Meteoritic Dust |
In chapter 10 we learn about meteors and their relationship to comets and asteroids. Unlike most objects studied by astronomers, meteorites can be studied directly. They can be collected and studied in a lab. You may come into contact with meteoritic material every day without even knowing it. (Find out more about this in the Activity.)
In fact, by some estimates as many as 100 million meteors enter the Earth's atmosphere daily. Almost all of these burn up, leaving the microscopic ashes we call meteoritic dust. Following the same estimate, up to 4 million tons of this material filters down to the surface of the Earth each year. This material may have an effect on the atmosphere, and may even serve as the nuclei for rain droplets, although this idea is not universally accepted.
In addition to this daily dose of mini-meteorites, a relative few reach the Earth's surface as larger particles, most of which appear, at first glance, little different from Earth rocks. It wasn't even until less than 200 years ago that meteorites were accepted as stones from heaven. President Jefferson, perhaps the most scientific president before physics-trained Jimmie Carter, could not believe that stones would fall from the sky. However, a shower of meteorites that fell on a French town in 1803 provided enough evidence to convince the scientific world that indeed these rocks were not of this world.
Meteors with no known origin, called sporadic meteors, can be seen any night. At some times during the year there are large numbers of meteors that come from a particular direction in the sky. These are called Meteor Showers, although the word "showers" is usually a strong over statement. Aside from a few special events, even the best normal meteor shower is more like a drizzle, and one or two meteors per minute is a great showing. Below is a table of the best and most reliable showers throughout the year:
| Date | Shower Name | Max. rate (single observer) | Constellation |
| Jan.2-4 | Quadrantids | 30-90 per hour | Bootes |
| May 2-7 | Eta Aquarids | 10-20 per hour | Aquarius |
| July 26-31 | South Delta Aquarids | 15-20 per hour | Aquarius |
| Aug. 10-14 | Perseids | 40-100 per hour | Perseus |
| Oct. 18-23 | Orionids | 15-20 per hour | Orion |
| Dec. 10-13 | Geminids | 50-95 per hour | Gemini |
| Dec 21-23 | Ursids | 20 per hour | Ursa Minor |
If the meteoroid that causes the display we call the meteor we see streaking through the sky, survives the trip through the atmosphere and lands on the Earth, we call it a meteorite. There are three basic types, with subcategories of each.
The most common type of meteorite is the stony variety: 
(Click the image for a bigger picture and more information.)
There also are iron meteorites.
The largest meteorite ever found (in Namibia) weighs 60 tons, but the vast majority of meteoritic material that reaches the surface of the Earth is tiny, even microscopic. You probably have had personal experience with some of this unearthly material, without even knowing it. You can even collect it, but let's leave that for the activity.
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In addition, there is a combination called stony-irons.
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