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1. What is a Comet?
Comets are small, fragile, irregularly shaped bodies composed of a mixture of non-volatile grains and frozen gases. They usually follow highly elongated paths around the Sun. Most become visible, even in telescopes, only when they get near enough to the Sun for the Sun's radiation to start subliming the volatile gases, which in turn blow away small bits of the solid material. These materials expand into an enormous escaping atmosphere called the coma, which becomes far bigger than a planet, and they are forced back into long tails of dust and gas by radiation and charged particles flowing from the Sun. Comets are cold bodies, and we see them only because the gases in their comae and tails fluoresce in sunlight (somewhat akin to a fluorescent light) and because of sunlight reflected from the solids. Comets are regular members of the solar system family, gravitationally bound to the Sun. They are generally believed to be made of material, originally in the outer part of the solar system, that didn't get incorporated into the planets - leftover debris, if you will. It is the very fact that they are thought to be composed of such unchanged primitive material that makes them extremely interesting to scientists who wish to learn about conditions during the earliest period of the solar system.
Comets are very small in size relative to planets. There average diameters usually range from 750m or less to about 20 km. Recently, evidence has been found for much larger distant comets, perhaps having diameters of 300 km or more, but these sizes are still small compared to planets. Planets are usually more or less spherical in shape, usually bulging slightly at the equator. Comets are irregular in shape, with their longest dimension often twice the shortest. - The best evidence suggests that comets are very fragile. Their tensile strength (the stress they can take without being pulled apart) appears to be only about 10kg/m² You could take a big piece of cometary material and simply pull it in two with your bare hands, something like a poorly compacted snowball.
Comets, of course, must obey the same universal laws of motion as do all other bodies. Where the orbits of planets around the Sun are nearly circular, however, the orbits of comets are quite elongated. Nearly 100 known comets have periods (the time it takes them to make one complete trip around the Sun) five to seven Earth years in length. Their farthest point from the Sun (their aphelion) is near Jupiter's orbit, with the closest point (perihelion) being much nearer to Earth. A few comets like Halley have their aphelion's beyond Neptune (which is six times as far from the Sun as Jupiter). Other comets come from much farther out yet, and it may take them thousands or even hundreds of thousands of years to make one complete orbit around the Sun. In all cases, if a comet approaches near to Jupiter, it is strongly attracted by the gravitational pull of that giant among planets, and its orbit is perturbed (changed), sometimes radically. This is part of what happened to comet Shoemaker-Levy 9.
The nucleus of a comet, which is its solid, persisting part, has been called an icy conglomerate, a dirty snowball, and other colourful but even less accurate descriptions. Certainly a comet nucleus contains silicates akin to some ordinary Earth rocks in composition, probably mostly in very small grains and pieces. Perhaps the grains are glued together into larger pieces by the frozen gases. A nucleus appears to include complex carbon compounds and perhaps some free carbon, which make it very black in colour. Most notably, at least when young, it contains many frozen gases, the most common being ordinary water. In the low pressure conditions of space, water sublimes, that is, it goes directly from solid to gas - just like dry ice does on Earth. Water probably makes up 75-80% of the volatile material in most comets. Other common ices are carbon monoxide (CO), carbon dioxide (CO2), methane (CH4), ammonia (NH3), and formaldehyde (H2CO). Volatiles and solids appear to be fairly well mixed throughout the nucleus of a new comet approaching the Sun for the first time. As a comet ages from many trips close to the Sun, there is evidence that it loses most of its ices, or at least those ices anywhere near the nucleus surface, and becomes just a very fragile old rock in appearance, indistinguishable at a distance from an asteroid.
A comet nucleus is small, so its gravitational pull is very weak. You could run and jump completely off of it (if you could get traction). The escape velocity is only about 1 m/sec (compared to 11 km/sec on Earth). As a result, the escaping gases and the small solid particles (dust) that they drag with them never fall back to the nucleus surface. Radiation pressure, the pressure of sunlight, forces the dust particles back into a dust tail in the direction opposite to the Sun. A comet's tail can be tens of millions of Kilometres in length when seen in the reflected sunlight. The gas molecules are torn apart by solar ultraviolet light, often losing electrons and becoming electrically charged fragments or ions. The ions interact with the wind of charged particles flowing out from the Sun and are forced back into an ion tail, which again can extend for millions of Kilometres in the direction opposite to the Sun. These ions can be seen as they fluoresce in sunlight.
Every comet then really has two tails, a dust tall and an ion tail. If the comet is faint, only one or neither tail may be detectable, and the comet may appear just as a fuzzy blob of light, even in a big telescope. The density of material in the coma and tails is very low, lower than the best vacuum that can be produced in most laboratories. In 1986 the Giotto spacecraft flew right through the coma of Comet Halley only a few hundred Kilometres from the nucleus. Though the coma and tails of a comet may extend for tens of millions of kilometres and become easily visible to the naked eye in Earth's night sky, as Comet West's were in 1976, the entire phenomenon is the product of a tiny nucleus only a few Kilometres across.
The nucleus is a very poor reflector of light and rarely visible in even the largest telescope, if not lost completely in the glare of the coma. A great deal was learned when the European Space Agency, the Soviet Union, and the Japanese sent spacecraft to fly by Comet Halley in 1986. For the first time, actual images of an active nucleus were obtained and the composition of the dust and gases flowing from it was directly measured. Early in the next century the Europeans plan to send a spacecraft called Rosetta to rendezvous with a comet and watch it closely for a long period of time. Even this sophisticated mission is not likely to tell scientists a great deal about the interior structure of comets, however. Therefore, the opportunity to reconstruct the events that occurred when Shoemaker-Levy 9 split and to study those that occurred when the fragments were destroyed in Jupiter's atmosphere was uniquely important
2. The Motion of Comets
Comets necessarily obey the same physical laws as every other object. They move according to the basic laws of motion and of universal gravitation discovered by Newton in the 17th century (ignoring very small relativistic corrections). If one considers only two bodies - either the Sun and a planet, or the Sun and a comet - the smaller body appears to follow an elliptical path or orbit about the Sun, which is at one focus of the ellipse.
Eccentricity is a mathematical measure of departure from circularity. A circle has zero eccentricity, and most of the planets have orbits which are nearly circles. Only Pluto and Mercury have eccentricities exceeding 0.1. Comets, however, have very large eccentricities, often approaching one, the value for a parabola. Such highly eccentric orbits are just as possible as circular orbits, as far as the laws of motion are concerned.
The Solar System consists of the Sun, nine planets, numerous satellites and asteroids, comets, and various small debris. At any given time the motion of any solar system body is affected by the gravitational pulls of all of the others. The Sun's pull is the largest by far, unless one body approaches very closely to another, so orbit calculations usually are carried out as two-body calculations (the body in question and the Sun) with small perturbations (small added effects due to the pull of other bodies). In 1705 Halley noted in his original paper predicting the return of his comet that Jupiter undoubtedly had serious effects on the comet's motion, and he presumed Jupiter to be the cause of changes in the period (the time required for one complete revolution about the Sun) of the comet. (Comet Halley's period is usually stated to be 76 years, but in fact it has varied between 74.4 and 79.2 years during the past 2,000 years.) In that same paper Halley also became the first to note the very real possibility of the collision of comets with planets, but stated that he would leave the consequences of such a contact or shock to be discussed by the Studies of Physical Matters.
In the case of Shoemaker-Levy 9 we have the perfect example both of large perturbations and their possible consequences. The comet was fragmented and perturbed into an orbit where the pieces hit Jupiter one period later. In general one must note that Jupiter's gravity (or that of other planets) is perfectly capable of changing the energy of a comet's orbit sufficiently to throw it clear out of the Solar System (to give it escape velocity from the Solar System) and has done so on numerous occasions. This is exactly the same physical effect that permits using planets to change the orbital energy of a spacecraft in so-called gravity assisted manoeuvres such as were used by the Voyager spacecraft to visit all the outer planets except Pluto. One of Newton's laws of motion states that for every action there is an equal and opposite reaction. Comets expel dust and gas, usually from localised regions, on the sunward side of the nucleus. This action causes a reaction by the comets' nucleus, slightly speeding it up or slowing it down. Such effects are called non gravitational forces and are simply rocket effects, as if someone had set up one or more rocket motors on the nucleus. In general both the size and shape of a comet's orbit are changed by the non-gravitational forces - not by much but by enough to totally confound all of the celestial mechanics experts of the 19th and early 20th centuries. Comet Halley arrived at its point closest to the Sun (perihelion) in 1910 more than three days late, according to the best predictions. Only after F. L. Whipple published his icy conglomerate model of a degassing nucleus in 1950 did it all begin to make sense. The predictions for the time of perihelion passage of Comet Halley in 1986, which took into account a crude model for the reaction forces, were off by less than five hours.
Much of modern physics is expressed in terms of conservation laws, laws about quantities which do not change for a given system. Conservation of energy is one of these laws, and it says that energy may change form, but it cannot be created or destroyed. Thus the energy of motion (kinetic energy) of Shoemaker-Levy 9 was changed largely to thermal energy when the comet was halted by Jupiter's atmosphere and destroyed in the process. When one body moves about another in the vacuum of space, the total energy (kinetic energy plus potential energy) is conserved.
Another quantity that is conserved is called angular momentum. In the first paragraph of this section, it was stated that the geometric constants of an ellipse are its semimajor axis and eccentricity. The dynamical constants of a body moving about another are energy and angular momentum. The total (binding) energy is inversely proportional to the semimajor axis. If the energy goes to zero, the semimajor axis becomes infinite and the body escapes. The angular momentum is proportional both to the eccentricity and the energy in a more complicated way, but, for a given energy, the larger the angular momentum the more elongated the orbit.
Comets simply are bodies which in general have more angular momentum per unit mass than do planets and therefore move in more elongated orbits. Sometimes the orbits are so elongated that, because we can observe only a small part of them, they cannot be distinguished from a parabola, which is an orbit with an eccentricity of exactly one. In very general terms, one can say that the energy determines the size of the orbit and the angular momentum the shape.
Appendix C - The Probability Of Collisions with Earth
Most bodies in the Solar System with a visible solid surface exhibit craters. On Earth we see very few because geological processes such as weathering and erosion soon destroy the obvious evidence. On bodies with no atmosphere, such as Mercury or the Moon, craters are everywhere. Without going into detail, there is strong evidence of a period of intense cratering in the solar system that ended about 3.9 billion years ago. Since that time cratering appears to have continued at a much slower and fairly uniform rate. The cause of the craters is impacts by comets and asteroids. Most asteroids follow sensibly circular orbits between the planets Mars and Jupiter, but all of these asteroids are perturbed, occasionally by each other and more regularly and dramatically by Jupiter. As a result some find themselves in orbits that cross that of Mars or even Earth. Comets on the other hand, as noted in Section 2, follow highly elongated orbits that often come close to Earth or other major bodies to begin with. These orbits are greatly affected if they come anywhere near Jupiter. Over the eons every moon and planet finds itself in the wrong place in its orbit at the wrong time, many times, and suffers the insult of a major impact.
Earth's atmosphere protects us from the multitude of small debris, the size of grains of sand or pebbles, thousands of which pelt our planet every day. The meteors in our night sky are visible evidence of bodies of this type burning up high in the atmosphere. In fact, up to a diameter of about 10 m (33 ft.) Most stony meteoroids are destroyed in the atmosphere in a terminal explosion. Obviously some fragments do reach the ground, because we have stony meteorites in our museums. Such falls are known to cause property damage from time to time. On October 9, 1992, a fireball was seen streaking across the sky all the way from Kentucky to New York. A 12 kg stony meteorite (chondrite) from the fireball fell in Peekskill, New York, punching a hole in the rear end of an automobile parked in a driveway and coming to rest in a shallow depression beneath it. Falls into a Connecticut dining room and an Alabama bedroom are other well documented incursions in this century. A 10m body typically has the kinetic energy of about five Hiroshima Atomic bombs, however, and the shock wave it creates can do considerable damage even if nothing but comparatively small fragments survive to reach the ground.
Many fragments of a 10m iron meteoroid will reach the ground. The only well studied example of such a fall in recent times took place in the Sikhote-Alin Mountains of eastern Siberia on February 12, 1947. About 150 tons of fragments reached the ground, the largest intact fragment weighing 1745 kg. The fragments covered an area of about 1-2 km^2, within which there were 102 craters greater than 1 m in diameter, the largest of them 26.5 m (87 ft), and about 100 more smaller craters. If this small iron meteorite had landed in a city, it obviously would have created quite a stir. The effect of the larger pieces would be comparable to having a supersonic auto suddenly drop in! Such an event occurs about once per decade somewhere on Earth, but most of them are never recorded, occurring at sea or in some remote region such as Antarctica. It is a fact that there is no record in modern times of any person being killed by a meteorite.
It is the falls larger than 10 m that start to become really worrisome. The 1908 Tunguska event was a stony meteorite in the 100-m class. The famous meteor crater in northern Arizona, some 4,000 ft. in diameter and 600 ft. deep, was created 50,000 years ago by a nickel-iron meteorite perhaps 60 m in diameter. It probably survived nearly intact until impact, at which time it was pulverised and largely vaporised as joules of kinetic energy were rapidly dissipated. An explosion equivalent to some 15 million tons of TNT creates quite a bang! Falls of this class occur once or twice every 1,000 years.
There are now over 100 ring-like structures on Earth recognised as definite impact craters. Most of them are not obviously craters, their identity masked by heavy erosion over the centuries, but the minerals and shocked rocks present make it dear that impact was their cause. The Ries Crater in Bavaria is a lush green basin some 25 km (15 miles) in diameter with the city of Nordlingen in the middle. Fifteen million years ago a 1,500-m (5,000-ft.) asteroid or comet hit there, excavating more than a trillion tons of material and scattering it all over central Europe. This sort of thing happens about once every million years or so. Another step upward in size takes us to Chicxulub, an event that occurs once in 50-100 million years. Chicxulub is the largest crater known which seems definitely to have an impact origin, but there are a few ring-like structures that are 2-3 times larger yet about which geologists are suspicious.
There are now more than 150 asteroids known that come nearer to the Sun than the outermost point of Earth's orbit. These range in diameter from a few meters to about 8 km. A working group chaired by D. Morrison estimates that there are some 2,100 such asteroids larger than 1 km and perhaps 320,000 larger than 100 m, the size that caused the Tunguska event and the Arizona meteor crater. An impact by one of the latter in the wrong place would be a great catastrophe, but it would not threaten civilisation. An impact by an 8-km object is in the mass extinction category. In addition there are many comets in the 1 to 10-km class, 15 of them in short- period orbits that pass inside Earth's orbit, and an unknown number of long period comets. Virtually any short-period comet among the 100 or so not currently coming near to Earth could become dangerous after a close passage by Jupiter.
An individual's chance of being killed by a meteorite is ridiculously small as compared to death by lightning, volcanism, earthquake, or hurricane, to say nothing of the multitude of human-aided events. That small probability was unlikely to have been any consolation to the dinosaurs, however. For this reason astronomers today are conducting ever-increasing searches for all of the larger asteroids that could become dangerous. Once discovered, with a few years of warning, there is every reason to believe that a space mission could be mounted to shove them aside.
The above articles are taken from the NASA database, accessed via the Internet.
