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Chapter 8 Pluto, Comets, and Space Debris Introduction We have learned about the Solar System’s giant planets, which range in size from about 4 to about 11 times the diameter of the Earth. We have seen that our Solar System has a set of terrestrial planets, which range in size from the Earth down to 40 per cent the diameter of the Earth. This size range includes the four inner planets as well as seven planetary satellites. Introduction The remaining object that has long had the name “planet,” Pluto, is only 20 per cent the diameter of Earth but is still over 2300 km across, so there is much room on it for interesting surface features. Recently, additional objects like it, but smaller, have been found in the outer reaches of the Solar System. We shall see how we determined Pluto’s odd properties, and what the other, similar objects are. Introduction Besides the planets and their moons, many other objects are in the family of the Sun. The most spectacular, as seen from Earth, are comets (see figure). Bright comets have been noted throughout history, instilling great awe of the heavens. Comets have long been seen as omens, usually bad ones. As Shakespeare wrote in Julius Caesar, “When beggars die, there are no comets seen; The heavens themselves blaze forth the death of princes.” Introduction Asteroids, which are minor planets, and chunks of rock known as meteoroids, are other residents of our Solar System. We shall see how they and the comets are storehouses of information about the Solar System’s origin. Asteroids, meteoroids, and comets are suddenly in the news as astronomers are finding out that some come relatively close to the Earth. We are realizing more and more that collisions of these objects with the Earth can be devastating for life on Earth. Every few hundred thousand years, one large enough to do very serious damage should hit, and every few tens of millions of years, an enormous collision can produce a mass extinction of life on Earth. Apparently, a comet or an asteroid caused the dinosaurs to become extinct some 65 million years ago. Introduction Should we be worrying about asteroid, meteoroid, or comet collisions? Should we be monitoring the sky around us better? Should we be planning ways of diverting an oncoming object if we were to find one? 8.1 Pluto Pluto, the outermost known planet, is a deviant. Its elliptical orbit is the most out of round (eccentric) and is inclined by the greatest angle with respect to the Earth’s orbital plane (the “ecliptic” plane, defined in Chapter 4), near which the other planets revolve. Pluto was closest to the Sun in 1989 and moved farther away from the Sun than Neptune in 1999. So Pluto is still relatively near its closest approach to the Sun out of its 248-year period, and it appears about as bright as it ever does to viewers on Earth. Pluto’s elliptical orbit is so eccentric that part lies inside the orbit of Neptune. It hasn’t been as bright for over 200 years. It is barely visible through a medium-sized telescope under dark-sky conditions. 8.1 Pluto The discovery of Pluto was the result of a long search for an additional planet that, together with Neptune, was believed to be slightly distorting the orbit of Uranus. Finally, in 1930, Clyde Tombaugh, hired at age 23 to search for a new planet because of his experience as an amateur astronomer, found the dot of light that is Pluto (see figure). It took him a year of diligent study of the photographic plates he obtained at the Lowell Observatory in Arizona. From its slow motion with respect to the stars over the course of many nights, he identified Pluto as a new planet. 8.1a Pluto’s Mass and Size Even such basics as the mass and diameter of Pluto are very difficult to determine. It had been hard to deduce the mass of Pluto because to do so was, at first, thought to require measuring Pluto’s effect on Uranus, a far more massive body. (The orbit of Neptune, known for less than a hundred years at the time Pluto was discovered, was too poorly known to be of much use.) Moreover, Pluto has made less than one revolution around the Sun since its discovery, thus providing little of its path for detailed study. As recently as 1968, it was mistakenly concluded that Pluto had 91 per cent the mass of the Earth, instead of the correct value of 0.2 per cent. 8.1a Pluto’s Mass and Size The situation changed drastically in 1978 with the surprise discovery (see figure) that Pluto has a satellite. The moon was named Charon, after the boatman who rowed passengers across the River Styx to the realm of Pluto, god of the underworld in Greek mythology. (Its name is informally pronounced “Shar´on,” similarly to the name of the discoverer’s wife, Charlene, by astronomers working in the field.) The presence of a satellite allows us to deduce the mass of the planet by applying Newton’s form of Kepler’s third law (Chapter 5). Charon is 5 to 10 per cent of Pluto’s mass, and Pluto is only 1/500 the mass of the Earth, ten times less than had been suspected just before the discovery of Charon. 8.1a Pluto’s Mass and Size Pluto’s rotation axis is nearly in the ecliptic, like that of Uranus. This is also the axis about which Charon orbits Pluto every 6.4 days. Consequently, there are two five year intervals during Pluto’s 248-year orbit when the two objects pass in front of (that is, occult) each other every 3.2 days, as seen from Earth. Such mutual occultations were the case from 1985 through 1990. When we measured their apparent brightness, we received light from both Pluto and Charon together (they are so close together that they appeared as a single point in the sky). Their blocking each other led to dips in the total brightness we received. 8.1a Pluto’s Mass and Size From the duration of fading, we deduced how large they are. Pluto is 2300 km in diameter, smaller than expected, and Charon is 1200 km in diameter. Charon is thus half the size of Pluto. Further, it is separated from Pluto by only about 8 Pluto diameters, compared with the 30 Earth diameters that separate the Earth and the Moon. So Pluto/Charon is almost a “double-planet” system. 8.1a Pluto’s Mass and Size The rate at which the light from Pluto/Charon faded also gave us information that revealed the reflectivities (albedoes) of their surfaces, since part of the surface of the blocked object remained visible most of the time. The surfaces of both vary in brightness (see figure). Pluto seems to have a dark band near its equator, some markings on that band, and bright polar caps. 8.1a Pluto’s Mass and Size In 1990, the Hubble Space Telescope took an image that showed Pluto and Charon as distinct and separated objects for the first time, and they can now be viewed individually by telescopes on Mauna Kea in Hawaii (see figure, top) and elsewhere where the “seeing” is exceptional. The latest Hubble views show that Pluto has a dozen areas of bright and dark, the finest detail ever seen on Pluto, whose diameter is smaller than that of the United States (see figure, below). 8.1a Pluto’s Mass and Size But we don’t know whether the bright areas are bright because they are high clouds near mountains or low haze and frost. We merely know that there are extreme contrasts on Pluto’s surface. If we were standing on Pluto, the Sun would appear over a thousand times fainter than it does to us on Earth. Consequently, Pluto is very cold; infrared measurements show that its temperature is less than 60 K. From Pluto, we would need a telescope to see the solar disk, which would be about the same size that Jupiter appears from Earth. 8.1b Pluto’s Atmosphere Pluto occulted—passed in front of and hid—a star on one night in 1988. Astronomers observed this occultation to learn about Pluto’s atmosphere. If Pluto had no atmosphere, the starlight would wink out abruptly. The observations showed that the starlight diminished gradually and unevenly. Any atmosphere would make the starlight diminish more gradually. Thus Pluto’s atmosphere has layers in it. Another such occultation wasn’t observed until 2002, when (again) Pluto was seen to make the star wink out for a minute or so on two separate occasions. 8.1b Pluto’s Atmosphere From the 1988 occultation, astronomers were also able to conclude that the bulk of Pluto’s atmosphere is nitrogen. A trace of methane must also be present, since the methane ice on Pluto’s surface, detected from its spectrum, must be evaporating. Still, Pluto’s atmospheric pressure is very low, only 1/100,000 of Earth’s. The data from the first occultation seemed to show a change at a certain height in Pluto’s atmosphere, leading to the deduction that either the atmosphere had a temperature inversion or that the lower atmosphere contained a lot of dust. The lone high-quality scan obtained in July 2002 showed no such change at a certain height in the rate at which the star’s light was dimming as it passed through Pluto’s atmosphere. 8.1b Pluto’s Atmosphere Then, in August 2002, a group of scientists, of which one of the authors (J.M.P.) was a member, succeeded in observing an occultation of a star by Pluto on ten different telescopes, several of them on Mauna Kea (see figures). J.M.P.’s team from Williams College obtained a thousand data points in a 5-minute interval of the occultation, part of a 20minute data run. Further work in 2005 on a similar occultation of a star but this time by Pluto’s moon, Charon, gave the MIT-Williams College consortium success on all but one of the five telescopes in South America they used. 8.1b Pluto’s Atmosphere Our Pluto results showed an expansion of its atmosphere, which would result from a global warming since 1988. Perhaps some contribution to that warming comes from the changing orientation of Pluto’s darker spots with respect to incoming solar radiation. We also saw some bright spikes in the light curve, which could be signs of waves or turbulence in Pluto’s atmosphere. Further, observations from several telescopes showed that Pluto’s atmosphere is not quite round, undoubtedly resulting from strong winds. Our Charon results pinned down its size, and therefore density, better than ever before, but even the high-timeresolution observations did not show an atmosphere. 8.1b Pluto’s Atmosphere As Pluto goes farther from the Sun, as it is now doing, its atmosphere is generally predicted to freeze out and snow onto the surface. Though some calculations indicate that this might not be so, it is still possible that if we want to find out about the atmosphere, we had better get a spacecraft there within a decade or two, or we’ll have to wait another 200 years for the atmosphere to form again. NASA’s New Horizons mission, after a period of on-again, off-again for funding reasons, is a small satellite that at the time of this writing is scheduled to be launched in 2006 and to reach Pluto a decade later. Its investigators used Hubble to find two additional, small (under 100-km) moons of Pluto. 8.1c What Is Pluto? From Pluto’s mass and radius, we calculate its density. Since ices have even lower densities than Pluto, Pluto must be made of a mixture of ices and rock. Its composition is more similar to that of the satellites of the giant planets, especially Neptune’s large moon Triton, than to that of Earth or the other inner planets. Ironically, now that we know Pluto’s mass, we calculate that it is far too small to cause the deviations in Uranus’s orbit that originally led to Pluto’s discovery. The discrepancy probably wasn’t real: It turns out to be about 2 g/cm3, twice the density of water and less than half the density of Earth. The wrong mass had been assumed for Neptune when predicting the orbit of Uranus. The discovery of Pluto was purely the reward of Clyde Tombaugh’s hard work in conducting a thorough search in a zone of the sky near the ecliptic. 8.1c What Is Pluto? Pluto, with its moon and its atmosphere, has some similarities to the more familiar planets. Increasingly, Pluto is being identified with a newly discovered set of objects in the outer Solar System, which we will now study. Pluto remains strange in that it is so small next to the giants, and that its orbit is so eccentric and so highly inclined to the ecliptic. Is Pluto even a planet? It is so small, so low in mass, and in such an inclined orbit with respect to the eight inner planets that perhaps it should only be called an asteroid, a “Kuiper-belt object,” or a “Trans-Neptunian Object.” As we will see in the next section, another such object even bigger than Pluto turned up in 2005. Should both be called planets, leaving the possibility that we may soon know of even more? Or should Pluto be demoted to asteroid or the mere status of a Trans-Neptunian Object? As of this writing, the matter is undecided. 8.2 Kuiper-belt Objects Beyond the orbit of Neptune, a population of icy objects with diameters of a few tens or hundreds of kilometers is increasingly being found. The planetary astronomer Gerard Kuiper (pronounced koy´per) suggested a few decades ago that these objects would exist and should be the source of many of the comets that we see. As a result, these objects are now known as the Kuiperbelt objects, or, less often, Trans-Neptunian Objects. 8.2 Kuiper-belt Objects The Kuiper belt is probably about 10 A.U. thick and extends from the orbit of Neptune about twice as far out (see figure). About 1000 Kuiper-belt objects have been found so far, and tens of thousands larger than 100 km across are thought to exist. The objects may be left over from the formation of the Solar System. 8.2 Kuiper-belt Objects They are generally very dark, with albedoes of only about 4 per cent. Still, Pluto is one of the largest of the Kuiper belt objects, so much larger than most of the others that it is covered with frost. Triton may have initially been a similar object, subsequently captured by Neptune. A Kuiper-belt object larger than Pluto’s moon Charon was found in 2001, about half of Pluto’s diameter. One that may be even somewhat larger was found in 2002, though the uncertainty limits of these two Kuiper-belt objects overlap. Pluto, by contrast, has an albedo of about 60 per cent. The newer one, tentatively and unofficially named Quaoar (pronounced “kwa-whar”) after the Indian tribe that inhabited today’s Los Angeles, was even imaged with the Hubble Space Telescope, so we have a firmer grasp of its diameter, 1300 km, slightly over half that of Pluto. The size, in turn, gives us the albedo (12 per cent), which is larger than had been assumed for Kuiper-belt objects. 8.2 Kuiper-belt Objects David Jewitt of the University of Hawaii and Jane Luu, now at MIT’s Lincoln Lab, have been the discoverers of most of the known Kuiper-belt objects. They found the first one in 1992 and they and several other astronomers are looking for more. Michael Brown of Caltech and his colleagues stunned the world in July 2005, as this book was going to press, with their discovery of an outersolar-system object even larger than Pluto (see figures). Initially named 2003 UB313, it was first sighted in 2003 but not confirmed until 2005. 8.2 Kuiper-belt Objects The object is now 97 A.U. out from the Sun, more than twice as far out as Pluto. Undoubtedly, it was thrown into that highly inclined orbit after a close gravitational encounter with Neptune. Is it a 10th planet? It takes over 500 years to orbit the Sun. Its orbit is tilted an incredible 44°, taking it so high out of the ecliptic that no previous planet hunter found it. That is really a matter of semantics, but words can count. Keep in touch with this book’s website or with other sources to find out the latest on it. 8.2 Kuiper-belt Objects A few objects may once have been Kuiper-belt objects but now come somewhat closer to the Sun, crossing the orbits of the outer planets. About 100 of these “centaur” objects a few hundred kilometers across may exist. Since they are larger and come closer to the Earth and Sun than most Kuiper-belt objects, we can study them better. On at least one, a coma (typical of comets, as we will soon see) was seen, so these centaurs are intermediate between comets and asteroids. NASA’s New Horizons mission is to go to some Kuiper-belt objects after it visits Pluto. Our MIT-Williams consortium certainly hopes to pick up an occultation of a star by one or more of these Kuiper-belt objects, which would accurately determine its diameter and albedo. 8.3 Comets Nearly every decade, a bright comet appears in our sky. From a small, bright area called the head, a tail may extend gracefully over one-sixth (30°) or more of the sky. The tail of a comet is always directed roughly away from the Sun, even when the comet is moving outward through the Solar System. Although the tail may give an impression of motion because it extends out only to one side, the comet does not move noticeably with respect to the stars as we casually watch during the course of a night. With binoculars or a telescope, however, an observer can accurately note the position of the comet’s head and after a few hours can detect that the comet is moving at a slightly different rate from the stars. 8.3 Comets Still, both comets and stars rise and set more or less together (see figure). Within days, weeks, or (even less often) months, a bright comet will have become too faint to be seen with the naked eye, although it can often be followed for additional months with binoculars and then for additional months with telescopes. 8.3 Comets Most comets are much fainter than the one we have just described. About two dozen new comets are discovered each year, and most become known only to astronomers. If you should ever discover a comet, and are among the first three people to report it to the International Astronomical Union Central Bureau for Astronomical Telegrams at the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, it will be named after you. Hundreds of comets that go very close to the Sun or even hit it, destroying themselves, have been discovered by (and named after) the Solar and Heliospheric Observatory (SOHO) spacecraft, since it can uniquely monitor a region of space too close to the Sun to be seen from Earth given our daytime blue skies. 8.3a The Composition of Comets At the center of a comet’s head is its nucleus, which is composed of chunks of matter. The most widely accepted theory of the composition of comets, advanced in 1950 by Fred L. Whipple of the Harvard and Smithsonian Observatories, is that the nucleus is like a “dirty snowball.” It may be made of ices of such molecules as water (H2O), carbon dioxide (CO2), ammonia (NH3), and methane (CH4), with dust mixed in. 8.3a The Composition of Comets The nucleus itself is so small that we cannot observe it directly from Earth. Radar observations have verified in several cases that it is a few kilometers across. The rest of the head is the coma (pronounced coh´ma), which may grow to be as large as 100,000 km or so across (see figure). The coma shines partly because its gas and dust are reflecting sunlight toward us and partly because gases liberated from the nucleus get enough energy from sunlight to radiate. 8.3a The Composition of Comets The tail can extend 1 A.U. (150,000,000 km), so comets can be the largest objects in the Solar System. But the amount of matter in the tail is very small—the tail is a much better vacuum than we can make in laboratories on Earth. The dust tail is caused by dust particles released from the ices of the nucleus when they are vaporized. Many comets actually have two tails (■ Fig. 8 –11). The dust particles are left behind in the comet’s orbit, blown slightly away from the Sun by the pressure of sunlight hitting the particles. As a result of the comet’s orbital motion, the dust tail usually curves smoothly behind the comet. 8.3a The Composition of Comets The gas tail is composed of gas blown outward from the comet, at high speed, by the “solar wind” of particles emitted by the Sun (see our discussion in Chapter 10). As puffs of gas are blown out and as the solar wind varies, the gas tail takes on a structured appearance. It follows the interplanetary magnetic field. Each puff of matter can be seen. A comet—head and tail together—contains less than a billionth of the mass of the Earth. It has jokingly been said that comets are as close as something can come to being nothing. 8.3b The Origin and Evolution of Comets It is now generally accepted that trillions of tail-less comets surround the Solar System in a sphere perhaps 50,000 A.U. (that is, 50,000 times the distance from the Sun to the Earth, or almost 1 light-year) in radius. This sphere, far outside Pluto’s orbit, is the Oort comet cloud (named after the Dutch scientist Jan Oort). The total mass of matter in the cloud may be only 1 to 10 times the mass of the Earth. In current models, most of the Oort cloud’s mass is in the inner 1000 to 10,000 A.U. 8.3b The Origin and Evolution of Comets Occasionally one of these comets leaves the comet cloud. Currently, astronomers tend to think that gravity from the disk of our Milky Way Galaxy does most of the tugging. In the early years of the Oort model, it was thought that sometimes the gravity of a nearby star tugged an incipient comet out of place. In any case, the comet generally gets directly ejected from the Solar System, but in some cases the comet can approach the Sun. The comet’s orbit may be altered, sometimes into an elliptical orbit, if it passes near a giant planet, most frequently Jupiter. Because the comet cloud is spherical, comets are not limited to the plane of the ecliptic, which explains why one major class of comets comes in randomly from all directions. 8.3b The Origin and Evolution of Comets Another group of comets has orbits that are much more limited to the plane of the Solar System (Earth’s orbital plane). They probably come from the Kuiper belt beyond the orbit of Neptune, a flatter distribution of objects ranging from about 25 to 50 A.U. We seem to discover more of these Kuiper-belt-origin comets than we expect compared with Oort-cloud-origin comets. Perhaps the discrepancy has to do with the way comets die. New calculations show that since so few dormant comets are found, the comets must mainly break up and disappear. Maybe Oort-cloud comets, coming from so far out in the Solar System, change temperature regimes so much more quickly than Kuiper-belt comets that they are preferentially disrupted. 8.3b The Origin and Evolution of Comets Until recently, astronomers tended to say that the long-period comets, those with orbital periods longer than 200 years, came from the Oort cloud while comets with periods shorter than 200 years came from the Kuiper belt (see figure). Part of the reason for this division was merely that we had observed comet orbits reliably for only about 200 years. Most of the long-period comets have semimajor axes close to 20,000 A.U., 5000 times the 40 A.U. semimajor axis of Pluto’s orbit. 8.3b The Origin and Evolution of Comets This radius corresponds to the peak of the Oort cloud, and comets from there are considered “new.” However, once comets are dislodged from the Oort cloud and come into the inner Solar System, the semimajor axes of the orbits of these “returning” comets are reduced. The short-period comets, those with periods less than 200 years, were divided into “Jupiter-family” comets, whose orbits were made so small by encounters with Jupiter that their periods were less than 20 years, and “Halley-type” comets, which suffered less influence by Jupiter. 8.3b The Origin and Evolution of Comets A new comet classification basically depends on the influence of Jupiter. One of the two major classes consists of those that come from all directions. Comets in the other major class are called “ecliptic,” since the comet orbits are aligned close to the plane of the Solar System, the ecliptic plane (see figure), rather than being highly tilted. Almost all of these come from the Oort cloud. Almost all of these ecliptic comets come from the Kuiper belt. In the new scheme, fewer comets change their classifications over time. Notice that comets on highly eccentric orbits spend most of their time far away from the Sun, an excellent example of Kepler’s second law (Chapter 5). 8.3b The Origin and Evolution of Comets As a comet gets closer to the Sun than those distant regions, the solar radiation begins to vaporize the ice in the nucleus. The tail forms, and grows longer as more of the nucleus is vaporized. Even though the tail can be millions of kilometers long, it is still so tenuous that only 1/500 of the mass of the nucleus may be lost each time it visits the solar neighborhood. Thus a comet may last for many passages around the Sun. But some comets hit the Sun and are destroyed (see figure). 8.3b The Origin and Evolution of Comets We shall see in the following section that meteoroids can be left in the orbit of a disintegrated comet. Some of the asteroids, particularly those that cross the Earth’s orbit, may be dead comet nuclei. In recent years, a handful of asteroids—notably Chiron in the outer Solar System—have shown comas or tails, making them comets; conversely, a few comets have died out and seem like asteroids. So we may have misidentified some of each in the past. 8.3b The Origin and Evolution of Comets How did comets get where they are? We will say more about the formation of the Solar System in Chapter 9. There, we will see that there were many small particles that clumped together in the early eras. Some of these clumps interacted gravitationally with other clumps and even with Jupiter and other planets as they were formed. Many of these clumps were ejected from the region of their formation, often where the asteroid belt now is between Mars and Jupiter, and wound up forming the Oort comet cloud. Other clumps were already beyond the orbit of Neptune, where fewer interactions took place. Those clumps formed the Kuiper belt. 8.3b The Origin and Evolution of Comets Because new comets come from the places in the Solar System that are farthest from the Sun and thus coldest, they probably contain matter that is unchanged since the formation of the Solar System. So the study of comets is important for understanding the birth of the Solar System. Moreover, some astronomers have concluded that early in Earth’s history, the oceans formed when an onslaught of water-bearing comets collided with Earth, although this view is still controversial. 8.3c Halley’s Comet In 1705, the English astronomer Edmond Halley (Halley is pronounced to rhyme with “Sally,” and not with “say´lee”) (see figure) applied a new method developed by his friend Isaac Newton to determine the orbits of comets from observations of their positions in the sky. He reported that the orbits of the bright comets that had appeared in 1531, 1607, and 1682 were about the same. Moreover, the intervals between appearances were approximately equal, so Halley suggested that we were observing a single comet orbiting the Sun, and he accounted for the slightly different periods with Newton’s law of gravity from interactions with planets. 8.3c Halley’s Comet Halley predicted that this bright comet would again return in 1758. Its reappearance on Christmas night of that year, 16 years after Halley’s death, was the proof of Halley’s hypothesis (and Newton’s method). The comet has thereafter been known as Halley’s Comet (see figure). Since it was the first known “periodic comet” (i.e., the first comet found to repeatedly visit the inner parts of the Solar System), it is officially called 1P, number 1 in the list of periodic (P) comets. 8.3c Halley’s Comet It seems probable that the bright comets reported every 74 to 79 years since 240 b.c. were earlier appearances of Halley’s Comet. Halley’s Comet came especially close to the Earth during its 1910 return, and the Earth actually passed through its tail. The fact that it has been observed dozens of times endorses the calculations that show that less than 1 per cent of a cometary nucleus’s mass is lost at each passage near the Sun. Many people had been frightened that the tail would somehow damage the Earth or its atmosphere, but the tail had no noticeable effect. Even then, most scientists knew that the gas and dust in the tail were too tenuous to harm our environment. 8.3c Halley’s Comet The most recent close approach of Halley’s Comet was in 1986. Since we knew long in advance that the comet would be available for viewing, special observations were planned for optical, infrared, and radio telescopes. It was not as spectacular from the ground in 1986 as it was in 1910, for this time the Earth and comet were on opposite sides of the Sun when the comet was brightest. For example, spectroscopy showed many previously undetected ions in the coma and tail. When Halley’s Comet passed through the plane of the Earth’s orbit, it was met by an armada of spacecraft. The best was the European Space Agency’s spacecraft Giotto (named after the 14th-century Italian artist who included Halley’s Comet in a painting), which went right up close to Halley. Giotto’s several instruments also studied Halley’s gas, dust, and magnetic field from as close as 600 km from the nucleus. 8.3c Halley’s Comet The most astounding observations were undoubtedly the photographs showing the nucleus itself (see figure, bottom left), which turns out to be potato-shaped (see figure, bottom right). It is about 16 km in its longest dimension, half the size of Manhattan Island. 8.3c Halley’s Comet The “dirty snowball” theory of comets was confirmed in general, but the snowball is darker than expected. Further, the evaporating gas and dust is localized into jets that are stronger than expected. It is as black as velvet, with an albedo of only about 3 per cent. They come out of fissures in the dark crust. We now realize that comets may shut off not when they have lost all their material but rather when the fissures in their crusts close. Giotto carried 10 instruments in addition to its camera. Among them were mass spectrometers to measure the types of particles present, detectors for dust, equipment to listen for radio signals that revealed the densities of gas and dust in the coma, detectors for ions, and a magnetometer to measure the magnetic field. 8.3c Halley’s Comet About 30 per cent of Halley’s dust particles are made only of hydrogen, carbon, nitrogen, and oxygen (see figure). This simple composition resembles that of the oldest type of meteorite. It thus indicates that these particles may be from the earliest years of the Solar System. 8.3c Halley’s Comet Many valuable observations were also obtained from the Earth. For example, radio telescopes were used to study molecules. Water vapor is the most prevalent gas, but carbon monoxide and carbon dioxide were also detected. The comet was bright enough that many telescopes obtained spectra (see figure). 8.3c Halley’s Comet The next appearance of Halley’s Comet, in 2061, again won’t be spectacular. Not until the one after that, in 2134, will the comet show a long tail to earthbound observers. Fortunately, though Halley’s Comet is predictably interesting, a more spectacular comet appears every 10 years or so. When you read in the newspaper that a bright comet is here, don’t wait to see it another time. Some bright comets are at their best for only a few days or a week. 8.3d Comet [Shoemaker-]Shoemaker-Levy 9 A very unusual comet gave thrills to people around the world. In 1993, Eugene Shoemaker, Carolyn Shoemaker, and David Levy discovered their ninth comet in a search with a wide-field telescope at the Palomar Observatory. (The authors of this book like to give each Shoemaker individual credit for the discovery, as in the chapter subheading, though the comet is generally and formally called Shoemaker-Levy 9.) This comet looked weird—it seemed squashed. 8.3d Comet [Shoemaker-]Shoemaker-Levy 9 Higher-resolution images taken with other telescopes, including the Hubble Space Telescope (see figure), showed that the comet had broken into bits, forming a chain that resembled beads on a string. Even stranger, the comet was in orbit not around the Sun but around Jupiter, and would hit Jupiter a year later. Apparently, several decades earlier the comet was captured in a highly eccentric orbit around Jupiter, and in 1992, during its previous close approach, it was torn apart into more than 20 pieces by Jupiter’s tidal forces. 8.3d Comet [Shoemaker-]Shoemaker-Levy 9 Telescopes all around the world and in space were trained on Jupiter when the first bit of comet hit. The site was slightly around the back side of Jupiter, but rotated to where we could see it from Earth after about 15 minutes. When they could view Jupiter’s surface, they saw a dark ring (see figure on next slide). Even before then, scientists were enthralled by a plume rising above Jupiter’s edge. Infrared telescopes detected a tremendous amount of radiation from the heated gas. Over a period of almost a week, one bit of the comet after another hit Jupiter, leaving a series of Earth-sized rings and spots as Jupiter rotated. The largest dark spots could be seen for a few months even with small backyard telescopes. (On one of the April 2005 solar eclipse cruises, David Levy sometimes wore a T-shirt that said “My comet crashed.”) 8.3d Comet [Shoemaker-]Shoemaker-Levy 9 8.3d Comet [Shoemaker-]Shoemaker-Levy 9 The dark material showed us the hydrocarbons and other constituents of the comet. Spectra showed sulfur and other elements, presumably dredged up from lower levels of Jupiter’s atmosphere than we normally see. The biggest comet chunk released the equivalent of 6 million megatons of TNT—100,000 times more than the largest hydrogen bomb. Had any of the fragments hit Earth, they would have made a crater as large as Rhode Island, with dust thrown up to much greater distances. Had the entire comet (whose nucleus was 10 km across) hit Earth at one time, much of life could have been destroyed. So Comet Shoemaker-Levy 9 made us even more wary about what may be coming at us from space. 8.3e Recently Observed Comets In 1995, Alan Hale and Thomas Bopp independently found a faint comet, which was soon discovered to be quite far out in the Solar System. Its orbit was to bring it into the inner Solar System, and it was already bright enough that it was likely to be spectacular when it came close to Earth in 1997. It lived up to its advance billing (see figure). 8.3e Recently Observed Comets Telescopes of all kinds were trained on Comet Hale-Bopp, and hundreds of millions of people were thrilled to step outside at night and see a comet just by looking up. Modern powerful radio telescopes were able to detect many kinds of molecules that had not previously been recorded in a comet. Occasionally, other bright comets, such as C /2002 C1, Comet Ikeya-Zhang (see figure), turn up and are fun to watch. 8.3f Spacecraft to Comets NASA’s Deep Space 1 mission flew close to Comet 19P/Borrelly in 2001. This comet’s surface, and therefore probably the surfaces of comet nuclei in general, was rougher and more dramatic than expected. It obtained more detailed images of the bowling-pin-shaped nucleus (see figure) than even Giotto’s views of Halley’s nucleus. Deep Space 1 found smooth, rolling plains that seem to be the source of the dust jets, which are more concentrated than Halley’s. Darkened material, perhaps extruded from underneath, covers some regions and accentuates grooves and faults. Borrelly’s albedo in these places is less than 1 per cent, while Borrelly’s overall albedo is only 4 per cent. 8.3f Spacecraft to Comets Borrelly is thought to have originated in the Kuiper belt, in contrast to Halley’s Comet’s origin in the Oort cloud. Still, compared with Halley, Borrelly gives off relatively little water, perhaps because so much of its surface is inactive. This difference would explain why Halley’s Comet gives off many carbon compounds while Borrelly gives off more water and ammonia than carbon. Scientists have yet to explain why the solar wind is deflected around Borrelly’s nucleus in an asymmetric fashion. The center of the plasma in Borrelly’s coma is some 2000 km off to the side, as strange as if a supersonic jet’s shock wave were displaced far to the airplane’s side. 8.3f Spacecraft to Comets NASA’s Stardust mission, launched in 1999, went to Comet Wild 2 (pronounced Vilt-too), a periodic comet with a sixyear orbit. When it got there in 2004, it not only photographed the comet but also gathered some of its dust. It carries an extremely lightweight material called aerogel (see figure), and flew through the comet with the aerogel exposed so that the comet dust could stick in it. Stardust’s orbit will bring it back near Earth in January 2006, when it will parachute the aerogel down to the Utah desert. (A parachute that didn’t open in a 2004 mission to gather solar wind particles, Genesis, makes everybody worried.) 8.3f Spacecraft to Comets A major European Space Agency spacecraft, Rosetta, was launched in 2004 to orbit with a comet for some years and to land a probe on the comet’s nucleus in 2014. It will use three gravity assists from Earth and one from Mars to reach the comet, passing asteroids (2867) Steins in 2008 and (21) Lutetia in 2010, both in the asteroid belt, on the way. (Asteroids are discussed in Section 8.5.) It is heading for Comet 67P/Churyumov-Gerasimenko. Rosetta will drop a lander, Philae, onto the comet’s nucleus. Just as the Rosetta Stone, now in the British Museum, enabled Egyptian hieroglyphics to be deciphered by having the same text in three scripts (hieroglyphics, Demotic, and Greek), scientists hope that the Rosetta spacecraft will prove to be the key to deciphering comets. (Philae was an island in the Nile on which an obelisk was found that helped to decipher the hieroglyphics of the Rosetta Stone.) 8.3f Spacecraft to Comets Rosetta is to orbit the comet at an altitude of only a few kilometers, mapping its surface and making other measurements, for 18 months, including the comet’s closest approach to the Sun and therefore, it is hoped, its increasing activity. The lander is to work for some weeks, taking photographs and drilling into the surface. NASA’s Deep Impact spacecraft crashed a 370-kg projectile into Comet Tempel 1 in 2005. The remainder of the spacecraft studied the impact, which should have formed a footballfield-sized crater some 7 stories deep. Astronomers were at telescopes all around the Earth, and were using telescopes in space like Hubble, to record the impact (see figure). 8.4 Meteoroids There are many small chunks of matter orbiting in the Solar System, ranging up to tens of meters across and sometimes even larger. When these chunks are in space, they are called meteoroids. When one hits the Earth’s atmosphere, friction and the compression of air in front of it heat it up—usually at a height of about 100 km—until all or most of it is vaporized. Such events result in streaks of light in the sky (see figure), which we call meteors (popularly, and incorrectly, known as shooting stars or falling stars). When a fragment of a meteoroid survives its passage through the Earth’s atmosphere, the remnant that we find on Earth is called a meteorite. Counting even tiny meteorites, whose masses are typically a milligram, some 10,000 tons of this interplanetary matter land on Earth’s surface each year. 8.4a Types and Sizes of Meteorites Space is full of meteoroids of all sizes, with the smallest being most abundant. Most of the small particles, less than 1 mm across, may come from comets. The large particles, more than 1 cm across, may generally come from collisions of asteroids in the asteroid belt (see Section 8.5). Tiny meteorites less than a millimeter across, micrometeorites, are the major cause of erosion (what little there is) on the Moon. Micrometeorites also hit the Earth’s upper atmosphere all the time, and remnants can be collected for analysis from balloons or airplanes or from deep-sea sediments. They are often sufficiently slowed down by Earth’s atmosphere to avoid being vaporized before they reach the ground. 8.4a Types and Sizes of Meteorites Some of the meteorites that are found have a very high iron content (about 90 per cent); the rest is nickel. These iron meteorites are thus very dense— that is, they weigh quite a lot for their volume (see figure). 8.4a Types and Sizes of Meteorites Most meteorites that hit the Earth are stony in nature. Because they resemble ordinary rocks (see figure) and disintegrate with weathering, they are not easily discovered unless their fall is observed. That difference explains why most meteorites discovered at random are made of iron. But when a fall is observed, most meteorites recovered are made of stone. Some meteorites are rich in carbon, and some of these even have complex molecules like amino acids. 8.4a Types and Sizes of Meteorites A large terrestrial crater that is obviously meteoritic in origin is the Barringer Meteor Crater in Arizona (see figure, left). It resulted from what was perhaps the most recent large meteoroid to hit the Earth, for it was formed only about 50,000 years ago. Every few years a meteorite is discovered on Earth immediately after its fall. The chance of a meteorite landing on someone’s house or car is very small, but it has happened (see figure, below)! 8.4a Types and Sizes of Meteorites Often the positions in the sky of extremely bright meteors are tracked in the hope of finding fresh meteorite falls. The newly discovered meteorites are rushed to laboratories in order to find out how long they have been in space by studying their radioactive elements. Over 10,000 meteorites have been found in the Antarctic, where they have been well preserved as they accumulated over the years. Though the Antarctic ice sheets flow, the ice becomes stagnant in some places and disappears, revealing meteorites that had been trapped for over 10,000 years. 8.4a Types and Sizes of Meteorites Some odd Antarctic meteorites are now known to have come from the Moon or even from Mars. Recall that in Chapter 6 we even discussed controversial evidence for ancient primitive life-forms on Mars, found in one such meteorite. As of mid-2005, the conclusion hasn’t been entirely ruled out, but few scientists accept it. As the late Carl Sagan said, “Extraordinary claims require extraordinary evidence,” and the evidence from this meteorite is not convincing, at least not yet. Meteorites that have been examined were formed up to 4.6 billion years ago, the beginning of the Solar System. The relative abundances of the elements in meteorites thus tell us about the solar nebula from which the Solar System formed. In fact, up to the time of the Moon landings, meteorites and cosmic rays (charged particles from outer space) were the only extraterrestrial material we could get our hands on. 8.4b Meteor Showers Meteors sometimes occur in showers, when meteors are seen at a rate far above average. Meteor showers are named after the constellation in which the radiant, the point from which the meteors appear to come, is located. The most widely observed—the Perseids, whose radiant is in Perseus—takes place each summer around August 12 and the nights on either side of that date. The best winter show is the Geminids, which takes place around December 14 and whose radiant is in Gemini. 8.4b Meteor Showers On any clear night a naked-eye observer with a dark sky may see a few sporadic meteors an hour—that is, meteors that are not part of a shower. (Just try going out to a field in the country and watching the sky for an hour.) During a shower, on the other hand, you may typically see one every few minutes. Meteor showers generally result from the Earth’s passing through the orbits of defunct or disintegrating comets and hitting the meteoroids left behind. (One meteor shower comes from an asteroid orbit.) 8.4b Meteor Showers Though the Perseids and Geminids can be counted on each year, the Leonid meteor shower (whose radiant is in Leo) peaks every 33 years, when the Earth crosses the main clump of debris from Comet TempelTuttle. On November 17/18, 1998, one fireball (a meteor brighter than Venus) was visible each minute for a while (see figure), and on November 17/18, 1999 through 2001, thousands of meteors were seen in the peak hour. We will now have to wait until about 2031 for the next Leonid peak. The visibility of meteors in a shower depends in large part on how bright the Moon is; you want as dark a sky as possible. Meteors are best seen with the naked eye; using a telescope or binoculars merely restricts your field of view. 8.5 Asteroids The nine known planets were not the only bodies to result from the gas and dust cloud that collapsed to form the Solar System 4.6 billion years ago. Thousands of minor planets, called asteroids, also resulted. We detect them by their small motions in the sky relative to the stars (see figure). Most of the asteroids have elliptical orbits between the orbits of Mars and Jupiter, in a zone called the asteroid belt. It is thought that Jupiter’s gravitational tugs perturbed the orbits of asteroids, leading to collisions among them that were too violent to form a planet. 8.5 Asteroids Asteroids are assigned a number in order of discovery and then a name: (1) Ceres, (16) Psyche, and (433) Eros, for example. Often the number is omitted when discussing well-known asteroids. Though the concept of the asteroid belt may seem to imply a lot of asteroids close together, asteroids rarely come within a million kilometers of each other. Occasionally, collisions do occur, producing the small chips that make meteoroids. 8.5a General Properties of Asteroids Only about 6 asteroids are larger than 300 km in diameter. Hundreds are over 100 km across (see figure), roughly the size of some of the moons of the planets, but most are small, less than 10 km in diameter. Perhaps 100,000 asteroids could be detected with Earthbased telescopes; automated searches are now discovering asteroids at a prodigious rate. Yet all the asteroids together contain less mass than the Moon. 8.5a General Properties of Asteroids Spacecraft en route to Jupiter and beyond travelled through the asteroid belt for many months and showed that the amount of dust among the asteroids is not much greater than the amount of interplanetary dust in the vicinity of the Earth. So the asteroid belt is not a significant hazard for space travel to the outer parts of the Solar System. Asteroids are made of different materials from each other, and represent the chemical compositions of different regions of space. The asteroids at the inner edge of the asteroid belt are mostly stony in nature, while the ones at the outer edge are darker (because they contain more carbon). Most of the small asteroids that pass near the Earth belong to the stony group. Three of the largest asteroids belong to the high-carbon group. A third group is mostly composed of iron and nickel. 8.5a General Properties of Asteroids The differences may be telling us about conditions in the early Solar System as it was forming and how the conditions varied with distance from the young Sun. Many of the asteroids must have broken off from larger, partly “differentiated” bodies in which dense material sank to the center (as in the case of the terrestrial planets; see our discussion in Chapter 6). The path of the Galileo spacecraft to Jupiter sent it near the asteroid (951) Gaspra in 1991 (see figure). It detected a magnetic field from Gaspra, which means that the asteroid is probably made of metal and is magnetized. 8.5a General Properties of Asteroids Galileo passed the asteroid (243) Ida in 1993, and discovered that the asteroid has an even smaller satellite (see figure), which was then named Dactyl. Other double asteroids have since been discovered, and astronomers newly recognize the frequency of such pairs. For example, ground-based astronomers found a 13-km satellite orbiting 200-kmdiameter (45) Eugenia every five days. (Note that Eugenia’s low number shows that it was one of the first asteroids discovered.) 8.5b Near-Earth Objects Some asteroids are far from the asteroid belt; their orbits approach or cross that of Earth. We have observed only a small fraction of these types of Near-Earth Objects, bodies that come within 1.3 A.U. of Earth. The Near Earth Asteroid Rendezvous (NEAR) mission passed and photographed the main-belt asteroid (253) Mathilde in 1997. The existence of big craters that would have torn a solid rock apart, and the asteroid’s low density, lead scientists to conclude that Mathilde is a giant “rubble pile,” rocks held together by mutual gravity. 8.5b Near-Earth Objects NEAR went into orbit around (433) Eros on Valentine’s Day, 2000 (see figures), when it was renamed NEAR Shoemaker after the planetary geologist Eugene Shoemaker. Eros was the first near-Earth asteroid that had been discovered. It is 33 km by 13 km by 13 km in size. NEAR Shoemaker photographed craters, grooves, layers, housesized boulders, and a 20-km-long surface ridge. 8.5b Near-Earth Objects The existence of the craters and ridge, which indicates that Eros must be a solid body, disagrees with the previous suggestions of some scientists that most asteroids are mere rubble piles as Mathilde seems to be. The impact that formed the largest crater, 8 km across and now named Shoemaker, is thought to have formed most of the large boulders found across Eros’s surface. Eros’s density, 2.4 g /cm3, is comparable to that of the Earth’s crust, about the same as Ida’s, and twice Mathilde’s. From orbit, NEAR Shoemaker’s infrared, x-ray, and gamma-ray spectrometers measured how the minerals vary from place to place on Eros’s surface. The last of these even survived the spacecraft’s landing on Eros (see figures), and radioed back information about the composition of surface rocks. 8.5b Near-Earth Objects Scientists analyzing the data have found abundances of elements similar to that of the Sun and of a type of primitive meteorite known as chondrites that are the most common type of meteorite found on Earth. NEAR Shoemaker’s observations show that Eros was probably broken off billions of years ago from a larger asteroid as a uniformly dense fragment. They have concluded that Eros is made of primitive material, unchanged for 4.5 billion years, so we are studying the early eras of the Solar System with it. This solidity contrasts with Mathilde’s rubble-pile nature. Besides providing much detailed information, the close-up studies of these objects are allowing us to verify whether the lines of reasoning we use with ground-based asteroid observations give correct results. 8.5b Near-Earth Objects Near-Earth asteroids (see figure) may well be the source of most meteorites, which could be debris of collisions that occurred when these asteroids visit the asteroid belt. Eventually, most Earth-crossing asteroids will probably collide with the Earth. Statistics show that there is a 1 per cent chance of a collision of this tremendous magnitude per millennium. Over 1000 of them are greater than 1 km in diameter, and none are known to be larger than 10 km across. This rate is pretty high on a cosmic scale. Such collisions would have drastic consequences for life on Earth. 8.5b Near-Earth Objects Smaller objects are a hundred times more common, with a 1 per cent chance that an asteroid greater than 300 m in diameter would hit the Earth in the next century. Such a collision could kill thousands or millions of people, depending on where it lands. The question of how much we should worry about NearEarth Objects hitting us is increasingly discussed, including at a meeting sponsored by the United Nations. Even Hollywood movies have been devoted to the topic, though at present we can’t send out astronauts to deflect or break up the objects the way the movies showed. 8.5b Near-Earth Objects Several projects are under way to find as many Near-Earth Objects as possible. Current plans are to map 90 per cent of them in the next couple of decades, and the pace of discovery is accelerating. Several projects use CCD detectors, repetitive scanning, and computers to locate asteroids and are discovering thousands each year, some of which are Near-Earth Objects.