AST 101: Introduction to Astronomy


Meteors: small bodies transversing the atmosphere, and (generally) burning up.
Meteorites: Meteors that survive the atmosphere and land on the Earth.
Meteoroids: small objects outside the Earth's atmosphere.

Meteorites are debris floating around the solar system. There are 3 basic types of meteorites:

It is possible to determine the origins of meteorites from their orbits (seen as they enter the atmosphere). Most chondrites and irons originate in the asteroid belt. In fact, most have compositions that match the asteroids. Most of these probably formed during collisions between asteroids.

Near Earth Asteroids

Asteroid 2004 FH

Early in the morning of March 19, 2004, asteroid 2004 FH flew by at a distance of 42,500 kilometers, about 1/9 the distance to the moon. It's size: about 30 meters. A rock this big passes this close every 2-3 years. Most are never seen. Had it hit the Earth, it would most likely have burned up in the atmosphere as a bolide.

Asteroid 1950 DA

In April 2002, astronomers re-observed an asteroid, called Asteroid 1950 DA, which was discovered in 1950. The new observations led to a refinement in the orbit, which showns that there is a 0.3% probability that the asteroid will hit the Earth on March 16, 2880.

This, of course, was news. You can find a number of reports on the web: here are links to reports on 1950 DA from Sky and Telescope; the BBC; ABC news; CBS news; and Newsday.

1950 DA is 1.1 km in diameter; were it to hit the Earth it would cause major damage. It is about 1/10 the diameter of the object that caused the K-T mass extinction (the one that wiped out the dinosaurs). With 1/1000 the volume, or mass, of the K-T impactor, it would hit with about 1/1000 the energy of the K-T impactor. The energy released would be the equivalent of about 100,000 megatons. This may not cause a mass extinction event, but it would be close.

The current estimate is that there is a 1/300 chance that 1950 DA will hit Earth on 3/16/2880. As more observations are obtained, the orbit will be further refined, and the odds will change. Most likely, 1950 DA will miss in 2880, but eventually it will hit the Earth.

Asteroid 2002 EM7

On March 8 2002, a small asteroid, 2002 EM7, passed by the Earth at a distance of 480,000 km, about 1.2 times the distance to the Moon. It was not discovered until 4 days later, because it approached us from the sunward side. The asteroid is about 70m in diameter.

Every year about 100 objects of this size pass by the Earth within this distance. Most are never seen. About 1 in every 10,000 hits the Earth. (Think of the Earth as a bullseye inside a target with a radius of the distance that 2002 EM7 missed us by. The Earth, with a radius of 6000 km, covers about 0.00016 of the area of that circle. Random tossing will hit the bullseye once every 6400 times.) You can expect an object this size to hit about once per century. An object about this size exploded above Tunguska, Siberia, in 1908, flattening trees within about 20 km of the impact site.

An object about this size missed the Earth by about 60 miles in 1972 over the Grand Tetons.

Asteroid 2004 MN4

Asteroid 2004 MN4 will miss the Earth on April 13, 2029 ... but only by about 18,600 miles!.

On the Lookout

There are estimated to be 500-1000 Near Earth Objects (NEOs) larger than 1 km diameter which have a chance of hitting the Earth.
322 are now known.

A number of efforts are underway to find the rest. These include:

When it Hits

Barringer Crater, outside Flagstaff Arizona. Diameter = 4000 feet; depth = 700 feet. Age: about 50,000 years. Size of impactor: about 50 meters. Energy released: 20 megatons.