Comets are usually discovered as faint, fuzzy patches of light on the sky while still several astronomical units away from the Sun. Traveling in a highly elliptical orbit with the Sun at one focus, a comet brightens and develops an extended tail as it nears the Sun. (The name "comet" derives from the Greek word kome, meaning "hair.") As the comet departs from the Sun's vicinity, its brightness and itstail diminish until it once again becomes a faint point of light receding into the distance. Like the planets, comets emit no visible light of their own—they shine by reflected (or reemitted) sunlight.

COMET APPEARANCE AND STRUCTURE

The various parts of a typical comet are shown in Figure 14.7. Even through a large telescope, the nucleus, or main solid body, of a comet is no more than a minute point of light. A typical cometary nucleus is extremely small—only a few kilometers in diameter. During most of the comet's orbit, far from the Sun, only this frozen nucleus exists. When a comet comes within a few astronomical units of the Sun, however, its icy surface becomes too warm to remain stable. Part of it becomes gaseous and expands into space, forming a diffuse coma ("halo") of dust and evaporated gas around the nucleus. The coma becomes larger and brighter as the comet nears the Sun. At maximum size, the coma can measure 100,000 km in diameter—almost as large as Saturn or Jupiter.

Figure 14.7 (a) Diagram of a typical comet, showing the nucleus, coma, hydrogen envelope, and tail. The tail is not a sudden streak in time across the sky, as in the case of meteors or fireworks. Instead, it travels through space along with the rest of the comet (so long as the comet is sufficiently close to the Sun for the tail to exist). (b) Halley's comet in 1986, about 1 month before perihelion.

Engulfing the coma, an invisible hydrogen envelope, usually distorted by the solar wind, stretches across millions of kilometers of space. The comet's tail, however, most pronounced when the comet is closest to the Sun and the rate of sublimation of material from the nucleus is greatest, is much larger still, sometimes spanning as much as 1 A.U.* From Earth, only the coma and tail of a comet are visible to the naked eye. Despite the size of the tail, however, most of the light comes from the coma; most of the comet's mass resides in the nucleus.

*(Sublimation is the process by which a solid changes directly into a gas without passing through the liquid phase. Frozen carbon dioxide—dry ice—is an example of a solid that undergoes sublimation rather than melting and subsequent evaporation. In space, sublimation is the rule, rather than the exception, for the behavior of ice when exposed to heat.)

Two types of comet tails may be distinguished. The ion tails are approximately straight, often made up of glowing, linear streamers like those seen in Figure 14.8(a). Their spectra show emission lines of numerous ionized molecules—molecules that have lost some of their normal complement of electrons—including carbon monoxide, nitrogen, and water among many others. (Sec. 4.2) The dust tails are usually broad, diffuse, and gently curved (Figure 14.8b). They are rich in microscopic dust particles that reflect sunlight, making the tail visible from afar.

Figure 14.8 (a) A comet with a primarily ion tail. Called comet Giacobini—Zinner and seen here in 1959, its coma measured 70,000 km across; its tail was well over 500,000 km long. (b) Photograph of a comet having (mostly) a dust tail, showing both its gentle curvature and inherent fuzziness. This is comet West, in 1976, whose tail stretched 13° across the sky.

The tails are in all cases directed away from the Sun by the solar wind (the invisible stream of matter and radiation escaping the Sun). Consequently, as depicted in Figure 14.9, the tail always lies outside the comet's orbit and actually leads the comet during the portion of the orbit that is outbound from the Sun.

Figure 14.9 Diagram of part of the orbit of a typical comet. As the comet approaches the Sun it develops an ion tail, which is always directed away from the Sun. Closer in, a curved dust tail, also directed generally away from the Sun, may also appear. Notice that although the ion tail always points directly away from the Sun on both the inbound and the outgoing portions of the orbit, the dust tail has a marked asymmetry, always tending to "lag behind" the ion tail.

The ion tails and dust tails differ in shape because of the different responses of gas and dust to the forces acting in interplanetary space. Every tiny particle in space in our solar system—including those in comet tails—follows an orbit determined by gravity and the solar wind. If gravity alone were acting, the particle would follow the same curved path as its parent comet, in accordance with Newton's laws of motion. (Sec. 2.7) If the solar wind were the only influence, the tail would be swept up by it and would trail radially outward from the Sun. The ion tails are much more strongly influenced by the solar wind than by the Sun's gravity, so those tails always point directly away from the Sun. The heavier dust particles have more of a tendency to follow the comet's orbit, giving rise to the slightly curved dust tails.

COMET ORBITS

Comets that survive a close encounter with the Sun—some break up entirely—continue their outward journey to the edge of the solar system. Their highly elliptical orbits take many comets far beyond Pluto, perhaps even as far as 50,000 A.U., where, in accord with Kepler's second law, they move more slowly and so spend most of their time. (Sec. 2.4) The majority of comets take hundreds of thousands, some even millions, of years to complete a single orbit around the Sun. However, a few short-period comets (conventionally defined as those having orbital periods of less than 200 years) return for another encounter within a relatively short time. According to Kepler's third law, the short-period comets do not venture far beyond the distance of Pluto at aphelion.

Unlike the orbits of the other solar system objects we have studied so far, the orbits of comets are not necessarily confined to within a few degrees of the ecliptic plane. Short-period comets do tend to have prograde orbits lying close near the ecliptic, but long-period comets exhibit all inclinations and all orientations, both prograde and retrograde, roughly uniformly distributed in all directions from the Sun.

Astronomers believe that the short-period comets originate beyond the orbit of Neptune, in a region of the solar system called the Kuiper belt (after Gerard Kuiper, a pioneer in infrared and planetary astronomy). A little like the asteroids in the inner solar system, most Kuiper belt comets move in roughly circular orbits between about 30 and 100 A.U. from the Sun, never venturing inside the orbits of the jovian planets. Occasionally, however, a close encounter between two comets, or (more likely) the cumulative gravitational influence of one of the outer planets, "kicks" a Kuiper belt comet into an eccentric orbit that brings it into the inner solar system, and into our view. The observed orbits of these comets reflect the flattened structure of the Kuiper belt.

What of the long-period comets? How do we account for their apparently random orbital orientations? Only a tiny portion of a typical long-period cometary orbit lies within the inner solar system, so it follows that for every comet we see, there must be many more similar objects at great distances from the Sun. On these general grounds, many astronomers reason that there must be a huge "cloud" of comets far beyond the orbit of Pluto, completely surrounding the Sun. This region, which may contain trillions of comets, of total mass comparable to the mass of the inner planets, is named the Oort cloud, after the Dutch astronomer Jan Oort, who first wrote (in the 1950s) of the possibility of such a vast and distant reservoir of inactive, frozen comets. The Kuiper belt and the orbits of some typical Oort cloud comets are sketched in Figure 14.10.

Figure 14.10 (a) Diagram of the Oort cloud, showing a few cometary orbits. Most Oort cloud comets never come close to the Sun. Of all the orbits shown, only the most elongated ellipse represents a comet that will actually enter the solar system (which is smaller than the dot at the center of the figure on this scale) and possibly become visible from Earth. (b) The Kuiper belt, believed to be the source of the short-period comets.

The observed orbital properties of long-period comets, have led researchers to believe that the Oort cloud may be up to 100,000 A.U. in diameter. Like those of the Kuiper belt, however, most of the comets of the Oort cloud never come anywhere near the Sun. Indeed, Oort cloud comets rarely approach even the orbit of Pluto, let alone that of Earth. Only when the gravitational field of a passing star happens to deflect a comet into an extremely eccentric orbit that passes through the inner solar system do we actually get to see one of these objects. Because the Oort cloud surrounds the Sun in all directions, instead of being confined near the ecliptic plane like the Kuiper belt, the long-period comets we see can come from any direction in the sky. Despite their great distances and long orbital periods, however, the Oort cloud comets are still gravitationally bound to the Sun. Their orbits are governed by precisely the same laws of motion that control the planets.

A VISIT TO HALLEY'S COMET

Probably the most famous comet of all is Halley's comet. (Two more recent and widely publicized contenders for that title are described in Interlude 14-2.) In 1705 the British astronomer Edmund Halley realized that the 1682 appearance of this comet was not a one-time event. Basing his work on previous sightings of the comet, Halley calculated its path and found that the comet orbited the Sun with a period of 76 years. He predicted its reappearance in 1758. Halley's successful determination of the comet's trajectory and his prediction of its return was an early triumph of Newton's laws of motion and gravity. Although Halley did not live to see his calculations proved correct, the comet was named in his honor.

Once astronomers knew the comet's period, they traced its appearances back in time. Historical records from many ancient cultures show that Halley's comet has been observed at every passage since 240 b.c. A spectacular show, the tail of Halley's comet can reach almost a full astronomical unit in length, stretching many tens of degrees across the sky. Figure 14.11(a) shows Halley's comet as seen from Earth in 1910. Its most recent appearance, in 1986 (Figure 14.11b and also Figure 14.7b), was not ideal for terrestrial viewing, but the comet was closely scrutinized by spacecraft. The comet's orbit is shown in Figure 14.12; its next scheduled visit to the inner solar system is in 2061.

Figure 14.11 (a) Halley's comet as it appeared in 1910. Top, on May 10, with a 30° tail, bottom, on May 12, with a 40° tail. (b) Halley, upon return and photographed with higher resolution, on March 14, 1986.

Figure 14.12 Halley's comet has a smaller orbital path and a shorter period than most comets, but its orbital orientation is not typical of a short-period comet. Sometime in the past the comet must have encountered a jovian planet (probably Jupiter itself), which threw it into a tighter orbit that extends not to the Oort cloud but merely a little beyond Neptune. Halley applied Newton's law of gravity to predict this comet's return.

When Halley's comet rounded the Sun in 1986, a small armada of spacecraft launched by the USSR, Japan, and a group of western European countries went to meet it. One of the Soviet craft, Vega 2, traveled through the comet's coma, coming to within some 8000 km of the nucleus. Using positional knowledge of the comet gained from the Soviet craft encounter, the European Giotto spacecraft (named after the Italian artist who painted an image of Halley's comet not long after its appearance in the year a.d. 1301) was navigated to within 600 km of the nucleus. This was a daring trajectory, since at 70 km/s—the speed of the craft relative to the comet—a colliding dust particle becomes a devastating bullet. Debris did in fact damage Giotto's camera, but not before it sent home a wealth of data. Figure 14.13 shows Giotto's view of the comet's nucleus, along with a sketch of its structure.

Figure 14.13 (a) The Giotto spacecraft resolved Halley's comet, showing its nucleus to be very dark, although heavy dust in the area obscured any surface features. Resolution here is about 50 m—half the size of a football field. At the time this image was made, in March 1986, the comet was within days of perihelion, and the Sun was toward the top. The brightest parts of the images are jets of evaporated gas and dust spewing from the comet's nucleus. (b) A diagram of Halley's nucleus, showing its size, shape, jets, and other physical and chemical properties.

The results of the Halley encounters were somewhat surprising. Halley's nucleus is an irregular, potato-shaped object, larger than astronomers had estimated. Spacecraft measurements showed it to be 15 km long by as much as 10 km wide. Also, the nucleus appeared almost jet black—as dark as finely ground charcoal. This solid nucleus was enveloped by a cloud of dust, which scattered light throughout the coma. Partly because of this scattering and partly because of dimming by the dust, no spacecraft was able to discern much surface detail on the nucleus.

The visiting spacecraft found direct evidence for several jets of matter streaming from the nucleus. Instead of evaporating uniformly from the whole surface to form the comet's coma and tail, gas and dust apparently vent from small areas on the sunlit side of Halley's nucleus. The force of these jets may be largely responsible for the comet's 53-hour rotation period. Like maneuvering rockets on a spacecraft, such jets can cause a comet to change its rotation rate and even to veer away from a perfectly elliptical orbit. Astronomers had hypothesized the existence of these nongravitational forces on the basis of slight deviations from Kepler's laws observed in some cometary trajectories. However, only during the Halley encounter did astronomers actually see these jets at work.

PHYSICAL PROPERTIES OF COMETS

The mass of a comet can sometimes be estimated by watching how it interacts with other solar system objects or by determining the size of the nucleus and assuming a density characteristic of icy composition. These methods yield typical cometary masses ranging from 10¹² to 10¹⁶ kg, comparable to the masses of small asteroids. A comet's mass decreases with time because some material is lost each time the comet rounds the Sun. For comets that travel within an astronomical unit of the Sun, this evaporation rate can reach as high as 10³⁰ molecules per second—about 30 tons of cometary material lost for every second the comet spends near the Sun (within Earth's orbit, say). Astronomers have estimated that this loss of material will destroy Halley's comet in about 5000 orbits, or 40,000 years.

In seeking the physical makeup of a cometary body itself, astronomers are guided by the observation that comets have dust that reflects light, as well as gas that emits spectral lines of hydrogen, nitrogen, carbon, and oxygen. Even as the atoms, molecules, and dust particles boil off, creating the coma and tail, the nucleus itself remains a cold mixture of gas and dust, hardly more than a ball of loosely packed ice with a density of about 100 kg/m³ and a temperature of only a few tens of kelvins. Experts now consider cometary nuclei to be largely made of dust particles trapped within a mixture of methane, ammonia, and ordinary water ice. (These constituents should be fairly familiar to you as the main components of most of the small moons in the outer solar system, discussed in Chapters 12 and 13.) Because of this composition, comets are often described as "dirty snowballs."

A COMETARY IMPACT

In July 1994, skywatchers were treated to an exceedingly rare event that greatly increased our knowledge of comet composition and structure—the collision of a comet (called Shoemaker—Levy 9, after its discoverers) with the planet Jupiter! When it was discovered in March 1993, Shoemaker—Levy 9 appeared to have a curious, "squashed" appearance. Higher-resolution images (see Figure 14.14a) revealed that the comet's flattened nucleus was really made up of several pieces, the largest no more than 1 km across. All the pieces were following the same orbit, but they were spread out along the comet's path like a string of pearls a million kilometers long.