Asteroids and Comets

Small rocky objects that orbit around the sun.

In addition to the sun and the planets, our solar system consists of all sorts of interesting objects. Asteroids and comets are two kinds of bodies that orbit our sun. Scientists are continuously studying these objects and learning more about them. Let's take a closer look.

Asteroids are rocky structures without atmospheres, and as far as we know, they are also without life. They orbit the sun like planets, but they are much smaller than planets. 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 millions of asteroids of different sizes within this belt. The largest asteroids are hundreds of miles across while the smaller ones are only about 30 feet across. The total mass of all the asteroids combined is still less than that of our moon.

Asteroids are left over from a time long ago when our solar system formed. About 4.6 billion years ago, a big cloud of gas and dust collapsed and formed the sun and the planets. Some of the leftover material became space rocks, or asteroids. Since asteroids formed at the same time as other objects in our solar system, scientists can study them to learn about the history of planets and the sun.

Asteroids are made of various materials. Some are composed of clay, while others are made up of nickel or iron. Like other space bodies, asteroids rotate as they revolve around the sun. Each has its own day and year cycle which varies from asteroid to asteroid. Most are irregularly shaped, although a few are spherical. 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 out of the asteroid belt and send them hurling through space into the paths of other planets' orbits. Asteroids hit planets, moons and other asteroids regularly. Most asteroids entering the Earth's atmosphere break apart before they can do any harm. Scientists carefully monitor near-Earth asteroids that approach Earth’s orbit and may pose a possible impact hazard.

Ceres is the largest object in the asteroid belt. It was discovered in 1801, the first asteroid to be found. However, it has now been classified as a dwarf planet. Ceres is about 950 kilometers (590 miles) in diameter. It takes 1682 Earth days (4.6 Earth years) for Ceres to make one trip around the sun, Ceres is especially interesting to scientists because it is one of the few space objects we know of that has water. It is possible that Ceres has or once had conditions that could support life.

Vesta is a giant asteroid also found in the asteroid belt. It is not as large as Ceres, but is significant in size at 329 miles (530 kilometers) across. It is covered with craters and troughs. Unlike most known asteroids, Vesta has a crust, mantle, and core, much like Earth. Learn about other asteroids that are of special interest to science.

Vesta and Ceres were the subjects of an important NASA mission known as Dawn. The Dawn spacecraft explored and photographed Vesta and Ceres between 2011 and 2016. Find out more about Dawn's extraordinary mission. NASA’s OSIRIS-REx visited near-Earth asteroid Bennu in 2018-2021, and delivered a sample of Bennu’s rocks and dust to Earth in September 2023. Now the spacecraft is on its way to another asteroid called Apophis. Asteroids will continue to be studied by scientists in future missions.

Like asteroids, comets orbit the sun and are also leftovers from the formation of the solar system. But they are made up of different materials. Comets have been known for a long time as “dirty snowballs.” But scientists are changing their definition of comets because comets are so much more complex than that.

Most comets exist in an area known as the Kuiper Belt which lies beyond Neptune. These comets are known as short-period comets, because they take less than 200 years to orbit the sun. The even more distant Oort Cloud, which exists at the outer edge of the solar system, contains comets known as long-period comets. These can take millions of years to make one trip around the sun. Learn more about the Kuiper Belt and the Oort Cloud at NASA’s website.

A comet is made of a solid, frozen nucleus containing dust, ice, and frozen gases such as methane, ammonia, and carbon dioxide. The nucleus is usually less than 10 miles (16 kilometers) across. 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. When a comet orbits near the sun, it starts heating up. As the frozen matter melts, the gases and dust create a fuzzy cloud around the nucleus known as a coma. As the comet moves closer to the sun, sun radiation and high-speed solar particles can blow the coma dust and gases into a bright tail stretching behind the comet. Tails can extend a million or more miles behind the traveling comet. In fact, the word “comet” comes from the Greek word for “long-haired.” Astronomers have found that comets actually have two tails, known as the dust tail and ion tail, both directed away from the sun. Most comets are too far away to be seen without a powerful telescope, but sometimes comets pass close enough that people on Earth can see them.

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 by humans 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. It will next be seen on Earth in 2061. Most people feel very fortunate if they get to see Halley's comet during their lifetime. Learn about other well-known comets at NASA’s comet site.

Comets are an important source of scientific study. They can give scientists clues about the formation of our solar system. It is believed that much of the water on Earth was brought here by comets early in the history of the solar system. Comets may also have brought carbon-based chemical compounds considered to be the building blocks of life, to Earth.

In recent years, scientists have had the chance to study comets more deeply. Several probes have flown close to comets and taken close-up pictures. In 2014, for the first time in history, a probe landed on the surface of a comet. Rosetta-Philae, a joint mission of NASA and the European Space Agency, dropped a lander on the nucleus of Comet 67P Churyumov-Gerasimenko and orbited the comet for two years as it made its journey around the sun.

Like asteroids, comets can impact planets and their moons. In 1994, the comet Shoemaker-Levy 9 collided with Jupiter in an astronomical event that scientists around the world were able to observe. The event lasted six days and was the first solar-system impact that had ever been recorded.

When discussing comets and asteroids, we also talk about meteoroids. Meteoroids are fragments of asteroids or comets. When one asteroid collides with another, small pieces break off and become meteoroids. Meteoroids can also be found in the tails of comets. These particles can trail behind the comet and get stranded in space. Sometimes meteoroids are referred to as space junk or debris. Most meteoroids are not very large – often no bigger than a particle of dust or a pebble. Some, however, can be much larger.

When meteoroids enter Earth’s atmosphere at high speed and burn up, they are called meteors. We see a streak of light and sometimes refer to meteors as “shooting stars” or “falling stars,” but they are not really stars at all. Almost all meteors burn up in our atmosphere and never hit the surface of Earth. Our moon, however, has no atmosphere to break up the material so collisions with the moon’s surface are common.

Meteors can be seen on most clear nights, if you are patient and keep watching. When there are many more meteors than usual, it is known as a meteor shower. Some meteor showers occur at regular intervals as the Earth passes through the trail of dusty debris left by a comet. For example, in August each year the Earth passes through a debris field left by the Swift-Tuttle comet, resulting in the annual Perseid meteor shower. During the Perseids, you may see 50-100 meteors per hour, with each meteor being a tiny piece of the comet.

If any part of the meteoroid survives the fall and lands on the Earth’s surface, it is called a meteorite. It may look like an ordinary rock, but a meteorite really does come from outer space. Most meteorites found on Earth come from shattered asteroids, although a few come from our moon or from Mars. About 50,000 meteorites have been found on Earth. Scientists collect and study meteorites to learn how old they are and where they came from. Because meteorites contain some of the original material from when planets were formed, scientists can learn about early conditions in our solar system’s history and about the building blocks of planets.

Impact craters are scars left on the surface of a planet or a moon where an asteroid, meteorite, or fragments of comets have crashed. Asteroids themselves often show impact craters. Our moon is covered in these scars that can be seen from your own backyard. All those spots on the moon viewed as dark, round dents or bright bowls are impact craters.

Earth’s atmosphere protects our planet against most collisions from space objects, but Earth does have a small number of craters from past impacts. A famous crater found in Arizona is about 50,000 years old and is .06 miles (1 kilometer) in diameter. One 180-mile (300 km)-wide crater is found at Chicxulub in the Yucatan Peninsula of Mexico. The crater is the result of a large asteroid impact 65 million years ago that may be the reason for the extinction of dinosaurs. The impact is thought to have thrown so much rock, dust, and debris into the atmosphere, that it changed the climate worldwide and severely damaged the environment, leading to the extinction of about 75% of animals of Earth at the time.

In 2013, a house-sized meteoroid entered Earth’s atmosphere over Russia and blew apart several miles above the ground. The explosion generated shock waves that damaged buildings and trees over 200 square miles (518 square kilometers.) Although collisions and impact craters on Earth are rare, they have happened in the past and will probably happen again someday.

There is still much to be learned about asteroids and comets. Every year, new asteroids and comets are discovered and named. Space probes and satellites send photographs and data that scientists analyze to learn more about these objects in our solar system. For example, early studies of the samples from asteroid Bennu, recently collected by NASA spacecraft, show evidence of water and high-carbon content, which could indicate the building blocks of life. The asteroid’s rocks and dust may provide clues about how our solar system formed, how life began, and how water and ingredients for life got to Earth.

Other scientists focus on impact craters found throughout our solar system. We know that asteroid fragments have crashed into planets and moons in the past, changing their geology and physical geography. By studying impact craters on different planets and moons, we can learn more about asteroid activity throughout time. Just like detectives, scientists follow clues about what happened when the solar system was young to understand the story of our beginnings.

Scientists at NASA’s Center for Near Earth Object Studies (CNEOS) monitor asteroid and comet orbits and predict their odds of impacting Earth. A valuable tool for detecting possible impact hazards is known as radar. By bouncing transmitted signals off objects in space, scientists can learn about potentially dangerous asteroids and develop ways to mitigate possible impacts. Other scientists monitor possible collisions with spacecraft or satellites in space so that such impacts can be avoided.

Scientists also study asteroids and comets to understand the challenges and resources they could present for future human expansion in space. Could we find new sources of raw materials that could be used in space exploration? Could asteroids or comets be used as refueling stations for spacecraft someday? No one knows for sure! If you are interested in exploration beyond Earth, there will be many opportunities for you in the future. You can begin now by observing the night sky and learning more about careers in space science.