Asteroids and Comets

Small rocky objects that orbit around the sun.

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.

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 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 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.

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.”

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.

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. 

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.

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.

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.

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.