This is the online edition of In the Beginning: Compelling Evidence for Creation and the Flood
(7th Edition) by Dr. Walt Brown. The online version of the book is designed to be read online.
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How Comets Move
Most comets travel on long, oval paths called ellipses that bring them near the Sun and then swing them back out into deep space. [See Figure 149 on page 273.] The point nearest the Sun on an elliptical orbit is called its perihelion. At perihelion, a comet’s speed is greatest. After a comet passes perihelion and begins moving away from the Sun, its velocity steadily decreases until it reaches its farthest point from the Sun—called its aphelion. (This is similar to the way a ball thrown up into the air slows down until it reaches its highest point.) Then the comet begins falling back toward the Sun, gaining speed until it again reaches perihelion.
Figure 145: What Is Jupiter’s Family? About 60% of all short-period comets have aphelions 4–6 AU from the Sun. (A comet’s aphelion is its farthest point from the Sun.) Because Jupiter travels in a nearly circular orbit that lies near the center of that range (5.2 AU from the Sun), those comets are called Jupiter’s family. (Comets in Jupiter’s family do not travel with Jupiter; those comet and Jupiter have only one orbital characteristic in common—aphelion distance.) Is Saturn, which lies 9.5 AU from the Sun, collecting a family? See the “aphelion scale” directly above each planet.
Why should comets cluster into families defined by aphelions? Why is Jupiter’s family so large? No doubt, Jupiter’s gigantic size has something to do with it. Notice how large Jupiter is compared to other planets and how far each is from the Sun. (Diameters of the Sun and planets are magnified relative to the aphelion scale.)
Short-Period Comets. Of the almost 1,000 known comets, 205 orbit the Sun in less than 100 years. They are called short-period comets, because the time for each to orbit the Sun once, called the period, is short—less than 100 years.24 Short-period comets usually travel near Earth’s orbital plane, called the ecliptic. Almost all (190) are prograde; that is, they orbit the Sun in the same direction as the planets. Surprisingly, about 60% of all short-period comets have aphelions near Jupiter’s orbit.25 They are called Jupiter’s family. [See Figure 145.]
To understand better what is meant by “Jupiter’s family,” look briefly at Figure 150 on page 275. While comets A, B, and C orbit the Sun, only A and B are in Jupiter’s family, because their farthest point from the Sun, their aphelion, is near Jupiter’s orbit.
How Jupiter collected its large family of comets presents major problems, because comets falling toward the Sun from the outer solar system would be traveling too fast as they zip inside Jupiter’s orbit. To slow them down so they could join Jupiter’s family would require such great deceleration forces that the comets would have to pass very near planets. But those near passes could easily tear comets apart or eject them from the solar system.26
Also, comets in Jupiter’s family run an increased risk of colliding with Jupiter or planets in the inner solar system, or being expelled from the solar system by Jupiter’s gigantic gravity. Therefore, they have a life expectancy of only about 12,000 years.27 This presents three possibilities: (1) Jupiter’s family formed less than about 12,000 years ago, (2) the family is resupplied rapidly by unknown processes, or (3) the family had many more comets prior to about 12,000 years ago—perhaps thousands of times as many. Options (2) and (3) present a terrible collection problem. In other words, too many comets cluster in Jupiter’s family, precisely where few should gather or survive for much longer than about 12,000 years. Why?
Long-Period Comets. Of the 659 comets with periods exceeding 700 years, fewer than half (47%) are prograde, while the rest (53%) are retrograde, orbiting the Sun “backwards”—in a direction opposite that of the planets. Because no planets have retrograde orbits, we must ask why so many long-period comets are retrograde, while few short-period comets are.
Intermediate-Period Comets. Only 50 comets have orbital periods between 100 and 700 years. So, we have two completely different populations of comets—short-period and long-period—plus a few in between.
Figure 146: An Early Lesson in Conservation of Energy. At the top of his swing, my grandson Preston has a minimum of kinetic energy (energy of motion) but a maximum of potential energy (energy of height). At the bottom of his swing, where he moves the fastest, he will convert potential energy into kinetic energy. In between, he has some of both.
Eventually, friction converts both forms of energy into heat energy, slowing the swing, and making Preston unhappy. Comets also steadily exchange kinetic and potential energy, but do so with essentially no frictional loss.
Energy. A comet falling in its orbit toward the Sun exchanges “height above” the Sun for additional speed—just as a ball dropped from a tall building loses elevation but gains speed. Moving away from the Sun, the exchange reverses. A comet’s energy has two parts: potential energy, which increases with the comet’s distance from the Sun, and kinetic energy, which increases with speed. Kinetic energy is converted to potential energy as the comet moves away from the Sun. The beauty of these exchanges is that the sum of the two energies never changes if the comet is influenced only by the Sun; the total energy is conserved (preserved).
However, if a comet orbiting the Sun passes near a planet, energy is transferred between them. What one gains, the other loses; the energy of the comet-planet pair is conserved. A comet falling in the general direction of a planet gains speed, and therefore, energy; moving away from a planet, it loses speed and energy. We say that the planet’s gravity perturbs (or alters) the comet’s orbit. If the comet gains energy, its orbit lengthens. The closer the encounter and more massive the planet, the greater the energy exchange. Jupiter, the largest planet, is 318 times more massive than Earth and causes most large perturbations. In about half of these planetary encounters, comets gain energy, and in half they lose energy.
Figure 147: Energies of Long-Period Comets. The tall red bar represents 465 comets with extremely high energy—comets that could, in theory, travel far from the Sun, such as 2,000 AU, 10,000 AU, 50,000 AU, or infinity. These comets, traveling on long, narrow ellipses that are almost parabolas, are called near-parabolic comets. Those who believe that this tall bar locates the source of comets usually represent this broad (actually infinite) range as “50,000 AU” and say that comets are falling in from those distances. Because near-parabolic comets fall in from all directions, this possible comet source is called the “Oort shell” or “Oort cloud,” named after Jan Oort who proposed its existence in 1950. (No one has detected the Oort cloud with a telescope or any other sensing device.)29 Actually, we only can say that 71% of the long-period comets, those represented by the red bar, are falling in with similar and very large energies.
As a comet “loops in” near the Sun, it interacts gravitationally with planets, gaining or losing energy. The green line represents parabolic orbits, the boundary separating elliptical orbits from hyperbolic orbits (i.e., closed orbits from open orbits). If a comet gains enough energy to nudge it to the right of the green line, it will be expelled from the solar system forever. This happened with the few outgoing hyperbolic comets represented by the short, black bar. Incoming hyperbolic comets have never been seen 30—a very important point. About half of all comets will lose energy with each orbit, so their orbits shorten, making collisions with the planets and Sun more likely and vaporization from the Sun’s heat more rapid. So, with each shift to the left (loss of energy), a comet’s chance of survival drops. Few long-period comets would survive the many gravity perturbations needed to make them short-period comets. However, there are about a hundred times more short-period comets than one would expect based on all the gravity perturbations needed.31 (Short-period comets would be far to the left of the above figure.)
If planetary perturbations acted on a steady supply of near-parabolic comets for millions of years, the number of comets in each interval should correspond to the shape of the yellow area.32 The small number of actual comets in that area (shown by the blue bars) indicates the deficiency of near-parabolic comets that have made subsequent trips into the inner solar system. Question: Where are the many comets that should have survived their first trip but with slightly less energy? Hasn’t enough time passed for them to show up? After only millions of years, blue bars should more or less fill the yellow area. Figure 147 shows us that the evidence which should be clearly seen if comets have been orbiting the Sun for only millions of years—let alone billions of years—does not exist. In other words, near-parabolic comets have not been orbiting the Sun for millions of years.
Notice the tall red bar. If these 465 near-parabolic comets had made many previous orbits, their gravitational interaction with planets would have randomly added or subtracted considerable energy, flattening and spreading out the red bar. As you can see, near-parabolic comets are falling in for the first time.33 Were they launched in a burst from near the center of the solar system, and are they just now returning to the planetary region again, falling back from all directions? If so, how did this happen?
* The horizontal axis represents 1/a, a proxy for energy per unit mass. The term “a” is a comet’s semimajor axis. Each cell has a width of 10-3(1/AU).
If a comet gains enough energy (and therefore speed), it will escape the solar system. Although the Sun’s gravity pulls on the comet as it moves away from the Sun, that pull may decrease so fast with distance that the comet escapes forever. The resulting orbit is not an ellipse (a closed orbit), but a hyperbola (an open orbit). (See Figure 148.) The precise dividing line between ellipses and hyperbolas is an orbit called a parabola. Most long-period comets travel on long, narrow ellipses that are almost parabolas. They are called near-parabolic comets. If they had just a little more velocity, they would permanently escape the solar system on hyperbolic orbits.
Figure 148: A Shot Fired Around the World. Imagine standing on a tall mountain rising above the atmosphere. You fire a bullet horizontally. If its speed is just right, and very fast, it will “fall” at the same rate the spherical Earth curves away. The bullet would be launched in a circular orbit (blue) around Earth. In other words, the bullet would “fall” around the Earth continuously. Isaac Newton first suggested this surprising possibility in 1687. It wasn’t until 1957 that the former Soviet Union demonstrated this with a satellite called Sputnik I.
If the bullet were launched more slowly, it would eventually hit the Earth. If the bullet traveled faster, it would be in an oval or elliptical orbit (red).28 With even more speed, the orbit would not “loop around” and close on itself. It would be an “open” orbit; the bullet would never return. The green orbit, called a parabolic orbit, represents the boundary between open and closed orbits. With any greater launch velocity, the bullet would travel in a hyperbolic orbit; with any less, it would be in an elliptical orbit. These orbits will be discussed in more detail later. Understanding them will help us discover how comets came to be.
Separate Populations. Few comets with short periods will ever change into near-parabolic comets, because the large boost in energy needed is apt to “throw” a comet across the parabola boundary, expelling it permanently from the solar system. The energy boost would have to “snuggle” a comet up next to the parabola boundary without crossing it.34 Likewise, few long-period comets will become short-period comets, because comets risk getting killed with each near pass of a planet. This would be especially true if such dangerous activity went on for millions of years in the “heavy traffic” of the inner solar system.
While all planets travel near Earth’s orbital plane (the ecliptic), long-period and intermediate-period comets have orbital planes inclined at all angles. However, short-period comets usually travel near the ecliptic. Comet inclinations change only slightly with most planet encounters.35 Because very few short-period comets can become long-period comets, and vice versa, most must have begun in their current category.