This stony meteorite was found in Antarctica. It is thought to have formed 4.55 billion years ago, at the same time as planet formation. Image courtesy of NASA/JPL.
Meteorites are pieces of interstellar debris that survive passage through the Earth's atmosphere and land on the surface. These "stones from heaven" represent the only first-hand evidence we have about the conditions of the formation and early history of our solar system. They are therefore very valuable!
Meteorites are different from meteors!! Meteorites are what fall to the ground - you can pick them up. Meteors burn up in the Earth's atmosphere and never reach the ground.
There are several different classes of meteorites:
This stony meteorite was found in Antarctica. It is thought to have formed 4.55 billion years ago, at the same time as planet formation. Image courtesy of NASA/JPL.
This iron meteorite also was found in Antarctica. It is probably the metallic core of an asteroid that broke apart. Image courtesy of NASA/JPL.
This meteorite is a sample of the crust of the asteroid Vesta. We know it is from Vesta because it contains a large amount of the mineral pyroxene - the exact same proportion as is found in Vesta. It landed on Earth in 1960.
This meteorite is the famous Martian meteorite that was believed to contain evidence for past life on Mars. It was ejected from the surface of Mars by a large impact 16 million years ago, and it arrived at Earth, falling in Antarctica, 13,000 years ago. We know it is from Mars because the air bubbles trapped inside the rock exactly match the chemical composition of the Martian atmosphere.
Electron microscope images of the Martian meteorite ALH 84001 (pictured above) reveal unusual tube-like structures. Their size is less than 1% the width of a human hair. They are (were?) thought to be microfossils of bacteria inside the rock. This is now under debate, because scientists have also found a way to make these structures without any life forms involved.
The main reason astronomers are interested in finding and characterizing Near Earth Objects is to be able to respond to the threat of an impact. The size of an object determines the amount of destruction it will cause on Earth. The good news is that the largest impactors, which would cause mass extinctions, are much more rare than smaller impactors.
There has been an ongoing debate about what killed the dinosaurs 65 million years. The two competing theories have been 1) a large impact, which caused mass extinctions, or 2) global climate change caused by increased volcanic activity. One problem with the impact theory was that there was no evidence for the giant impact. Recently, the crater was located, underground and underwater, near the Yucatan Peninsula. The Chicxulub crater provides the "smoking gun" for the impact theory.
This map shows the three-dimensional gravity field variations near the Yucatan Peninsula. The basin has been buried by sediments, so it was not detected until modern technology was able to probe the sub-surface layers.
You can read more about the dinosaur-killing debate here.
This is a very comprehensive site about the risk of impacts.
In case you are not convinced that impacts are still going on in the solar system, we have evidence from an unusual source. The Solar and Heliospheric Observatory (SOHO) spacecraft takes daily pictures of the Sun's corona, or outermost region.
These images reveal comets that streak towards the Sun, eventually impacting the Sun and being lost forever. SOHO has discovered more than a hundred comets this way! Even YOU can discover comets without ever going to a telescope - all you have to do is pour through the daily SOHO data that are available on the internet and look for them! There is a web page where you can report your discoveries.
Here is a link to a movie of the Peekskill meteorite coming in for a landing - its trajectory through the Earth's atmosphere stretched from Kentucky to New York, where it landed on a parked car. [Go to "Multimedia Gallery" and find the MPEG file at the bottom of that page.]
NASA is committed to finding all of the hazardous NEO's out there! In 1998 it started a program to find 90% of all NEO's 1 km in diameter or larger. Here is a progress report:
Source: http://neo.jpl.nasa.gov/neo/number.html
There are only a handful of telescopes that are dedicated to this research. Here are their discovery statistics:
Source: http://neo.jpl.nasa.gov/missions/stats.html
How big is the threat, really? Here is a table that may help answer that question. It estimates the chances of an individual dying in the United States due to various causes:
| Cause of Death | Chances |
| Motor vehicle accident | 1 in 100 |
| Murder | 1 in 300 |
| Fire | 1 in 800 |
| Firearms accident | 1 in 2,500 |
| Asteroid/comet impact (lower limit) | 1 in 3,000 |
| Electrocution | 1 in 5,000 |
| Asteroid/comet impact | 1 in 20,000 |
| Passenger aircraft crash | 1 in 20,000 |
| Flood | 1 in 30,000 |
| Tornado | 1 in 60,000 |
| Venomous bite or sting | 1 in 100,000 |
| Asteroid/comet impact (upper limit) | 1 in 250,000 |
| Fireworks accident | 1 in 1 million |
| Food poisoning by botulism | 1 in 3 million |
| Drinking water with EPA limit of TCE* | 1 in 10 million |