asteroid Ida and its moon

Asteroids are irregularly shaped, rocky objects that orbit our Sun. When viewed from Earth through a telescope they look like stars, which is how they got their name. Asteroid means star-like body.

Most asteroids are approximately 300 km (186 miles) in diameter. The largest asteroid that we know of is Ceres with a diameter of 940 km (584 miles) (Chaisson and McMillan, 2005).

Most asteroids are located in a region of space between the orbit of Mars and Jupiter called the asteroid belt. Scientists believe that these asteroids are rocks that were never able to combine and form into a planet due to Jupiter's strong gravitational pull.

The NASA image of Asteroid Ida (S-type) and its moon Dactyl was taken by the Galileo spacecraft. Ida is 56 km in diameter (35 miles) and Dactyl is only 1.5 km (1 mile) in diameter. Scientists believe that Ida is at least a billion years old. Before the discovery of Ida's moon Dactyl, astronomers didn't know that asteroids could have their own satellites.

Types of Asteroids

Asteroids are classified by their composition and reflectivity. The most common type are C-type asteroids. C-types are dark non-reflective asteroids that contain a high amount of carbon.

The second most common are the S-type asteroids. These contain contain a high amount of silicate material and reflect much more light than the C-type asteroids.

The least common type are M-type asteroids, which contain high amounts of nickel and iron. These asteroids are highly reflective.

Will an Asteroid Ever Hit Earth?

Most of us have seen movies about the threat of an asteroid about to collide with Earth, but could it really happen?

Most asteroids stay within the orbits of Mars and Jupiter. However, some asteroids do enter the vaccinity of Earth's orbit. These are called Near Earth Objects or Earth-Crossing Asteroids.

These Near Earth Objects are called Apollo or Aten asteroids. Apollo asteroids have orbits that intersect with Earth's orbit and Aten asteroids have orbits that are located within Earth's orbit (Moche, 2004). Astronomers have discovered over 2,600 of these Near Earth Objects (Chaisson and McMillan, 2005), and 624 of these have been classified as potentially hazardous asteroids (Maran, 2005).

There is evidence that asteroids have collided with Earth in the past. In fact, most scientists believe that an asteroid impact with Earth may have been responsible for the extinction of the dinosaurs. There are also a few large impact craters on Earth that scientists believe were caused by asteroid collisions.

Astronomers now scan the sky looking for asteroids that could impact Earth so that we would have time to prepare or somehow knock the asteroid into a different orbit so that it doesn't hit Earth. This is another reason why the space program is so important. With the high-tech telescopes that astronomers have available to them, we'd know in plenty of time if an asteroid was headed for Earth so that something could be done about it.

In fact, NASA's Jet Propulsion Laboratory has a new site where you can find information on the location of near Earth Asteroids: NASA Asteroid Watch.

In the last 2 decades there have been asteroids that have come fairly close to Earth and at least one asteroid that came within 105,000 km (65,244 miles) of Earth (Chaisson and McMillan, 2005).

Most of these Near Earth asteroids are very small (1 km or less than 1 mile in diameter), but even an asteroid of this size could have devasting effects if it collided with Earth.


Chaisson, E. and McMillan, S. (2005). Astronomy Today. Pearson Prentice Hall: Upper Saddle River, NJ.

Maran, S. P. (2005). Astronomy for Dummies. Wiley: Indianapolis, IN.

Moche, D. L. (2004). Astronomy. Wiley: Hoboken, NJ.

Photo of Asteroid Ida and moon Dactyl: Courtesy of NASA/JPL-Caltech.