14.3: Comets

Learning Objectives

By the end of this section, you will be able to:

• Characterize the general physical appearance of comets
• Explain the range of cometary orbits
• Describe the size and composition of a typical comet’s nucleus
• Discuss the atmospheres of comets
• Summarize the discoveries of the Rosetta mission

Comets differ from asteroids primarily in their icy composition, a difference that causes them to brighten dramatically as they approach the Sun, forming a temporary atmosphere. In some early cultures, these so-called “hairy stars” were considered omens of disaster. Today, we no longer fear comets, but eagerly anticipate those that come close enough to us to put on a good sky show.

Appearance of Comets

A comet is a relatively small chunk of icy material (typically a few kilometers across) that develops an atmosphere as it approaches the Sun. Later, there may be a very faint, nebulous tail, extending several million kilometers away from the main body of the comet. Comets have been observed from the earliest times: accounts of comets are found in the histories of virtually all ancient civilizations. The typical comet, however, is not spectacular in our skies, instead having the appearance of a rather faint, diffuse spot of light somewhat smaller than the Moon and many times less brilliant. (Comets seemed more spectacular to people before the invention of artificial lighting, which compromises our view of the night sky.)

Like the Moon and planets, comets appear to wander among the stars, slowly shifting their positions in the sky from night to night. Unlike the planets, however, most comets appear at unpredictable times, which perhaps explain why they frequently inspired fear and superstition in earlier times. Comets typically remain visible for periods that vary from a couple of weeks to several months. We’ll say more about what they are made of and how they become visible after we discuss their motions.

Note that still images of comets give the impression that they are moving rapidly across the sky, like a bright meteor or shooting star. Looking only at such images, it is easy to confuse comets and meteors. But seen in the real sky, they are very different: the meteor burns up in our atmosphere and is gone in a few seconds, whereas the comet may be visible for weeks in nearly the same part of the sky.

Comet Orbits

The study of comets as members of the solar system dates from the time of Isaac Newton, who first suggested that they orbited the Sun on extremely elongated ellipses. Newton’s colleague Edmund Halley (see the Edmund Halley: Astronomy’s Renaissance Man feature box) developed these ideas, and in 1705, he published calculations of 24 comet orbits. In particular, he noted that the orbits of the bright comets that had appeared in the years 1531, 1607, and 1682 were so similar that the three could well be the same comet, returning to perihelion (closest approach to the Sun) at average intervals of 76 years. If so, he predicted that the object should next return about 1758. Although Halley had died by the time the comet appeared as he predicted, it was given the name Comet Halley (rhymes with “valley”) in honor of the astronomer who first recognized it as a permanent member of our solar system, orbiting around the Sun. Its aphelion (furthest point from the Sun) is beyond the orbit of Neptune.

We now know from historical records that Comet Halley has actually been observed and recorded on every passage near the Sun since 239 BCE at intervals ranging from 74 to 79 years. The period of its return varies somewhat because of orbital changes produced by the pull of the giant planets. In 1910, Earth was brushed by the comet’s tail, causing much needless public concern. Comet Halley last appeared in our skies in 1986 (Figure 13.16), when it was met by several spacecraft that gave us a wealth of information about its makeup; it will return in 2061.

Figure 13.16 : Comet Halley. This composite of three images (one in red, one in green, one in blue) shows Comet Halley as seen with a large telescope in Chile in 1986. During the time the three images were taken in sequence, the comet moved among the stars. The telescope was moved to keep the image of the comet steady, causing the stars to appear in triplicate (once in each color) in the background. (credit: modification of work by ESO)

Voyagers in Astronomy: Edmund Halley: Astronomy’s Renaissance Man

Edmund Halley (Figure 13.17), a brilliant astronomer who made contributions in many fields of science and statistics, was by all accounts a generous, warm, and outgoing person. In this, he was quite the opposite of his good friend Isaac Newton, whose great work, the Principia (see Orbits and Gravity), Halley encouraged, edited, and helped pay to publish. Halley himself published his first scientific paper at age 20, while still in college. As a result, he was given a royal commission to go to Saint Helena (a remote island off the coast of Africa where Napoleon would later be exiled) to make the first telescopic survey of the southern sky. After returning, he received the equivalent of a master’s degree and was elected to the prestigious Royal Society in England, all at the age of 22.

In addition to his work on comets, Halley was the first astronomer to recognize that the so-called “fixed” stars move relative to each other, by noting that several bright stars had changed their positions since Ptolemy’s publication of the ancient Greek catalogs. He wrote a paper on the possibility of an infinite universe, proposed that some stars may be variable, and discussed the nature and size of nebulae (glowing cloudlike structures visible in telescopes). While in Saint Helena, Halley observed the planet Mercury going across the face of the Sun and developed the mathematics of how such transits could be used to establish the size of the solar system.

In other fields, Halley published the first table of human life expectancies (the precursor of life-insurance statistics); wrote papers on monsoons, trade winds, and tides (charting the tides in the English Channel for the first time); laid the foundations for the systematic study of Earth’s magnetic field; studied evaporation and how inland waters become salty; and even designed an underwater diving bell. He served as a British diplomat, advising the emperor of Austria and squiring the future czar of Russia around England (avidly discussing, we are told, both the importance of science and the quality of local brandy).

In 1703, Halley became a professor of geometry at Oxford, and in 1720, he was appointed Astronomer Royal of England. He continued observing Earth and the sky and publishing his ideas for another 20 years, until death claimed him at age 85.

Figure 13.17 : Edmund Halley (1656–1742). Halley was a prolific contributor to the sciences. His study of comets at the turn of the eighteenth century helped predict the orbit of the comet that now bears his name.

Only a few comets return in a time measureable in human terms (shorter than a century or two), like Comet Halley does; these are called short-period comets. Many short-period comets have had their orbits changed by coming too close to one of the giant planets—most often Jupiter (and they are thus sometimes called Jupiter-family comets). Most comets have long periods and will take thousands of years to return, if they return at all. As we will see later in this chapter, most Jupiter-family comets come from a different source than the long-period comets (those with orbital periods longer than about a century).

Observational records exist for thousands of comets. We were visited by two bright comets in recent decades. First, in March 1996, came Comet Hyakutake, with a very long tail. A year later, Comet Hale-Bopp appeared; it was as bright as the brightest stars and remained visible for several weeks, even in urban areas (see the image that opens this chapter, Figure 13.1) Since then, there have been few comets visible to the naked eye, and astronomers (both professional and amateur) eagerly await such a visitor.

Table 13.2 lists some well-known comets whose history or appearance is of special interest.

Table 13.2: Some Interesting Comets

Name	Period	Significance
Great Comet of 1577	Long	Tycho Brahe showed it was beyond the Moon (a big step in our understanding)
Great Comet of 1843	Long	Brightest recorded comet; visible in daytime
Daylight Comet of 1910	Long	Brightest comet of the twentieth century
West	Long	Nucleus broke into pieces (1976)
Hyakutake	Long	Passed within 15 million km of Earth (1996)
Hale–Bopp	Long	Brightest recent comet (1997)
Swift-Tuttle	133 years	Parent comet of Perseid meteor shower
Halley	76 years	First comet found to be periodic; explored by spacecraft in 1986
Borrelly	6.8 years	Flyby by Deep Space 1 spacecraft (2000)
Biela	6.7 years	Broke up in 1846 and not seen again
Churyumov-Gerasimenko	6.5 years	Target of Rosetta mission (2014–16)
Wild 2	6.4 years	Target of Stardust sample return mission (2004)
Tempel 1	5.7 years	Target of Deep Impact mission (2005)
Encke	3.3 years	Shortest known period

The Comet’s Nucleus

When we look at an active comet, all we normally see is its temporary atmosphere of gas and dust illuminated by sunlight. This atmosphere is called the comet’s head or coma. Since the gravity of such small bodies is very weak, the atmosphere is rapidly escaping all the time; it must be replenished by new material, which has to come from somewhere. The source is the small, solid nucleus inside, just a few kilometers across, usually hidden by the glow from the much-larger atmosphere surrounding it. The nucleus is the real comet, the fragment of ancient icy material responsible for the atmosphere and the tail (Figure 13.18).

Figure 13.18 : Parts of a Comet. This schematic illustration shows the main parts of a comet. Note that the different structures are not to scale.

The modern theory of the physical and chemical nature of comets was first proposed by Harvard astronomer Fred Whipple in 1950. Before Whipple’s work, many astronomers thought that a comet’s nucleus might be a loose aggregation of solids, sort of an orbiting “gravel bank,” Whipple proposed instead that the nucleus is a solid object a few kilometers across, composed in substantial part of water ice (but with other ices as well) mixed with silicate grains and dust. This proposal became known as the “dirty snowball” model.

The water vapor and other volatiles that escape from the nucleus when it is heated can be detected in the comet’s head and tail, and therefore, we can use spectra to analyze what atoms and molecules the nucleus ice consists of. However, we are somewhat less certain of the non-icy component. We have never identified a fragment of solid matter from a comet that has survived passage through Earth’s atmosphere. However, spacecraft that have approached comets have carried dust detectors, and some comet dust has even been returned to Earth (see Figure 13.19). It seems that much of the “dirt” in the dirty snowball is dark, primitive hydrocarbons and silicates, rather like the material thought to be present on the dark, primitive asteroids.

Figure 13.19 : Captured Comet Dust. This particle (seen through a microscope) is believed to be a tiny fragment of cometary dust, collected in the upper atmosphere of Earth. It measures about 10 microns, or 1/100 of a millimeter, across. (credit: NASA/JPL)

Since the nuclei of comets are small and dark, they are difficult to study from Earth. Spacecraft did obtain direct measurements of a comet nucleus, however, in 1986, when three spacecraft swept past Comet Halley at close range (see Figure 13.20). Subsequently, other spacecraft have flown close to other comets. In 2005, the NASA Deep Impact spacecraft even carried a probe for a high-speed impact with the nucleus of Comet Tempel 1. But by far, the most productive study of a comet has been by the 2015 Rosetta mission, which we will discuss shortly.

Figure 13.20 : Close-up of Comet Halley. This historic photograph of the black, irregularly shaped nucleus of Comet Halley was obtained by the ESA Giotto spacecraft from a distance of about 1000 kilometers. The bright areas are jets of material escaping from the surface. The length of the nucleus is 10 kilometers, and details as small as 1 kilometer can be made out. (credit: modification of work by ESA)

The Comet’s Atmosphere

The spectacular activity that allows us to see comets is caused by the evaporation of cometary ices heated by sunlight. Beyond the asteroid belt, where comets spend most of their time, these ices are solidly frozen. But as a comet approaches the Sun, it begins to warm up. If water (H₂O) is the dominant ice, significant quantities vaporize as sunlight heats the surface above 200 K. This happens for the typical comet somewhat beyond the orbit of Mars. The evaporating H₂O in turn releases the dust that was mixed with the ice. Since the comet’s nucleus is so small, its gravity cannot hold back either the gas or the dust, both of which flow away into space at speeds of about 1 kilometer per second.

The comet continues to absorb energy as it approaches the Sun. A great deal of this energy goes into the evaporation of its ice, as well as into heating the surface. However, recent observations of many comets indicate that the evaporation is not uniform and that most of the gas is released in sudden spurts, perhaps confined to a few areas of the surface. Expanding into space at a speed of about 1 kilometer per second, the comet’s atmosphere can reach an enormous size. The diameter of a comet’s head is often as large as Jupiter, and it can sometimes approach a diameter of a million kilometers (Figure 13.21).

Figure 13.21 : Head of Comet Halley. Here we see the cloud of gas and dust that make up the head, or coma, of Comet Halley in 1986. On this scale, the nucleus (hidden inside the cloud) would be a dot too small to see. (credit: modification of work by NASA/W. Liller)

Most comets also develop tails as they approach the Sun. A comet’s tail is an extension of its atmosphere, consisting of the same gas and dust that make up its head. As early as the sixteenth century, observers realized that comet tails always point away from the Sun (Figure 13.22), not back along the comet’s orbit. Newton proposed that comet tails are formed by a repulsive force of sunlight driving particles away from the head—an idea close to our modern view.

Figure 13.22 : Comet Orbit and Tail. The orientation of a typical comet tail changes as the comet passes perihelion. Approaching the Sun, the tail is behind the incoming comet head, but on the way out, the tail precedes the head.

The two different components that make up the tail (the dust and gas) act somewhat differently. The brightest part of the tail is called the dust tail, to differentiate it from a fainter, straight tail made of ionized gas, called the ion tail. The ion tail is carried outward by streams of ions (charged particles) emitted by the Sun. As you can see in Figure 13.23, the smoother dust tail curves a bit, as individual dust particles spread out along the comet’s orbit, whereas the straight ion tail is pushed more directly outward from the Sun by our star’s wind of charged particles

Figure 13.23 : Comet Tails. (a) As a comet nears the Sun, its features become more visible. In this illustration from NASA showing Comet Hale-Bopp, you can see a comet’s two tails: the more easily visible dust tail, which can be up to 10 million kilometers long, and the fainter gas tail (or ion tail), which is up to hundreds of millions of kilometers long. The grains that make up the dust tail are the size of smoke particles. (b) Comet Mrkos was photographed in 1957 with a wide-field telescope at Palomar Observatory and also shows a clear distinction between the straight gas tail and the curving dust tail. (credit a: modification of work by ESO/E. Slawik; credit b: modification of work by Charles Kearns, George O. Abell, and Byron Hill)

Link to Learning

These days, comets close to the Sun can be found with spacecraft designed to observe our star. For example, in early July, 2011, astronomers at the ESA/NASA’s Solar and Heliospheric Observatory (SOHO) witnessed a comet streaking toward the Sun, one of almost 3000 such sightings. You can also watch a brief video by NASA entitled “Why Are We Seeing So Many Sungrazing Comets?”

The Rosetta Comet Mission

The 1976 Giotto flyby of Comet Halley, which gave us the first picture of an active comet nucleus, was a watershed in our understanding of comets. In the 1990s, European scientists decided to design a much more ambitious mission that would match orbits with an incoming comet and follow it as it approached the Sun. They also proposed that a smaller spacecraft would actually try to land on the comet. The 2-ton main spacecraft was named Rosetta, carrying a dozen scientific instruments, and its 100-kilogram lander with nine more instruments was named Philae.

The Rosetta mission was launched in 2004. Delays with the launch rocket caused it to miss its original target comet, so an alternate destination was picked, Comet Churyumov-Gerasimenko (named after the two discoverers, but generally denoted 67P). This comet’s period of revolution is 6.45 years, making it a Jupiter-family comet.

Since the European Space Agency did not have access to the plutonium-fueled nuclear power sources used by NASA for deep space missions, Rosetta had to be solar powered, requiring especially large solar panels. Even these were not enough to keep the craft operating as it matched orbits with 67P near the comet’s aphelion. The only solution was to turn off all the spacecraft systems and let it coast for several years toward the Sun, out of contact with controllers on Earth until solar energy was stronger. The success of the mission depended on an automatic timer to turn the power back on as it neared the Sun. Fortunately, this strategy worked.

In August 2014, Rosetta began a gradual approach to the comet nucleus, which is a strangely misshapen object about 5 kilometers across, quite different from the smooth appearance of Halley’s nucleus (but equally dark). Its rotation period is 12 hours. On November 12, 2014, the Philae lander was dropped, descending slowly for 7 hours before gently hitting the surface. It bounced and rolled, coming to rest under an overhang where there was not enough sunlight to keep its batteries charged. After operating for a few hours and sending data back to the orbiter, Philae went silent. The main Rosetta spacecraft continued operations, however, as the level of comet activity increased, with steamers of gas jetting from the surface. As the comet approached perihelion in September 2015, the spacecraft backed off to ensure its safety.

The extent of the Rosetta images (and data from other instruments) far exceeds anything astronomers had seen before from a comet. The best imaging resolution was nearly a factor of 100 greater than in the best Halley images. At this scale, the comet appears surprisingly rough, with sharp angles, deep pits, and overhangs (Figure 13.24).

Figure 13.24 : Comet 67P’s Strange Shape and Surface Features. (a) This image from the Rosetta camera was taken from a distance of 285 kilometers. The resolution is 5 meters. You can see that the comet consists of two sections with a connecting “neck” between them. (b) This close-up view of Comet Churyumov-Gerasimenko is from the Philae lander. One of the lander’s three feet is visible in the foreground. The lander itself is mostly in shadow. (credit a: modification of work by ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA; credit b: modification of work by ESA/Rosetta/Philae/CIVA)

The double-lobed shape of 67P’s nucleus has been tentatively attributed to the collision and merger of two independent comet nuclei long ago. The spacecraft verified that the comet’s dark surface was covered with organic carbon-rich compounds, mixed with sulfides and iron-nickel grains. 67P has an average density of only 0.5 g/cm³ (recall water in these units has a density of 1 g/cm³.) This low density indicates that the comet is quite porous, that is, there is a large amount of empty space among its materials.

We already knew that the evaporation of comet ices was sporadic and limited to small jets, but in comet 67P, this was carried to an extreme. At any one time, more than 99% of the surface is inactive. The active vents are only a few meters across, with the material confined to narrow jets that persist for just a few minutes (Figure 13.25). The level of activity is strongly dependent on solar heating, and between July and August 2015, it increased by a factor of 10. Isotopic analysis of deuterium in the water ejected by the comet shows that it is different from the water found on Earth. Thus, apparently comets like 67P did not contribute to the origin of our oceans or the water in our bodies, as some scientists had thought.

Figure 13.25 : Gas Jets on Comet 67P. "(a and b) Jet activity was photographed by the Rosetta spacecraft near perihelion. You can see a jet suddenly appearing; it was active for only a few minutes. (c) This spectacular photo, taken near perihelion, shows the active comet surrounded by multiple jets of gas and dust. (credit a, b: modification of work by ESA/Rosetta/MPS; credit c: modification of work by ESA/Rosetta/NAVCAM)

Link to Learning

The European Space Agency is continuing to make interesting short videos illustrating the challenges and results of the Rosetta and Philae missions. For example, watch “Rosetta’s Moment in the Sun” to see some of the images of the comet generating plumes of gas and dust and hear about some of the dangers an active comet poses for the spacecraft.

Comets in History

Comets have often been written about and portrayed in history. Some of the earliest comet observations were recorded by the Chinese on oracle bones. Oracle bones were pieces of turtle shells and bones used for a form of divination. Aristotle thought and proposed that comets were an atmospheric phenomena, in other words, not in our solar system or in space.

Comets were considered bad omens until the 16th century. These omens would include crop failures, diseases, deaths of royalty, or other such catastrophic events. Pliny the Elder (23-79 AD), a Roman natural philosopher, thought comets were the cause of political issues and death. Halley’s Comet made a passage at the same time during the Battle of Hastings in 1066.

In the more-advanced and scientific approaches to comets, Tycho Brahe measured the parallax of a comet in 1577, which showed it was outside the atmosphere of the Earth. Isaac Newton derived his Law of Universal Gravitation, for which he showed the occasional parabolic orbits of objects. He was able to correlate a parabolic orbit to a comet in 1680. Edmund Halley applied Newton’s Law of Universal Gravitation to a series of comet appearances over some 350 years. Three of these comet appearances had similar orbits; Halley hypothesized it was the same comet. Based on this, Halley predicted the comet would return in 1758 or 1759; the comet did appear as Halley predicted and was named Halley’s Comet.

Active Comets

Often called dirty snowballs, comets are really more like snowy dirtballs. Presently, there are around 5,000 known comets; with new comets being discovered all of the time. The number of known comets is much smaller in number than the known number of asteroids. Many comets orbit in the Kuiper Belt and the Oort Cloud. Comet orbits are often highly elliptical, so much so that these orbits are sometimes described as cigar-shaped. Comet orbits around the sun can be a short period (<200 years) or a long period (>200 years).

Comets are named after their discoverers, which is a tradition that spans centuries. And hundreds of years ago some European astronomers made their living discovering comets.

We have seen a comet impact a planet; Comet Shoemaker-Levy 9 impacted Jupiter with 21 cometary fragments in July 1994. Gene and Carolyn Shoemaker and David Levy discovered the comet.

Comets are composed of Carbon dioxide (CO2), water ices, silicates, and organic material. The materials at the surface of the comet are of a talc-like fineness, as seen up close on Comet Tempel-1. Comets are the sources for the majority of our meteor shower “material;” the fine dust comets leave behind like bread crumbs as they orbit the sun.

The comet’s nucleus is the solid, core structure of a comet. The coma or head is the fuzzy haze that surrounds the comet’s nucleus. The tail may be made of gas, dust, or both. The coma and tailare what we see from Earth. The sizes of comet nucleus have been observed up to around 30 kilometers or 20 miles in diameter. When a comet gets close to the sun, they begin to heat up and outgas, making for the coma and sometimes a tail.

Tempel-1 cometPublic Domain | Image courtesy of NASA.

The behavior of comets is often unpredictable, due to their interactions with the sun, and other solar system dynamics. Comets, which have made numerous passages around the sun, appear to produce less dust and gas, thus less-impressive tails. Often first-time inner solar system comets can break up when passing near the sun, which was the unfortunate fate of Comet ISON in 2013.

The true nucleus of a comet has been seen six times: Halley (1986), Borrelly (2001), Wild-2 (2004), Tempel-1 (2004 and 2005), Hartley-2 (2010), and Churyumov-Gerasimenko (2014). If all goes according to plan, the European Space Agency’s Rosetta spacecraft will release a lander, called Philae, in November 2014. Philae will land on Comet Churyumov-Gerasimenko, providing us with a wealth of data.

Public Domain | Image courtesy of NASA.

Public Domain | Image courtesy of NASA.

CC SY-SA 3. 0 | Image courtesy of Wikimedia Author: Soerfm.

Public Domain | Image courtesy of NASA.

Famous Comets

Halley's Comet

Perhaps the best-known comet, Halley’s Comet, has been a regular visitor through the solar system for thousands of years. Halley is pronounced like Valley … hæli. The comet has been observed and recorded since at least 240 BC. Records of Halley’s passages were made by the Chinese, Babylonians, and medieval Europeans. Recall, Edmund Halley was the first astronomer to determine that Halley’s comet was the same comet that passed by Earth every 75-76 years. The 1910 Earth passage of Halley’s Comet brought much scientific revelation, and much fear. Cyanogen gas was discovered by astronomers during the 1910 passage, leading to mass panic among the public. People were selling comet pills, comet umbrellas, and gas masks.

Halley’s Comet was the first comet to be visited by spacecraft, in 1986; Vega 1 (USSR) and Giotto (European Space Agency). And, interestingly, it turns out that in the last 2,000 years the least-visible or poorest passage of Halley’s Comet to Earth was in 1986. The next close passage of Halley’s Comet will be July 2061; so mark your calendars!

Halley’s Comet in 1910Public Domain

The Comet That Hit Jupiter

Except for seeing meteors enter our atmosphere, no contemporary astronomer had ever observed an impact on another world until 1994. Comet Shoemaker–Levy 9, S-L 9, was discovered in March 1993 by astronomers Eugene and Carolyn Shoemaker and David Levy. Carolyn’s first impression of the discovery photo was it “looked like a squashed comet. ” This description was due to the fact that the comet had been broken apart by the gravitational pull of Jupiter. The largest cometary fragment was 2 kilometers or 1. 2 miles across.

Public Domain | Image courtesy of NASA.

After determining the comet’s orbit, it was found that Shoemaker–Levy 9 fragments would impact Jupiter over several days, 22-26 July 1994. The question became if the impacts would be visible to astronomers and orbiting satellites, like the Hubble Space Telescope.

Over the five-day period, twenty-one (21) distinct impacts were observed by satellites like the Hubble Space Telescope, as well as Earth-based telescopes. Fireballs resembling an atomic bomb mushroom cloud were seen at Jupiter’s horizon. Dark spots were seen in the upper atmosphere of Jupiter after impact; these were visible even in small amateur telescopes. Some of these spots were as big as Earth. The black impact features were visible on Jupiter for months; some likened them to a black eye. Due to the S-L 9 series of impacts, astronomers were better able to explain rows or chains of craters found on the Moon and other objects, like Jupiter’s moon Ganymede.

NASA Hubble Space TelescopePublic Domain | Image courtesy of NASA.

NASA Hubble Space TelescopePublic Domain | Image courtesy of NASA.

NASA Galileo SpacecraftPublic Domain | Image courtesy of NASA.

Learning Objectives

By the end of this section, you will be able to:

• Describe the traits of the centaur objects
• Chronicle the discovery and describe the composition of the Oort cloud
• Describe trans-Neptunian and Kuiper-belt objects
• Explain the proposed fate of comets that enter the inner solar system

The comets we notice when they come near Earth (especially the ones coming for the first time) are probably the most primitive objects we can study, preserved unchanged for billions of years in the deep freeze of the outer solar system. However, astronomers have discovered many other objects that orbit the Sun beyond the planets.

Centaurs and TNOs

In the outer solar system, where most objects contain large amounts of water ice, the distinction between asteroids and comets breaks down. Astronomers initially still used the name “asteroids” for new objects discovered going around the Sun with orbits that carry them far beyond Jupiter. The first of these objects is Chiron, found in 1977 on a path that carries it from just inside the orbit of Saturn at its closest approach to the Sun out to almost the distance of Uranus (Figure 13.26). The diameter of Chiron is estimated to be about 200 kilometers, much larger than any known comet.

Figure 13.26 : Chiron’s Orbit. Chiron orbits the Sun every 50 years, with its closest approach being inside the orbit of Saturn and its farthest approach out to the orbit of Uranus.

In 1992, a still-more-distant object named Pholus was discovered with an orbit that takes it 33 AU from the Sun, beyond the orbit of Neptune. Pholus has the reddest surface of any object in the solar system, indicating a strange (and still unknown) surface composition. As more objects are discovered in these distant reaches, astronomers decided that they will be given the names of centaurs from classical mythology; this is because the centaurs were half human, half horse, and these new objects display some of the properties of both asteroids and comets.

Beyond the orbit of Neptune lies a cold, dark realm populated by objects called simply trans-Neptunian objects (TNOs). The first discovered, and best known, of these TNOs is the dwarf planet Pluto. We discussed Pluto and the New Horizons spacecraft encounter with it in Rings, Moons, and Pluto. The second TNO was discovered in 1992, and now more than a thousand are known, most of them smaller than Pluto. In late 2018, a team of astronomers announced the discovery of the most distant TNO found so far, orbiting at 120 Astronomical Units, or 3 ½ times the distance of Pluto; they nicknamed it “farout.”

The largest ones after Pluto—named Eris, Makemake, and Haumea—are also classed as dwarf planets. Except for their small size, dwarf planets have many properties in common with the larger planets. Pluto has five moons, and two moons have been discovered orbiting Haumea and one each circling Eris and Makemake.

The Kuiper Belt and the Oort Cloud

TNOs are a part of what is called the Kuiper belt, a large area of space beyond Neptune that is also the source of many comets. Astronomers study the Kuiper belt in two ways. New, more powerful telescopes allow us to discover many of the larger members of the Kuiper belt directly. We can also measure the composition of short-period comets that are thought to originate in the Kuiper belt, where small gravitational perturbations from Neptune can gradually shift their orbits until they can penetrate the inner solar system. More than a thousand Kuiper belt objects have been discovered, and astronomers estimate that there are more than 100,000 with diameters larger than 100 kilometers, in a disk extending out to about 50 AU from the Sun.

Following its successful flyby of Pluto, the New Horizons spacecraft explored the Kuiper Belt. The team eventually identified a member of the belt, 2014 MU16 (subsequently named Arrokoth), that could be reached by a slight change in the spacecraft trajectory. New Horizons flew past Arrokoth on January 1, 2019, at a distance of only 3,500 km, obtaining images and other data. The target turned out to be a contact binary, something new and unexpected (see photo).

By following their orbits backward, we can calculate that the aphelia (points farthest from the Sun) of newly discovered comets typically have values near 50,000 AU (more than a thousand times farther than Pluto). This clustering of aphelion distances was first noted by Dutch astronomer Jan Oort, who, in 1950, proposed an idea for the origin of those comets that is still accepted today (Figure 13.27).

Figure 13.27 : Jan Oort (1900–1992). (a) Jan Oort first suggested that there might be a reservoir of frozen chunks, potential comet nuclei, at the edge of the region of the Sun’s gravitational influence. (b) The first color image of the Kuiper Belt Object named Arrokoth, taken at a distance of 137,000 kilometers from the New Horizons spacecraft on January 1, 2019. (credit (a): This image is copyright by the Leiden Observatory; credit (b): NASA/JHU/SWRI)

It is possible to calculate that a star’s gravitational sphere of influence—the distance within which it can exert sufficient gravitation to hold onto orbiting objects—is about one third of its distance to the nearest other stars. Stars in the vicinity of the Sun are spaced in such a way that the Sun’s sphere of influence extends a little beyond 50,000 AU, or about 1 light-year. At such great distances, however, objects in orbit about the Sun can be perturbed by the gravity of passing stars. Some of the perturbed objects can then take on orbits that bring them much closer to the Sun (while others might be lost to the solar system forever).

Oort suggested, therefore, that the new comets we were seeing were examples of objects orbiting the Sun near the edge of its sphere of influence, whose orbits had been disturbed by nearby stars, eventually bringing them close to the Sun where we can see them.¹ The reservoir of ancient icy objects from which such comets are derived is now called the Oort cloud.

Astronomers estimate that there are about a trillion $({10}^{12})$ comets in the Oort cloud. In addition, we estimate that about 10 times this number of icy objects could be orbiting the Sun in the volume of space between the Kuiper belt (which is gravitationally linked to Neptune) and the Oort cloud. These objects remain undiscovered because they are too faint to be seen directly and their orbits are too stable to permit any of them to be deflected inward close to the Sun. The total number of icy or cometary objects in the outer reaches of our solar system could thus be on the order of 10 trillion $({10}^{13})$, a very large number indeed.

What is the mass represented by ${10}^{13}$ comets? We can make an estimate if we assume something about comet sizes and masses. Let us suppose that the nucleus of Comet Halley is typical. Its observed volume is about 600 km³. If the primary constituent is water ice with a density of about 1 g/cm³, then the total mass of Halley’s nucleus must be about $6\times {10}^{14}$ kilograms. This is about one ten billionth $({10}^{-10})$ of the mass of Earth.

If our estimate is reasonable and there are ${10}^{13}$ comets with this mass out there, their total mass would be equal to about 1000 Earths—comparable to the mass of all the planets put together. Therefore, icy, cometary material could be the most important constituent of the solar system after the Sun itself.

Example 13.1: Mass of the Oort Cloud Comets

Suppose the Oort cloud contains 10¹² comets with an average diameter of 10 km each. Let’s estimate the mass of the total Oort cloud.

Solution

We can start by assuming that typical comets are about the size of Comets Halley and Borrelly, with a diameter of 10 km and a density appropriate to water ice, which is about 1 g/cm³ or 1000 kg/m³. We know that $\text{density}=$, the volume of a sphere, $V=\frac{4}{3}\text{π}{R}^3,$ and the radius, $R=\frac{1}{2}D.$ Therefore, for each comet,

$$\begin{array}{cc}\hfill \text{mass}& =\text{density}\times \text{volume}\hfill \\ & =\text{density}\times \frac{4}{3}\text{π}{\left(\frac{1}{2}D\right)}^{}\hfill \end{array}$$

Given that $10\text{km}={10}^4$, each comet’s mass is

$$\begin{array}{cc}\hfill \text{mass}& =1000{\text{kg}}^{}{\text{/m}}^3\times \frac{4}{3}\times 3.14\times \frac{1}{8}\times {\left({10}^4\right)}^{}{3}^{}{\text{m}}^3\hfill \\ & \approx {10}^{15}\text{kg}\hfill \\ & =\hfill \end{array}$$

To calculate the total mass of the cloud, we multiply this typical mass for one comet by the number of comets:

$$\begin{array}{cc}\hfill \text{total mass}& ={10}^{15}\text{kg}\text{/}\text{comet}\times {10}^{12}\text{comets}\hfill \\ & =\hfill \end{array}$$

Exercise \(\PageIndex{1}\)

How does the total mass we calculated above compare to the mass of Jupiter? To the mass of the Sun? (Give a numerical answer.)

Answer

 

The mass of Jupiter is about $1.9\times {10}^{27}\text{kg}$. The mass of the Oort cloud calculated above is ${10}^{27}$ kg. So the cloud would contain about half a Jupiter of mass. The mass of the Sun is $2\times {10}^{30}$. This means the Oort cloud would be
$\frac{{10}^{27}\text{kg}}{\left(2\times {10}^{30}\text{kg}\right)}=0.0005\times \text{the mass of the}$

 

Centaurs

In the past few years, astronomers were closely observing asteroids that had characteristics of comets, and comets, which seemed more like asteroids. Astronomers determined they had found a new group of objects: the Centaurs. These objects are named after the mythological beings that were a mixture of human and horse. Centaurs behave with characteristics of both asteroids and comets; the formerly clean line between comets and asteroids was gone.

It is thought that perhaps 44,000 centaurs exist. Most centaurs have been found between Jupiter and Neptune, crossing the orbit or orbits of one of the Gas Giants. Several varying characteristics have been observed with identified centaurs being studied. What has been found so far is water ice on their surfaces, exhibiting comet-like tails, and the fundamental make-up of asteroids and not comets. There may be some relation between the centaurs and the Kuiper Belt Objects.

Probable the most notable centaur is Chiron ; an approximately 110-kilometer-wide object that displays characteristics of both an asteroid and the nucleus of Halley’s Comet. Saturn’s moon Phoebe might be a captured centaur. No confirmed centaur has been photographed up close.

Public Domain | Image courtesy of NASA.

 

The Kuiper Belt

Early Evolution of the Planetary System

Comets from the Oort cloud help us sample material that formed very far from the Sun, whereas the short-period comets from the Kuiper belt sample materials that were planetesimals in the solar nebula disk but did not form planets. Studies of the Kuiper belt also are influencing our understanding of the early evolution of our planetary system.

The objects in the Oort cloud and the Kuiper belt have different histories, and they may therefore have different compositions. Astronomers are therefore very interested in comparing detailed measurements of the comets derived from these two source regions. Most of the bright comets that have been studied in the past (such as Hyakutake and Hale-Bopp) are Oort cloud comets, but P67 and several other comets targeted for spacecraft measurements in the next decade are Jupiter-family comets from the Kuiper belt (see Table 13.2).

The Kuiper belt is made up of ice-and-rock planetesimals, a remnant of the building blocks of the planets. Since it is gravitationally linked to Neptune, it can help us understand the formation and history of the solar system. As the giant planets formed, their gravity profoundly influenced the orbits of Kuiper belt objects. Computer simulations of the early evolution of the planetary system suggest that the gravitational interactions between the giant planets and the remaining planetesimals caused the orbit of Jupiter to drift inward, whereas the orbits of Saturn, Uranus, and Neptune all expanded, carrying the Kuiper belt with them.

Another hypotheses involves a fifth giant planet that was expelled from the solar system entirely as the planetary orbits shifted. Neptune’s retrograde (backward-orbiting) moon Triton (which is nearly as large as Pluto) may have been a Kuiper belt object captured by Neptune during the period of shifting orbits. It clearly seems that the Kuiper belt may carry important clues to the way our solar system reached its present planetary configuration.

The Fate of Comets

Any comet we see today will have spent nearly its entire existence in the Oort cloud or the Kuiper belt at a temperature near absolute zero. But once a comet enters the inner solar system, its previously uneventful life history begins to accelerate. It may, of course, survive its initial passage near the Sun and return to the cold reaches of space where it spent the previous 4.5 billion years. At the other extreme, it may collide with the Sun or come so close that it is destroyed on its first perihelion passage (several such collisions have been observed with space telescopes that monitor the Sun). Sometimes, however, the new comet does not come that close to the Sun but instead interacts with one or more of the planets.

Link to Learning

SOHO (the Solar and Heliospheric Observatory) has an excellent collection of videos of comets that come near the Sun. At this site, comet ISON approaches the Sun and is believed to be destroyed in its passage.

A comet that comes within the gravitational influence of a planet has three possible fates. It can (1) impact the planet, ending the story at once; (2) speed up and be ejected, leaving the solar system forever; or (3) be perturbed into an orbit with a shorter period. In the last case, its fate is sealed. Each time it approaches the Sun, it loses part of its material and also has a significant chance of collision with a planet. Once the comet is in this kind of short-period orbit, its lifetime starts being measured in thousands, not billions, of years.

A few comets end their lives catastrophically by breaking apart (sometimes for no apparent reason) (Figure 13.29). Especially spectacular was the fate of the faint Comet Shoemaker-Levy 9, which broke into about 20 pieces when it passed close to Jupiter in July 1992. The fragments of Shoemaker-Levy were actually captured into a very elongated, two-year orbit around Jupiter, more than doubling the number of known jovian moons. This was only a temporary enrichment of Jupiter’s family, however, because in July 1994, all the comet fragments crashed unto Jupiter, releasing energy equivalent to millions of megatons of TNT.

Figure 13.29 : Breakup of Comet LINEAR. (a) A ground-based view with much less detail and (b) a much more detailed photo with the Hubble Space Telescope, showing the multiple fragments of the nucleus of Comet LINEAR. The comet disintegrated in July 2000 for no apparent reason. (Note in the left view, the fragments all blend their light together, and can’t be distinguished. The short diagonal white lines are stars that move in the image, which is keeping track of the moving comet.) (credit a: modification of work by the University of Hawaii; credit b: modification of work by NASA, Harold Weaver (the Johns Hopkins University), and the HST Comet LINEAR Investigation Team)

As each cometary fragment streaked into the jovian atmosphere at a speed of 60 kilometers per second, it disintegrated and exploded, producing a hot fireball that carried the comet dust as well as atmospheric gases to high altitudes. These fireballs were clearly visible in profile, with the actual point of impact just beyond the jovian horizon as viewed from Earth (Figure 13.30). As each explosive plume fell back into Jupiter, a region of the upper atmosphere larger than Earth was heated to incandescence and glowed brilliantly for about 15 minutes, a glow we could detect with infrared-sensitive telescopes.

Figure 13.30 : Comet Impact on Jupiter. (a) The “string” of white objects are fragments of Comet Shoemaker-Levy 9 approaching Jupiter. (b) The first fragment of the comet impacts Jupiter, with the point of contact on the bottom left side in this image. On the right is Jupiter’s moon, Io. The equally bright spot in the top image is the comet fragment flaring to maximum brightness. The bottom image, taken about 20 minutes later, shows the lingering flare from the impact. The Great Red Spot is visible near the center of Jupiter. These infrared images were taken with a German-Spanish telescope on Calar Alto in southern Spain. (credit a: modification of work by ESA; credit b: modification of work by Tom Herbst, Max-Planck-Institut fuer Astronomie, Heidelberg, Doug Hamilton, Max-Planck-Institut fuer Kernphysik, Heidelberg, Hermann Boehnhardt, Universitaets-Sternewarte, Muenchen, and Jose Luis Ortiz Moreno, Instituto de Astrofisica de Andalucia, Granada)

After this event, dark clouds of debris settled into the stratosphere of Jupiter, producing long-lived “bruises” (each still larger than Earth) that could be easily seen through even small telescopes (Figure 13.31). Millions of people all over the world peered at Jupiter through telescopes or followed the event via television or online. Another, smaller, impact feature was seen on Jupiter in summer 2009 (and six more since), indicating that the 1994 events were by no means unique. Seeing these large, impact explosions on Jupiter helps us to appreciate the disaster that would happen to our planet if we were hit by a comet or asteroid.

Figure 13.31 : Impact Dust Cloud on Jupiter. These features result from the impact of Comet Shoemaker-Levy 9 with Jupiter, seen with the Hubble Space Telescope 105 minutes after the impact that produced the dark rings (the compact back dot came from another fragment). The inner edge of the diffuse, outer ring is about the same size as Earth. Later, the winds on Jupiter blended these features into a broad spot that remained visible for more than a month. (credit: modification of work by H. Hammel, MIT, and NASA/ESA)

For comets that do not meet so dramatic an end, measurements of the amount of gas and dust in their atmospheres permit us to estimate the total losses during one orbit. Typical loss rates are up to a million tons per day from an active comet near the Sun, adding up to some tens of millions of tons per orbit. At that rate, a typical comet will be gone after a few thousand orbits. This will probably be the fate of Comet Halley in the long run.

Earlier we mentioned that comets can also escape the solar system. In 2019, astronomers discovered comet 2I/Borisov, which came from a planetary system other than ours. It is only the second interstellar object we have found, and the first that is rich in volatile materials such as water and carbon compounds. The presence of an atmosphere allowed its composition to be studied. Borisov contains much more carbon monoxide than normal solar system comets, which tells us that it was probably formed in a cooler location, perhaps around a red dwarf star.

Link to Learning

Watch this 4-minute NASA retrospective on the collision of Comet Shoemaker-Levy 9 with Jupiter History Channel video to learn more.

Footnotes

• 1We now know that not every comet we see originates in the Kuiper Belt or Oort Cloud. In 2017 and 2019, astronomers discovered two “interstellar comets,” (named 1I/Oumuamua and 2I/Borisov) whose orbits indicated that they came from outside the solar system! (Oumoumua means scout or messenger in Hawaiian.)

The Oort Cloud

Attribution

https://phys.libretexts.org/Bookshel...9.11%3A_Comets

https://phys.libretexts.org/Bookshel...haracteristics

https://phys.libretexts.org/Bookshel...ets_in_History

https://phys.libretexts.org/Bookshel..._Halleys_Comet

https://phys.libretexts.org/Bookshel...pacts_a_Planet

https://phys.libretexts.org/Bookshel...oids_or_Comets

https://phys.libretexts.org/Bookshel...-Haired_Comets

https://phys.libretexts.org/Bookshel...elated_Objects