by The Great Christ Comet- Revealing the True Star of Bethlehem (retail) (epub)
Long-Period and Short-Period Comets
Astronomers like to divide comets into two major groups: long-period and short-period ones. Long-period comets take more than 200 years to complete a single orbit, while short-period comets have an orbital period of 200 years or less. In terms of performance and productivity, long-period comets are wild and powerful in contrast to short-period comets, which are tame and weak.107
Long-Period Comets
Many of the individual comets observed over the last few hundred years have been in extraordinarily long orbits with periods in the hundreds, thousands, and even millions of years. Indeed almost all the great comets in history are long-period comets.
Long-period comets have no particular preference for the plane of Earth’s orbit, with many cutting across it at very sharp angles. They may have any inclination from 0° to 180°. Accordingly, they may journey around the Sun in a prograde (counterclockwise) or retrograde (clockwise) fashion. They are widely considered to originate in a nearly spherical region extending halfway to the nearest star, known as the Oort Cloud.
These long-period comets, hailing from the deep and dark reaches of the solar system, far from the Sun’s light and warmth, have the potential to become notably bright as they approach the Sun, because they contain a high concentration of volatiles.108 They are visible for longer and at greater distances than short-period comets. Some are large, and a small percentage are giant size.
With respect to their constitution, long-period comets also tend to be more fragile and susceptible to fragmentation.109
The brightest long-period comets in history tend to be ones that have made close approaches to the Sun.110 Some, known as sungrazers, come perilously close to the Sun at perihelion. From their population come many of the brightest and most spectacular comets in history.111 One example of a sungrazer is Comet Ikeya-Seki, which has a period of 877 years and in October 1965 came as close as 0.008 AU to the Sun (by comparison, the Moon is 0.00257 AU from Earth). Some sungrazers clearly belong to the same family. For example, the Kreutz Sungrazing Group consists of comets with high-inclination and 600- to 1,100-year orbits.112 One member of this group, the Great September Comet of 1882, became a stunning daytime comet around perihelion time, partly because it fragmented when exposed to the raw pressure of the Sun’s gravitational pull and its fierce heat (see fig. 5.23). The Kracht Group consists of comets that come as close as 0.047 AU from the Sun and have a relatively low inclination (roughly 13.4 degrees). The Great Comet of 1680 was not related to either of these groups of comets, but nevertheless came to within 0.0062 AU of the Sun.
FIG. 5.23 A sketch of the Great September Comet of 1882, as seen from Cairo, Egypt. It peaked at apparent magnitude -17 or -15 to -20, and was seen in broad daylight. Image credit: The Graphic (November 4, 1882): 477.
Passing this close to the Sun may prove catastrophic for a comet. Comet ISON (which had a perihelion distance of 0.0124 AU), for example, did not survive the Sun’s scalding and gravitational pull as it made its way around the solar sphere on Thanksgiving Day 2013, but simply disintegrated. Many sungrazers share this fate.113 Those that survive the close encounter with the Sun do so because they are relatively large (over 2 km in diameter) and structurally sound and because they are hurtling so fast at that point in their orbit—the effect is similar to the rapid movement of a finger through the flame of a candle.114 As Fred Schaaf comments, “If the Moon orbited Earth at such a speed, we would see it complete its orbit and go through its entire set of phases in less than an hour. If Earth traveled around the Sun at this velocity, each season would last about 3 days, and the year would complete in less than 2 weeks. . . . No other enduring, discrete, macroscopic object in our solar system travels anywhere near so fast.”115
Short-Period Comets
Short-period comets spend much or all of their time in the inner solar system. They include a wide range of comets and may be subdivided into two main categories, Halley-type (more than 20-year periods but less than or equal to 200-year periods) and Jupiter-type (20-year periods or less).
Halley-type comets are, of course, named after Halley’s Comet. This comet has been observed since 239 BC and perhaps even earlier. Its orbital period has consistently been 75–80 years. Like long-period comets, Halley-type comets may travel around the Sun in either a prograde or a retrograde revolution. However, in general they are less steeply inclined than long-period comets.
Most Jupiter-family comets are very narrowly inclined to the ecliptic and are prograde,116 and have orbits that are greatly indebted to Jupiter’s gravitational influence. In short, Jupiter, acting like a pinball flipper,117 is able to fling narrowly inclined, prograde, long-period comets that make close passes by it into short orbits. Thereafter the orbits of these “captured” comets are perturbed by frequent close encounters with Jupiter.
Within the Jupiter family is the group of Encke-type comets. The shape and orientation of these comets’ orbits around the Sun are modified by long-range interactions with Jupiter, but their orbits no longer bring them into close approaches to Jupiter.118 Indeed their entire orbits are entirely within Jupiter’s orbit.119
Over time, it seems that a comet’s nucleus may form a crust that seals its remaining volatiles inside.120 As a consequence, it may cease to react to the Sun’s heat and therefore no longer develop a coma and tails. Since the nucleus itself is darker than freshly laid asphalt, it ceases to be visible to naked-eye observers on Earth and is liable to be mistaken for an asteroid.121 Astronomers had classified 4015 Wilson-Harrington an asteroid but then discovered some old images from decades beforehand that revealed that it had formerly sported a gas tail.122 Comets that cease reacting to the Sun may be either dormant (that is, generally inactive, but occasionally flaring into life for a limited period when some of their volatiles are freshly exposed) or extinct (that is, devoid of volatiles and therefore never reacting to the Sun). Comet Encke, with its orbital period of 3.3–3.5 years, burst to life in 1786, but scholars have been unable to find a single reference to it in the historical records stretching back over two millennia,123 most likely because it was dormant most, if not all, of that time.124 The comet is now, it would seem, in the last decades of its current phase of activity.
It should also be noted that, due to fragmentation, each Jupiter-family comet is probably little more than a small kernel of a larger original progenitor comet.125
Orbital Elements
The orbit of a comet at any particular point in time, and therefore its place within the dome of the sky, may be fully known if we have six pieces of technical information, known as the orbital elements. The six elements are the closest distance that the object comes to the Sun (perihelion distance) in AU (q) and the time when this occurs (T), the eccentricity (e) of the cometary orbit, the inclination of the plane of that orbit relative to the plane of the ecliptic (i), the point where the cometary orbit crosses the plane of the ecliptic as the comet moves from the south to the north (the longitude of the ascending node) (Ω), and the angular distance from there to the perihelion point (the argument of perihelion) (ω).126
FIG. 5.24 The orbital elements of a comet. Earth’s orbital plane is known as the ecliptic. Image credit: Sirscha Nicholl.
You can insert these pieces of information with respect to any comet into planetarium software such as Starry Night® Pro,127 Redshift,128 or Project Pluto’s Guide129 and follow the orbital course of the comet.
In order for the six orbital elements to be calculated approximately, at least three good-quality observations of a cometary apparition must be made. The more observations on which a cometary orbit is based and the longer the length of time they span, the more accurate the orbital elements will be.130
Whenever a set of orbital elements is determined, they may remain valid for only a relatively brief window of time, becoming increasingly unreliable as one moves forward or backwards in time due to gravitational and, to a lesser extent, nongravitational effects. We shall now consider thes
e two factors briefly in turn.
Gravitational Effects
When a comet comes close to a planet either on the way toward or away from the inner solar system, it is gravitationally perturbed. This can have a significant effect on its orbit. Jupiter has the greatest gravitational pull in the solar system and so is the chief “bully” of the comet population.131 For example, it was an encounter with Jupiter in April 1996 that changed the period of Hale-Bopp from 4,269 years to 2,534 years.132 Saturn is likewise capable of seriously perturbing comets that venture too close. Other planets such as Uranus and Neptune may also act to change the speed and trajectory of a comet.
Jupiter may also throw comets out of the solar system altogether, or cause comets to split or disintegrate. In one particularly famous case, that of D/1993 F2 (Shoemaker-Levy 9), the comet, after getting trapped in an ever-decreasing short-period orbit around Jupiter, was split into pieces under tidal forces when it made a close approach to the gas giant in 1993. Then the resultant objects spread out around the orbit and collided with Jupiter the next time they passed it, in July 1994. Working out the effect of planetary perturbations on a given comet’s orbit is a complex business, undertaken only by those who specialize in the field of solar system dynamics.
Nongravitational Effects
The most important nongravitational effect on a comet is outgassing. Comet Encke’s orbital period shortened from 3.5 years at the end of the eighteenth century to 3.3 years in the 1970s. From that point it stabilized. The reason for 2P/Encke’s acceleration was that its spin axis was tilted in such a way that the nucleus rotated in a direction opposite to its orbital motion.133 As a result, the rocket effect of its outgassing sped the comet up.134
Fragmentation and Destruction of Comets
Comets are relatively fragile objects and sometimes break up, whether due to the explosive release of internal pressure, collisions with small solar system bodies, and/or the gravitational pull of Jupiter or the Sun. This can result in boulders (e.g., Hyakutake) or significant fragments (e.g., 73P/Schwassmann–Wachmann) being thrust away from the nucleus or the wholesale disintegration of the nucleus (e.g., C/1999 S4).135
As we have already seen, some comets climax their career by being swallowed up by the Sun or by colliding with a planet.
Comets and Meteoroid Streams
Comets are responsible for most meteor showers and meteor storms. Due to outgassing and/or fragmentation, comets deposit along their orbital course a stream of dust particles, stones, and some boulders.
As soon as each dust particle has been ejected from the nucleus, it orbits the Sun in its own path, which is, naturally, almost identical to that of the parent comet. As a result, there are ribbons of particles and little stones journeying around the Sun on similar orbits, subject to the effects of the planets’ gravitational pull. Over time the orbits of these particles, or meteoroids, evolve and the meteoroids spread out, so that the ribbons become convoluted and contorted.136 These ribbons (or groups of ribbons) of meteoroids on evolving orbits are called meteoroid streams. The primary way we come to discover their existence is when they cross the plane on which Earth orbits, about 1 AU away from the Sun, and Earth passes through them (fig. 5.25).
FIG. 5.25 How meteoroid streams may give rise to meteor showers and storms. The meteoroids in a meteoroid stream are crossing Earth’s orbit when Earth is present. This image is not to scale. Image credit: Sirscha Nicholl, using an image of Earth at night from NASA Earth Observatory.
Essentially, meteors are bright streaks in the night sky that occur when meteoroids crash into Earth’s upper atmosphere. The meteoroids begin to heat up when encountering resistance at 130–150 km altitude. By the time they have reached 100 km they are bright enough to be visible on Earth as “shooting stars.” By 75 km, most have disappeared. Larger meteoroids, however, are able to penetrate further through the atmosphere and tend to produce brighter, bigger meteors. Especially bright meteors are called fireballs; their brightness will be equal to or greater than that of the planets Jupiter or Venus.137
When the trails of a number of meteors seem to point back to one particular point in the sky, called a radiant, they are regarded as constituting a “meteor shower.” Whenever astronomers judge that more than 1,000 meteors would have been observable per hour in a dark sky had the radiant been at the zenith (the imaginary point immediately above an observer’s head), they classify the phenomenon as a “meteor storm.”
During the great meteor storm of 1833, hundreds of thousands of meteors seemed to radiate from the head of Leo the lion, causing many observers in North America to imagine that the stars were falling from the sky, that the heavens were on fire, and that the world was coming to an end. This and other Leonid meteor storms before and since then were due to a dense section of meteoroids along the meteoroid stream parented by the Halley-type comet Tempel-Tuttle. Meteor storms radiating from the constellation Andromeda in 1872 and 1885 were obviously linked to the splitting and disintegration of the Jupiter-family Comet Biela over the previous few decades. Meteor showers and storms are usually related to Jupiter-family or Halley-type comets. However, some meteor outbursts have been identified with long-period comets with orbital periods of less than 10,000 years that come within 0.1 AU of Earth.138
Most meteors, however, are “sporadic,” that is, they are not associated with any particular shower. Many undoubtedly did in the distant past belong to clearly defined meteor showers, but it has now been so long since they were last replenished with fresh debris that the streams are too diffuse to be identified as showers.
FIG. 5.26 The radiant of the Andromedid Meteor Storm on November 27, 1872. Chromolith by F. Méneux. From Amédée Guillemin, Le Ciel, notion élémentaire d’Astronomie physique (Paris: Librairie Hachette et Cie, 1877), 623.
Great Comets
Some comets set themselves apart from the majority by virtue of their sheer magnificence—their brightness and largeness and/or length seen against the backdrop of a dark sky. Such comets are classified as “great comets.”
There is widespread agreement regarding the attributes that render a comet “great.”139
First, a comet may make a close pass by the Sun. Chief among those attaining to greatness primarily because they came close to the Sun are the Kreutz Sungrazers. For example, the sungrazing Great March Comet of 1843 (see figs. 5.27–28) came to within 0.005 AU of the Sun (about 130,000 km from the Sun’s surface),140 traveling so fast (560 km a second) that it made its way three-fourths of the way around the Sun in less than 12 hours,141 and attaining to -7 to -10 magnitude.
FIG. 5.27 A painting of the magnificent Great Comet of 1843. From Amedée Guillemin, The World of Comets, ed. and trans. James Glaisher (London: Sampson Low, Marston, Searle, & Rivington, 1877), opposite page 152. Image credit: Patrick Moore Collection, www.patrickmoorecollection.com.
FIG. 5.28 An impression of the 1843 Comet by D. A. Hardy. Image credit: The Patrick Moore Collection, www.patrickmoorecollection.com.
Second, a comet may make a close approach to Earth. The closer a comet comes to our planet, the brighter it seems to human observers—even normally dim objects can become remarkably bright when they make close passes by Earth. Among the great comets notable for coming near to Earth is the Great Comet of 1861, which came as close as 0.13 AU to Earth, and Comet Hyakutake, which came to within 0.1 AU of Earth on March 25, 1996.
Third, a comet may develop a large, bright coma. Since large nuclei tend to have more volatiles to react to the Sun and hence become more active, they usually form bigger comas. Among those considered great primarily because of the large size of their comas are the Great Comet of 1811 (see figs. 5.29–31) and Hale-Bopp in 1996–1997, neither of which came close to the Sun or Earth.
Fourth, a comet may sport an eye-catchingly long tail. A great comet will generally have a tail that is 10 degrees or more in length. The Great Comet of 1618 (C/1618 W1) and Tebbutt’s Comet of 1861 are two striking examples of comets rendered great because
of their impressively long tails, 104 and 120 degrees respectively.
Fifth, a comet should offer the general public in the more populated northern hemisphere good viewing opportunities, preferably in a dark sky (at least 16–20 degrees from the Sun) in the hours after sunset, and should capture the public’s attention. In addition, the comet should be clearly visible to the naked eye for a significant period of time.
Almost all great cometary apparitions are associated with long-period comets. The only short-period comet whose apparitions may be considered “great” is Halley’s Comet.
FIG. 5.29 The Great Comet of 1811, with its prominent pseudonucleus, as drawn by William Henry Smyth. From George F. Chambers, The Story of the Comets (Oxford: Clarendon, 1909), 130 fig. 47.
FIG. 5.30 The Great Comet of 1811. A painting by C. H. R. Schreiber, “Komet von 1811 der Burg Katz,” in Bibliothek des allgemeinen und praktischen Wissens, vol. 4, ed. Emanuel Müller-Baden (Berlin: Deutsches Verlagshaus Bong, 1912). This was one of a number of comets that developed a coma that appeared larger than the Sun or Moon. Source: Wikimedia Commons.
FIG. 5.31 The Great Comet of 1811 as seen on October 15, 1811, from near Winchester, England. An engraving by H. R. Cook based on a drawing by Abraham Pether. Image credit: The Patrick Moore Collection, www.patrickmoorecollection.com.
Ancient Cometary Records
While the places of the planets and Sun and Moon could be established by the ancients by means of calculation, the unpredictability of comets meant that they had to be observed and careful records of each stage of their apparition kept. We are fortunate that the Babylonians and Far Easterners made astronomical observations on a regular basis and over a long period. Their reason for doing so was largely their belief in astrology.142
