Asteroid vs Comet

From active asteroids to dormant comets, rubble piles to remnant cores, main belt comets and manx comets to ambiguous interstellar objects, this article provides a brief overview of the smaller objects in our planetary zoo—the main differences and similarities between asteroids and comets.

But if it’s meteors you’re looking for, read Meteoroids, Meteors and Meteorites.

This article contains the following sections:
• So what is the difference between an asteroid and a comet?
• A continuum of compositions?
• The distribution of asteroids
• The distribution of comets
• The A to X of comets
• Straddlers
• Interstellar asteroid vs comet
• The A to Z of asteroids

So what is the difference between an asteroid and a comet?

A simple distinction is this:

Comets are loose collections of ice, dust and rocky particles, usually on highly eccentric (elliptical) orbits that take them to the outer reaches of the Solar System and beyond, and in close to the Sun where the volatile ices and dust they contain outgas to produce a tail and a gravitationally unbound atmosphere known as a coma. The coma and tail can extend for hundreds and thousands of kilometers from the comet’s nuclei which is only a fraction of the size of the glow of the comet’s coma observed from Earth.

Comets spend most of their time in the outer Solar System far from the Sun which is why they still contain volatile material, unlike the majority of asteroids. Comet orbital periods range from a few years to thousands and millions of years, or they may orbit the Sun once and never come back if they are on parabolic or hyperbolic orbits (more on that later). Cometary orbits cross the ecliptic (plane of Earth’s orbit) at almost any angle, unlike asteroid orbits which are inclined at significantly smaller angles.

Asteroids are composed of rock and metal, either as solid coherent bodies or loosely bound rubble piles. Most asteroids are on orbits that keep them in the inner Solar System orbiting in the Main Belt (at around 2.2–3.3 AU) between Mars (at 1.5 AU) and Jupiter (at 5.2 AU) and on orbits that are less elliptical (more circular) than comets.

An estimate of the number of asteroids that exist in the asteroid belt is hard to give, but astronomers believe there may be up to 2 million asteroids larger than 1 km and millions of smaller ones. Almost half of the mass of the Main Belt is provided by four large asteroids: 1 Ceres (946 km), 4 Vesta (525 km), 2 Pallas (512 km) and 10 Hygiea (431 km). There are also a not insignificant number of small asteroids that reside closer to Earth (within the orbit of Mars at 1.5 AU)—the so-called near-Earth objects (NEOs). These are categorised into Apollo, Aten, Atira and Amor types depending on whether their orbits are Earth-crossing or entirely within or entirely outside Earth’s orbit.

Asteroids which have a minimum orbit intersection distance (MOID) with Earth of 0.05 AU or less (where AU is an astronomical unit and 1 AU is the Earth-Sun distance of roughly 150 million km) and that have an absolute magnitude (H) of 22.0 or less are deemed to be potentially hazardous asteroids. (Note that H is a way of estimating the relative size of the asteroid because, in general, smaller H means brighter and therefore a larger diameter.)

There are also number of objects which have been classified as both asteroid and comet or are currently ambiguous in nature and have been assigned to categories such as active asteroids, main belt comets, manx comets and extinct comets.

At first glance, the stunning blue object in the featured image at the very top of this article could be mistaken for that of a comet with its long double tail, but it’s an active asteroid—the inner Solar System asteroid 6478 Gault (see NASA/ESA/ESO). Gault is sporadically to persistently active (depending on what you read here or here) and has a clear tail composed of dust shed from the asteroid which is slowly disintegrating, with the dust in the tail sorted by grain size (largest close to the asteroid and fine grained further downstream). Active asteroids reside in the inner and outer Solar System—further out in the Solar System, the Centaur asteroid 2060 Chiron (aka comet 95P/Chiron) intermittently displays a coma of comet-like outgassing and other activity.

Some active objects in the main belt have also been described as main belt comets, which may or may not be comet interlopers among the asteroids, for example comet 133P/Elst–Pizarro aka asteroid 7968 Elst–Pizarro (an object which was first reported here). However, the terms main belt comet and active asteroid have perplexingly often been used interchangeably, even if the activity of the object is not due to that generally associated with comets, such as sublimation of ice (although the activity of 133P is thought to be due to sublimation of ice). Read this short clarification by one of the scientists who coined both terms (active asteroid vs main belt comet) or read this 2012 paper on the subject.

Although the surface of most of the so-called active asteroids appear to be made of dark, low albedo material similar to comets, and belong to the primitive C, D, P and other dark spectral types in asteroid taxonomy (more on that later), some active asteroids are made of brighter, higher albedo material and belong to the silicate-rich S spectral types. The activity shown by the dark asteroids is thought to be due to outgassing of volatile material, just like that shown by comets. The activity shown by brighter types is thought to be dust created by impacts or due to rotational breakup as a result of the YORP effect (both of which effects have been suggested for the activity shown by asteroid 6478 Gault).

Generally, the type of activity shown by active asteroids may range from rubble pile disintegration (by rotational instability or otherwise), dust ejection by meteoroid impacts, electrostatic repulsion, thermal stress fracturing or dehydration cracking with release of volatiles, or radiation pressure sweeping up loose material, or sublimation of ice whether due to the subsurface being uncovered by impacts or otherwise.

There are also extinct (dormant) comets—lumps of inert rock showing no cometary activity which are on comet-like orbits but resemble dark asteroids. In particular, the Damocloids which are thought to be the now inactive nuclei of Halley-type comets and which have orbits ranging from a few AU to hundreds (and in a few cases thousands) of AU. In the inner Solar System, 3200 Phaethon is on an inclined and elliptical comet-like orbit but the object resembles a dark asteroid and develops a dust tail when close to the Sun (and it gets closer to the Sun than any other named asteroid). The Japanese Space Agency (JAXA) will launch the DESTINY+ mission in 2022 to fly by and investigate this ambiguous object.

A new category named manx comet has also been coined to describe asteroid-type objects resembling long period comets coming in from the Oort Cloud (a hypothesied region at the edge of the Solar System). These objects appear to be composed of brighter material generally associated with inner Solar System asteroids and display only subdued cometary activity. The two examples observed so far are C/2014 S3 PANSTARRS and C/2013 P2 PANSTARRS, which you can read about in Science and ADS, respectively. The unusual object 1996 PW is another reported asteroid-like body coming in from the Oort Cloud on a comet-like orbit.

A continuum of compositions?

So is it that asteroids and comets are the rocky (rock+dust+metal) and icy (ice+dust++) end members of a continuum of compositions for which there is no distinct boundary?

The following diagram shows a distinction between different types of asteroids and comets using a parameter called Tisserand’s parameter (T_j):

Tisserand’s parameter is based on the particular object’s orbital elements (semi-major axis, eccentricity and inclination of the orbit) with respect to a perturbing body. In our case, Jupiter is the planet which has the greatest dynamical influence on inner Solar System asteroids and comets, so the parameter is known as Tisserand’s parameter with respect to Jupiter (T_j),

Tisserand’s parameter is used to determine whether or not the orbiting body (in our case, asteroid or comet) is the same one observed before and after an encounter with the perturbing body (in our case, Jupiter), because when you plug the orbital elements into the Tisserand equation you get a number (in our case, T_j) which remains almost unchanged before and after the encounter.

The value of T_j also helps to distinguish between asteroids and comets because the relative eccentricity, size and range of inclination angles of comet orbits are much greater than that of asteroids—the value of T_j obtained tends to be above 3 for asteroids and below 3 for comets. There is also a distinction between long and short period comets and between Damocloids and dormant comets at around 2, as shown in the diagram above.

The distribution of asteroids

The graphic below shows the distribution by mass of the different asteroid spectral types that exist in the asteroid belt between Mars and Jupiter, where concentrations of different types of asteroidal material can be found.

As mentioned above, the mass distribution is dominated by the four largest asteroids (Ceres, Vesta, Pallas, and Hygiea). But whether or not you consider those four objects, you can see that the brighter asteroids (dominated by S types) populate the inner Main Belt and gradually reduce in abundance further outwards, with darker types (D, P, C) becoming dominant in the outer Main Belt and thereafter, although the general distribution within the Main Belt is not entirely distinct between darker and brighter types.

The superb graphic above (and perhaps my favourite graphic of any planetary science paper) is from a 2014 paper by Francesa DeMeo and Benoit Carry. Their main purpose is to use their distribution to explain the evolution of the Solar System, but the graphic serves well for the purposes of my post. You need a subscription to Nature to read the full article but here is the pre-print (so not in final typeset form and all the figures are at the end, but it’s free to read and well worth the read).

The distribution of comets

Comets are categorised into different types, based on their orbital period which determines where they mostly reside. There are short period comets which have periods less than 200 years and long period comets which have periods of 200 to several thousand years that take them out into the far reaches of the Solar System and beyond. There are also parabolic and hyperbolic comets which, by definition of the shape of the open orbits, are non-periodic comets that will orbit the Sun once and never come back as they have enough excess velocity (hyperbolic higher than parabolic) to escape the Sun’s gravitational pull and will be slingshot out of the Solar System.

The short period comets are categorised into different types: Encke-type comets which have periods of around 3.3 years (based on the type example of comet 2P/Encke); main belt comets (if any, and which are not active asteroids) with periods of around 3 to 6 years; Jupiter Family comets which have periods of less than 20 years and named because their orbits are determined by the gravitational influence of Jupiter. These tend to have low orbital inclinations so are also known as ecliptic comets; and Halley-type comets which have periods of 20 to 200 years (the type example being 1P/Halley) and have a much larger range of orbital inclinations so also form part of the group of near-isotropic comets.

Short period comets are thought to be sourced from the reservoir of icy Trans-Neptunian Objects (TNOs) that reside beyond the orbit of Neptune (30 AU) in the Scattered Disc in unstable orbits that allow them to be perturbed by the giant planets and forced inwards towards the Sun, where they become active comets.

Long-period comets are thought to be sourced from the Oort Cloud, a diffusely-populated sphere of icy objects theorised to exist beyond the edge of the Solar System which can be perturbed by massive objects outside the Solar System. The inner part of the Oort Cloud known as the Hills Cloud is thought to be a densely-populated ring of icy objects that feeds the outer spherical cloud and which together feed long-period comets into the Solar System. Comets perturbed from the spherical cloud can have (almost) any inclination with respect to Earth’s orbital plane (hence known as near-isotropic comets) and can orbit in either direction—either prograde like Earth, or retrograde which is effectively a result of the extent of the inclination angle.

One well-known example of a long-period comet is C/1995 O1 (Hale-Bopp) which has an orbital period of 2,530 years and reappeared as a bright naked eye comet in 1997.

A well-known near-parabolic comet is C/1910 A1, the so-called Great Daylight Comet of 1910. This is the comet which stole the limelight from the return of the short period Halley’s Comet whose apparition was due three months later (and which together inspired the first comet disaster movie, the 1916 silent film The End Of The World).

One of the fastest hyperbolic comets ever observed is C/1980 E1 (Bowell) which flew by the Sun in 1982 and is currently at around 70 AU and never to return (unless otherwise perturbed by a massive object).

Although no spacecraft or telescope has ever imaged the Oort Cloud, it may start somewhere around 1,000 to 5,000 AU and extend out to 100,000 AU or beyond, with the Hills Cloud extending up to about 20,000 AU within it. The following graphic puts these distances into perspective with respect to where we reside in the Solar System and everything else referred to so far in this article.

The A to X of comets

Comets have been given the following prefixes by the International Astronomical Union (IAU): P for a periodic comet and C for a non-periodic comet. However, there is some confusion, because some people use periodic comet to mean only short period comets, whereas others use periodic comet to mean short and long period comets. This means some long period comets have a C (non-periodic) designation, even though they are periodic.

It seems the confusion may be historic and may mean ‘periodic in our lifetime‘—after all, there’s no chance of us seeing a long period comet with a 200-year period twice in our lifetime—but then there is no chance of us seeing any of the short period comets with periods greater than at most 100 years in a lifetime either. These designations will remain perplexing until the nomenclature is clarified by the IAU.

The other prefixes used for comets are: D for a comet that has disappeared or disintegrated; X for comets with no reliable orbit calculation, so mainly historical comets and many hyperbolic comets; A for objects re-designated from comet to asteroid or other minor body; and a newly introduced class I for an interstellar object, meaning a body not gravitationally bound to a star, whether a comet, asteroid or (to use a term much loved by movie-makers) a rogue planet.

Straddlers

Damocloids, which have orbits over a wide region from a few AU to many hundreds of AU (but mostly less than that of Neptune’s 30 AU orbit) may be the extinct or dormant nuclei of the short-period Halley-type comets.

Centaurs, which reside in the region between Jupiter (5.2 AU) and Neptune (30.1 AU) have characteristics of both asteroids and comets and may originate from the Kuiper Belt. Centaurs have unstable orbits which can be perturbed by interaction with the outer planets: if they have Neptune-crossing orbits they are classified as Centaurs and if perturbed into Jupiter-crossing orbits and subsequently develop cometary activity they are re-classified as short-period Jupiter Family comets.

One object classified as a Centaur, 514107 Ka’epaoka’awela (dubbed Bee-Zed following its provisional designation of 2015 BZ₅₀₉), is in a stable orbit between 3.2 and 7.1 AU, co-orbital with Jupiter. The orbit is retrograde, unlike most objects in the Solar System which orbit in the same direction as the Sun. Because of this stability and direction, Bee-Zed is therefore thought by some to be an interstellar object captured very early in the history of the Solar System.

Interstellar asteroid vs comet

Only two interstellar objects have, however, so far been confirmed as such: these are the mysterious object designated I1/’Oumuamua which was discovered in 2017 and exo-comet I2/Borisov which was discovered in 2019.

Both objects are on hyperbolic trajectories, although the eccentricity of Borisov’s orbit is more extreme than that of ‘Oumuamua—my sketch below gives the general idea.

There has been a great deal of debate as to what ‘Oumuamua is and suggestions have included comet, asteroid and a whole lot more, so its designation has changed a few times accordingly. Some astronomers estimate it to be a few hundred metres long and a few tens of metres in diameter (so about ten times as long as it is wide, like a cigar). Others suggest it’s no cigar, but more flat, like a pancake—um…like a light sail.

‘Oumuamua’s closest approach to the Sun was about 0.25 AU, well within Mercury’s orbit. In contrast, Borisov is estimated to be about 500 metres in diameter and its closest approach to the Sun was about 2 AU, passing between the orbits of Mars and Jupiter, near the inner margin of the asteroid belt.

I1/’Oumuamua was originally designated C/2017 U1 when thought to be a comet (exo-comet) based on its speed and hyperbolic trajectory, but it showed no cometary activity and was re-designated as an asteroid (exo-asteroid) and designated A/2017 U1, and also found to be tumbling wildly. It was subsequently re-designated as an object (interstellar object I/2017 U1) due to the ambiguity. Then it was again suggested to be a comet based on an observed small, non-gravitational acceleration attributed to subdued outgassing. Then it was suggested to be a disintegrating comet, or perhaps an asteroid, or maybe a contact binary asteroid, or possibly the remnant of a shredded planet. Currently, it is not thought to be a comet.

Let’s just say the jury is out on classifying this intriguing object. Other suggestions have included: a cloud of dust particles (ruled out due to the fact that such a dust bunny would fall apart when heated on its journey close to the Sun); a hydrogen iceberg (hydrogen-infused ejecta from the core of an interstellar molecular cloud whereby sublimation of the hydrogen ice would propel the body…but the mass-loss would be too great); a nitrogen iceberg chipped of a Pluto-like exo-planet (but apparently there’s not enough nitrogen in the Milky Way galaxy to make many such icebergs of this size, for us by chance to see one of them); or a clump of dark matter (but the Solar System didn’t fall apart, so that was ruled out).

If it’s not a natural phenomenon and if humans didn’t create it, then someone else made it: alien artefact. And so there’s the suggestion that it’s an alien light sail (to explain the object’s extreme relative dimensions and its extreme brightness, with solar radiation pressure providing the non-gravitational acceleration); or it’s some other alien space probe. Sadly not confirmed, but not ruled out either. Fingers-crossed.

The classification of I2/Borisov has proved less troublesome, since it was observed to possess an unmistakable coma and tail almost 14 times the diameter of the Earth. Unfortunately, the nucleus of our second interstellar visitor has now been observed to be fragmenting as it speeds away, so it could be bye-bye Borisov in more ways than one.

The A to Z of asteroids

Earlier in this article, I mentioned spectral types when I talked about the distribution of asteroids. If you have nothing better to do and would relish a nice, long read about asteroid spectral types, classification systems and taxonomies, read my article History of Asteroid Classification which not only provides an overview of what constitutes the different asteroid spectral types, but provides an account of the evolution of the multitude of perplexing A to Z of asteroid classification systems that have popped in and out of existence over the last nearly 50 years. You’ll need to commit to get through it.

So that’s the end of the article Asteroid vs Comet. For more graphics, click here.