Comet

• 1. Introduction

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  A comet can stretch an arc of up to 30 degrees across the night sky, the span of sixty full Moons. That is what a comet looks like when it is close and bright. Yet the object behind that spectacle is small. The solid core, the nucleus, measures only a few hundred meters to tens of kilometers across, a loose collection of ice, dust, and small rocky particles. For most of its life it stays frozen and dark in the cold reaches beyond Neptune. Then something pulls it toward the Sun, and it transforms.

  This is the story of how an icy lump becomes one of the most dramatic sights in the sky. It is also the story of how humans read that sight. Comets have been observed and recorded since ancient times by many cultures and religions. They were taken as omens of doom, as the smoke of human sins, and finally as predictable bodies obeying the law of gravity. What turns a small Solar System body into a glowing apparition with a tail that can stretch beyond one astronomical unit? Where do these objects come from, and where do they go? And why did the panic they once caused give way to spacecraft chasing them down?

• 2. Dirty Snowball

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  In 1950, Fred Lawrence Whipple proposed that comets were icy objects containing some dust and rock, rather than rocky objects containing some ice. This "dirty snowball" model soon became accepted. Cometary nuclei are an amalgamation of rock, dust, water ice, and frozen carbon dioxide, carbon monoxide, methane, and ammonia. The nuclei also carry organic compounds, which may include methanol, hydrogen cyanide, formaldehyde, ethanol, ethane, and perhaps long-chain hydrocarbons and amino acids.

  After NASA's Deep Impact mission watched a probe strike Comet 9P/Tempel 1 in July 2005, some researchers suggested "icy dirtballs" fit better than snowballs. Research in 2014 went further, describing comets as like "deep fried ice cream". Their surfaces are dense crystalline ice mixed with organic compounds, while the interior ice is colder and less dense.

  The outer surfaces of cometary nuclei are among the least reflective objects in the Solar System. The Giotto space probe found that the nucleus of Halley's Comet reflects about four percent of the light that falls on it. Deep Space 1 found that Comet Borrelly's surface reflects less than 3.0 percent. By comparison, asphalt reflects seven percent. Solar heating drives off lighter volatile compounds and leaves behind larger organic compounds that are very dark, like tar or crude oil.

  Because of their low mass, comet nuclei do not become spherical under their own gravity, so they have irregular shapes. Known comets have an estimated average density of 0.6 grams per cubic centimeter. The nucleus of Halley's Comet measures roughly 15 by 8 by 8 kilometers. In 2009, the amino acid glycine was confirmed in comet dust recovered by NASA's Stardust mission.

• 3. Coma And Tails

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  When a comet passes close to the Sun it warms and releases gases, a process called outgassing. The streams of dust and gas form a huge, extremely thin atmosphere around the comet called the coma. Water makes up to 90 percent of the volatiles that flow out of the nucleus. The coma can grow thousands or millions of kilometers across, even though the solid nucleus is generally less than 60 kilometers wide. About a month after an outburst in October 2007, comet 17P/Holmes briefly had a tenuous dust atmosphere larger than the Sun. The Great Comet of 1811 had a coma roughly the diameter of the Sun.

  A comet may have a dust tail, an ion tail, both, or neither. The dust tail contains particles about the size of human hairs, and radiation pressure from the Sun pushes them away into a gentle arc. The ion tail contains molecule-sized particles ionized by the Sun, then captured by the magnetic field created by the interaction between those ions and the solar wind. The ion tail points directly away from the Sun, following magnetic field lines. When Earth passes through a comet's orbital plane, an antitail pointing the opposite direction may appear.

  In 1996, comets were found to emit X-rays, which greatly surprised astronomers because X-ray emission is usually associated with very high-temperature bodies. Thomas E. Cravens was the first to propose an explanation, in early 1997. Highly charged solar wind ions steal electrons from cometary atoms in a process called charge exchange, and the de-excitation releases X-rays and far ultraviolet photons. Uneven heating can also blow gases out of a weak spot like a geyser. In 2010 it was revealed that sublimation of frozen carbon dioxide can power such jets, and infrared imaging of Hartley 2 showed them carrying dust grains into the coma.

• 4. Where Comets Live

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  Active, visible comets last only a few hundred thousand years on their unstable orbits. The vast majority wait in the cold, far from the Sun. Short-period comets originate in the Kuiper belt or its associated scattered disc, which lie beyond the orbit of Neptune. Long-period comets are thought to come from 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 named after the Dutch astronomer Jan Hendrik Oort, who hypothesized its existence. Some estimates place its outer edge between 100,000 and 200,000 astronomical units. The region splits into a spherical outer cloud of 20,000 to 50,000 astronomical units and a doughnut-shaped inner cloud, the Hills cloud, of 2,000 to 20,000 astronomical units. The inner Oort cloud is named after Jack G. Hills, who proposed its existence in 1981. Models predict the inner cloud holds tens or hundreds of times as many cometary nuclei as the outer halo.

  The reservoir of comet-like bodies in the Oort cloud is about one trillion. Against that number, the count of known comets stands at 4,584. Gravitational perturbations from passing stars and the galactic tide can set a long-period comet in motion toward the Sun. Jupiter is the source of the greatest perturbations, being more than twice as massive as all the other planets combined. Roughly one comet per year is visible to the naked eye, though many are faint and unspectacular.

• 5. Orbits And Origins

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  Comets usually travel on highly eccentric elliptical orbits, with periods that range from several years to potentially several millions of years. Encke's Comet sits at the shorter extreme, with an orbit that does not reach Jupiter. Short-period comets with periods under 20 years and low inclinations are called Jupiter-family comets. Those like Halley, with periods between 20 and 200 years and inclinations from zero to more than 90 degrees, are Halley-type comets. The aphelion of Halley's Comet lies a little beyond the orbit of Neptune.

  Three objects have been discovered with eccentricity significantly greater than one, marking an origin outside the Solar System: 1I/ʻOumuamua, 2I/Borisov, and 3I/ATLAS. ʻOumuamua, 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. Yet changes to its trajectory suggested outgassing, indicating it is probably a comet. 2I/Borisov, with an estimated eccentricity of about 3.36, has a coma and is considered the first detected interstellar comet. 3I/ATLAS has an eccentricity of about 6.1 and also has a coma.

  Comet C/1980 E1 had an orbital period of roughly 7.1 million years before its 1982 perihelion passage. A 1980 encounter with Jupiter accelerated it, giving it the largest eccentricity of any known solar comet, 1.057. Exocomets beyond the Solar System have also been detected and may be common in the Milky Way. The first exocomet system was found around Beta Pictoris, a very young A-type main-sequence star, in 1987. A total of 11 such systems have been identified.

• 6. From Omens To Orbits

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  From ancient sources, such as Chinese oracle bones, comets are known to have been noticed for millennia. Until the sixteenth century they were usually read as bad omens: deaths of kings, coming catastrophes, or even attacks by heavenly beings. Aristotle, who lived from 384 to 322 BCE, was the first known scientist to fold comets into a structured cosmological theory. He believed they were atmospheric phenomena, confined within the sphere of the Moon and separated from the heavens. His view held through much of the Middle Ages.

  Seneca the Younger questioned Aristotle in the 1st century CE, arguing that comets move regularly, resist the wind, and need not stay within the zodiac. Pliny the Elder, in the same century, tied comets to political unrest and death, describing their tails as "long hair" or "long beard". The word comet itself derives from the Greek for wearing long hair. In 1301, the Italian painter Giotto became the first person to accurately portray a comet, placing Halley's Comet in the role of the Star of Bethlehem in his Adoration of the Magi.

  Tycho Brahe and Michael Maestlin measured the parallax of the Great Comet of 1577 and showed that comets must exist outside Earth's atmosphere, at least four times more distant than the Moon. Isaac Newton, in his Principia Mathematica of 1687, proved that a body moving under an inverse square law traces a conic section, and he fit the comet of 1680 to a parabolic orbit. In 1705, Edmond Halley applied Newton's method to 23 apparitions between 1337 and 1698. He saw that the comets of 1531, 1607, and 1682 shared similar orbits and predicted a return in 1758 to 1759. Three French mathematicians, Alexis Clairaut, Joseph Lalande, and Nicole-Reine Lepaute, later refined that date to within a month.

• 7. Fear And Fascination

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  Fear of comets as acts of God and signs of impending doom was highest in Europe from 1200 to 1650 CE. The year after the Great Comet of 1618, Gotthard Arthusius published a pamphlet declaring it a sign that the Day of Judgment was near. He listed ten pages of comet-related disasters, including "earthquakes, floods, changes in river courses, hail storms, hot and dry weather, poor harvests, epidemics, war and treason and high prices".

  By 1700 most scholars had concluded that such events happened whether a comet was seen or not. In 1711, William Whiston used Edmond Halley's records to claim the Great Comet of 1680 had a periodicity of 574 years and had caused the worldwide flood in the Book of Genesis. His announcement revived fear of comets for another century, now as direct threats rather than mere signs. Spectroscopic analysis in 1910 found the toxic gas cyanogen in the tail of Halley's Comet, sparking panicked buying of gas masks and quack "anti-comet pills" and "anti-comet umbrellas".

  The dread survived into modern memory. The writer Mark Twain correctly speculated that he would "go out with the comet" in 1910, his life bracketed by its appearances. Mary Chapin Carpenter later dedicated the song "Halley Came to Jackson" to the life of Eudora Welty. The appearance of Comet Hale-Bopp in 1997 triggered the mass suicide of the Heaven's Gate cult, a grim echo of the old belief in comets as harbingers of world-altering change.

• 8. Reaching The Comets

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  On the 12th of November 2014, the Philae lander touched down on Comet Churyumov-Gerasimenko, the first time a spacecraft had ever landed on such an object. Philae was carried there by the European Space Agency's Rosetta probe, which orbited the comet. Instruments on the lander found at least sixteen organic compounds at the surface, four of them detected on a comet for the first time: acetamide, acetone, methyl isocyanate, and propionaldehyde. Results showed that 67P/Churyumov-Gerasimenko has no magnetic field, suggesting magnetism may not have shaped the early formation of planetesimals.

  Other missions cracked comets open and caught their dust. In July 2005, NASA's Deep Impact probe blasted a crater on Comet Tempel 1 to study its interior, and the results suggested most of a comet's water ice sits below the surface. Renamed EPOXI, the spacecraft flew past Comet Hartley 2 on the 4th of November 2010. In 2001, Deep Space 1 imaged Comet Borrelly and found its surface hot and dry, between 26 and 71 degrees Celsius. Data from the Stardust mission showed that material from the tail of Wild 2 was crystalline and could only have formed at temperatures over 1000 degrees Celsius, deep in the hot inner Solar System.

  In 1986, an armada of spacecraft, including ESA's Giotto and the Soviet Union's Vega 1 and Vega 2, flew through the coma of Halley's Comet and photographed its nucleus. Comets keep blurring into their rocky cousins. On the 22nd of January 2014, ESA scientists reported water vapor on the dwarf planet Ceres, the largest object in the asteroid belt. As one of those scientists put it, "The lines are becoming more and more blurred between comets and asteroids."