Every few years, on average, an unusually bright comet will blaze forth in the night sky becoming visible to the unaided eye and sporting a graceful tail resembling long hair blowing in the wind. In fact, the word comet is derived from the Greek name aster kometes, meaning “long-haired star”. But a comet is not anything like a star. Their dramatic display emanates from a relatively small, blackened chunk of ice and dust, comparable to large city in size.
Unlike the planets, the comets can appear almost anywhere in the sky, remain visible for a few weeks or months, and then vanish into the darkness. Astronomers call this period of visibility an “apparition”. During its apparition, a comet changes its shape, often from night to night.
Comets used to frighten people, filling ancient minds with awe and terror. They upset the natural order, by moving into the otherwise placid firmament. By their unexpected arrivals, these celestial intruders seemed to upset the natural order of the otherwise placid firmament, and to presage changes in the order of things on Earth, such as the death of rulers, wars and other disasters.
Halley demystified comets by showing that at least one of them travels in an elongated orbit around the Sun. He found that the orbit of the comet of 1682 was similar to those of comets observed in 1607 (by Johannes Kepler, 1571-1630) and in 1531 (by Petrus Apianus, 1495-1552). All three comets moved around the Sun in retrograde orbits with a similar orientation. Halley also knew that the Great Comet of 1456 had traveled in the retrograde direction, and he concluded that all four comets were returns of the same comet in a closed elliptical orbit around the Sun with a period of about 76 years. Halley confidently predicted its return in 1758, noting that he would not live to see it. After the comet was re-discovered, on Christmas night of the predicted year, Halley’s achievement was acknowledged, albeit posthumously, by naming it Comet Halley.
After its 1910 apparition, Comet Halley moved away from the Sun into the outer darkness, arriving in 1948 at the remotest part of its orbit at 35 AU, or at 35 times the Earth’s distance from the Sun. The comet then turned the direction of its course, and began falling back toward the heart of the solar system with ever-increasing speed. It reached perihelion or its closest distance from the Sun, on 9 February 1986. Comet Halley and the Earth were then on opposite sides of the Sun, so this was among the least favorable apparitions for observing the comet with the unaided eye. Nevertheless, it still became one of the most thoroughly studied apparitions in the history of comet research, including visits by six spacecraft.
Comets are primitive bodies that formed at the same time as the Sun and planets about 4.6 billion years ago. But once they come close enough to be seen, comets begin to fall apart and they must eventually vanish from sight, often in less than a million years after first sighting. So comets are very old, but once they swing near the Sun they do not last very long. This means that ancient reservoirs must be furnishing the inner solar system with new comets. They come from two reservoirs, one that is very far away, at the fringe of the outer solar system, and a nearer one at the edge of the planetary realm. These small icy worlds have been hibernating in the cold outer reaches of space ever since the formation of the solar system.
The long-period and short-period comets
Most discovered comets have arrived near the Sun from distant regions far beyond the major planets. They have very elongated trajectories that take them back to the distant regions they came from. These comets are known as the long-period comets, with orbital periods larger than 30 Earth years. The long-period comets are observed in the inner part of the solar system just once, arriving unannounced and unpredicted. As you might expect, they come from very far away, at the outer fringes of the solar system.
Some known comets, about 150 in all, have appeared more than once during the past two centuries. These are the short-period comets, which revolve around the Sun with orbital periods of less than 30 Earth years. They are sometimes distinguished by putting a number and the letter “P” before their name, with the short-period comet number corresponding to the order of recognition. The short-period comets are seen time and again, trapped in tight orbits within the planetary realm. Most of them have low orbital inclinations near the plane of the Earth’s orbit, with mean distances from the Sun of just a few times that of the Earth, or a few AU. It is these short-period comets whose regular returns we are able to predict, and which we can examine in detail with spacecraft.
The Oort cloud
The size and orientation of the trajectories of long-period comets can be explained if they come from a remote, spherical shell belonging to the outer parts of the solar system. This vast comet repository is known as the Oort cloud, named after the Dutch astronomer Jan H. Oort (1900-1992) who first postulated its existence. Since the long-period comets approach the Sun from enormous distances, of 100,000 AU or more, the Oort cloud has a diameter of up to twice this size. By way of comparison, the average distance between the Earth and the Sun is just 1 AU, about 150 billion meters. And because long-period comets enter the planetary realm at all possible angles, with every inclination to the Earth’s orbital plane, they must come from a spherical shell. This would also explain the fact that long-period comets move in all directions. Roughly half of them move along their trajectories in the retrograde direction, opposite to the orbital motion of the planets.
But how do comets fall from the Oort comet cloud to the heart of the solar system? The distant comets are only weakly bound to the solar system, and are easily perturbed by the gravitation of nearby, moving objects, which throw some of the comets back into the planetary system. The random gravitational jostling of individual stars passing nearby, for example, knocks some of the comets in the Oort cloud from their stable orbits, either injecting them into interstellar space or gradually deflecting their paths toward the Sun. Every one million years, about a dozen stars pass close enough to stir up the cometary objects, sending a steady trickle of comets into the inner solar system on very long elliptical orbits. A giant interstellar molecular cloud can also impart a gravitational tug when it moves past the comet cloud, helping to jostle some of them out of their remote resting-place. Tidal forces generated in the cloud by the disk of our Galaxy, the Milky Way, also help to feed new long-period comets into the planetary region. As time goes on, the accumulated effects of these tugs will send a few comets in toward the Sun – or outward to interstellar space. If the several hundred new comets observed during recorded history have been shuffled into view by the perturbing action of nearby stars or molecular clouds, then there are at least one hundred billion, or 1011, comets in the Oort cloud. There may be a trillion, 1012, or even ten trillion, 1013, of them. This large population of unseen comets can sustain the visible long-period comets and persist without serious depletion for many billions of years, until long after the Sun expands to consume Mercury and boil the Earth’s oceans away.
The Kuiper belt
The Oort cloud cannot easily explain the comets with the shortest periods, the so-called Jupiter-family comets with periods less than 20 Earth years. These comets have relatively small orbits tilted only slightly from the orbital plane of the Earth, and they usually move in the same prograde direction as the planets. Unlike their longer-period cousins, the motions of the Jupiter-family comets resemble those of the planets. The main source of these comets is thought to be a ring of small icy objects at the outer edge of the planetary realm, just beyond the orbit of Neptune and a thousand times closer than the Oort cloud. It is known as the Kuiper belt, named after the Dutch-American astronomer Gerard P. Kuiper (1905-1973) who predicted its existence in 1951. The name Edgeworth-Kuiper belt is used in the United Kingdom, acknowledging Kenneth E. Edgeworth’s (1880-1972) proposal of the belt’s existence in 1943.
The density in this outer region of the primeval planetary disk was so low that the small objects did not coalesce into a single larger planet. They instead formed the flattened Kuiper belt of 100 million to 10 billion, or 108 to 1010, small frozen worlds that have remained there for billions of years.
Once a comet is launched into the planetary realm, from either the Kuiper belt or the Oort cloud, it may not stay on the same trajectory. Its orbit can be transformed if it passes near Jupiter, the most massive of planets. The giant planet’s gravity can perturb the comet into a new elliptical orbit around the Sun.
All of the comets in the Oort cloud, and most of those in the Kuiper belt, are invisible. They are the nuclei of comets that are seen only when they come near the Sun. Each nucleus is the solid, enduring part of a comet. It is just a gigantic ball of frozen water ice and other ices laced with darker dust and pieces of rock. Light from the distant Sun is much too feeble to warm the comet ices, which remain frozen solid at the low temperatures in the remote comet reservoirs. When a comet nucleus emerges from the deep freeze of outer space and moves toward the Sun, the increased solar heat causes the comet’s surface material to sublimate, with gases escaping through fissures in the crust of the nucleus. At the low pressure conditions of space, the solid ice goes directly into gas without passing through a liquid state, in a process called sublimation, just as dry ice does on Earth and water ice in the right terrestrial circumstances. The escaping gases also carry along dust particles. The gas and dust make the comet grow in size, enabling it to be seen.
If a comet nucleus provides such huge quantities of gas and dust, and still survives for hundreds or thousands of trips near the Sun, then it ought to be mainly composed of water ice. The fact that the outer layers of periodic comets start releasing material near 3 AU from the Sun suggests that water ice dominates their nucleus, since the temperature of the Sun’s radiation at 3 AU is approximately that required to vaporize water ice. Ices of other possible molecules begin to sublimate off the nucleus at much lower temperatures and greater distances from the Sun. The vaporization of these more volatile substances, such as carbon dioxide, methane, and ammonia, may initiate the production of gas and dust in the new comets.
No two comets ever look identical, just as no two snowflakes are alike, but most comets have basic features in common. When they emerge from the deep freeze of outer space and move toward the Sun, the increased solar heat eventually causes their ices to sublimate and blow dust away with the escaping gas. The comet then becomes visible as an enormous moving patch of light. This glowing, misty ball of light is called the coma, the Latin word for “hair”. One or more tails can eventually stream from the coma, in a direction away from the Sun.
Comet gas and dust are initially ejected primarily in the general direction of the Sun; solar forces push them into tails that flow away from the Sun. As a result, a comet travels headfirst when approaching the Sun and tail first when moving away from it.
At the heart of a comet’s coma lies a nucleus of solid material, no more than 10 thousand meters across. The nucleus can be directly imaged from spacecraft that pass near it. This has been accomplished two times so far, measuring oblong shapes of about this size.
With a mass density somewhere between that of water ice and rock, or between 1,000 and 3,000 kilograms per cubic meter, the comet nucleus would have a mass of just 1015 kilograms or less. They are less than one billionth the mass of the Earth, which weighs in at 5.97 x 1024 kilograms. The visible coma, or head, is a spherical cloud of gas and dust that has emerged from the nucleus, which it surrounds like an extended atmosphere. The coma sometimes reaches a billion meters in size, which is about as large as the Sun, and they are usually become bigger than the Earth.
A vast hydrogen cloud, containing hydrogen atoms that emit ultraviolet radiation, envelops the coma and nucleus. Observations of this glow – invisible to the eye – indicate that the hydrogen halo can be ten billion, or 1010, meters across, or about ten times bigger than the Sun. The atomic hydrogen is produced when water molecules, released from the comet nucleus, are torn apart by energetic sunlight. The relatively light hydrogen atoms travel at high speed to great distances before they are also ionized by the Sun’s energetic light and swept away by its winds.
Some comets show two types of tails at the same time. There are the long, straight blue ion tails and the shorter, curved yellow dust tails. The gases liberated by a comet nucleus become ionized by the action of solar ultraviolet radiation and emit a faint blue light by fluorescence. The dust tail shines only by reflecting yellow sunlight. Since the individual dust particles enter slightly different orbits of their own, the dust tail often spreads out into a fan shape. An individual comet may have a dust tail, an ion tail, both types of tail, and no tail at all.
But what are the solar forces that blow the gas and dust into comet tails? The gentle pressure of sunlight pushes the tiny, solid dust grains along curved paths as the comet moves through space. When the Sun’s light bounces off the dust particles, it gives them a little outward push, called radiation pressure, and this forces them into the dust tails. For larger solid particles, comparable in size to sand or pebbles, the Sun’s gravitational pull overcomes the radiation pressure, and so these particles stay near the orbital path of the comet and they do not enter the dust tails.
A solar wind of electrically charged particles and magnetic fields propels and constrains the ions on straight paths away from the Sun. The solar wind, which continuously flows away from the Sun’s surface, also accelerates the ions to high velocities. Thus, the ion tail acts like a windsock and, in fact, the existence of the solar wind was hypothesized from observations of comets before the age of space exploration. Spacecraft have now confirmed these predictions, and have permitted measurements of the electrons and protons that are blown away from the Sun, carrying the solar magnetic fields with them.
The gas lost from a comet is ionized by ultraviolet sunlight, producing an ionosphere that envelops the comet nucleus. Magnetic fields carried by the solar wind are unable to penetrate the ionosphere, so they pile up in front of it and drape around it to form nearly parallel, adjacent magnetic field lines that point toward and away from the Sun. Guided and constrained by these folded magnetic field lines, the comet ions are pushed away from the Sun by the much faster solar wind particles, forming a straight, blue ion tail.
But the interplanetary magnetism extending from the Sun is divided into sectors that point in opposite directions, toward and away from the star. When a comet crosses from one sector to another, the magnetism that envelops its ion tail becomes pinched and the comet loses the tail, somewhat like a tadpole. But unlike a tadpole, the comet soon grows another ion tail.
To sum up, a comet’s anatomy consists of a concealed nucleus, an Earth-sized or Sun-sized coma, a vast hydrogen cloud, and two types of tails, the dust and ion tails. But a comet’s anatomy is not a static thing, for comets are always changing shape. All of the comet tails grow when the comet approaches the Sun, and shrink when the comet moves away from the Sun. There is no such thing as a typical comet tail. They differ in shape, size and structure. Some comets have multiple tails, some have only one tail, and others have no tail.
An international flotilla of six spacecraft, belonging to four space agencies, flew by Comet Halley in March 1986, to examine the gas and dust in the vicinity of the comet and to photograph its nucleus.
The European spacecraft Giotto obtained the best images, with the highest resolution. The surface of the nucleus was charcoal black, reflecting only about 4 percent of the sunlight that falls on it. Bright jets were spewing gas and dust from the comet’s sunlit side, but there wasn’t much white ice in site.
The images obtained with the camera on Giotto showed that the nucleus of Comet Halley has an elongated, irregular shape with dimensions of 16 x 8.5 x 8.2 thousand meters, about the size of Paris or Manhattan. For a mass density about that of water, this volume corresponds to a mass of about 1015 kilograms or 1000 billion tons. A varied, lumpy topography was seen, with craters, valleys, hills, and mountains.
We now know that Comet Borrelly also has a black heart, with a nucleus that is just as dark and unreflective as that of Comet Halley. The Deep Space 1 spacecraft was directed toward this comet after completing its primary mission of flight-testing an ion engine and 11 other advanced technologies. On 22 September 2001, the spacecraft whizzed by Comet Borrelly at a distance of just 2.2 million meters, revealing an irregular chunk of rock and ice, about 8 thousand meters long and perhaps 4 thousand meters wide. It is covered with a dark, carbon-rich slag that reflects only about 4 percent of the incident sunlight, on average; comparable to the reflectivity of the powdered toner used in laser printers. The surface of this nucleus also has a rugged terrain, with mountains, valleys, deep fractures and smooth rolling terrain where jets of gas and dust have apparently polished the surface. Some of the jets vastly exceed the nucleus in length, resulting in an asymmetric coma that was offset from the center of the nucleus by up to 2 million meters.
A comet’s nucleus also rotates. Typical rotation periods are a few hours to a few days. Observations of Comet Halley, for example, indicate that it rotates around its longest axis once every 7.4 days, and that it wobbles about its shortest axis once every 2.2 days or 53 hours. As the nucleus rotates, new regions turn to face the Sun, heat up and become active, while others face away from the Sun and momentarily turn off their activity.
The gas and dust streaming off the sunlit side of a comet nucleus initially heads toward the Sun, before being swept back into the comet tails. The expelled material pushes the comet in the opposite direction, making it arrive sooner or later than expected. A similar recoil effect explains the darting action of a small balloon when it is released, as well as the forward thrust of a rocket engine. The thrust of the rocket-like, comet jets either pushes the comet along in its orbit or slows it down.
Once a comet enters the inner solar system, it returns again and again on a relentless voyage of continual decay and disintegration. Most comets are consumed by their own emissions, blowing themselves away as they sublimate the ice that holds them together. Sooner or later most short-period comets will either fall apart or turn into a dark rocky corpse that looks like an asteroid. It’s just part of the aging process. The only way to avoid this fate is to be thrown out of the solar system through close passage to Jupiter, or to collide with one of the planets or the Sun. Comets are very fragile, with little internal strength and a very low mass density. Some comet nuclei are crumbly, fluffy structures with mass densities less than that of solid ice and far less than solid rock. The central pressure of their nucleus is probably comparable to that under a thick layer of blankets. So it is little wonder that some comets have been observed to break up as the result of tidal forces from either the Sun or Jupiter. They pull on the near side of the comet a little more than the far side, tearing the comet apart. If you could get a piece of a comet in your bare hands, it would most likely fall apart it.
As an example, shortly after its closest approach to the Sun, the Great September Comet (1882 II) divided into four or more pieces stretched along nearly the same orbit like a string of pearls. The nucleus of Comet West also split into pieces when it passed near the Sun in 1976. The pieces of the split nucleus have too little mass to pull themselves together gravitationally. So once a nucleus splits, its pieces remain forever separated. The jets of escaping gas kick them away from each other, and they continue to drift farther apart.
Earth’s cosmic dusting
Cosmic dust is everywhere. It is in the air we breathe, the food that we eat, and the water we drink. Some of the dust that has been spawned by comets and asteroids even enters our own hair. The smallest dust particles are so tiny that the air slows them down rather than burning them up. Hundreds of them have been collected from the stratosphere and examined in the terrestrial laboratory, often with the fragile, porous structure expected of comet debris.
The cosmic dust particles that we examine today are time capsules that may date back to the origin of the solar system. They have probably not been significantly altered from the moment of their creation. The delicate primordial comet dust is therefore thought to preserve a record of chemical conditions at the time of planet formation, and it may even contain the ashes of stars that existed before the Sun was born.
Nights of the shooting stars
Although meteor showers are commonly called shooting stars, they are not stars, but fragile material from comets. In addition to spewing off small dust particles, which can drift down to the ground, comets also expel larger particles ranging in size from sand grains to pebbles. This debris burns up when it enters our atmosphere, producing visible meteors.
When just one of the comet particles rubs against the air, it vaporizes in a streak of light, producing the luminous trail of a meteor. And when many fall into the dark night sky, they produce meteor showers.
From the luminous path of a meteor, it is possible to determine the incoming particle’s orbital path around the Sun, and in most cases the orbits are similar to those of comets. A comet ejects the particles along its orbital path as it loops around the Sun, and this material continues to revolve around our star, something like the ice particles that circle Saturn in its ring. The swarm of comet material is called a meteoroid stream. And when the Earth passes though one of these streams, it intercepts some of the orbiting particles that enter our atmosphere and create a meteor shower.
When a meteor shower includes large numbers of shooting stars, the trails appear to intersect and emanate from a distant point called the radiant. But meteors that appear to diverge from a point are actually moving on parallel paths, just as parallel railroad tracks seem to come from a point on the distant horizon. Meteor showers are named after the constellation in which their radiant appears.
Astronomers have separated the small objects in the solar system into two main categories based on their telescopic appearance. Known as the comets and asteroids, they differ in composition, orbits, and beginnings. Most comets are dirty balls of ice with very elongated orbits that take them out to the distant reaches of the solar system, moving at every possible angle to the Earth’s orbital planet. The asteroids are lumps of rock confined mostly to nearly circular orbits relatively close to the Sun, in the main belt between Mars and Jupiter, moving in the same plane and direction as the planets. Comets sublimate surface ice when they travel close to the Sun, emitting gas and dust, but asteroids do not emit anything wherever they are.
Recent discoveries have blurred the boundaries between these two classes of small objects. There are comets that behave like asteroids, and asteroids that act like comets. A handful of bodies have a dual personality, behaving like both a comet and an asteroid. So there is some overlap between the two categories, and our strict definitions have to be relaxed to take the anomalies into account.