What are asteroids made of? Why is there an asteroid belt? What are Earth-crossing asteroids? What should we do if an asteroid is headed for the Earth?

asteroid Gaspra

An asteroid is a small rocky object that orbits the Sun. Asteroids vary widely in size, from very small particles to objects that are hundreds of miles across.

Gaspra, pictured above, is a typical asteroid, about 20 kilometers long. It resides in the asteroid belt, a region between Mars and Jupiter that houses between one and two million asteroids larger than one kilometer in diameter.


By definition, asteroids orbit the Sun, not a planet. This asteroid, Ida, actually has a small moon orbiting it. Ida is about 13 miles long. Asteroids are typically oblong and somewhat cratered and covered with a fine icy dust.

asteroid Itokawa

This asteroid, Itokawa, is different from most asteroids, in that it exhibits no cratering. We believe it is a conglomerate of chunks of rocks and ice. Measurements from ESO’s New Technology Telescope show that the density is not constant throughout this asteroid, which supports the hypothesis.

asteroid belt

This diagram shows the general distribution of asteroids in the asteroid belt, and the groups of asteroids that orbit in Jupiter's orbit, ahead and behind the massive planet.


Early astronomers speculated that the asteroid belt consisted of pieces of a planet that somehow exploded, possibly via collision. We now think this was very unlikely, since the total mass of all of the asteroids in the asteroid belt totals much less than Earth's moon. It is much more probable that the gravitational field of Jupiter kept a planet form forming in that region.

Dwarf planet Ceres

The largest object in the asteroid belt is the dwarf planet Ceres with a diameter of almost 600 miles. Ceres was the first object discovered in the asteroid belt, in 1801 by Giuseppe Piazzi. Ceres was classified as a planet for about fifty years after its discovery. However, soon after its discovery, other objects were discovered in the region between Mars and Jupiter, and it became evident that many objects resided there. Since the sizes of these objects varied, and some were very small, people started to realize that a new class of objects had been discovered, they could not all be considered planets. Ceres was reclassified as an asteroid, along with many other objects in the asteroid belt.


In 2006, a debate arose as to whether or not Pluto should be considered a planet. At that time, the classification of Ceres was reconsidered, and Ceres was reclassified as a member of a new group of objects called dwarf planets. Ceres was the first dwarf planet to be visited by a space probe, when NASA's Dawn spacecraft visited it in 2015.


Notice that Ceres is spherical, rather than oblong, like asteroids in the asteroid belt. Ceres is spherical because it has enough mass that the gravitational force dominates the electromagnetic force. Every massive particle, like an atom, is attracted gravitationally to every other atom. Atoms also feel electromagnetic forces between them. Even though the electromagnetic attraction between charged particles is many times stronger than the gravitational force between massive particles, the gravitational force dominates if the body is massive enough, because every particle attracts every other particle. That is what gives it a spherical shape. As for the electromagnetic force, only particles with unlike charge are attracted to each other.

Earth-crossing asteroids

Some asteroids have very elongated orbits, which take them inside Earth's orbit as they travel around the Sun. These asteroids are known as Earth-crossing asteroids. Usually, when they cross the orbital path of Earth, we are in another position somewhere around the Sun. Occasionally, an asteroid will collide with Earth.

Tiny objects from space bombard Earth all the time; every day more than 100 tons of dust and sand-sized particles strike the Earth. The larger the size of the object, the less often the impact. An object that is a few meters long strikes Earth about once a year. When this happens, it creates a fireball as the object burns up in the atmosphere.


About once per century, an impact occurs that is large enough to leave a significant crater. Once every thousand years, a large impact like the one that created the Tunguska crater occurs.


Once every few million years, a very large impact occurs, that could possibly threaten civilization and/or cause a major extinction event.

orbits of known Earth-crossing asteroids

This graphic shown the orbits of known Earth-crossing asteroids, currently over 1,700 objects, that are deemed Potentially Hazardous Asteroids because they are at least 460 feet across and they pass within 4.7 million miles of Earth's orbit. These asteroids are being tracked and analyzed, so that their paths can be predicted with increasing precision.

asteroid widget

Want to know if an asteroid may be getting close? There's an app for that! This Asteroid Watch widget shows the five asteroids or comets that will be in closest approach. It displays the date of closest approach, diameter of the object, relative size and distance from Earth, and displays a web page with info about the object upon clicking the encounter date.


For reference regarding the above close encounter distances, the average distance between Earth and the moon is about 239,000 miles.

gravitational tractor

What should we do if an asteroid appears to be heading for Earth? Your first thought might be that we should try to blow it up. There are potential hazards involved with this option. First, we don't understand well how asteroids are made. There are indications that they vary in density inside, so knowing how to detonate one would be tricky. Also, it would not stop many of the pieces from continuing onward to impact Earth. They would be smaller, but if we used a nuclear bomb to blow up the asteroid, the pieces would become radioactive.


Perhaps a better idea would be to use a gravitational tractor. This artist's concept of a 20-ton spacecraft near an asteroid illustrates the idea. The massive spacecraft would gravitationally attract the asteroid just by being in close proximity, and use the gravitational force to tow it to a new trajectory. This approach would require quite a large window in time; the craft would need to encounter the asteroid well before it got near Earth since the weak  gravitational force would need time to slowly tug the asteroid.