Comets vs. asteroids: Here are 7 differences between the two very common objects

Comets and asteroids are both very common objects in our solar system. But, what exactly is the difference?
Christopher McFadden
Comet or asteroid? What's the difference?
Comet or asteroid? What's the difference?

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  • Asteroids and comets were formed during the birth of our solar system.
  • Both have distinct shapes and compositions and tend to come from different places in the Solar System.
  • But, there are some objects that are blurring the lines between them.

Sometime around 4.5 billion years ago, the genesis of the Solar System saw the eventual creation of the Sun and all the planets we are so familiar with. But it also gave birth to many much smaller things too, like comets and asteroids.

While most stay where they formed, sometimes these bodies can be pulled out of their homes by gravitational forces to come and visit us in the inner Solar System. Some even collide with planets and moons from time to time.

But both are distinctly different things, as you are about to find out.

What is a comet and where do they come from?

In short, comets are large ice and dust-based bodies that orbit the Sun. But there is, of course, a lot more to them than that.

Comets vs. asteroids: Here are 7 differences between the two very common objects
Artist's impression of a comet,

Comets are ancient objects that date from the birth of our Solar System, around 4.5 billion years ago. They are perhaps best known for their long, streaming tails.

However, the majority of comets are located far out in our Solar System. Some are found in the Kuiper Belt, a vast disc of icy objects located beyond the orbit of Neptune. This is where short-period comets, which take less than 200 years to orbit the Sun, are thought to originate.

The Oort Cloud, a theoretical sphere-shaped shell encircling the solar system at a distance of around 100,000 astronomical units (AU)(around 100,000 times the distance between Earth and the Sun, is thought to be home to vast quantities of additional comets. Because of how much longer it takes them to orbit the Sun, these are known as long-period comets.

It is thought that some long-period comets could take as long as 30 million years to complete one orbit of the Sun. The comet with the longest known period completes one orbit of the Sun in excess of 250,000 years!

Comets can be wrenched from their homes in the Kuiper Belt (or Oort Cloud) by the gravitational pull of a planet or star. If strong enough, this can cause a comet to be steered toward the Sun and the inner planets of the Solar System.

This process results in the comets effectively being diverted from their usual course. If you were to track them for some time, their orbits would resemble lengthy, stretched ovals.

The comet swings around behind the Sun as it approaches the Sun with increasing speed before turning around and returning to its original direction.

Some comets can even collide with the Sun and atomize, never to be seen again.

Comets vs. asteroids: Here are 7 differences between the two very common objects
A comet in the night's sky.

But, for those few comets that survive the ordeal and make it to the inner Solar System, we here on Earth are usually in for a visual treat for a short period of time.

What are the main parts of a comet?

Every comet has a nucleus, which is its solid, frozen core. This ball of ice and dust is usually less than 10 miles (16 kilometers) across, which is roughly the size of a small town. Although, in April 2022, the Hubble Space Telescope imaged a comet nucleus with a diameter of just under 80 miles - the largest ever seen by astronomers.

According to experts, comets in the Kuiper Belt or Oort Cloud are really little more than frozen nuclei.

However, a comet begins to warm up as it approaches the Sun. The ices eventually start to transform into a gas. This may also cause gas jets to erupt from the comet, ejecting dust along with them. The freely escaping gases and dust form the coma, which surrounds the nucleus.

For much the same reason, this process also leads to the development of tails on comets.

A brilliant, curving tail that spreads for millions of miles behind the comet is created as dust and gases stream away from the comet's nucleus under the influence of solar radiation pressure.

More recently, astronomers have discovered that comets actually have two distinct tails when they look closely. One is formed from dust and appears white or yellow in color.

This dust tail follows the comet in a wide, gently curving arc. The other tail, called an ion tail, is bluish and is composed of ions or gas molecules that have an electrical charge. These tails always face away from the Sun.

Have we ever visited a comet?

Comets have piqued people's curiosity for thousands of years, with some becoming returning visitors over the ages. While they are visually stunning, it is often difficult to actually "see" the nucleus of each comet due to it being obscured by the coma's gas and dust that surrounds it.

This has held back our complete understanding of the nature of comets. However, in recent years, a number of spacecraft have had the opportunity to closely examine comets.

Comets vs. asteroids: Here are 7 differences between the two very common objects
Computer generated image of Rosetta probe on comet surface.

In fact, NASA’s Stardust mission collected samples from Comet Wild 2 (pronounced like “Vilt Two” in English) and brought them back to Earth. Scientists found those particles to be rich in hydrocarbons, which are chemicals we consider the “building blocks” of life.

The European Space Agency's Rosetta mission, which included a number of NASA instruments, has also managed to examine Comet 67P Churyumov-Gerasimenko. After deploying a lander, Philae, on the comet's nucleus, Rosetta spent two years orbiting it and relaying signals from Philae.

Images also revealed Comet 67P to be a rocky object with active surface activity. Rosetta/Philae also found complex molecules on the comet that could be the key building blocks of life.

What are some facts about comets?

  • Comets orbit the Sun in elliptical paths – just like the planets. The path of a comet though is far more elliptical than that of any planet.
  • Comets vs. asteroids: Here are 7 differences between the two very common objects
    Comet over a forest at night.
  • Comets are believed to originate in one of two regions – the Oort Cloud, or the Kuiper Belt found beyond the orbit of Neptune.
  • A comet has four components: a nucleus, a coma, a dust tail, and an ion tail.
  • The nucleus of a comet contains the vast majority of its total mass.
  • The tail of a comet is the result of solar winds that blow the gas particles directly away from the Sun.
  • Comets have a halo when they move close to the Sun. What happens is that solar radiation vaporizes the ice and gas in the comet, which forms a halo around it. The halo is known as the comet coma.
  • A comet’s dust tail is a trail of dust and rocky material that is left behind as it moves along its orbit path.
  • Comets vs. asteroids: Here are 7 differences between the two very common objects
    Comet over a city at night.
  • The Oort cloud is an outer region of the Solar System 50,000-150,00 times the distance from the Sun to Earth that is believed to contain countless dormant comets. Some of the comets that originate here have orbits lasting millions of years.
  • The Kuiper Belt is a ring of dormant comets located just beyond the orbit of Neptune. The comets originating here have orbits lasting hundreds of years or less.
  • The most famous comet is probably Halley’s Comet. It has been observed since at least 240 B.C. Its orbit makes it visible from Earth every 76 years. It was named after the British astronomer Edmond Halley.
  • Comets vs. asteroids: Here are 7 differences between the two very common objects
    Comets have fascinated humans for millennia.
  • There are over 3,000 currently known comets. Scientists believe that there are up to one billion comets in our solar system.
  • A great comet is considered one that is bright enough to be visible from Earth without the need for a telescope. Approximately one great comet passes by every ten years.
  • While most comets are small, with average diameters from 2,460 feet (750 meters) or less to about 12 miles (20 kilometers), some have been found that are much larger. One prime example is UN271, which appears to be approximately 75 miles (120 km) across.

What are some famous comets?

There are several famous comets, with some being known to ancient humans and some only recently appearing. Below are the most well-known comets.

The most famous of all is Halley’s Comet, which is visible to the naked eye. Although it has been spotted since ancient times, including in 1531, 1607, and 1682, it was Edmond Halley who suggested these were actually all the same comet. He made his observation in 1705 while studying the orbits of several known comets and comparing his observations with previous records of comets in the past.

Halley quickly realized that one comet, in particular, must be the same one observed several times over the previous centuries. As a result of Halley’s discovery, the comet was named after him. Halley’s Comet is visible every 75 to 76 years.

Comets vs. asteroids: Here are 7 differences between the two very common objects
Comet Hale-Bopp in 1997.

Next is the comet Hale-Bopp. Comet Hale-Bopp was also visible to the naked eye and may be best known to people for the much-publicized Heaven's Gate cult that believed God was an advanced alien traveling in a spaceship in the trail of the Hale-Bopp comet. Comet Hale-Bopp was last visible in 1997 and will not be visible again for approximately 2,300 years. It is named after its co-discoverers, Alan Hale and Thomas Bopp.

Another famous, albeit now no longer existing, comet is Shoemaker-Levy 9. Shoemaker-Levy 9, also known as SL 9, was comprised of a string of active cometary nuclei that was captured by the gravity of Jupiter and subsequently began an orbit about the planet.

The orbit of SL 9 assumed about Jupiter was, however, highly irregular and as a result of this irregularity SL 9 crashed into Jupiter in a brilliant display during the week of July 16, 1994. Shoemake-Levy 9 is named after its discoverers, Gene Shoemaker, Carolyn Shoemaker, and David Levy.

What is an asteroid?

In brief, asteroids are small, stony objects that orbit the Sun and which are too small to be classified as planets.

In our Solar System, there are lots of asteroids (the currently known asteroid count is over 1 million), with the majority of them found in the main asteroid belt, which is located between the orbits of Mars and Jupiter.

However, they are not just isolated to this belt. For instance, some asteroids, called trojans, share an orbit with a planet.

They do not collide with the planet because they are located at the Lagrangian points, where the gravitational pull from the Sun and the planet are balanced. A large number of trojan asteroids are associated with Jupiter. There are also Mars and Neptune trojans, and in 2011, an Earth trojan was discovered.

Where do asteroids come from?

Most astronomers believe that asteroids are bits of leftover stuff from the formation of our solar system. This occurred roughly 4.6 billion years ago when a large cloud of gas and dust collapsed in on itself. When this occurred, the majority of the material fell to the cloud's core and created the Sun.

Comets vs. asteroids: Here are 7 differences between the two very common objects
Artistic representation of the Solar System.

The rest of the stuff in this cloud began to condense and form planets too. Those bits of material that are now the asteroid belt apparently never got the chance to develop fully into a planet or planets.

This formation process, just like the planets, was not homogenous across the entire solar system, so, like planets, asteroids come in all different sizes (and shapes).

For this reason, no two asteroids are alike since they formed in various places at various distances from the Sun. However, they tend to share some characteristics: -

  • Unlike planets, asteroids tend not to be circular. Their shapes are often irregular and jagged.
  • Many asteroids are smaller than pebbles, yet some have diameters of hundreds of miles/kilometers.
  • The majority of asteroids are composed of various types of clay and silicate rock, although some also contain metals like nickel and iron.

What can we learn from asteroids?

Asteroids can teach scientists a lot about the formation of planets and the Sun because they formed at the same time as our solar system. By examining meteorites, which are minute pieces of asteroids that have passed through our atmosphere and come to rest on Earth, scientists can learn more about asteroids.

Comets vs. asteroids: Here are 7 differences between the two very common objects
An artist's rendering of NASA's Near Earth Asteroid Rendezvous (NEAR) spacecraft at asteroid Eros.

We can also observe and study them when they fly near the Earth. For example, the NEAR Shoemaker spacecraft actually managed to visit and land on the asteroid Eros, which is relatively close to Earth.

Then, in 2011, the Dawn spacecraft visited the asteroid belt. It observed and orbited the dwarf planet Ceres and the enormous asteroid Vesta.

The OSIRIS-REx spacecraft was launched by NASA in 2016 to investigate the asteroid Bennu, which is close to Earth. The spacecraft reached Bennu in 2018 and will return a sample of pebbles and dust from the asteroid's surface to Earth in 2023.

In September 2023, OSIRIS-REx's sample container will touch down in the Utah desert. In order to learn more about how planets develop and how life first began, scientists will then remove the container and analyze the minerals and dust inside.

What are some facts about asteroids?

We've covered a lot of ground, well space, above, but if you are hungry for more information on asteroids, then here are some select facts.

1. There are millions of asteroids out there, perhaps more

Comets vs. asteroids: Here are 7 differences between the two very common objects
Artist's impression of the asteroid belt.

There are many, many, asteroids in our solar system alone. Scientists haven't been able to count every single one, but they doubtless count in the millions, probably billions.

A large number of them exist in the asteroid belt. Interestingly, their presence there has a lot to do with Jupiter. When this giant planet formed, millions of space rocks left over from the formation of the Solar System were imprisoned by Jupiter's powerful gravity.

This is believed to be the main mechanism that prevented the formation of additional planetary bodies between Mars and Jupiter.

These millions of asteroids, which range greatly in size, orbit the Sun in broad elliptical paths.

2. Some asteroids can have their own little moons and rings

Comets vs. asteroids: Here are 7 differences between the two very common objects
Asteroid with several little moons.

Asteroids have weak gravity because they are small in comparison to bodies like planets. However, when a smaller rock enters the vicinity of a larger one, the larger one pulls, the smaller one and can transform it into a little moon.

In 2013, researchers discovered that an asteroid might potentially contain rings too, just like planets. Compared to the rings of, say, Saturn, these obviously pale in comparison to size and scale, but are effectively the same thing, only in miniature.

Under the right circumstances, a tail-like extension of an asteroid can also be created by the fusion of two or more tiny asteroids.

3. Some asteroids became moons

Asteroids occasionally cross paths with, strike, or smash into planets as they circle the Sun. Some of them are attracted by the gravity of a planet and can become trapped forever as a tiny moon.

This is how some of the moons of Mars, Jupiter, Saturn, Uranus, and Neptune are thought to have formed.

4. Asteroids have an abundance of stuff for us to exploit, should we wish

Comets vs. asteroids: Here are 7 differences between the two very common objects
We could mine asteroids in the future.

Asteroids are abundant in metals, silica, and carbon. Some of them even feature water ice that has been trapped in the debris pile.

Astronomers hypothesize that some of these essential elements were transferred to the planets during the early days of the solar system when asteroids routinely crashed with them. They contend that deposits from asteroids may have served as the initial catalyst for the emergence of life on Earth.

In an effort to harvest the mineral reserves on asteroids, researchers are actively seeking methods to explore them further. While this consists of only probes at present, it is possible that in the future we could use asteroids as sort of space-based resource supermarkets in the future.

5. Asteroids are usually oddly shaped

Comets vs. asteroids: Here are 7 differences between the two very common objects
No two asteroids are the same.

Asteroids typically range in size from 329 miles (530 kilometers) in diameter, like Vesta, to less than 33 feet (10 meters) across. Not only that, but they are typically irregularly shaped due to their stony composition and minimal gravitational pull.

Layers of dust have also coated the majority of their surfaces, too. The average surface temperature of an asteroid is roughly -70 degrees Celsius, and they are unable to support an atmosphere.

6. Some special asteroids even have water gullies

On the asteroid, Vesta, water trails known as gullies have actually been spotted by scientists in 2015. A water trail is created in the rocks when a minor asteroid collides with a larger one and melts the water ice that has been trapped there.

How are comets and asteroids different?

In short, comets are dirty snowballs that usually come from the Oort Cloud, while most asteroids are rocky bodies that tend to come from within the asteroid belt. However, there are some overlaps between the two that can blur the lines somewhat.

Comets vs. asteroids: Here are 7 differences between the two very common objects
Meteor shower.

In either case, both are tiny objects that orbit the Sun and were both created very recently (about 4.5 billion years ago). However, the way they formed differed enough to make them distinctly different things.

Let's take a look at the main differences.

1. Asteroids are typically rocky bodies from the asteroid belt

Comets vs. asteroids: Here are 7 differences between the two very common objects
Rocky asteroid approaching Earth.

Asteroids, for example, are typically small objects that are leftover from the formation of the solar system. They are typically rocky, can come in a wide range of sizes, and may also include metals, clays, and stony materials, similar to the composition of the Earth itself. They also generally have bumpy and jagged surfaces, which some have compared to potatoes.

2. Comets, on the other hand, may come from the Oort Cloud and are icy

Comets vs. asteroids: Here are 7 differences between the two very common objects
The distance from the Oort cloud to the interior of the Solar System.

Unlike asteroids, many comets originate in the Oort Cloud, a part of the solar system located well beyond Pluto's orbit. The orbits of comets are often far longer than those of asteroids, occasionally extending into the solar system's outermost regions where these small things might be disturbed by nearby stars.

A comet may then dive inward toward our Sun if that occurs. Comets are icier than asteroids because they form in the extreme cold of the outer solar system. They have the moniker "dirty snowballs" since they are made of rock and ice.

Comets develop their distinctive gas and dust tails when they approach the Sun near.

3. Asteroids tend to be named by an international panel, comets after their discoverers

The task of naming asteroids is under the purview of the International Astronomical Union. According to NASA, the nomenclature is not very strict when it comes to naming asteroids. As a result, asteroids, like 3505 Byrd, 8749 Beatles, or Mr. Spock, may celebrate people, musicians, or characters from TV and film. Some names are more serious-minded, such as the seven asteroids named for the crew of the Space Shuttle Columbia killed in 2003.

4. Comets tend to have tails; asteroids typically don't

Comets typically have one or both of two types of tails. When the comet approaches the Sun at a distance closer than Mars' orbit, a gas tail typically begins to form. The other tail, a dust tail, is made up of tiny solid dust particles.

5. But it can get tricky to make a distinction between the two

Comets vs. asteroids: Here are 7 differences between the two very common objects
A comet flying past Earth.

Not all asteroids and comets fit neatly into the main criteria listed above. Other comets occur with orbits that are far closer to the Sun than those that originate from the Oort Cloud.

So-called "short-period comets" are all comets with orbital periods of fewer than 200 years. The vast majority of short-period comets orbit the planets in the same general direction. The most well-known comet of them all, Halley's Comet, is one prominent exception to this rule.

6. Comets can also make micro asteroids, but not vice versa

Comets vs. asteroids: Here are 7 differences between the two very common objects
Artist’s concept of asteroid 3200 Phaethon.

The distinction between the two can get even more complicated in some circumstances. For example, comets typically serve as the parent objects for the annual meteor showers.

Comets are fragile objects that frequently leave a trail of frozen debris in their wake as they approach the Sun. Every year, Earth travels across numerous old comet trails like these. We witness meteor showers because cometary debris penetrates our atmosphere and burns up.

For instance, debris from Halley's Comet form the Orionids and Eta Aquariids meteor showers. The Leonid meteor shower comes from Comet Tempel-Tuttle. However, not all meteor showers are caused by comets.

An enigmatic object known as 3200 Phaethon serves as the source of the Geminid meteor shower, which is visible every December. This object, which is sometimes referred to as a "rock-comet," resembles an asteroid-comet hybrid in many aspects.

7. There are even hybrids between comets and asteroids out there

Because of objects like 3200 Phaethon, the distinction between asteroids and comets is becoming blurred by a number of objects.

The Hubble Space Telescope acquired images of asteroid 311P/PANSTARRS and found that it had not one but six tails! In addition to being a main-belt comet, the peculiar asteroid may potentially be surrounded by a satellite.

Space is definitely a very strange place.

And that comet and asteroid fans is your lot for today.

So, the basic difference between comets and asteroids comes down to their composition, place of origin, orbits, and morphology. Most asteroids are rocky bodies that lie within the asteroid belt, while comets are dirty snowballs and can come from the Oort Cloud.

But, in recent years, the clear distinction between the two is becoming increasingly blurred. Heavens knows how many more comet-asteroid hybrids will be discovered in future years.

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