AST 101: Introduction to Astronomy

Meteorites

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

• Stony meteorites. Most of these are chondrites, names for the chondrules, small refractory grains or crystals trapped in a rocky matrix. These are the most common. They look like ordinary terrestrial rocks.
  The achondrites have differentiated: they lack chondrules. They resemble terrestrial basalt (magmatic rock). Some appear to be Lunar or Martian in origin.
  The carbonaceous chondrites are primitive, undifferentiated objects, and are thought to be left over from the beginning of the Solar system in more-or-less the same state they were formed. They contain carbon-bearing compounds, and their volatiles suggest they have never been heated nor compressed. They have ages of about 4.6 billion years.

• Irons are mostly iron and nickel. They are differentiated, meaning they have been processed in the interior of massive bodies. Most identified metorites are Irons, because they look so unlike regular rocks.

• Stony-Irons. These rare meteorites are combinations of Irons and chondrites.

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.

Asteroid 2004 FH

Asteroid 1950 DA

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.

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.

Asteroid 2004 MN4

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:

• Project Spacewatch (U. Arizona)
• NEAT (JPL)
• LINEAR (MIT)
• LONEOS (Lowell Observatory)

When it Hits