Minor Planets

Minor planets were once considered to be the 'vermin of the solar system'. They got in the way of astronomers studying the stars and galaxies. We now know that minor planets may give us important clues about the formation of the solar system. The number of minor planets for which we have orbits has increased dramatically over the last ten years. All of these objects have been known as asteroids, although the more correct term would be minor planet.


The First Minor Planets

   In the year 1801 Giuseppe Piazzi found himself assigned to be one of the Celestial Police. The idea was to have a group of observers who would scan their particular section of the sky in order to discover new objects in the solar system.
    Unfortunately, Piazzi did not know of his assignment until after he had already spotted a moving point of light on the first day of that year. In little more than a month, he became ill, and could not follow the new object long enough to determine its orbit. It took the mathematical genius of Carl Friederich Gauss to develop a new way to establish the orbit of newly discovered objects.  Using Gauss' technique the object was recovered once again before the year came to a close. Piazzi had decided to name the new object Ceres, after the Goddess of Agriculture and protector of Sicily.

       Soon after the discovery of Ceres, three more objects were discovered in similar orbits.Pallas, Juno and Vesta made it clear that these were not planets. So all four were eventually demoted to minor planets.
Classes of Minor Planets

There are thousands of objects circling the Sun in orbits that are between Jupiter and Mars, for the most part. Some of these objects have organized themselves into distinct groups, based largely on their orbits. There are locations in the solar system called Kirkwood gaps where one is not likely to find any minor planets due to the gravitational influence of other bodies.  Some of these objects do, in fact, cross Earth's orbit such as the Atens.   Amors come close to Earth but do not cross our orbit. The group that should be of greatest interest to us are the Apollo asteroids. The Trojan asteroids lead and follow Jupiter in its orbit. These minor planets are at two of Jupiter's gravitationally stable Lagrangian points. Here is a list of those minor planets that are not in the main belt.

    Minor planets can also be grouped according to their physical characteristics.  The three types most often referred to are C, S, and M types.

Some Views of Minor Planets
   What does your typical minor planet look like? Look here for an image of the minor planet Gaspra. Click on the small image on the top left of this page to see a big version of the image.   Recently we got a very close look at the minor planet Eros from the NEAR spacecraft. The recently imaged minor planet Toutatis has an interesting shape. It is likely two minor planets that have gently merged. For true weirdness, take a look at this recent image of the minor planet Itokawa , taken by the Japanese spacecraft Hayabusa.

The Physical Nature of Minor Planets

   Most minor planets rotate in just a few hours; however, there are some notable exceptions. Do any minor planets have moons? Yes, there are many asteroids that are now suspected to be binary.

Impact Hazards:  Could the Earth Be In Danger?

   The prospect of an asteroid hitting the Earth has been in the news a lot recently. One object, 2004 MN4, will come very close to us in 2029. Two recent big Hollywood movies have exploited this interest. However, what are the chances of something actually hitting the Earth? Go here to find out for yourself. What would we do if an impact were a certainty?

    On the other hand, perhaps you would like to slam one those big rocks into the Earth yourself and see what happens. Go ahead; the people at the University of >Maryland have a way for you to do it! Have fun!

   How about the other inner planets, Mercury, Venus and Mars? Are they in danger of being hit? Check here.




    Here is a service that will give you an ephemeris for any minor planet you may choose.

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Updated November 20, 2009