MIRA's Field Trips to the Stars Internet Education Program
Back to the MIRA Website | Field Trips to the Stars Home > Introduction to the Solar System > The Asteroids


Where are They? - What are They? - Collisions with Earth?

Asteroids are small, rocky objects which orbit the Sun along with all the other planets. They are also commonly called minor planets (and rarely as planetoids) because they are considerably smaller than ordinary planets. Astronomers estimate that there are more than 100,000 asteroids in our solar system although only about 8000 have been cataloged as of mid-1997.

The very first asteroid, called (1) Ceres, was discovered by accident in 1801 by Giuseppe Piazzi. While making a map of faint stars, Piazzi noticed one object whose position moved over the course of several nights. Careful mathematical analysis later revealed that this faint object orbits the Sun at an average distance of 2.77 Astronomical Units with a period of 4.6 years. Piazzi named it Ceres (pronounced SEE-reez) after the Roman goddess of agriculture.

Since this first discovery, many such asteroids have been found in a similar manner -- by searching for faint objects which move relative to the background stars. Newly discovered asteroids are initially given a designation by date, and are only officially named and numbered (in order of discovery) after their orbital properties have been accurately determined. Nearly every conceivable human name is now associated with an asteroid, check here to see if you share your name with a minor planet!

Where Are They?

Diagram courtesy of the Minor Planet Center (MPC) operated at the Smithsonian Astrophysical Observatory.
The vast majority of asteroids are located between the orbits of Mars and Jupiter (or between 1.5 and 5.2 Astronomical Units). According to the Titius-Bode law, this is the predicted position for a planet but instead we find these thousands of small rocky objects. This region, between Mars and Jupiter, is called the Asteroid Belt.

To the right, the locations of most of the asteroids, as they were on March 7, 1997, are shown (black dots) relative to the Sun (yellow dot) and the orbits of the Earth (blue dot), Mars (red dot), and Jupiter (green dot). All of the planets and asteroids shown on this diagram orbit the Sun in a counterclockwise direction. Note that some asteroids, called the Trojans, lie along Jupiter's orbit just ahead and behind that giant planet. The asteroid at the lower-left corner is called Chiron, and the one at the top-left is named Don Quixote.

The elliptical orbits of some notable asteroids are shown here; each of these asteroids has been imaged (close-up) either by passing spacecraft or using radar techniques from Earth. As you can see, the paths taken by these asteroids are considerably more eccentric than the majority of planets. In some cases, the asteroids even cross the orbit of the Earth, these are discussed further below. Besides their orbital motion around the sun, the asteroids also rotate - sometimes quite irregularly.

What Are They?

Unlike the major planets, the minor planets are mostly quite small. Ceres, the first one found and the largest known asteroid, is only 1000 kilometers in diameter -- or about one-third the size of the Moon. Furthermore, estimates for the sum total mass of all the asteroids yield a value of only a few percent that of the moon! This fact, together with their orbital properties and varying compositions, suggest that the asteroids originated from material left-over from the formation of the solar system; they are not likely to be remnants of a once larger planet.

In terms of their compositions, there are three main classes of asteroids: S-types, C-types, and M-types. The S-types, for "stony", are made-up of Silicate materials and are brighter in appearance (higher albedos, roughly 15 percent). The C-types are mainly composed of Carbon and have relatively low albedos (2 to 5 percent). And finally, the M-types are "metallic" and fall between the S and C types in terms of brightness with albedos of about 10 percent. The C and S-type asteroids are the most common; only 5 percent of the total number of asteroids are M-types.


Again, unlike the predominantly spherical planets, the asteroids have very irregular shapes. The majority of asteroids are not large enough to pull themselves into a spherical configuration via gravity. Hence their unusual shapes are determined mainly through collisions with other asteroids. Close-up images obtained from spacecraft show that Gaspra is oblong, whereas Mathilde is almost round with some large indentations, and Castalia looks like two muffins stuck together! The cratered surfaces of asteroids clearly demonstrate that collisions have occurred. In some cases, these collisions may have caused the break-up of larger asteroids into many smaller pieces. One possible example of this phenomena is the extraordinary asteroid, Ida which has a small satellite, Dactyl. The discovery of this first binary asteroid came as quite a surprise to scientists in 1995; Dactyl was found from images of Ida taken in 1993 when the Galileo spacecraft flew past it en route to Jupiter.

Collisions with Earth?

The aptly named Meteor Crater in Arizona. It is 1200 m wide and 170 m deep.
Some families of asteroids (those with similar orbits) cross the orbit of the Earth. These are called the Apollo asteroids, or Earth Crossing Asteroids. Some of the closest are even dubbed near-Earth Asteroids (or NEAs). They have often been the subject of science-fiction stories due to their chances of colliding with the Earth. In fact, NEA collisions with Earth are expected to occur once every few million years. One of the most dramatic pieces of evidence for such collisions exists in the form of a giant impact crater located in the Arizona desert. Similar collisions may have caused some massive extinction events on Earth -- such as the end of the dinosaurs 65 million years ago. The reason for such devastation is not just the impact itself, but the profound after effects resulting from the collision. The most damaging effect being an opaque dust cloud which would be released into the atmosphere; this dust cloud would block most of the essential light we receive from the sun and cause massive die-outs of vegetation.

It's fairly obvious therefore, why recent news of an asteroid -- on a collision course with the Earth -- became such a scare! On March 12, 1998 the Central Bureau for Astronomical Telegrams announced that the asteroid called 1997 XF11 was going to come very close to, or even hit the Earth in thirty years' time -- on October 26, 2028. Fortunately, as astronomers obtained more accurate data on this NEA's orbit, they later determined that 1997 X11 was not likely to collide with the earth -- but would pass no closer than 600,000 miles away. This is more than double the distance between the Earth and the Moon and is considerably safer than the original distance estimate of passage within 30,000 miles. These reports have demonstrated the need for obtaining confirming measurements of such critical information before making announcements which can have such frighteningly global effects. This episode has also heightened the public's awareness of the importance of NEAs and may lead to better plans for dealing with possible impacts in the future.


<<< PreviousNext >>>