Page Sections

Where They Are
What They Are
Meteorites - Downed Asteroids


Apollo: Asteroids with a semi-major axis greater than 1 AU but perihelion less than 1.017 AU.

Amor: Asteroids that come close to Earth but do not cross its orbit. They generally lie near Mars

Asteroid Belt: A region of space between the orbits of Mars and Jupiter where most of the solar system's asteroids lie.

Aten: Asteroids with a semi-major axis less than 1 AU.

Centaur: Asteroids that lie between the orbit of Jupiter and Neptune.

Hirayama Family:

Kirkwood Gap:

Near-Earth Asteroid: Apollo, Amor, and Aten asteroids.

Trans-Neptunian Object: Asteroids (or comets) that lie beyond the orbit of Neptune.

Trojans: Asteroids that share a planet's orbit, lying at either the L4 or L5 point.

Vulcanoid: Asteroids that lie interior to Mercury's orbit. No members have ever been discovered, so this is a hypothetical group.

External Links

List of Known Asteroids



Asteroids are relatively small, rocky bodies that tend to be irregularly shaped. They can be found throughout the solar system but the vast majority lie in the asteroid belt between the orbits of Mars and Jupiter. Literally hundreds of thousands of asteroids have been cataloged, and there are many more that have yet to be discovered.

Asteroids that cross planets can pose a severe danger to the target they hit. Every solid body in the solar system - except for Jupiter's moon Io - bear the record of having been hit by asteroids throughout their history. It is believed that an asteroid hitting Earth resulted in the death of the dinosaurs approximately 65 million years ago.


The first asteroid to be discovered is also the largest asteroid - 1 Ceres. When naming asteroids, they are given both a number and then either a name or a designation. They are numbered in the order they are discovered.

1 Ceres was discovered on January 1, 1801, by the Italian astronomer Giuseppe Piazzi. ...

As more asteroids were discovered, it became clear to the astronomical community that these were not "planets" in the traditional sense, but something else entirely. Their disks could not be resolved in telescopes, unlike the planets, since they were too small. Astronomer William Herschel suggested the term "asteroid" from the ancient Greek language, meaning "star-like," or "star-shaped." They are also known occasionally as "minor planets."

One of the breakthroughs in the understanding of the asteroid belt came with American astronomer Daniel Kirkwood, who studied the distribution of the locations or known asteroids. In 1866, he published the paper On the Theory of Meteors, in which he suggested that there were gaps in the spatial number density of asteroids in the main belt. In other words: Gaps. These gaps can easily be seen in the present-day known distribution of asteroids (see the graphs below in the Data section). Today, they are known as Kirkwood Gaps in his honor. They are formed at locations of "gravitational resonance" with Jupiter, where the asteroid's orbit and Jupiter's orbit line up periodically and Jupiter acts to nudge it away from that location.

An important pioneer in understanding asteroid orbital elements was the Japanese astronomer Kiyotsugu Hirayama (1874-1943). His work led to the identification of asteroids that travelled in groups that posessed similar motions, eccentricities, and orbital inclinations, that he believed could not be random chance. One of his first papers on the subject was published in 1918, "Groups of asteroids probably of common origin," in the Annales de l'Observatoire astronomique de Tokyo, but also published in The Astronomical Journal.

In this seminal paper, he identified three main groups of asteroids which he named after the largest asteroid in the group: Koronis Family (after 158 Koronis), Eos Family (after 221 Eos), and Themis Family (after 24 Themis). He suggested that these formed when an asteroid broke up but the individual framents had velocity dispersions much smaller than the original did. Thus, over time, they would spread out in the original asteroid's orbit, but they would remain in that orbit, forming a family. Over the next several years, he continued to identify families of asteroids.


Where They Are

The majority of asteroids are located between the orbits of Mars and Jupiter in what is known as the "asteroid belt," or the "main asteroid belt." Asteroids that are members are sometimes referred to as "main belt asteroids." This region holds over 400,000 cataloged asteroids.

Besides main belt asteroids, there are also groupings of asteroids that share orbits with the planets. These are known as trojan asteroids. Jupiter itself has over 2000 trojans, while others are known to share Mars' and Neptune's orbits. These share the planet's orbit, either trailing or leading the planet, and they are gravitationally locked.

There are other asteroids that are not locked between planets nor that share their orbits. Asteroids that lie beyond Neptune are known as trans-Neptunian objects, though these are likely to be more the composition of comets than asteroids. Over 1200 have been identified. Asteroids that lie between Jupiter and Neptune are known as Centaurs. There is a hypothesized group of asteroids that lie between the Sun and Mercury called the Vulcanoids, but these have never been discovered. They are believed to exist because dynamical models show that region is stable, and we have yet to find regions of the solar system that would be stable that do not have asteroids.

There are three groups of near-Earth asteroids (also known as near-Earth objects (NEOs)). Apollo asteroids have a semi-major axis (one of the orbital elements) that is larger than Earth's, but a perihelion (closest approach to the Sun) of less than 1.017 AU (the Earth-Sun distance is 1 AU). A second group, Amor asteroids, by definition do not cross Earth's orbit. They orbit beyond Earth but still can come close to it. Most tend to orbit near Mars. A third group, Aten asteroids, is defined as an asteroid that has a semi-major axis smaller than 1 AU. However, since most of these have highly eccentric (very elliptical) orbits, they generally do cross Earth's orbit.

What They Are


Meteorites - Downed Asteroids



The following is a list of the missions that have finished, are currently in operation, or are planned to be lanuched to explore asteroids. A brief summary is displayed, but you can click on the name of the mission to be taken to a page detailing the mission.

Near-Earth Asteroid Rondezvous - Shoemaker (NEAR-Shoemaker) ~ 1996-2001 ~ NEAR was a mission to study the NEA 433 Eros from orbit. The mission was successful and terminated when it landed at the end of February, 2001.


The following graphs were created from the data from Lowell Observatory. It was downloaded on June 14, 2008, and contains information for 410,517 asteroids. Click on any graph for a larger version.

  • Absolute Magnitude
  • Semi-Major Axis
  • Eccentricity
  • Orbital Inclination
  • Semi-Major Axis vs. Eccentricity
  • Semi-Major Axis vs. Inclination

The absolute magnitude of an object is a measure of how bright it is relative to the star Vega. Vega is defined to have an absolute magnitude of 0 in all colors; anything brighter than Vega is a negative absolute magnitude, and anything fainter is a positive magnitude. Magnitudes are on a logarithmic scale such that for every increase in 2.5 magnitudes, the object is 10 times fainter. So, a star that is 10th magnitude means that it is 10,000 times fainter than Vega.

The absolute magnitude of asteroids is very faint, which is a result of them being very small and not reflecting a lot of sunlight. The absolute magnitude histogram is more a reflection of how good our detection technology is than what the actual distribution of the brightness of asteroids is.

The actual distribution of magnitudes is probably more of an exponential, considering that there are many more smaller asteroids than larger ones. There is also a slightly larger tail towards the bright end (lower numbers) of magnitudes. This is an artifact again of our detection methods: It is much easier to spot a bright object than a faint one, and so we know of many more bright asteroids than faint ones.

Asteroids are found throughout the solar system. However, in the interest of readability, I have isolated in the graph just the solar system out to Jupiter, since the vast majority of known asteroids lie within that region. This allows a certain structure to the location distribution of the asteroids to be discerned.

At about 5.2 AU, there is a marked increase in the number of asteroids. These asteroids are Jupiter's Trojans, the asteroids that orbit the Sun with Jupiter. They are not moons of Jupiter, they just share its orbit around the Sun. From about 3.2 AU in to about 1.8 AU is the realm of the main asteroid belt. Almost all known asteroids orbit within this region between Mars and Jupiter.

The next graph shows an expanded view of the main asteroid belt, from 1.5 to 3.5 AU. The data have been re-binned with intervals of 0.002 AU. It is clear from this chart that there are many gaps in the distribution of the asteroids, and in some cases almost no asteroids are found in the locations. These are called Kirkwood Gaps, and they arise because of resonances with Jupiter. (Martian resonances are inconsequential compared with those from Jupiter.)

At a distance of 2.49 AU from the Sun, Jupiter's gravitational pull on an object is only about 0.3% of the Sun's. Over time, though, this pull adds up and serves to nudge an object away from that region, but this will really only work if the pull is repeated at a regular interval. 2.49 AU happens to lie at a distance from the Sun such that an object's year there will be exactly 1/3 of Jupiter's. This means that once every three years, an asteroid in that location will feel a tug from Jupiter, and over time, this will remove it from that location. This is known as a 3:1 resonance.

There are many other resonances that happen, and based upon how often an asteroid feels Jupiter's tug, the relative strength of the resonance is established. The resonance is especially effective at nudging an asteroid if the tug occurs in the same location in the asteroid's orbit and only in that location, so the 2, 3, 4, and 5 to 1 resonances are the strongest. The major resonances are:

  • 5:1 ~ 1.78 AU
  • 9:2 ~ 1.91 AU
  • 4:1 ~ 2.06 AU
  • 7:2 ~ 2.26 AU
  • 3:1 ~ 2.50 AU
  • 8:3 ~ 2.70 AU
  • 5:2 ~ 2.82 AU
  • 7:3 ~ 2.96 AU
  • 9:4 ~ 3.03 AU
  • 2:1 ~ 3.28 AU

The asteroid belt is bounded by the 5:1 and 2:1 resonance. Asteroids that are pushed outside of these resonances will generally become planet-crossing asteroids.

The eccentricity of an orbit is how much it varies from a perfect circle. A stable orbit can have an eccentricity anywhere from a perfect circle with an eccentricity of 0, up to a highly elliptical orbit with an eccentricity up to (but not including) 1. If an orbit had an eccentricity of 1, it would be parabolic and escape from the system. If it were larger than 1, it would be hyperbolic and also escape from the system.

Earth's eccentricity is 0.017, while Jupiter's is 0.094. In our solar system, the planet with the largest eccentricity is Mercury with 0.205 (Pluto's is larger at 0.244, but it is not considered a planet any longer). The planet with the lowest eccentricity is Venus with 0.007. Unless there is some gravitational tugging (such as with the Galilean Satellites) that keeps an orbit eccentric, orbits will usually circularize with time.

As shown in this diagram, the average eccentricity is about 0.11. In general, asteroids' orbits are far from perfect circles. This is probably due to perturbations from the planets - especially Jupiter - and collisions. A collision between asteroids can dramatically alter their orbits, though over time they should average out to circularize the orbit. The eccentricity can also be an effect of sampling. As discussed in the section on semi-major axis vs. eccentricity, objects that are farther away from the Sun are more likely to be found by Earth-based searches if they have a large eccentricity that takes them much closer to Earth than they usually are.

When the solar system formed, most of the solar nebula evolved into a disk, and from this the planets, moons, asteroids, comets, and Sun formed. Because of this, the planets all generally orbit in the same plane. The largest difference is Mercury which orbits 7° from Earth. (Pluto's orbit is ~17° off from Earth's, but it is no longer considered a planet.) The rest of the planets orbit within 2.5° of Earth's orbital plane.

The orbital inclination histogram here shows that asteroids are another exception to this rule. The average orbital inclination is about 8.2°.

A probable mechanism for creating the - sometimes very large - orbital inclinations are collisions. When two asteroids hit each other, their orbits can be changed dramatically. And, since the inclination of orbits is much less subject to gravitational perturbations than other orbital elements, they really don't have any driving force to put them back into an orbit with the rest of the solar system.

Another factor is that when a rather large asteroid is fragmented into smaller pieces, the pieces tend to stay close to the original orbit, which creates a "family" of asteroids (Hirayama Family). This increases the raw number of asteroids with a given orbital inclination.

This semi-logarithmic scatter plot of eccentricity as a function of the semi-major axis shows mostly what is discussed in the other sections. Visible are the general asteroid trends, such as the large number of main belt asteroids, trojans, a sprinkling of Centaurs, and the TNOs. Most asteroids have an eccentricity relatively low, with higher eccentricities tapered off.

Two important new observations, however, can be made. The first is from the asteroids that orbit within Mars' orbit, about 1.5 AU from the Sun. There are many fewer asteroids in this region than in the main belt, but on average, their eccentricities are much higher. This is a strong indication that they did not form in these locations, but that they were pushed or pulled into this region. When that happens, the asteroid is likely to return to its original orbit distance for at least part of its orbit, until it evolves more. This will cause it to have a high eccentricity, for during part of its orbit it will be much closer to the Sun than during another part of it.

For the Kuiper Belt Objects beyond the orbit of Neptune, there is also a large increase in eccentricity. This is probably an artifact of how we detect them. Since we are limited by how much light we can detect from an asteroid, the closer it is to us, the more likely we are to detect it. So, if an object were to have a semi-major axis of 100 AU and lie on a circular orbit, then it might be too faint to detect. But if it were on an elliptical orbit and this brought it within 35 AU of the Sun, then we would receive about 8 times more light from it, and so we would be more likely to detect it. So, objects that lie far from the Sun are much more likely to be detected today if they have highly eccentric orbits.

The first graph is a semi-logarithmic scatter plot with orbital inclination plotted against semi-major axis. This is a much truer picture of what the asteroid distribution would actually "look like" in space than the eccentricity vs. semi-major axis.

Probably the most important feature of this plot is that the Hirayama Families become apparent. This is the name given to "families" of asteroids, which are asteroids that tend to share the same orbital characteristics, and they probably had a common parent body that was destroyed during a collision. Japaneses astronomer Kiyostugu Hirayama, in 1918, was the first astronomer to suggest that they existed.

The second graph is an enlarged section of the orbital inclination vs. semi-major axis. It has been made into a number density image by creating a 2-D histogram. The 2 AU range has been divided into 0.005 AU increments, and the inclination of 0°-50° has been divided into 0.25° increments. The number of asteroids per bin is based upon the color displayed in the color bar in the upper left corner. The reason the data are displayed this way is that, even with dots only one pixel large, over 400,000 asteroids would make the graph look like a lot of red dots.

With this chart, it now becomes clear that there are some obvious clusters of asteroids, as indicated by the larger number density in certain areas. These groupings are the Hirayama Families.

Each Hirayama family is named for the main asteroid in the group. Some of the major ones are:

  • Hungarians (~1.9 AU at ~22°)
  • Floras (~2.2 AU at ~6°)
  • Phocaea (~2.4 AU at ~22°)
  • Koronis (~2.9 AU at ~1°)
  • Eos (~3 AU at ~11°)
  • Themis (~3.1 AU at ~1°)
  • Cybeles (~3.4 AU at ~ 4°)
  • Hildas (~4.0 AU at ~8°)

It is also fairly apparent from this image that there is a resonance at 2 AU, 2.5 AU, 2.8 AU, 2.9 AU, and 3.3 AU, since these are distances at which the number density of asteroids is almost 0.



Asteroid data downloaded from

Hirayama, K. (1918) "Groups of Asteroids Probably of Common Origin." The Astronomical Journal, 31, p. 185-188.

Kirkwood, D. (1866) "On the theory of meteors." Proceedings of American Association for the Advancement of Science for 1866, p. 8-14.

-- (1882) "On the distributions of the minor planets." The Observatory, 5, p. 194-198.