Asteroids and Comets

Asteroid and Comet Facts

Asteroids and Comets ['as-tə-rȯidz] [\ən(d) ][ \'kä-mətz]

Small rocky objects that orbit around the sun.

Asteroids and Comets

Leonid Meteor

Our solar system is filled with all sorts of interesting objects. Scientists are continuously studying these objects. Asteroids and comets are two kinds of bodies that orbit our sun and that scientists are still learning more about. Recent scientific studies have put asteroids and comets in the news. Let's take a closer look.


Ateroid image
Asteroid Image courtesy of NASA

Asteroids are rocky structures without atmospheres, and as far as we know, also without life. Most of the asteroids in our solar system are located in a group known as the Asteroid Belt which is located between Mars and Jupiter. There are hundreds of varying sizes of asteroids within this belt. The largest asteroid is still smaller than our moon while the smallest are only about 20 feet across. Asteroids are made of various matter, from nickel to iron to volcanic-type materials. Each has its own day and year cycle which varies from asteroid to asteroid. Most are irregularly shaped and are not much more than a rock. Some of the asteroids have moons and some even have rings in the same way that Saturn has rings

The gravitational pull from the orbits of Mars and Jupiter can force asteroids and asteroid debris to be sent hurdling through space into the paths of the planets' orbits. Asteroids hit planets and their moons regularly. About once per year, Earth's atmosphere is hit by a car-sized asteroid that burns up before it can do any harm.

Ceres image
Ceres Image courtesy of JPL

Ceres is the largest of the asteroids and has even been classed as a dwarf planet. It was the first of the asteroids to be discovered. Ceres was found in 1801 by Giuseppe Piazzi. Ceres is about 950 kilometers (590 miles) in diameter. It takes 1680 Earth days for Ceres to make one trip around the sun. Here are two views of Ceres.

Vesta image
Vesta Image courtesy of JPL

Vesta is a giant asteroid also found in the asteroid belt. It is not as large as Ceres but is significant in size and a great curiosity to scientists. It and Ceres are the subjects of a NASA mission known as Dawn. Dawn is a spacecraft that visited and photographed Dawn and Ceres. It arrived at Vesta on July 16, 2011. It arrived at Ceres to take a close look on March 6, 2015. Find out more about Dawn's mission. See images of what Dawn found.


Comets have for a long time been known as “dirty snowballs.” But scientists are changing their definition of comets because comets are so much more complex than that. The word comet comes from the Greek word for “long-haired.”

A comet is made of a nucleus which can contain dust, ice, methane, ammonia, and carbon dioxide along with many other components. It is thought that much of the water on Earth was brought here by comets early in the history of the solar system. Most comets exist in an area known as the Kuiper Belt which lies beyond Neptune. The even more distant Oort Cloud also contains comets. Learn more about the Kuiper Belt and the Oort Cloud at NASA's website.

Comet orbit diagram
Comet orbit diagram courtesy of ESA - European Space Agency

Comets orbit the sun in huge paths which can bring them into a route near the sun. This happens only rarely or in intervals that repeat every few decades or centuries. Most comets are too small to be seen without a telescope as the comet itself might be only 10 miles across. When a comet orbits near the sun, a coma can sometimes be seen as the frozen matter melts from the sun's heat and vents off creating a fuzzy cloud around the nucleus. As the comet moves closer to the sun the coma changes into a tail of venting gases and dust which follows behind. Tails can extend a million or more miles behind the traveling comet. The tail reflects light from the sun and the venting gases can absorb energy from the sun. A comet can have more than one tail. The tail can also leave a debris field of meteoroids. In August each year, the Earth passes through such a field left by the Swift-Tuttle comet. This is known as the Perseid Meteor Shower and meteors can be seen as they skip across Earth's atmosphere. Meteors are often called “falling stars.”

Giotto images of Halley's comet
Giotto images of Halley's comet courtesy of ESA - European Space Agency

Probably the most famous of all comets is Halley's comet which can be seen every 75 or 76 years as it passes by Earth. While the comet had been seen for thousands of years, it was not known that it was the same one passing by over and over again. Early stargazers thought they were witnessing different comets passing by. Edmond Halley was the first to determine that the comet that had been seen in the years 1531, 1607 and 1682 was actually the same one. He predicted the next visit in 1758 but did not live long enough to see it. The comet was later named after him. It did appear again in 1758 as Halley had predicted and again in 1986 and will be seen here on Earth in 2061. Most people feel very fortunate if they get to see Halley's comet during their lifetime. See pictures of Halley's Comet.

Rosetta comet
67P/Churyumov-Gerasimenko Image courtesy of ESA - European Space Agency

For the first time in history, a probe landed on the surface of a comet. On November 12, 2014, Comet 67P/Churyumov-Gerasimenko was the location for a new scientific exploration. The purpose of this probe, known as the Rosetta Mission, is to study the comet's behavior and how its matter changes as it passes near the sun. The Rosetta Mission is a joint project between NASA and the ESA (European Space Agency). Before the Rosetta Mission, several probes flew close to other comets and took up-close pictures. Learn more about the Rosetta Mission and see pictures from some of the other comet probes.

Like asteroids, comets can impact planets and their moons. In 1994, the comet Shoemaker-Levy 9 collided with Jupiter in an astronomical event that had the entire world watching. The impact took days and was the first ever solar-system impact that had been recorded. 

Shoemaker-Levy 9 collision
Shoemaker-Levy 9 image courtesy of JPL – artist rendition

Impact Craters

The Earth's moon
The Moon

Impact craters are scars left on the surface of a planet or a moon where an asteroid or a meteorite has crashed. Even the surface of asteroids can show impact craters. Our moon is covered in these scars that can be seen from your own backyard. All of those “spots” viewed as dark, round, dents, or bright star-like spots are impact craters. Earth also has a number of impact craters. One famous crater is found in Arizona and is over two miles in circumference.

One impact crater – found at Chicxulub in the Yucatan Peninsula of Mexico is theorized to be the reason for the extinction of dinosaurs. The impact of a meteorite, an asteroid, or a comet is thought to have thrown so much rock, dust, and debris into the atmosphere, that it changed the climate worldwide and caused much damage to the environment.

meteor crater
Arizona crater image courtesy of NASA


It is hard to talk about asteroids and comets without some mention of meteoroids. They are related in many ways. Meteoroids are the result of solar system junk or garbage. True meteoroids are not very large – usually no bigger than a particle of dust, but can also reach kilometers in size. They can come from asteroids or comets.

meteorite streaking across the sky
image courtesy of NASA

As mentioned earlier, meteoroids can be found in the tails of comets. These particles are debris from the comet itself. They can trail behind the comet and get stranded in space. Sort of like garbage that blows out of the back of a moving pickup truck.

Other meteoroids are tiny asteroids that have left the asteroid belt and are drifting around in space. When the gravity from a planet or a moon pulls the asteroid close it can impact the surface or the atmosphere.

iron meteorite on mars
iron meteorite on mars image courtesy of JPL

Meteoroids actually get classified with three separate names depending upon what they are doing. The debris is called a meteoroid as it exists out in space.

A meteor is a flash of light created when it heats up in the atmosphere. We see these as falling stars. They usually burn up from this friction and never hit the surface. Our moon has no atmosphere to break up the material so there is a collision with the surface of the moon.

If any part of the object survives this fall and lands on the earth's surface, it is called a meteorite. Scientists collect meteorites to study their matter.

Fun Links

Explore comets and asteroids with the Ready Jet Go! gang in these fun videos from PBS.

Learn about the Dawn Spacecraft and its mission to explore two of the largest asteroids in the solar system. images taken by Dawn.

Learn about the Rosetta Mission and see pictures from some of the other comet probes.

Visit to learn more aboout asteroids and comets!

NASA has a great site about asteroids for kids. And one about comets too. Yes, and meteoroids!

The European Space Agency has a kids' site for you to learn more about asteroids.

The Main Asteroid Belt is located between Mars and Jupiter. Learn more about the Asteroid Belt.

What can the Hubble Space Telescope teach us about comets?

Learn about comets and meteors from the European Space Agency.

Star Child is a NASA site for kids – learn more about asteroids, comets and any other space topic you wish to investigate. Check out Level 1 and Level 2.

NASA Space Place has great information about asteroids, comets, and meteors, as well as fun activities and games for you.

Top 10 Questions

Thanks to Brian Jackson, assistant professor of physics, Boise State University; and Camille Eddy, microgravity team leader, Boise State University for the answers.

  1. How are asteroids formed?

    Asteroids are formed from the leftover material after the sun and the planets formed. So they are collections of rock and dust that agglomerated, or formed into a mass, over millions of years ago. This occurred a long time ago, right at the birth of our solar system. (From Katrina at Liberty Elementary School in Boise)

  2. How frequently do comets pass the Earth?

    Comets enter the inner solar system every few months. Some years there are many comets entering in a single month. (From Ashley at Lewis and Clark Middle School in Meridian)

  3. Has a comet ever hit our moon, and how many?

    Comets impact the moon quite a bit. There are a lot of impact craters that you can see on the moon, and that happened from different meteorites, comets, and asteroids hitting the moon. (From Caden at Cynthia Mann Elementary School in Boise)

  4. What was the first ever comet's name?

    The first comet was called the Great Comet of 1680. It was also called Kirch's comet, and it was observed around 1680. Then it went away. However, we are still tracking it today. As of a couple of years ago, it was about 23 astronomical units (the distance between the Earth and the sun) away from the sun. (From Simran at Dalton Elementary School in Dalton Gardens)

  5. What are inside asteroids?

    The largest asteroids are actually big enough that they differentiate the interior. So, deep in the interior of these asteroids are iron and other metals. As you move to the exterior, you encounter more and more rock. Some of the smallest asteroids are actually low in gravity and full of holes. This means they have a very high porosity. Therefore, in some cases, asteroids are full of metal and rocks. In other cases, they are full of empty space. (From William at Eagle Elementary School in Eagle)

  6. Can asteroids turn into comets?

    Sometimes scientists have a hard time differentiating between what are comets and what are asteroids. This is because comets contain ice and asteroids do not. After many times around the sun, comets will start to lose their ice. Generally, comets don't turn into asteroids, but sometimes it is hard to tell the difference between the two. (From Colin at Cynthia Mann Elementary School in Boise)

  7. Were dinosaurs wiped out when a huge comet or asteroid hit the Earth a long time ago?

    In the beginning of 1980, planetary scientists and astronomers found ample (more than enough) evidence to suggest that the Earth received an impact about 65 million years ago on the Yucatan Peninsula in Mexico. The impact was so large that it actually enshrouded (surrounded) the entire Earth in a cloud of dust and blocked out the sun for many weeks or even months. This led to an environmental cataclysm (a large scale, violent event) that ultimately culminated in the extinction of dinosaurs. (From Mia at Dalton Elementary School in Dalton Gardens)

  8. Is it possible to see asteroids and comets from Earth?

    Sometimes it's hard to see asteroids and comets from Earth with the unaided eye, or without a telescope. However, the easiest part of a comet to see would be its tail when it's closer to Earth. (From Jerred at Cynthia Mann Elementary School in Boise)

  9. Why do comets have tails?

    Comets contain ice and when they get closer to the sun, they tend to stream out those particles. They expel the particles, and that's what we see in orbit. (From Austin at A. J. Winters Elementary School in Montpelier)

  10. How big is the biggest comet?

    Comet Sarabat has a core that is about 100 miles wide. Comet McNaught-Russell has a core that's around 15 miles long, but its tail is about 139 million miles long. (From Alex at Taylor's Crossing Public Charter School in Idaho Falls)