Page 28911.4 Comets

A bright comet is a stunning sight, as illustrated by the chapter­opening figure. Sadly, such sights arenow rare because light pollution from our cities drowns out the view for most people. Comets have longbeen held in fear and reverence, and their sudden appearance and equally sudden disappearance after afew days—or, in some cases, weeks—have added to their mystery. Today we recognize them as visitorsfrom the most remote parts of the Solar System as well as being visitors from the remote past.

The Appearance and Structure of Comets

Comets consist of three main parts—the tail, the coma, and the nucleus (fig. 11.12). The largest part isthe long tail, a narrow column of dust and gas that may stretch across the inner Solar System for as muchas 100 million kilometers (nearly an AU!).

Figure 11.12Artist's depiction of the structure of a comet, showing the tiny nucleus, surrounding coma, and long tail.

The tail emerges from a cloud of gas called the coma, which may be some 100,000 kilometers indiameter (10 times or so the size of the Earth). However, despite the great volume of the coma and thetail, these parts of the comet contain very little mass. The gas and dust are extremely tenuous, and so acubic centimeter of the gas contains only a few thousand atoms and molecules. By terrestrial standards,this would be considered a superb vacuum. This extremely rarefied gas is matter that the Sun's heat hasboiled off the heart of the comet, its nucleus.

The comet nucleus is a block of ice and gases that have frozen in the extreme cold of the outer SolarSystem into an irregular mass whose diameter is typically about 10 kilometers. The nucleus of a comethas been described as a giant “iceberg” or “dirty snowball,” and it contains most of the comet's mass.

There is disagreement about how to pronounce Halley. Most astronomers say it as if it rhymes with Sally,but there is evidence that Halley himself pronounced his name as haw­lee

Most comets appear unexpectedly, often discovered by amateur astronomers with nothing but binoculars.They are too small to be seen in the outer Solar System, where they spend most of their time. They are

named for their discoverer, though one of the most famous, Comet Halley, got its name because SirEdmund Halley was the first to propose that it orbited the Sun like a planet. He predicted that the greatcomet of 1682 would reappear in 1759. It did, but Halley did not live to see his prediction verified.

Comet Halley also provided astronomers with their first close look at a comet nucleus when the Giottospacecraft approached to within 600 km of it. Launched by the European Space Agency as part of aninternational study of Comet Halley during its most recent return in 1986 (fig. 11.13A), Giotto wasnamed after an Italian artist who painted a Christmas scene with a comet in 1301, which may have beencomet Halley.

Figure 11.13Comet Halley in March 1986: (A) photographed from Earth: (B) its nucleus photographed from theGiotto spacecraft.Page 290

The Giotto spacecraft revealed the size and appearance of a comet nucleus for the first time. Despite itsicy composition, the nucleus of comet Halley is dark, as you can see in Figure 11.13B, which is one ofthe pictures made by the Giotto spacecraft. Astronomers think that the dark color comes from dust andcarbon­rich material (similar to that of the carbonaceous chondritic meteorites) coating the surface of thenucleus. Other visible features of the nucleus are its irregular shape and the jets of gas erupting from thefrozen surface. The jets form when sunlight heats and vaporizes the icy material. The irregular shape isprobably the outcome of uneven melting of the nucleus during passage by the Sun on previous orbits.

Formation of the Comet's Tails

Most comets are visible only when they are less than about 5 AU from the Sun, where solar heating isable to vaporize the ices into gases that escape to make the coma. The comet then appears through atelescope as a dim, fuzzy ball. For most of their orbits we cannot see them, but we can extrapolate therest of their orbits (see Astronomy by the Numbers: “Calculating Comet Halley's Orbit” on p. 292). Asthey get closer to the Sun, the tail forms. Actually, comets have not one but two tails (fig. 11.14), madeof different materials that flow off the nucleus.

Figure 11.14Comet Hale­Bopp in 1997, illustrating the two tails, one of dust, one of ions. The ion tail is affectedalmost exclusively by the solar wind, so it points nearly directly away from the Sun. By contrast, dustparticles feel the effect of the Sun's gravity as well as its radiation pressure. Responding to the gravity,the dust particles follow orbits around the Sun, but particles farther from the Sun orbit more slowly, sothe dust tail arcs behind the comet.

The gas evaporating off the nucleus of a comet is rapidly ionized by ultraviolet light from the Sun. It isthen caught up in a flow of ions that streams from the Sun into space, a flow called the solar wind. Thesolar wind blows away from the Sun at about 400 kilometers per second. It is very tenuous, containingonly a few atoms per cubic centimeter. But the material in the comet's coma is tenuous too, and the solarwind is dense enough to blow it into a long plume. This is known as the ion tail. The ion tail usually hasa bluish color and points directly away from the Sun. Magnetic fields in the solar wind enhance its effecton the comet's ion tail, helping to drag matter out of the coma and channel its flow, just as magneticfields in the Earth's atmosphere channel particles to form the aurora.

The second tail is made of dust. Sunlight striking dust grains imparts a tiny force to them, a processknown as radiation pressure. We don't feel radiation pressure when sunlight falls on us because theforce is tiny and the human body is far too massive to be shoved around by solar photons. However, themicroscopic dust grains in the coma do respond to radiation pressure and are pushed away from the Sun,as illustrated in figure 11.15. Smaller grains experience a bigger acceleration, but all the grains move inthe same direction, away from the Sun, forming a dust tail. Particles in the dust tail swing on a wider andtherefore slower orbit than the comet nucleus, so the tail curves slightly behind the comet's direction,although still away from the Sun.

Figure 11.15Sketch of how radiation pressure pushes on dust particles. Photons hit the dust, and their impact drivesthe dust away from the Sun, forming a dust tail. Sizes and distances are not to scale.ANIMATIONPage 291

The orientation of comet tails

Thus, two forces, radiation pressure and the solar wind, act on the cometary material to drive out tails.Because those forces are directed away from the Sun, the comet's tails always point away from the Sun,and the tail even points out ahead of the comet as it moves away from the Sun (fig. 11.16). It might helpyou understand this seemingly odd phenomenon if you think of a runner carrying a torch. If the air isstill, the smoke from the torch will, of course, trail behind the runner. However, if a strong wind isblowing at, say, 40 miles per hour, the smoke will be carried along in the direction of the wind regardlessof which way the runner moves. Likewise, the high velocity of the solar wind (400 kilometers per secondversus about 40 kilometers per second for the comet) carries the tail of a comet outward from the Sunregardless of the comet's motion. Therefore the tails always point away from the Sun.

Figure 11.16Sketch illustrating how radiation pressure and the solar wind make a comet's tail always pointapproximately away from the Sun. Sizes and distances are not to scale. Comets may orbit in any directionaround the Sun.ASTRONOMY by the numbersCALCULATING COMET HALLEY'S ORBIT

Sir Edmund Halley's discovery that comets orbit the Sun just like planets do means that it is possible toapply Kepler's laws to comet orbits. As an example, consider Comet Halley, which Sir Edmundestimated returns every 76 years to the inner Solar System.

First, we can use Kepler's third law to predict the semimajor axis of Comet Halley's orbit. We set P2 =a3, and because P = 76 years, we know that

Taking the cube root of each side, we have

So the semimajor axis of Comet Halley's orbit is 18 AU.

For most of its orbit, Comet Halley is invisible to us. Only when it comes within a few AU of the Sun isit visible. How far out does it go?

Recall that the semimajor axis is half of the long axis of the orbital ellipse (Kepler's first law), so the sumof Comet Halley's smallest distance and largest distance must be 2 × 18 AU = 36 AU. Observations ofthe comet indicate that it gets as close as about 0.5 AU from the Sun, so at its farthest point it must be 36− 0.5 = 35.5 AU from the Sun. Thus, it travels out beyond Neptune's orbit into the Kuiper Belt.

The Orientation of Comet Tails

Page 292Composition of Comets

The escaped gas from the comet offers astronomers a way to probe the comet's composition. The dustparticles reflect sunlight, and the gases emit light of their own by a process called fluorescence.Fluorescence is produced when light at one wavelength is converted to light at another wavelength. Afamiliar example is the so­called black light that you may have seen for illuminating posters. Black lightis really ultraviolet radiation that we have difficulty seeing because of its short wavelength. When suchultraviolet radiation falls on certain paints or dyes, the chemicals in the pigment absorb the ultravioletradiation and convert it into visible light.

A major part of a comet's light is created by fluorescence. A photon of ultraviolet—and thus energetic—radiation from the Sun lifts electrons in the atoms of the comet's gas molecules to an upper, excited levelin a single leap. The electron then returns to its original level in two or more steps, emitting a photoneach time it drops. The combined energy of these photons must equal that of the absorbed ultravioletphoton, to conserve energy. Thus, the energy of each emitted photon must be less than that of the originalultraviolet one. That smaller energy then gives them a longer wavelength, which we can see with oureyes. Thus, fluorescence creates the soft glow of the comet's ion tail. In addition, the spectrum of thefluorescing gas tells us of what the comet is made.

Spectra of gas in the coma and tail show that comets are rich in water, CO2, CO, and small amounts ofother gases that condensed from the primordial solar nebula. Evaporating water is broken up by solarultraviolet radiation to create oxygen and hydrogen gas, so most comets are surrounded by a large cloudof hydrogen.

Astronomers have recently gotten even closer looks at comets. The Stardust probe took pictures ofComet Wild 2 (fig. 11.17A) and collected samples of dust from near the comet. The samples weresuccessfully returned to Earth by parachute in early 2006. The dust particles included small crystals ofsilicate rock as well as a wide range of organic compounds, and some chemicals that require liquid waterin order to form. The EPOXI spacecraft made even more detailed pictures of Comet Hartley 2 (fig.11.17B and C), even observing chunks of ice ejected from the nucleus by jets of evaporating carbondioxide. Spectroscopic studies of the gas flowing off Comet Hartley have also confirmed that its isotopiccomposition closely matches that of Earth's oceans, suggesting that comets may have been a source ofEarth's water.

Figure 11.17(A) The nucleus of Comet Wild 2 (Wild is pronounced “vilt”). A longer exposure image is superimposed,showing jets of gas. (B) The nucleus of Comet Hartley 2. (C) A close­up image of chunks of ice blownoff Hartley 2 by jets of evaporating carbon dioxide.Page 293

During these various fly by missions, the gravitational pull of the comet nucleus has been measured, and

so it is possible to estimate a mass and density. There are some uncertainties in these measurements, butthe comet nuclei appear to typically have a density of about 0.6 grams per cubic centimeter. Thissuggests an icy composition that is not very tightly packed, more like a snowball that a chunk of ice.

A more intrusive comet sampling was made by NASA's Deep Impact mission, which in 2005 smashed a370­kilogram (800­lb) probe into Comet Tempel 1 at a relative speed of just over 10 kilometers persecond (about 23,000 mph). The impact was designed to break through the comet's outer crust and stir upand release dust and gas. The impact event is shown in figure 11.18. The first two images are from thepoint of view of the impact probe, which sent pictures up until a few seconds before it smashed into thesurface. The third image is from the main spacecraft, showing a cloud of very fine dust blasted out by theimpact, which created a crater about 150 meters across, which was imaged several years later by theStardust spacecraft. The spectra of the material blasted out by the impact showed the presence of waterand silicates as well as clays and other water­based crystals.

Figure 11.18Comet Tempel 1 before and after NASA's Deep Impact mission. (A) The comet nucleus minutes beforeimpact. (B) A composite image made from the impactor probe, showing the collision point. (C) Animage made by the main spacecraft about a minute after impact shows a spray of fine particles blastedout by the impact and brightly lit by sunlight. (D) The Stardust probe imaged the impact crater about 6years later.

If a comet passes by the Sun too often, the escape of its gas eventually erodes it away. Also, some cometsliterally fall into the Sun. For example, the SOHO satellite (which observes the Sun's outer atmosphere)has imaged dozens per year falling into the Sun. New comets show up frequently, so there must be asource to replace those devoured by the Sun, and it is to their origin that we now turn.

Page 294Origin of CometsANIMATION

Oort cloud and Kuiper belt

Astronomers think that most comets come from the Oort cloud, a swarm of trillions of icy bodiesthought to lie far beyond the orbit of Neptune, as we discussed in chapter 8. Astronomers think the Oortcloud formed from planetesimals that originally orbited near the giant planets and were tossed into theouter parts of the Solar System by the gravitational force of those planets. There, they form a sphericalshell that completely surrounds the Solar System and extends to perhaps as much as 150,000 AU fromthe Sun, as illustrated in figure 11.19. Astronomers deduce this shape for the Oort cloud from the manycomet orbits that are highly tilted with respect to the main plane of the Solar System. However, as wewill discuss later, some comets also seem to come from a flatter, less remote region—the so­calledKuiper belt, also shown schematically in figure 11.19. The Kuiper belt begins at about the orbit ofNeptune and extends to approximately 50 AU.

Figure 11.19Schematic drawing of the Oort cloud, a swarm of icy comet nuclei orbiting the Sun out to about 100,000AU. Also shown is the Kuiper belt, another source of comet nuclei, with a size exaggerated for clarity.

Oort Cloud and Kuiper Belt

Each comet nucleus moves along its own path, and those in the Oort cloud take millions of years tocomplete an orbit. With orbits so far from the Sun, these icy bodies receive essentially no heat from theSun, and calculations indicate that their temperature is a mere 3 K, or about –454°F. Thus, the gases andices remain deeply frozen.

Such cold and distant objects are invisible to us on Earth; so if we are to see a comet, its orbit mustsomehow be altered to carry it closer to us and the Sun. Astronomers think that such orbital changes mayarise from the chance passage of a star far beyond the outskirts of the Solar System or from tidal forcesexerted on the Oort cloud by the Milky Way. Such gravitational effects disturb the orbits of the cometnuclei in the Oort cloud, altering their paths and making some drop in toward the inner Solar System, asshown in figure 11.20. A single disturbance may shift enough orbits to supply comets to the inner SolarSystem for tens of thousands of years.

Figure 11.20Sketch of how a passing star alters the orbit of a comet nucleus. On its new orbit, the comet will pass bythe Sun and be visible from Earth. Although only one comet's orbit is shown, such encounters typicallywill affect many comets. Note: The distance and size scales are greatly exaggerated. No star presentlygets anywhere near this close to the Oort cloud.Page 295Short­Period Comets and the Kuiper Belt

Although most comets that we see from Earth swing by the Sun on orbits that will bring them back to theinner Solar System only after millions of years, a small number of short­period comets reappear at timeintervals less than 200 years. The origin of short­period comets is still under study. At one time it wasthought that they came from the Oort cloud but as they moved through the region of the Solar Systemcontaining the giant planets, their orbits were shifted by a close encounter with one of the planets intosmaller orbits with periods of centuries rather than millennia. Many astronomers think this is the origin ofComet Halley, which has a period of 76 years.

However, astronomers now think that the majority of short­period comets come from the icy nucleiorbiting beyond Neptune in the Kuiper belt. Support for this origin comes from the detection of hundredsof small, presumably icy, bodies orbiting near and somewhat beyond Pluto. Astronomers estimate thatthe Kuiper belt contains well over 30,000 icy objects bigger than 100 kilometers in diameter, and its totalmass may be hundreds of times larger than that of the asteroid belt between Mars and Jupiter. Thesefrozen objects are probably survivors of the Solar System's birth—icy planetesimals still orbiting in thedisk—but they are too far apart to form additional planets.

The outer radius of the Kuiper belt is uncertain. There appear to be few comets or larger bodies such asplutoids orbiting beyond 50 AU, but even the largest of these bodies is extremely difficult to detect.Much still remains to be learned about this remote part of our Solar System.

Fate of Short­Period Comets

A short­period comet's repeated orbits past the Sun gradually whittle it away: all the ices and gasesevaporate, and only the small amount of solid matter, dust and grit, remains. This fate is like that of asnowball made from snow scooped up alongside the road, where small amounts of gravel have been

packed into it. If such a snowball is brought inside, it melts and evaporates, leaving behind only the gritaccidentally incorporated in it. So too, the evaporated comet leaves behind in its orbit grit that continuesto circle the Sun.

A comet is a bit like the “Peanuts” cartoon character Pig­pen, who trails dirt wherever he goes.

As a comet orbits the Sun and its icy and gaseous material evaporates, it leaves in its path a trail of dustand small bits of solid material ejected from its nucleus. When we cross through or closely approach sucha trail, our planet is blasted by this microscopic debris that rains into our atmosphere, burning up, andcreating a meteor shower.

Page 296Meteor Showers

If you go outside on a clear night and have an unobscured view of the sky, you will see, on average, onemeteor every 15 or so minutes. Most of these meteors are stray fragments of asteroids that arrive at theEarth randomly. At some times of year, however, you may observe one every few minutes. Furthermore,if you watch such meteors carefully, you will see that they appear to come from the same generaldirection in the sky. Meteors of this type are part of what is called a meteor shower, which is made ofthe debris from a comet.

Meteor Shower

ANIMATION

A meteor shower

One of the best­known meteor showers occurs each year in mid­August. Peaking in intensity aroundAugust 13, meteors shoot through our atmosphere from a direction that lies toward the constellationPerseus. The meteors themselves have no association with Perseus. Rather, they are following an orbitaround the Sun that happens to move toward us from roughly that direction (fig. 11.21A), and the Earthhappens to cross their orbits in mid­August. Thus, at that time we encounter far more meteoroids thanusual.

Figure 11.21Sketch showing how (A) in mid­August, at the time of the Perseid meteor shower, the Earth is movingalong its orbit. When the Earth crosses the debris strewn along a comet's orbit, the scattered materialplunges into our atmosphere, producing (B) the diverging pattern of meteors characteristic of a meteor

shower. (Bodies and orbits are not to scale.)

This encounter creates an effect similar to what you observe when you drive at night through fallingsnow: the flakes seem to radiate from a point in front of you, the location of which depends on acombination of the direction and speed of both the wind and your car. Thus, during the time that theEarth crosses the path followed by the meteoroids, they seem to diverge from a common point (fig.11.21B), called the radiant. Meteor showers are generally named for the constellation from which theyappear to diverge, and appendix table 6, lists several of the brighter and more impressive showers andtheir dates. Each shower therefore marks when the Earth crosses the path of a comet.

On rare occasions, the Earth will pass through a particularly dense clump of material left by the comet. Ifthat happens, thousands of meteors per hour may spangle the sky. Such a display happened in November1966, when dawn observers on the West Coast of the United States and Canada saw literally dozens ofmeteors per second! Likewise, in November 2001 and 2002, observers were treated to a similar cosmicfireworks display as shown in the chapter's opening “What Is This?” figure.

Spectacles of this kind are one of the delights of astronomy. However, on even rarer occasions, far moresinister meteoritic events may occur.