Meteor Impacts: Orientation

The earth has been repeatedly hit by these objects. If this sounds like a highly unlikely event, consider this: every day the weight of the earth gains hundreds of tons through the in-fall of extraterrestrial material! Fortunately for humans, most incoming objects burn-up as they fall through earth's atmosphere.

Occasionally a few-mile-wide asteroid or comet punches through our atmosphere and strikes the earth with deadly force. Earth has numerous craters from these incredibly powerful events - some over 100 miles in diameter! The map below shows the location of nearly 200 impact structures that have been identified to date. If you would like to explore an impact crater in Arizona with aerial photos and maps click here to open a new window with the Microsoft Terraserver (The aerial photos at Terraserver are very large files. It may take a few minutes for them to download to your computer.)

Image courtesy of Natural Resources Canada, a Canadian government department specializing in energy, minerals and metals, forests and earth sciences.

An object that is only a few miles across, traveling at 80,000 miles per hour, has the ability to blast a tremendous crater in the earth's crust. The energy released from such an impact would be many times greater than the simultaneous detonation of all of the nuclear weapons in all of earth's arsenals. The effect on earth's life would be devastating.

These huge impacts are unusual events, but they have happened, and they will happen again in the future. Right now, numerous several-mile-across objects are traveling in orbits that cross earth's orbit on a regular basis. Those that are large enough could wipe-out life as we know it - something like the event that many scientists believe caused the extinction of dinosaurs. Experts have estimated that during your lifetime, the earth has one chance in 10,000 of being struck by an object that is large enough to threaten the extinction of large numbers of species.

Consider the heavily cratered surface of our moon. Each of those craters was created by an impact event. Compared to the moon, the earth occupies the same region of space, is a larger target, and has a gravitational "pull" that far exceeds that of the moon. These facts support the idea that the earth has been repeatedly impacted by large objects from space. The reason the earth does not have abundant craters is that they are quickly obscured by vegetation, weathering, erosion, volcanic activity, and other earth processes. For this first lesson, your job will be to learn the relevant facts about asteroids, comets, meteorites and the effects that the next large impact might have upon earth and its life.

Moon illuminated by earth shine. Even under these dimly illuminated conditions, nearly all of the features that you can see are impact craters.

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