Near Earth Asteroid / NEO Classifications Based on Locations
There are two main classes of asteroids:
Near Earth Asteroids / NEOs are in turn split up into 4 subcategories, technically, whereby you may want to get familiar with these terms.
Apollo, Amor, Aten, and Libration Asteroids Near Earth
The 3 main subcategories of Near Earth Asteroids are:
This figure shows an example of an Amor, an Apollo and an Aten orbit.
As of the beginning of 2012, there were approximately 8000 known near Earth asteroids, which were roughly about 54% Apollo, 37% Amor, 8% Aten, and 1% near Earth live comets (with tail and/or coma).
There is considerable bias to these statistics because asteroids close to the horizon are much more difficult to see from Earth based telescopes, which affects the Atens most of all.
There are estimated to be about 200,000 sizeable Apollo, Amor and Aten asteroids / NEOs altogether, and millions of Main Belt asteroids.
As you might have guessed, there is an additional class of asteroids. After all, the above says are two Earth-crossing types, Apollo and Aten, but listed only one non-crossing type, Amor, which stays outside Earth's orbit, i.e., always further from the Sun. There is also a type of asteroid which stays closer to the Sun and never crosses Earth's orbit. These are called Atira asteroids. We've yet to find one of interest for mining.
While most of the literature refers to the near Earth asteroids as NEAs or NEOs (objects), sometimes they're referred to as AAAs or "triple-As" for Apollo, Amor and Aten.
If you have a difficult time remembering all this, don't worry, it's not so important. It's just that you'll come across these names from time to time, so it's good to know what someone is talking about. It cannot be said that one type is generally better than another type. The attractiveness of an asteroid is better analyzed and stated in other terms. It's just sometimes nice to know the lingo.
The vast majority of Near Earth asteroids are thought to have originated in two ways:
It is generally estimated at this time that roughly 40% of the near Earth asteroids are captured comets, and the rest came from the asteroid belt.
Astrophysicists' calculations have led to a consensus that over the next 100 million years, most near-Earth asteroids will have been thrown back out by close gravitational encounters with the inner planets or will collide with the inner planets. Conversely, a new supply will be constantly generated from the Main Belt and incoming comets which will replace these losses.
The authority for naming stars, galaxies, planets, and other things is the International Astronomical Union (IAU). Under the IAU, the authority for asteroids and comets is the Minor Planet Center, which is located at the Smithsonian Astrophysical Observatory. Most of the funding for the Minor Planet Center actually comes from NASA.
There is actually an additional kind of asteroid, known by the category name as a Trojan, due to a process called libration.
Simply put, Earth Trojan asteroids are already captured by Earth, and orbit the Sun synchronized with Earth.
These asteroids are normally far from Earth, not in any circular nor ellipical orbit close to Earth. They are better described as orbiting the Sun in Earth's orbit, but some can stay about 60 degrees behind Earth in orbit around the Sun, whereas others stay around 60 degrees ahead of Earth, and some yo-yo around Earth in odd orbits. Some of these are the easiest to reach, but not all.
Much bigger planets like Jupiter and Saturn have a lot of big Trojan asteroids. Small planets like Earth have a difficult time gathering and keeping them for long.
For example, an asteroid was observed to be orbiting Earth in 2006 for several months. (At first, it was thought to be a spent fuel tank from an Apollo mission, but after a longer analysis, the trajectory ruled that out.) It was catalogued as asteroid "2006 RH120".
Further analysis by a group of astronomers who modeled asteroid orbits and numbers concluded that at any time, there were probably one or two very small asteroids orbiting the earth in a yo-yo way similar to this asteroid, of size around one meter, roughly the size of a clothes washing machine. These are just normally too faint to be seen. In addition, there are about a thousand tiny asteroids the size of a softball or larger orbiting the Earth. However, this is an early computer model.
Approximately a million tiny objects pass by Earth every year, but the vast majority don't actually get very close to Earth. Besides, if they were to enter Earth's atmosphere, they are mostly too small to hit the ground and do much damage, and most instead break apart high in the atmosphere. (See the section on meteors.)
However, a few of these asteroids are captured into the Earth-Moon system and orbit Earth for awhile, usually in oddly shaped orbits. They don't stay long, as gravitational perturbations by the Moon and other planets eventually kick them back out, with a typical lifetime of roughly a year. Some hit the Earth's atmosphere and constitute some of the meteors we see.
According to recent computer modeling, a 10 meter bus sized asteroid goes into orbit approximately once per 50 years, and a 100 meter size asteroid, nearly the size of a football stadium in volume, goes into orbit around Earth about once per 100,000 years. The latter would be visible to humans.
This analysis was performed by Mikael Granvik of the University of Finland, Jeremie Vaubaillon of the Observatoire de Paris, and Robert Jedicke of the Institute for Astronomy at the University of Hawaii, with the results published in the journal Icarus, in 2012.
Asteroid 2006 RH120 orbited the Earth for about one year, and was about 3 meters wide.
These small asteroids are very difficult to detect. However, if we had asteroid probes ready, this kind of material would be the most economical to retrieve for studying. These small asteroids of just a few meters in size are not large enough to be economically attractive to mine for materials, but they may be interesting for research and development purposes.
Asteroidal material which falls to the ground to become meteorites tends to be biased in composition to be the stronger rocks. Studying pristine asteroidal material still in space might prove to be interesting enough.
As discussed in the section on Transportation, orbital mechanics creates the possibility for "Trojan" asteroids which orbit the Sun with Earth but which stay around 60 degrees ahead or behind Earth, so that they also orbit Earth in a sense.
Other planets have Trojan asteroids, especially the big ones like Jupiter, Saturn, and Neptune, which have some very large Trojan asteroids. Even little Mars has Trojans, though Mars benefits from being much closer to the asteroid belt.
Scientists have long suspected that there are Earth Trojans, and for many years, some astronomers looked for Earth Trojans but did not find them. A major challenge is that Earth Trojans would be low on the horizon near the Sun, so very difficult to see from Earth based telescopes. Space based telescopes should fare much better, but we don't have many of those.
As telescopes became more sensitive and more telescopes were brought into service, a telescope eventually detected an Earth Trojan asteroid in 2010, named 2010 TK7, which is estimated to be approximately 300 meters in diameter -- a fairly massive body! It orbits the Sun about 80 million kilometers ahead of Earth. We detected it because its odd shaped orbit takes it further away from the Sun than most Trojans would go.
2010 TK7 is not so economical to get to because its orbit is so odd, inclined quite a lot relative to Earth's orbit. Other Trojans would be much more economical to visit, but they are still a bit difficult to detect.
When we get telescopes above Earth's atmosphere and dedicated to asteroid searches, we are sure to find a lot of good surprises. We're just not there quite yet, as of 2012.