 (peaks August 12-13 at about 60 meteors/hour)
- The Orionids -- October 15-November 29 (peaks October 21 at about 20 meteors/hour)
- The Leonids --November 13-20 (peaks November 17 at about 40 meteors/hour)
- The Geminids -- December 6-19 (peaks December 14 at about 60 meteors/hour)
A meteorite is a meteor that survives its entry into the Earth's atmosphere and comes to rest on the ground. These bodies are usually quite a bit larger than the dust particles discussed above. They have to be, because the intense heat created by their entry into the atmosphere melts and vaporizes much of their mass! Thus smaller particles are vaporized completely, while only a larger meteoroid** will be able to retain some of what it started with.
When a large meteoroid streaks through the sky towards earth, it creates a fireball that can be seen for hundreds of miles, even in the middle of the day. While the rock may be only one or two feet in diameter when it enters the atmosphere, the intense heat might produce a fireball hundreds of feet in diameter!
In addition, the shock waves generated by the intense speed of the body (up to 26 miles per second) create a type of sonic boom. Sometimes the sound of the meteorite -- which can be much like an explosion (or a series of explosions if the meteorite breaks apart as it falls to earth) -- isn't heard until a minute or two after the fireball has passed overhead.
One of the largest fireballs in recorded history was created by the Sikhote-Alin meteorite, which fell in 1947 in Siberia. Witnesses said the fireball was as big as the sun, and was so bright that it produced moving shadows as it passed overhead. The meteorite's fall resulted in no fewer than 122 craters, the largest of which was 85 feet in diameter and almost 20 feet deep.
The Sikhote-Alin meteorite gave scientists a perfect opportunity to study crater formation, among other things. They discovered that most meteorites large enough to create a crater were also too large to withstand their impact with the earth -- they shattered and/or vaporized into many smaller pieces.
A crater much closer to home -- Meteor Crater***-- is a fine example of this phenomenon. At 4150 feet in diameter, 2.4 miles in circumference, and 550 feet deep, it makes even the largest Sikhote-Alin crater seem little more than a puddle. (By the way, scientists theorize that the meteorite that formed this huge crater was all of 100 feet in diameter!) Most of the meteorites found there have been found at the perimeter of the crater and up to seven miles away.
In 1902, Daniel Barringer, the first person to suggest that Meteor Crater might be linked to the fall of a meteorite, took an interest in it because of the bits of nickel iron that had been found in its vicinity. He imagined he would make his fortune by mining nickel. Alas, he spent many years drilling in Meteor Crater, trying to find the main body of the meteorite that he was sure must be there, to no avail. We now know, of course, that the meteorite that formed that crater was far too large to have possibly survived the impact.
While Daniel Barringer ended up spending a fortune trying to mine in Meteor Crater, he was not quite as foolish one might think. Until he theorized a link between the crater and a meteorite fall, such a circumstance was thought to be impossible; it was generally thought that craters on earth could only be the result of volcanic activity. Largely because of the research carried out at Meteor Crater and in the Sikhote-Alin mountains of Siberia, there are now more than 150 recognized impact craters throughout the world.
Three Types of Meteorites
There are basically three main kinds of meteorites: stony meteorites, iron meteorites, and stony-iron meteorites. Stony meteorites (chondrites and achondrites) are by far the most common, making up 85-90% of all known meteorites. They can be composed of olivine, pyroxene, magnetite, serpentine, troilite, pyroxenes, and/or feldspar. They also contain a small but significant amount of nickel-iron alloy. Numerous stony meteorites (Correo meteorites) have been found in Valencia County, New Mexico.
Iron meteorites are probably the best known to the layperson; both the Meteor Crater and Sikhote-Alin meteorites are irons. These are composed mostly of nickel-iron alloys, hence their classification. Stony-iron meteorites (pallasites, mesosiderites, and lodranites are the least common, accounting for less than 2% of known meteorites, and arguably the most spectacular. In these meteorites, areas of iron-nickel alloy contrast sharply with portions of olivine, eucrite, diogenite, troilite, bronzite, and feldspar. The meteorite that fell outside of Portales, New Mexico in 1998 is likely a stony-iron.
Learn More about Meteorites
New Mexico boasts a fine research facility, the Institute of Meteoritics, at UNM's main campus in Albuquerque. The Institute maintains a small museum in Northrop Hall, which is open 9-4, Monday through Friday; admission is free. You can also visit the Institute's web site. Of special interest is their page on identifying meteorites.
*Meteor shower information is from O. Richard Norton's outstanding book on meteorites, Rocks from Space, supplemented from Gary W. Kronk's Meteor Observing Calendar.
**A meteoroid is what the object that creates a meteor is called while it's still in space.
***Meteor Crater is 40 miles east of Flagstaff, Arizona, just off I-40, and well worth a visit. You probably won't find any meteorites there (and aren't allowed to remove them, in any case), but you can take a pretty spectacular tour along the rim of the crater.