Comet: the story on HearLore

Comet

The solid core of a comet is not a gleaming jewel of ice, but a dark, porous lump of rock and dust that reflects less than four percent of the light that strikes it. This low reflectivity, comparable to fresh asphalt, hides the volatile ices that drive the comet's spectacular transformation when it nears the Sun. Composed of water ice, frozen carbon dioxide, carbon monoxide, methane, and ammonia mixed with rocky particles, these nuclei range from a few hundred meters to tens of kilometers across. The term dirty snowball was coined by Fred Whipple in 1950 to describe this composition, yet modern observations have refined the image to deep fried ice cream, where a dense crystalline crust covers a colder, less dense interior. The surface is generally dry and dusty, with a crust several meters thick that insulates the volatile materials beneath until solar heating forces them to vaporize. This process, known as outgassing, creates the massive atmosphere called the coma and the spectacular tails that have fascinated humanity for millennia. The nucleus of Halley's Comet, for instance, measures 15 by 8 by 8 kilometers and has a density of 0.6 grams per cubic centimeter, making it a fragile rubble pile rather than a solid chunk of ice. Even the smallest known comets, like 322P/SOHO, are only about 60 meters in diameter, yet they possess the same fundamental structure as their massive cousins. The discovery of amino acids like glycine in the dust of Comet 81P/Wild in 2009 and the detection of DNA components in meteorites in 2011 suggest that these dark, icy bodies may have delivered the building blocks of life to the early Earth. The Rosetta mission to 67P/Churyumov, Gerasimenko revealed that the nucleus has no magnetic field, challenging previous theories about the role of magnetism in the formation of planetesimals. The surface of 67P contains at least sixteen organic compounds, four of which were detected for the first time on a comet, including acetamide and methyl isocyanate. These findings confirm that comets are not just simple ice balls but complex chemical factories that have preserved the primordial ingredients of the solar system for billions of years.

Tails of Two Cities

As a comet approaches the Sun, the solar radiation and solar wind plasma act upon the nucleus to create two distinct tails that point in different directions. The ion tail, or type I tail, is made of gases that have been ionized by solar ultraviolet radiation and is pushed directly away from the Sun by the solar wind, following magnetic field lines. This tail can extend one astronomical unit, or 150 million kilometers, into space and is responsible for the discovery of the solar wind itself. The dust tail, or type II tail, consists of larger dust particles that are left behind near the comet's orbital path, forming a curved structure that follows the comet's trajectory. Sometimes, an antitail pointing in the opposite direction to the ion and dust tails can be seen when Earth passes through the comet's orbital plane. The interaction between the solar wind and the cometary ionosphere creates a bow shock, a region where the solar wind is slowed and deflected. This bow shock is wider and more gradual than the sharp planetary bow shocks seen at Earth, and the Rosetta spacecraft observed an infant bow shock at 67P/Churyumov, Gerasimenko that was asymmetric and wider than fully developed bow shocks. The ion tail is formed as a result of the ionization of particles in the coma, which attain a net positive electrical charge and create an induced magnetosphere around the comet. The comet and its induced magnetic field form an obstacle to outward flowing solar wind particles, and because the relative orbital speed of the comet and the solar wind is supersonic, a bow shock is formed upstream of the comet. In this bow shock, large concentrations of cometary ions, called pick-up ions, congregate and act to load the solar magnetic field with plasma, such that the field lines drape around the comet forming the ion tail. If the ion tail loading is sufficient, the magnetic field lines are squeezed together to the point where, at some distance along the ion tail, magnetic reconnection occurs, leading to a tail disconnection event. This was observed on the 20th of April 2007, when the ion tail of Encke's Comet was completely severed while the comet passed through a coronal mass ejection. The ion tail is formed as a result of the ionization of particles in the coma, which attain a net positive electrical charge and create an induced magnetosphere around the comet. The comet and its induced magnetic field form an obstacle to outward flowing solar wind particles, and because the relative orbital speed of the comet and the solar wind is supersonic, a bow shock is formed upstream of the comet. In this bow shock, large concentrations of cometary ions, called pick-up ions, congregate and act to load the solar magnetic field with plasma, such that the field lines drape around the comet forming the ion tail. If the ion tail loading is sufficient, the magnetic field lines are squeezed together to the point where, at some distance along the ion tail, magnetic reconnection occurs, leading to a tail disconnection event. This was observed on the 20th of April 2007, when the ion tail of Encke's Comet was completely severed while the comet passed through a coronal mass ejection. The ion tail is formed as a result of the ionization of particles in the coma, which attain a net positive electrical charge and create an induced magnetosphere around the comet. The comet and its induced magnetic field form an obstacle to outward flowing solar wind particles, and because the relative orbital speed of the comet and the solar wind is supersonic, a bow shock is formed upstream of the comet. In this bow shock, large concentrations of cometary ions, called pick-up ions, congregate and act to load the solar magnetic field with plasma, such that the field lines drape around the comet forming the ion tail. If the ion tail loading is sufficient, the magnetic field lines are squeezed together to the point where, at some distance along the ion tail, magnetic reconnection occurs, leading to a tail disconnection event. This was observed on the 20th of April 2007, when the ion tail of Encke's Comet was completely severed while the comet passed through a coronal mass ejection.

Common questions

What is the physical composition of a comet nucleus?

The solid core of a comet is a dark, porous lump of rock and dust that reflects less than four percent of the light that strikes it. This nucleus contains water ice, frozen carbon dioxide, carbon monoxide, methane, and ammonia mixed with rocky particles. Modern observations describe this structure as deep fried ice cream, where a dense crystalline crust covers a colder, less dense interior.

How do the ion tail and dust tail of a comet differ?

The ion tail, or type I tail, is made of gases ionized by solar ultraviolet radiation and is pushed directly away from the Sun by the solar wind. The dust tail, or type II tail, consists of larger dust particles that form a curved structure following the comet's orbital path. These two tails point in different directions and are created by the interaction of solar radiation and solar wind plasma with the comet nucleus.

Why were comets historically feared as omens of disaster?

For most of human history, comets were seen as terrifying omens of death and disaster rather than celestial bodies. The fear of comets as acts of God and signs of impending doom was highest in Europe from 1200 to 1650 CE. In 1910, erroneous newspaper reports inspired panic that toxic gas cyanogen in the tail of Halley's Comet might poison millions.

Where do short-period and long-period comets originate?

Short-period comets with orbital periods of less than 200 years originate in the Kuiper belt or its associated scattered disc beyond the orbit of Neptune. Long-period comets with periods ranging from 200 years to millions of years are thought to originate in the Oort cloud. The Oort cloud occupies a vast space starting from between 2,000 and 50,000 astronomical units from the Sun.

When was the dirty snowball model of comets proposed?

The term dirty snowball was coined by Fred Whipple in 1950 to describe the composition of comets. This model proposed that comets were icy objects containing some dust and rock rather than rocky objects containing some ice. The dirty snowball model became accepted and was supported by observations of spacecraft that flew through the coma of Halley's Comet in 1986.

What significant discovery did the Rosetta mission make about Comet 67P/Churyumov, Gerasimenko?

The Rosetta mission revealed that the nucleus of 67P/Churyumov, Gerasimenko has no magnetic field. On the 12th of November 2014, the Philae lander successfully landed on the comet's surface, the first time a spacecraft has ever landed on such an object. Instruments on the Philae lander found at least sixteen organic compounds at the comet's surface, four of which were detected for the first time on a comet.

The Great Fear

For most of human history, comets were not seen as celestial bodies but as terrifying omens of death and disaster. In the 13th century, the year after the Great Comet of 1618, Gotthard Arthusius published a pamphlet listing ten pages of comet-related disasters, including earthquakes, floods, epidemics, and war. The fear of comets as acts of God and signs of impending doom was highest in Europe from 1200 to 1650 CE, with Pope Callixtus III warning that comets were the thick smoke of human sins kindled by the hot and fiery anger of the Supreme Heavenly Judge. In 1910, when Earth passed through the tail of Halley's Comet, erroneous newspaper reports inspired a panic that the toxic gas cyanogen in the tail might poison millions, leading to the panicked buying of gas masks and quack anti-comet pills. The Great Comet of 1577 was seen by many, including well-known astronomers Tycho Brahe and Taqi ad-Din, and its observations led to several significant findings regarding cometary science. The Great Comet of 1811 had a coma roughly the diameter of the Sun, and the Great Comet of 1910 caused a slew of sensationalist publications of all sorts at each of its reappearances. The birth and death of some notable persons coincided with separate appearances of the comet, such as with writers Mark Twain, who correctly speculated that he'd go out with the comet in 1910, and Eudora Welty. In times past, bright comets often inspired panic and hysteria in the general population, being thought of as bad omens. More recently, during the passage of Halley's Comet in 1910, Earth passed through the comet's tail, and erroneous newspaper reports inspired a fear that cyanogen in the tail might poison millions, whereas the appearance of Comet Hale, Bopp in 1997 triggered the mass suicide of the Heaven's Gate cult. The fear of comets as acts of God and signs of impending doom was highest in Europe from 1200 to 1650 CE, with Pope Callixtus III warning that comets were the thick smoke of human sins kindled by the hot and fiery anger of the Supreme Heavenly Judge. In 1910, when Earth passed through the tail of Halley's Comet, erroneous newspaper reports inspired a panic that the toxic gas cyanogen in the tail might poison millions, leading to the panicked buying of gas masks and quack anti-comet pills. The Great Comet of 1577 was seen by many, including well-known astronomers Tycho Brahe and Taqi ad-Din, and its observations led to several significant findings regarding cometary science. The Great Comet of 1811 had a coma roughly the diameter of the Sun, and the Great Comet of 1910 caused a slew of sensationalist publications of all sorts at each of its reappearances. The birth and death of some notable persons coincided with separate appearances of the comet, such as with writers Mark Twain, who correctly speculated that he'd go out with the comet in 1910, and Eudora Welty. In times past, bright comets often inspired panic and hysteria in the general population, being thought of as bad omens. More recently, during the passage of Halley's Comet in 1910, Earth passed through the comet's tail, and erroneous newspaper reports inspired a fear that cyanogen in the tail might poison millions, whereas the appearance of Comet Hale, Bopp in 1997 triggered the mass suicide of the Heaven's Gate cult.

The Long Journey Home

Comets travel on highly eccentric elliptical orbits that take them from the deep freeze of the outer solar system to the warmth of the inner solar system. Short-period comets, with orbital periods of less than 200 years, originate in the Kuiper belt or its associated scattered disc, which lie beyond the orbit of Neptune. Long-period comets, with periods ranging from 200 years to thousands or even millions of years, are thought to originate in the Oort cloud, a spherical cloud of icy bodies extending from outside the Kuiper belt to halfway to the nearest star. The Oort cloud is thought to occupy a vast space starting from between 2,000 and 50,000 astronomical units from the Sun, and the region can be subdivided into a spherical outer Oort cloud and a doughnut-shaped inner cloud, the Hills cloud. The inner Oort cloud is also known as the Hills cloud, named after Jack G. Hills, who proposed its existence in 1981. Models predict that the inner cloud should have tens or hundreds of times as many cometary nuclei as the outer halo, and it is seen as a possible source of new comets that resupply the relatively tenuous outer cloud as the latter's numbers are gradually depleted. The Hills cloud explains the continued existence of the Oort cloud after billions of years. Long-period comets are set in motion towards the Sun by gravitational perturbations from passing stars and the galactic tide. Single-apparition or non-periodic comets have a hyperbolic or parabolic osculating orbit which allows them to permanently exit the Solar System after a single pass of the Sun. The Sun's Hill sphere has an unstable maximum boundary of 230,000 astronomical units, and only a few hundred comets have been seen to reach a hyperbolic orbit. Three objects have been discovered with an eccentricity significantly greater than one: 1I/Oumuamua, 2I/Borisov, and 3I/ATLAS, indicating an origin outside the Solar System. While Oumuamua, with an eccentricity of about 1.2, showed no optical signs of cometary activity during its passage through the inner Solar System in October 2017, changes to its trajectory, which suggests outgassing, indicate that it is probably a comet. On the other hand, 2I/Borisov, with an estimated eccentricity of about 3.36, has been observed to have the coma feature of comets, and is considered the first detected interstellar comet. 3I/ATLAS has an eccentricity of about 6.1, and also has a coma, indicating that it is also a comet. Comet C/1980 E1 had an orbital period of roughly 7.1 million years before the 1982 perihelion passage, but a 1980 encounter with Jupiter accelerated the comet giving it the largest eccentricity of any known solar comet with a reasonable observation arc. Comets not expected to return to the inner Solar System include C/1980 E1, C/2000 U5, C/2001 Q4, C/2009 R1, C/1956 R1, and C/2007 F1. Some authorities use the term periodic comet to refer to any comet with a periodic orbit, whereas others use it to mean exclusively short-period comets. Similarly, although the literal meaning of non-periodic comet is the same as single-apparition comet, some use it to mean all comets that are not periodic in the second sense. Early observations have revealed a few genuinely hyperbolic trajectories, but no more than could be accounted for by perturbations from Jupiter. Comets from interstellar space are moving with velocities of the same order as the relative velocities of stars near the Sun, a few tens of kilometers per second. When such objects enter the Solar System, they have a positive specific orbital energy resulting in a positive velocity at infinity and have notably hyperbolic trajectories. A rough calculation shows that there might be four hyperbolic comets per century within Jupiter's orbit, give or take one and perhaps two orders of magnitude.

The Science of Comets

The scientific understanding of comets has evolved from ancient superstition to precise measurement and direct sampling. In 1456, crude attempts at a parallax measurement of Halley's Comet were made, but were erroneous. Regiomontanus was the first to attempt to calculate diurnal parallax by observing the Great Comet of 1472, and his predictions were not very accurate, but they were conducted in the hopes of estimating the distance of a comet from Earth. In the 16th century, Tycho Brahe and Michael Maestlin demonstrated that comets must exist outside of Earth's atmosphere by measuring the parallax of the Great Comet of 1577. Within the precision of the measurements, this implied the comet must be at least four times more distant than from Earth to the Moon. Isaac Newton, in his Principia Mathematica of 1687, proved that an object moving under the influence of gravity by an inverse square law must trace out an orbit shaped like one of the conic sections, and he demonstrated how to fit a comet's path through the sky to a parabolic orbit, using the comet of 1680 as an example. In 1705, Edmond Halley applied Newton's method to 23 cometary apparitions that had occurred between 1337 and 1698. He noted that three of these, the comets of 1531, 1607, and 1682, had very similar orbital elements, and he was further able to account for the slight differences in their orbits in terms of gravitational perturbation caused by Jupiter and Saturn. Confident that these three apparitions had been three appearances of the same comet, he predicted that it would appear again in 1758, 59. Halley's predicted return date was later refined by a team of three French mathematicians: Alexis Clairaut, Joseph Lalande, and Nicole-Reine Lepaute, who predicted the date of the comet's 1759 perihelion to within one month's accuracy. On 1758 November 14, Alexis Clairaut announced to the Royal Academy of Sciences in Paris his prediction of the date at which Halley's comet would return. On 1759 April 7, the French astronomer Joseph-Nicolas Delisle announced to the Royal Academy of Sciences in Paris that he and his assistant Charles Messier had observed the return of Halley's comet, as predicted. De l'Isle subsequently admitted that the comet's return had first been seen by a German amateur astronomer and farmer, Georg Palitzsch. The story behind the rediscovery of Halley's comet was given by Joseph Lalande, who acknowledged the contributions of Madame Lepaute to predicting the return of Halley's comet. As early as the 18th century, some scientists had made correct hypotheses as to comets' physical composition. In 1755, Immanuel Kant hypothesized in his Universal Natural History that comets were condensed from primitive matter beyond the known planets, which is feebly moved by gravity, then orbit at arbitrary inclinations, and are partially vaporized by the Sun's heat as they near perihelion. In 1836, the German mathematician Friedrich Wilhelm Bessel, after observing streams of vapor during the appearance of Halley's Comet in 1835, proposed that the jet forces of evaporating material could be great enough to significantly alter a comet's orbit, and he argued that the non-gravitational movements of Encke's Comet resulted from this phenomenon. In 1950, Fred Lawrence Whipple proposed that rather than being rocky objects containing some ice, comets were icy objects containing some dust and rock. This dirty snowball model soon became accepted and appeared to be supported by the observations of an armada of spacecraft that flew through the coma of Halley's Comet in 1986, photographed the nucleus, and observed jets of evaporating material. On the 22nd of January 2014, ESA scientists reported the detection, for the first definitive time, of water vapor on the dwarf planet Ceres, the largest object in the asteroid belt. The detection was made by using the far-infrared abilities of the Herschel Space Observatory. The finding is unexpected because comets, not asteroids, are typically considered to sprout jets and plumes. According to one of the scientists, the lines are becoming more and more blurred between comets and asteroids. On the 11th of August 2014, astronomers released studies, using the Atacama Large Millimeter/Submillimeter Array for the first time, that detailed the distribution of HCN, HNC, and dust inside the comae of comets C/2012 F6 and C/2012 S1.

The Spacecraft Era

The modern era of comet science began with the Halley Armada, a collection of spacecraft missions that visited and/or made observations of Halley's Comet during its 1986 perihelion. The Space Shuttle Challenger was intended to do a study of Halley's Comet in 1986, but exploded shortly after being launched. The Deep Impact probe, launched in 2001, obtained high-resolution images of the surface of Comet Borrelly, which was found to be hot and dry, with a temperature of between 200 and 300 Kelvin, and extremely dark, suggesting that the ice has been removed by solar heating and maturation, or is hidden by the soot-like material that covers Borrelly. In July 2005, the Deep Impact probe blasted a crater on Comet Tempel 1 to study its interior. The mission yielded results suggesting that the majority of a comet's water ice is below the surface and that these reservoirs feed the jets of vaporized water that form the coma of Tempel 1. Renamed EPOXI, it made a flyby of Comet Hartley 2 on the 4th of November 2010. The Ulysses probe, launched in 1990, unexpectedly passed through the tail of the comet C/2006 P1 in 2007. The Stardust mission showed that materials retrieved from the tail of Wild 2 were crystalline and could only have been born in fire, at extremely high temperatures of over 1,000 Kelvin. Although comets formed in the outer Solar System, radial mixing of material during the early formation of the Solar System is thought to have redistributed material throughout the proto-planetary disk. As a result, comets contain crystalline grains that formed in the early, hot inner Solar System. This is seen in comet spectra as well as in sample return missions. More recent still, the materials retrieved demonstrate that the comet dust resembles asteroid materials. These new results have forced scientists to rethink the nature of comets and their distinction from asteroids. The Rosetta probe orbited Comet Churyumov, Gerasimenko, and on the 12th of November 2014, its lander Philae successfully landed on the comet's surface, the first time a spacecraft has ever landed on such an object in history. The Rosetta and Philae spacecraft show that the nucleus of 67P/Churyumov, Gerasimenko has no magnetic field, which suggests that magnetism may not have played a role in the early formation of planetesimals. Further, the ALICE spectrograph on Rosetta determined that electrons, within 1,000 kilometers above the comet nucleus, produced from photoionization of water molecules by solar radiation, and not photons from the Sun as thought earlier, are responsible for the degradation of water and carbon dioxide molecules released from the comet nucleus into its coma. Instruments on the Philae lander found at least sixteen organic compounds at the comet's surface, four of which, acetamide, acetone, methyl isocyanate and propionaldehyde, have been detected for the first time on a comet. The Halley Armada describes the collection of spacecraft missions that visited and/or made observations of Halley's Comet 1980s perihelion. The Space Shuttle Challenger was intended to do a study of Halley's Comet in 1986, but exploded shortly after being launched. The Deep Impact probe, launched in 2001, obtained high-resolution images of the surface of Comet Borrelly, which was found to be hot and dry, with a temperature of between 200 and 300 Kelvin, and extremely dark, suggesting that the ice has been removed by solar heating and maturation, or is hidden by the soot-like material that covers Borrelly. In July 2005, the Deep Impact probe blasted a crater on Comet Tempel 1 to study its interior. The mission yielded results suggesting that the majority of a comet's water ice is below the surface and that these reservoirs feed the jets of vaporized water that form the coma of Tempel 1. Renamed EPOXI, it made a flyby of Comet Hartley 2 on the 4th of November 2010. The Ulysses probe, launched in 1990, unexpectedly passed through the tail of the comet C/2006 P1 in 2007. The Stardust mission showed that materials retrieved from the tail of Wild 2 were crystalline and could only have been born in fire, at extremely high temperatures of over 1,000 Kelvin. Although comets formed in the outer Solar System, radial mixing of material during the early formation of the Solar System is thought to have redistributed material throughout the proto-planetary disk. As a result, comets contain crystalline grains that formed in the early, hot inner Solar System. This is seen in comet spectra as well as in sample return missions. More recent still, the materials retrieved demonstrate that the comet dust resembles asteroid materials. These new results have forced scientists to rethink the nature of comets and their distinction from asteroids. The Rosetta probe orbited Comet Churyumov, Gerasimenko, and on the 12th of November 2014, its lander Philae successfully landed on the comet's surface, the first time a spacecraft has ever landed on such an object in history. The Rosetta and Philae spacecraft show that the nucleus of 67P/Churyumov, Gerasimenko has no magnetic field, which suggests that magnetism may not have played a role in the early formation of planetesimals. Further, the ALICE spectrograph on Rosetta determined that electrons, within 1,000 kilometers above the comet nucleus, produced from photoionization of water molecules by solar radiation, and not photons from the Sun as thought earlier, are responsible for the degradation of water and carbon dioxide molecules released from the comet nucleus into its coma. Instruments on the Philae lander found at least sixteen organic compounds at the comet's surface, four of which, acetamide, acetone, methyl isocyanate and propionaldehyde, have been detected for the first time on a comet.