An Asteroid is defined as any of numerous small celestial bodies that revolve around the sun, with orbits lying particularly between Mars and Jupiter and characteristic diameters between a few and several hundred kilometers. They are generally described as large chunks of rock that may have broken off from other larger Asteroids, or possibly from a moon. The majority of Asteroids are found in the Asteroid Belt. There are also asteroids that could potentially harm Earth. The total mass of all asteroids is less than the mass of Earth's Moon. The largest asteroid by far is 1 Ceres. It is 933 km in diameter and makes up about 25% of the mass of all the asteroids combined. The next largest are 2 Pallas, 4 Vesta and 10 Hygiea which are between 400 and 525 kilometers in diameter. All other known asteroids are less than 340 kilometers across. Asteroids are classified in two distinct ways: by composition and orbital characteristics.

Classification by Location

Asteroids are minor planets, most of them in the Inner Belt. The larger ones being called planetoids. There are millions of asteroids, they are categorized in 3 main groups: C-Type (carbon rich), M-Type (metallic) and S-Type (silicate(or stony)).

  1. Main Belt: located between Mars and Jupiter roughly 2 - 4 AU
  2. Near Earth Asteroids (NEA's): ones that closely approach the Earth
  3. Trojans: located near Jupiter's Lagrange points (60 degrees ahead and behind Jupiter in its orbit).

Based on location, and thus their importance depending upon the situation, they are generally considered to be in one of 3 categories. Most asteroids are located in the "Main Belt", the broad area between Mars and Jupiter. This zone includes some of the most well known asteroids, like Ceres, Pallas, and Vesta. There is a group known as Trojans that are at the L4 and L5 points of Jupiter's orbit. Of most concern to Earthlings are the Near Earth Asteroids (NEA's). An NEA is any asteroid whose orbit crosses the orbit of the Earth. Since NEA's have the potential to collide with Earth, attempts have recently been initiated to catalog and track all as many as possible.

Classification by Composition

Based on spectra, which has to do with surface content, they are separated into 3 groups:

  1. C-group (contains C-type, B-type and G-type) have carbon
  2. S-type which contain silicon
  3. X-group (includes the metal M-type)

The composition of asteroids depend on their origins, and for the most part is poorly understood. Some of the NEA's are very likely comets that have petered out. Many NEA's may have been driven out of the asteroid belt by gravitational interactions with Jupiter. A few of the largest are roughly spherical and are very much like miniature planets. This includes Ceres with a diameter of 975 kilometers and Pallas and Vesta of over 500 kilometers. Then there are the not-so-grand rocks, just tens of meters across. Ceres appears to be composed of a rocky core covered by an icy mantle, whereas Vesta is thought to have a nickle-iron core, olivine mantle, and basaltic crust, and Hygiea appears to have a primitive composition of undifferentiated carbonaceous chondrite. The majority, however, are much smaller, oddly shaped, and are piles of rubble held together loosely by gravity. Some have moons or are co-orbiting pairs of binary asteroids. All three conditions, as well as scattered asteroid families, may be the result of collisions which disrupted a parent asteroid. Sadly, or luckily, in some opinions, NEA's only survive in their orbits for a few million years. They are eventually eliminated by orbital decay and accretion by the sun, collisions with the inner planets, or by being ejected from the solar system by near misses with planets.

Asteroid composition

Asteroids are layered, having heavy element cores and lighter element outer layers, similar to planets. It is common for asteroids to shatter into pieces, and pieces of shattered layered asteroids obviously comes from only one layer, just like pieces of planets blown into space. This means that their chemical element signature is not that of undifferentiated asteroids. And since undifferentiated asteroids are the standard by which traditional geology identifies asteroids that have once hit Earth (chemical signatures like "the iridium signature"), lots of asteroid impacts are therefore unknown. The low asteroid impact rate in present time can be explained by impacts being concentrated to time periods when a big asteroid have been recently shattered (and big asteroids tend to be layered).

Earth formation

Many asteroid impacts hitting Earth are undetected means that Earth have been hit by far more asteroids than previously thought. This means that asteroids may well have added major amounts of mass to Earth over millions of years. This means that Earth's gravity have increased significantly over geological time. Weaker gravity in the distant geological past can explain giant land animals (like big dinosaurs) and really big fliers (like big pterosaurs) in the distant geological past.


Giuseppe Piazzi (on Jan 1, 1801) discovered what he assumed was a comet. After determining its orbit, it became clear that he had found a small planet. He named it Ceres, after the Sicilian goddess of grain. He discovered Pallas, Vesta, and Juno within the next few years. By the end of the 19th century there were several hundred. By this point they were labeled asteroids, as opposed to planets. Ceres could be said to be the first body to lose its "Planetary" status. Several hundred thousand asteroids (1000s more each year) are now known. At least 26 are larger than 200 km in diameter. There are probably more than a million in the 1 km range. Eleven comets and asteroids have been explored by spacecraft so far.

There have been thousands of asteroid discoveries through the years. here is a video to give an idea of the number.


Asteroid Discovery From 1980 - 2010


The process of naming the asteroids is fairly simple, for an astronomer. A newly discovered asteroid is given a provisional designation - the year of discovery, an alphanumeric code indicating the half-month of discovery and the sequence within that half-month. Once the asteroid's orbit has been confirmed, it is given a number, and later an official name. Usually the names coincide with the asteroid's discoverer and its discovery date. Humans name asteroids for the simple purpose of keeping their observations organized.


  • the process could take decades
  • It begins with the discovery of a minor planet that cannot be identified with any already-known multiple-opposition or recent single-opposition object. When observations on two nights of a new object are available they are reported to the Minor Planet Center, which assigns a provisional designation to the object.
  • Guidelines for naming:
    • 16 characters or less in length (including any spaces or punctuation)
    • preferably one word
    • pronounceable (in at least one language)
    • non-offensive
    • not too similar to an existing name of a minor planet or natural planetary satellite

Well Known Asteroids

Five of the largest and well known asteroids are Ceres, Pallas, Gaspra, Vesta, and Eros.

  • 1 Ceres -- The largest body in the asteroid belt at about a fourth the diameter of the moon. It makes up more than a third of the estimated total mass of all the asteroids. Also it was added to the dwarf planet list in 2006 along with Pluto. It was named after Ceres the Roman Goddess of Agriculture.
  • 2 Pallas -- One of the largest asteroids discovered, Pallas is estimated to make up 7% of the total mass of the asteroid belt. It was named after Pallas-Athena the Goddess of Wisdom.
  • 951 Gaspra -- Estimated to to be about 200 million years old, Gaspra is believed to be made up of a mixture of rocky and metallic minerals. Its entire surface is covered with impact craters. It was named after Gaspra, a town in Crimea, Ukraine.
  • 4 Vesta -- This asteroid has one of the most diverse terrains of the larger asteroids. Scientists suggest that meteorite impacts, radioactive "shrapnel" and ancient lava flows shaped the complex surface of this asteroid. It was named after the Roman goddess of the Home and Hearth, Vesta.
  • 433 Eros -- Eros is an odd-shaped NEA, near-Earth asteroid, located outside the main asteroid belt. Some believe that Eros is a chunk knocked off another body, perhaps another asteroid. Its size is equal to almost double that of the Manhattan Island. It was named after Eros the Greek God of Love.

--A complete list of Asteroids can be found here:



  • The Planetary Report[(Visions of Exploration) Volume XXVIII Number 2 March/April 2008 p.5 ]
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