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Asteroids are being described and tracked, including ones that someday may strike Earth.

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Could human life end with an asteroid?

Asteroid impacts have played an enormous role in creating Earth and in altering the course of the evolution of life. It is most likely that an asteroid impact brought the end of the dinosaurs and many other lifeforms at the end of the Mesozoic. Could one asteroid do it again?


Asteroids are very small, rocky bodies that orbit the Sun. "Asteroid" means "star-like," and in a telescope, asteroids look like points of light, just like stars. Asteroids are irregularly shaped because they do not have enough gravity to become round. They are also too small to maintain an atmosphere, and without internal heat they are not geologically active (Figure below). Collisions with other bodies may break up the asteroid or create craters on its surface.

Picture of the asteroid 951 Gaspra

In 1991, Asteroid 951 Gaspra was the first asteroid photographed at close range. Gaspra is a medium-sized asteroid, measuring about 19 by 12 by 11 km (12 by 7.5 by 7 mi).

Asteroid impacts have had dramatic impacts on the shaping of the planets, including Earth. Early impacts caused the planets to grow as they cleared their portions of space. An impact with an asteroid about the size of Mars caused fragments of Earth to fly into space and ultimately create the Moon. Asteroid impacts are linked to mass extinctions throughout Earth's history.

The Asteroid Belt

Hundreds of thousands of asteroids have been discovered in our solar system. They are still being discovered at a rate of about 5,000 new asteroids per month. The majority of the asteroids are found in between the orbits of Mars and Jupiter, in a region called the asteroid belt, as shown in Figure below. Although there are many thousands of asteroids in the asteroid belt, their total mass adds up to only about 4% of Earth’s Moon.

The asteroid belt, along with the Hilda, Trojan, and Greek asteroids

The white dots in the figure are asteroids in the main asteroid belt. Other groups of asteroids closer to Jupiter are called the Hildas (orange), the Trojans (green), and the Greeks (also green).

Scientists think that the bodies in the asteroid belt formed during the formation of the solar system. The asteroids might have come together to make a single planet, but they were pulled apart by the intense gravity of Jupiter.

Near-Earth Asteroids

More than 4,500 asteroids cross Earth’s orbit; they are near-Earth asteroids. Between 500 and 1,000 of these are over 1 km in diameter.

Any object whose orbit crosses Earth’s can collide with Earth, and many asteroids do. On average, each year a rock about 5–10 m in diameter hits Earth (Figure below). Since past asteroid impacts have been implicated in mass extinctions, astronomers are always on the lookout for new asteroids, and follow the known near-Earth asteroids closely, so they can predict a possible collision as early as possible.

A painting of an asteroid impact with Earth

A painting of what an asteroid a few kilometers across might look like as it strikes Earth.

Asteroid Missions

Scientists are interested in asteroids because they are representatives of the earliest solar system (Figure below). Eventually asteroids could be mined for rare minerals or for construction projects in space. A few missions have studied asteroids directly. NASA’s DAWN mission explored asteroid Vesta in 2011 and 2012 and will visit dwarf planet Ceres in 2015.

A picture of the surface of 433 Eros

The NEAR Shoemaker probe took this photo as it was about to land on 433 Eros in 2001.

KQED: Asteroid Hunters

Thousands of objects, including comets and asteroids, are zooming around our solar system; some could be on a collision course with Earth. QUEST explores how these Near Earth Objects are being tracked and what scientists are saying should be done to prevent a deadly impact. 


  • Asteroids are small rocky bodies that orbit the Sun and sometimes strike Earth.
  • Most asteroids reside in the asteroid belt, between Mars and Jupiter.
  • Near-earth asteroids are the ones most likely to strike Earth, and scientists are always looking out for a large one that may impact our planet and cause problems.


  1. What is the reason there is a belt of asteroids between Mars and Jupiter?
  2. Why do scientists look for asteroids that might strike our planet?
  3. What do scientists hope to learn from missions to visit asteroids?

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asteroid Rocky objects larger than a few hundred meters that orbit the Sun.
asteroid belt Region between the orbits of Mars and Jupiter where many asteroids are found.
near-Earth asteroid Asteroids that orbit relatively close to Earth.

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