Most asteroids "reside" in the asteroid belt.  These are the so-called main-belt asteroids.  Nearly 10,000 such objects are known and catalogued, the biggest of which is Ceres. See  Is Pluto a Planet  for links to images of selected asteroids.

From these images of asteroids, it is clear that these objects share a common history of collisions and cratering with all other objects in the solar system.  Some asteroids appear to be double, or binaries (243 Ida, 3671 Dionysus), and others appear  to be loosely or barely attached double-objects (4769 Castalia, 4179 Toutatis).  Such doubles or almost doubles may be the result of collisions that either fractured a larger object or compacted two objects into a loosely bound, single object.

Classification of Asteroids by Orbital Characteristics

Here's a  map of objects in the inner solar system

Here's a  map of objects in the innermost region of the solar system

The number of such objects known is increasing rapidly (doubled in 6 months).

Why are Apollos, Atens and Amors of interest?

  1. collisions of near-Earth asteroids (NEAs) with Earth could influence/damage life on Earth
  2. by definition, meteorites must come from these families of objects
  3. near-Earth orbits make these objects "easy" to explore
  4. near-Earth orbits make these objects potentially useful natural resources (metals, water) to harvest on Moon or in orbit
  5. short lifetimes as planet-crossers teaches us that the asteroid belt exists as a reservoir for such objects

The future of planet-crossing asteroids

If we were to wait 10 million years and then survey the solar system, would there be any Earth- or Mars-crossing asteroids?

Certainly, some of the present population of earth-crossers will hit Earth, as we know meteorites hit the Earth all the time and we know of impact craters on the Earth that formed within the last ten thousand years (e.g., meteor crater in Arizona).  Only Earth-crossing asteroids can collide with Earth (or the Moon).  Such asteroids are on unstable orbits.  Over a few million years time, they either will collide with one of the terrestrial planets or experience a near miss.  Each near miss will change the object's orbit, bringing it potentially into an even more dangerous orbit relative to the Earth and possibly bringing the asteroid close to the Sun or Jupiter or putting it on a collision course with another asteroid or planet.  Whatever the actual outcome for any given asteroid, all Earth-crossers must have short "lifetimes" as Earth-crossers.

Similarly, all present Apollos, Atens and Amors will have experienced enough close encounters with Venus, Earth or Mars that they will have collided with one of these objects or been thrown into Mercury or the Sun or tossed out of the inner solar system where the giant planets will stir up their orbits even more.

So the answer would appear to be "no."

This answer leads us from science to philosophy.  One of the guiding philosophical principles of astronomy is the Copernican Principle, which states that unless there is a special justification, today is not a special or unique time in the history of the solar system, that the solar system today must look very much like it did 10 million years ago and as it will look like in 10 million years.  Therefore, just like comets or Centaurs, or the rings of giant planets, the populationof earth-crossing asteroids must continually be replenished from an asteroidal reservoir, most likely the asteroid belt.

Terrestrial Impact Craters

Images from Terrestrial Impact Craters, Second Edition, Compiled by Christian Koeberl and Virgil L. Sharpton. Follow this link for more information about these craters.

How is an impact crater identified (i.e., distinguished from a volcanic crater)?

Illustration of an  impact crater with a central ring , in this case on the far side of the moon.

Terrestrial  Impact Crater list , and another  Terrestrial Impact Structure list

Impact craters in Tennessee:
maps from Tennessee Department of Conservation
 map  of Kentucky
1. Flynn Creek Crater, diamter 3.55 km, age 360 +/0 20 MY 2. Wells Creek Crater, diameter 14 km, age 200 +/- 100 MY
3. Howell Structure
4. Middlesboro (KY), diameter 6 km, age 300 MY
Tunguska On June 30, 1908, a meteoroid exploded over Siberia. The resulting explosion flattened 2150 km2 of trees. The trees were burned on one side. No crater was formed.  This part of Siberia is one of most remote places on Earth and this was during a 20 year period of continuing war and revolution in Russia.  So the site wasn't explored for 20 years.

The explosion was heard 800 km away and shook the ground 600 km southwest, causing railroad tracks to vibrate and engineers to stop the trans-Siberian express.  Passengers got out and were pelted with smoldering rocks and a black, tar-like rain. Meteorologic stations in England recorded 20 minutes of wild fluctuations in barometric pressure, five  hours after the event.   Reportedly, the night sky was bright for several days.

Best modern estimates are that the Tunguska event  was caused by a stony asteroid 30-m in diameter, that exploded 8-10 km above the Earth's surface.  The explosion had the force equivalent to that of 20 hydrogen bombs, more than 1000 times more powerful than the atomic blast at Hiroshima in 1945.

Some anecdotal information, and a few more good pictures, can be found here.
Some eyewitness testimony can be found here.

Why did it explode above the surface and not make a crater? Stony asteroids (which contain no iron) are fairly weak objects.  When a stony asteroid strikes Earth's atmosphere, it is like a cracker hitting the surface of a pond.  Denser material might penetrate deep below the surface of the pond upon impact, but to a weak structure, the pond surface is too strong.  The asteroid can only penetrate the atmosphere to a height of 5-10 km before it shatters.  A comet should shatter well above 10 km while an iron-rich asteroid is more likely to reach the surface and form a crater.

Atmospheric Explosions

The pentagon listening stations report about 1 atmospheric explosion per month, caused by asteroids about 2 m in diameter.  Military records have observed them for 30 years. These are typically about 1 kiloton explosions.

About  once per year, a larger (6 m diameter object; 15 kiloton blast) event is detected. (see plot, linked below).

Close Approaches

Astronomers are actively searching for and cataloging orbits of near-earth asteroids.  The Minor Planet Center keeps a list of all objects that will make close approaches (within 0.2 AU) to the Earth in the next 33 years, in chronological order.  This list gives the close approaches chronologically and gives the distance of closest approach.

Close Approaches to the Earth: listed in order of approach distance through 2178.  The closest approach in 2001 occurred on January 15, at a distance of only 0.002 AU = 300,000 km, which is a little bit closer than the Moon is to Earth. (The mean distance to the moon is 0.0026 AU). The closest approach currently predicted will be in 2140, at only 0.00054 AU, i.e. only 0.21 times the distance to the moon!

Should we worry about asteroids colliding with Earth?

Calculations of energies of earthquakes, bomb explosions and impacts
NASA's Asteroid Comet Impact Hazards homepage
The Day the Sands Caught Fire, A Scientific American article on the Wabar impact crater

What are the odds of an impact?
             plot of Average Frequency of Impacts vs. size of meteorite

NASA estimates that ~1000 NEA larger than 1 km and probably 1 million larger than 50 m in diameter exist.  (50 m is a nominal threshold for an object that can penetrate Earth's atmosphere).  A 50 m object would impact with an energy equivalent to a 5 megaton bomb. A 2 km object would impact with 1 million megatons of energy.  A 2 km object would also leave  a large geological signature --- a 20 km diameter crater ---  and perhaps cause significant biological damage.

What is the evidence for this statistical assertion that one 2 km diameter object collides with the Earth every million years? Should we accept this as reasonable?

Since a 2 km object would generate a 20 km crater, we can look for craters in this size range on Earth. Statistically, since only 25% of the Earth's surface is land, we should find one 20 km crater per 4 million years, assuming all craters on land are findable and that the NASA estimate is correct.

From our listing of Terrestrial Impact Craters, above, we know that we have

So, we have evidence of about 4 craters in the ~20 km size range over an 18 MY time interval, or one every 4.5 MY. Almost certainly, we have missed some (in jungles and rain forests, under glacial cover, buried by sediments and destroyed by erosion), so this estimate is a lower limit to the cratering rate for craters of this size and is a good match to the estimate quoted by NASA.

Characteristics of Asteroids

We use reflectrance spectra to classify asteroids. Certain wavelengths of sunlight are absorbed and others reflected by the mineral grains or materials in the outer few microns of an asteroids surface.  From these spectra, we can determine the surfaced composition of the asteroids.  Overall, the asteroids show a compositional trend that is a function of distance from the Sun: the closest asteroids are more stoney and iron rich, the outer most asteroids are more carbon rich. This trend suggests a temperature dependent formation history, with asteroids further from the sun being more abundant in low temperature materials.


Class Composition Location Comment
E similar to chondrites inner edge of belt sources of most meteorites?
S, M stony, stony-iron central belt
V basalt Vesta, a few earth crossers volcanism?
C carbonaceous, hydrates outer belt, a > 2.7 AU most abundant asteroid type

Special cases

Rotation rates of asteroids

Rotation rates of asteroids are measured by their light curves. We measure the total amount of light reflected off the asteroid as a function of time.  If the object is perfectly spherical and all parts of the surface are covered by material with the same reflectance (albedo) properties, the measured light intensity will be constant; however, it the object is odd-shaped (imagine a potato) or has one dark side and one light side, the measured light intensity will vary periodically. The variational period is the rotation curve of the asteroid.

Most asteroids have rotation periods from 4 hours to 16 hours.  The fastest known is 2.87 hours (321 Florentina).

Harvesting an asteroid

Consider a 1 km diameter (spherical, for simplicity) stony-iron asteroid. This object has a volume of

Volume = 4/3 pi x (1000 m)3 = 4 x 109 m3
and a density of about 4000 kg/m3 , of which about 15% is iron or iron-nickel minerals, and therefore a mass of
Mass = 1.6 x 1013 kg
Assuming a price for raw iron ore of about $30/ton for materials that yield about 7% iron, the market value of this asteroid would be
Value = ($60/ton) x (1.6 x 1013 kg) x (1 ton/1000 kg)
Value = $100 x 1010 = $1 trillion
Many asteroids are likely to be pure iron-nickel objects.  Such an object of similar size would easily be worth well over 10 times the value of the stony-iron object.