Introducing Asteroids

Origin of Asteroids

Asteroids, also called minor planets, are rocky bodies that orbit the sun within our solar system. They range in size from several meters to hundreds of kilometers and are thought to be the remnants of planetesimals that were abundant during the formation of the solar system, some 4.5 billion years ago. Most of the ancient asteroids collided with the major planets, but some of them still exist today due to their location in areas of the solar system where the gravitational attraction of the large planets do not substantially influence them. Asteroids have orbits that are similar to, but generally more eccentric (out of round) than those of the planets. They also have greater inclinations, which means they travel further above and below the plain of the ecliptic than most of the planets. The greatest population of asteroids is found in the main asteroid belt located between the planets Mars and Jupiter. This, however, is not the only place where asteroids exist. Several populations of minor planets have been found within the orbital territory of the inner planets, including earth. This raises the concern of a possible impact that could be devastating to the fragile atmosphere that sustains life on our blue planet. The threat is real, but the chances of a major impact occurring during the average lifetime of a human is remote, but not zero. Such an impact would change life as we know it and could possibly wipe out life altogether on our planet.

History of Asteroid Discovery

The first asteroid was discovered on January 1, 1801 by an astronomer named Guiseppe Piazzi of Palermo, Sicily. It was named Ceres and is now known to be the largest asteroid in the solar system with a diameter of 1003 kilometers. An asteroid of this size is more accurately described as a minor planet since it has enough mass and therefore gravity to pull itself into a sphere. Early asteroid discoveries like this occurred by chance while astronomers were observing or searching for other objects. The asteroid Pallas, 608 kilometers in diameter, was discovered in 1802 and Vesta, 538 kilometers in diameter was discovered in 1807. By 1845 five such discoveries had been made and soon after organized searches began. The number of new finds increased to five per year by 1865, fifteen per year by 1895, twenty-five by 1910 and up to about 40 by 1930. Today the number of known asteroids measures in the thousands, including about 220 that are larger than 100 kilometers across.

Asteroid Composition

Regarding their physical nature, asteroids can be categorized into three major types. As well, there are a few minor ones. There is also a significant number of asteroids in the unknown category that will need further study. Here is a list of the major types:

  • C-Carbonaceous: These asteroids are very dark in color and reflect only a few percent of the sunlight that strikes them. They are believed to be the oldest asteroids, dating back to the formation of the solar system. Asteroids this old may contain the primordial chemistry of the original solar nebula and are therefore of great interest to planetary scientists.
  • S-Silicate: These asteroids are not as dark as carbonaceous types and reflect up to one fifth of the sunlight that strikes them. They have a slightly reddish color and are made of stoney silicates with moderate to high concentrations of iron and other rocky elements.
  • M-type: These asteroids are perhaps even more reflective than S-types and are thought to be the core nickel-iron remnants of larger asteroids that broke up due to collisions. They are neutral in color and appear to be composed mainly of metals.

Observing Asteroids

To find an asteroid in the night sky you will need a good star map where you can plot its location using the Right Ascension and Declination coordinates provided by an observing guide such as the RASC's Observer's Handbook. You will have to make sure that the information is accurate for the date that you will be observing because asteroids will change position against the background stars due to orbital motion. At certain points in their orbits they may even change position over a period of just a few hours. Asteroids are relatively small objects that will appear as stars in the night sky so; you will need to have a way of determining which star is actually the asteroid you are looking for. Usually a good finder chart will be enough to accurately identify the asteroid, especially if you know its current position and magnitude. If you are using a finder chart that does not contain enough accurate information, then you will need to use another method to determine which object is the asteroid. One good way of doing this is to draw the star field on paper, then return later to the same star field and check to see if any objects on your drawing have changed position. If one has moved, it will likely to be the asteroid you were looking for. The length of time for orbital motion to be detected will vary; so you may have to check back a few times before you are successful.

Choosing asteroids to observe

The first thing you need to determine before choosing an asteroid to observe is the limiting magnitude of the instrument you are using, as well as the limiting magnitude of that instrument in the location where you are going to observe. This can be done fairly easily by using a chart with the visual magnitudes of the stars clearly indicated on it. Here is a general guideline for various instruments.

  • Binoculars and small telescopes: Several asteroids are bright enough to be seen through small instruments. Generally, but with some exceptions, binoculars can see to eighth or ninth magnitude and small telescopes to eleventh or twelfth magnitude. The RASC Observer's Handbook is a good source of information about asteroids that will be magnitude ten or brighter each year.
  • Medium sized telescopes: Observer's with medium sized telescopes can seek out fainter objects in the twelfth to fourteenth magnitude range using data that is available from the Minor Planet Center in Cambridge, Mass., or through the Association of Lunar and Planetary Observers. You could also use a computer program such as The Earth Centered Universe that will also print very good finder charts.
  • Large Sized Telescopes: For those who have access to large telescopes, a great many asteroids can be observed. To find faint asteroids, a pre-printed finder chart will be necessary as well as a regular star map such as SkyAtlas 2000 or Uranometria. A pre-printed chart of a faint star field usually represents a very small field of view that is very difficult to find in the night sky. By using the regular charts and progressing from large fields of view to small fields of view it will be much easier to find the asteroid you are looking for. Computer programs such as Earth Centered Universe and others can print out finder charts containing stars as faint as fifteenth magnitude or more.

Sample Charts Available

For all instruments sample charts are available by clicking on the sample charts link in this section. You are welcome to print and use these charts to get started in the fascinating world of asteroid observation and discovery.


The Association of Lunar and Planetary Observer's has a very good web page in their Minor Planets Section about visually observing asteroids that you can go to for more information.

Asteroid Names

Many asteroids have names with a Canadian connection. The RASC maintains a definitive list here.

Last modified: 
Saturday, October 8, 2016 - 7:13am