The Great Christ Comet (20 page)

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.

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

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 Guide
129
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 these 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
).