Comets

573-581

I am like a slip of comet,
Scarce worth discovery, in some corner seen
Bridging the slender difference of two stars,
Come out of space, or suddenly engender'd
By heady elements, for no man knows;
But when she sights the sun she grows and sizes
And spins her skirts out, while her central star
Shakes its cocooning mists; and so she comes
To fields of light; millions of travelling rays
Pierce her; she hangs upon the flame-cased sun,
And sucks the light as full as Gideons's fleece:
But then her tether calls her; she falls off,
And as she dwindles shreds her smock of gold
Between the sistering planets, till she comes
To single Saturn, last and solitary;
And then she goes out into the cavernous dark.
So I go out: my little sweet is done:
I have drawn heat from this contagious sun:
To not ungentle death now forth I run

Gerard Manley Hopkins, 1864

The tranquility of the night sky is an illusion. The immutability of the heavens is equally a disguise. Even a casual watcher of the sky is treated, on a nightly basis, to a number of "fireworks". On a clear night a patient observer will see several "falling stars". Every few years or so a brilliant fireball will literally flash and thunder across the sky. Perhaps every two decades a wondrous pale wisp will grace the evening or morning skies for several weeks. All of these phenomena are related and yet in some ways quite distinct. In this section we turn our attention to comets. Comets Through the Ages Early astronomers up to the time of Galileo (1620) debated the location of comets. Some (Aristotle to Galileo) contended that comets were atmospheric phenomena. Others such as the Roman poet Seneca to Kepler, Brahe and Maestelin held that comets moved "among the planets" Kepler's discovery of the elliptical nature of orbits helped pave the way for a better understanding of cometary motion and the understanding that comets are distinct objects in our solar system. THe most significant historical episode was Newton's mathematical explanation of cometary orbits and Halley's prediction of the return of the Great Comet in 1757.

Figure 19.14 Comet west as seen in 1976 Click to enlarge. (copyright Peter Stättmayer of the Munich Public Observatory)

Halley's comet has been followed down through the ages and made a foreboding appearance during the Battle of Hastings and appears in the Bayeaux Tapestry. Every passage of this comet since 240 BC has been noted .Improvements in mathematical techniques during the early 19th century made the prediction of orbits more reliable and many comets were observed and catalogued.

The Properties of Comets

All comets share a set of common of properties that tell us not only where the come from but also what they are.

The Orbits of Comets

• Short Period: (such as Halley's) with periods from years to decades, low orbital inclination and prograde motion.
• Long Period: centuries to thousands of years (Hyakutake: Period 18 400 a), any inclination and just as likely retrograde as prograde.

Composition ("Geology") of Comets and Their Nuclei

Table 19.3 summarizes the main properties of comets - which would include a comet such as Comet West shown in Figure 19.14 or Hale-Bopp shown in Figure 19.15. Figure 19.15 is a roll-over image that shows the main parts of a comet: the nucleus, coma and dust and ion tails.

Comets consist of primarily volatile molecules making up various ices. They are aptly described as "dirty snowballs". As they approach the sun they begin to heat and the ices vaporize. Water ice evaporates and the H2O liberated is quickly broken down to hydrogen and oxygen gas. Major chemical constituents include: cyanide (HCN), Methyl cyanide (CH3CN), CN, CO, NH, OH  When far from the sun the comet is a cold, dark "ice ball" only a few km across. As it nears the sun it begins to "melt" and forms a nucleus. Eventually a coma extending as much as 100 000 km from the nucleus forms The comet becomes surrounded by a sheath of hydrogen gas that is easily excited to glow. The pale white light that we see is the result of fluorescence . Cometary tails can reach tens of millions of km in length! Comets can exhibit highly variable behaviour as the nucleus heats up. Gases can vent from the comet and perturb its orbit as well as dramatically change its appearance. Figure 19.16 shows a vent on the nucleus of Comet Halley as viewed by the Giotto satellite in 1986. In addition to this light is also reflected from the dusty part of the comet. In fact, as the tail of the comet grows it will often split into two distinct parts. The ion tail points straight back or forward always opposite the sun. It glows through fluorescence and is quite variable from night to night. The dust tail on the other hand can become distinctly arched and is seen as it trails along the orbit of the comet. It is the dust left behind by this tail that contributes to the meteoroid population Table 19.3 Summary of the composition and properties of comets
	Figure 19.15 Comet Hale-Bopp in the spring of 1997 (Image courtesy B. Martin, The King's University College Observatory)
	Figure 19.16 A gas vent on the nucleus of Comet Halley in 1986. Image from the Giotto satellite (Image courtesy NASA)

Example 19.5 Even though we can observe comets almost "around the clock" from many observatories and thus get excellent positional information, cometary orbits can be fickle things! The orbital characteristics of many short period comets need to "tweaked" from time to time - why is this?

Solution: Figure 19.16 gives the answer! Venting from comets occurs as the comets approach the Sun and heat. The gases that are expelled can act like small "thrusters" on the comet and cause small changes in the comet's trajectory. These events occur randomly on the nucleus and hence cannot be predicted. Over a number of orbits these changes lead to detectable deviations from the predicted orbit.

A "Dirty Snowball"

An apt description for a comet is that it is a dirty snowball. Figure 19.17 is an image of the nucleus of Comet Wild2 taken by the Stardust space probe which passed close to Wild2 in February 1999 and returned to Earth bearing a canister of cometary material collected as the probe passed through the tail and coma of the comet.

Figure 19.17 The nucleus of Comet Wild 2 as seen by the Stardust flyby mission in February 1999 (Image courtesy NASA),

The Comet Wild 2 samples contained some surprises! The mission was named "Stardust" in anticipation of what most astronomers thought comets should be like - ices impregnated with lots of "dust" that was part of the original solar nebula. While the samples from Wild 2 do contain very small amounts of pre-solar dust they also contain some very complex materials including chondrules and white, irregular shaped crystals called Calcium-Aluminum Inlcusions or CAIs. This paints a puzzling picture in which the comet appears to be composed of ices from the far reaches of the solar system and rocky materials which must have formed much closer to the Sun. This finding has profoundly challenged older notions of what comets are and how they formed. It now appears that comets must have formed much closer to the sun and then been transported (perhaps through violent collisions in the earthly solar system) to the outer parts of the solar system.

In addition the discovery of chondrules and CAIs in samples from Wild 2, astronomers have also detected complex organic molecules including glycine - one of the amino acids necessary for life!

Tales about Comet Tails

Many comets display two distinct kinds of tails. Figures 19.14, 19.15 and 19.18 show bright, white tails on comets West and Hale-Bopp as well as fainter, bluish tales. The bright white tails are the dust tails and usually have a characteristic bent or arc-like shape. This is caused by larger, dusty materials evaporating off the comet as it approaches the Sun and heats up. THe material continues to follow along with the comet and traces out the orbit of the comet. The blue tail consists of molecules that become ionized by solar radiation. These ionized molecules form a fainter, blue tail called the ion tail. The molecules are easily pushed by the pressure of the solar wind and radiation and hence tend to be much straighter and on a line radially outward from the Sun.

Figure 19.18 Dust and Ion Tails of Comet West. (copyright Peter Stättmayer of the Munich Public Observatory)

Origin and Distribution of Comets

Comets are most likely primordial ie - they formed early in the condensing solar nebula and represent very ancient material. The data from Comet Wild 2 suggest that formation occurred close to the Sun and that interaction events with the newly forming planets in the early solar system ejected cometary bodies to more remote parts of the solar system. The orbits of comets are usually highly elliptical which indicates that they travel deep into interstellar space and spend most of their time in the deepfreeze of space.

The solar system is believed to be surrounded by:

• a belt of comets just beyond the orbit of Neptune. This belt is called the Kuiper Belt after the American astronomer who predicted its existence.
• a cloud of comets called the Oort Cloud after Jan Oort - a Dutch astronomer who predicted this in the early part of this century. This cloud could contain up to 1 trillion comets and reaches about 100 000 AU in radius. At this distance nearby stars can perturb the Oort cloud enough to "kick" comets into toward the sun. This is shown in Figure 19.19.

Figure 19.19 Flash animation of the Oort Cloud

Example 19.6 The Oort Cloud extends from 10 000 to almost 100 000 AU from the Sun and is a vast reservoir of comets. Why, however, is it unlikely that the comets formed there?

Solution: There are at least two reasons:

1. The amount of material in the Solar Nebula at the current position of the Oort Cloud would have been far to sparse to build cometary nuclei
2. Comet Wild 2 shows that cometary nuclei contain materials that could only form close to the Sun - chondrules for example require extreme temperatures to form. Again this could not happen in the Oort cloud.

Why Should We Care About Comets?

Comets - Traveling Museums of the Early Solar System: There is ample reason to think that comets are made of material that formed very early in the history of the solar system. Analysis of cometary material will give us valuable insight into the earliest stages in our planetary system's formation. Cometary Collisions in the Past and the Future: In July 1994 we witnessed a spectacular event that has never before been observed but has likely been played out numerous times in the past and will, no doubt, occur in the future. That event was the collision of comet Schoemaker-Levy 9 with Jupiter. In the early history of the solar system we suspect that cometary collisions were frequent and it is now thought that large amounts of water and other light elements could have been deposited "special delivery" by cometary bodies.

Figure 19.20 HST images of the impact site created by Shoemaker-Levy 9 on the face of Jupiter. (Image courtesy NASA)

Comet Linear 2000/WM1

Comet Linear 2000/WM1 is an example of one of the many small comets that pass through the solar system each year. This comet was discovered by the Lincoln Near Earth Asteroid Research (LINEAR) program conducted by The Massachusetts Institute of Technology. This is a project designed to detect near-earth orbiting objects and to date has discovered more than 250 000 asteroids, over 2000 NEA's and more than 200 comets. This set of images was taken on the night of December 3, 2001 at The King's University College Observatory and shows the motion of the comet relative to the background stars over a 30 minute period. As we shall see in the next section, a collision with even a small comet such as LINEAR/WM1 could have catastrophic consequences for Earth.

Figure 19.21 Flash applet showing the motion of comet LINEAR/WM1 over a 30 minute period on the evening of December 3, 2001. (Image courtesy of The King's University College Observatory)

Practice

1. True or False?
   1. Most comets travel along circular orbits
   2. Cometary orbits usually lie in the orbital plane of the planets
   3. Comets can travel as much as 100 000 AU from the Sun in their orbits around the Sun
   4. Comets contain complex organic molecules as well as rocky material
   5. Comets formed at a much greater distance from the Sun than did the planets
   6. Comet tails contain deadly poison gases such as cyanide

What is wrong with the depiction of the tail of the comet in this picture? Sketch how you think the comet and its tail should be represented in this picture. What was the Stardust Mission and how has it radically changed our ideas about the formation of comets? What is the difference between the Kuiper Belt and the Oort Cloud?