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Comets in Perspective:
What the Comet Halley probes tell us
F. Glenn Graham

Nowhere is the ancient passion for astronomy any more evident than in the pre-Columbian ruins of Mesoamerica. In the Yucatan, and other parts of Mexico and Central America, ancient remains have been uncovered showing a vast system of cities and their satellites, a network of straight stone-paved highways between them, and ingenious astronomical alignments of pyramids and temples, themselves richly decorated with astronomical symbols. It seemed an appropriate setting, in introducing the topic of perception in ancient astronomy, to observe and to photograph for the reader (see cover) still another apparition in the heavens which has played a major role in ancient cosmologies - the temporary appearance of a cometary visitor. Then, as now, comets were a source of intense interest not only to the astronomers, but a matter of special attention among the general population as well. Over the ruins at Kabah in the Yucatan, Halley's comet of 1986 was not as dramatic as its appearance in 1910, but it was no less fascinating. The varied and changing appearance of comets, sometimes spectacular, during their relatively brief periods of visibility, always have stirred a flurry of speculation and debate. Today, we seek to know the origin, the composition, the orbit etc., with advanced technology to help find the proper scientific niche for comets in the scheme of creation. In the ancient world, however, the common perception of the visitor was wholly different. Here, the perspective was that a special messenger had arrived; an agent of the astronomical gods that ruled the cycles of time and controlled the destiny of Earth -an intelligent celestial being with its attention directed at humankind and a message for its prophets to interpret. Seeing the comet rise above the ruins of an ancient temple, one could only wonder if, in some ancient age, an earlier appearance comet Halley announced to its astral prophets the disaster that was to befall Mayan civilization. In the following article, astronomer F. Glenn Graham summarizes some of the scientific findings regarding this mysterious, recurrent visitor from the depths of space. -Ed.


There it was! Comet Halley appeared as a fuzzball in my 6-inch f/12 reflector on that magic April 19 comet party. Wayne B. stepped up to the eyepiece to have a look; as it was nearly opposite in the sky than the sun, we were looking at it head-on. It was Wayne's second sighting of Halley; much earlier in his life, in May 1910, he saw it tail-on from a Nevada ranch. Two other women, who had missed it as young girls in 1910, also saw it now for the first time. Ralph, who probably liberally imbibes carrot juice, could see it naked- eye.

Everywhere comet-watching get-togethers such as this are occurring. Halley is being watched by an international organization of astronomers and observatories and has been met by an international armada of interplanetary probes. Two are from Japan, two from the USSR, and one was sent by Western Europe. This international assembly of technology has sampled the energies of the comet and has photographed the nucleus of a comet for the first time. Even when behind the sun, Halley was watched by the Pioneer probe orbiting Venus, which had a grand view of our recurrent visitor at this time.

The results are still being sorted through. This article presents some of those results, and describes some of what we now know, and what we don't, about these visitors from the outer reaches of space.

What are comets?

Comets are small (dozens of kilometers in maximum dimension) objects that are absolutely out of equilibrium within the inner solar system. Their composition, predominated by volatiles, cannot exist in a stable state within the range of solar radiation fluxes present as far as the orbit of Saturn. When we see a comet from our Earth vantage point, we view it (with exceptions) while it is on a very small portion of its total orbit. The calculated orbits of comets show them to spend the major portion of their time beyond Saturn's orbit. The long tail and bright head we see in the more visible comets characterize an object very much out-of-equilibrium for a relatively short period of an orbit that may take the comet beyond the reaches of observation by existing instruments for two or three thousand years.

The elongated orbits of comets sometime carry them near the giant planets Jupiter, Saturn, and Uranus. Here, they may be gravitationally perturbed from their initial long-period orbits to shorter ones, or ejected from the solar system altogether. Both types of event actually have been observed to occur. All of

the shorter period comets, such as Encke and Halley, are believed to be comets whose orbits have been so modified. Encke is the shortest period comet known (3.3 years). It is also significantly depleted of volatiles and is not bright at all. It is visible only with a telescope. This depletion supports the perturbed orbit view. Halley, with a 76-year period, is carried under the solar system away from the giant planets. Coincidentally, the phases of the moon occur on the same dates in 1910 as in 1986 and this will again be so when Halley returns in 2062.

Once inside the orbital radius of Saturn, these cometary nuclei can absorb only so much solar energy before they evaporate all of their volatiles and break up; the end result is a meteor stream. Once again, this has been observed; Comet Biela broke apart before the eyes of 19th century astronomers to become the Andromedid meteor shower which itself depleted by century's end. This disintegration is also going on continuously at a lower rate: the Orionid and Aquarid meteor showers come from Comet Halley.

Sometimes, comets hit planets! In our own century, the Tunguska meteor impact was caused by a volatile body whose pre-impact orbit was consistent with its having been a piece of Comet Encke. Comets have been observed falling into the Sun as well, as recently as 1979. The frequency with which comets and large meteors have impacted the Earth is an important question for theories of biological evolution and perhaps human psychosocial development. Historically, people have feared comets intensely. What reason, dim in human history, is responsible for this seemingly irrational response?

[*!* Image: This space-probe photograph of Halley's comet was taken on March 14, 1986 at 00:06 UTC from a distance of about 18,000 km. the Sun illuminates the comet from the lower right corner of the scene as indicated by the arrow in the inset. In the upper left corner one sees a roughly circular, very dark area having a diameter of 4 km. That area actually is part of a larger elliptical region of 15 km. in the longest dimension and at least 8 km. in the shortest, which appears to be the solid nucleus. It is somewhat larger than expected.

From the right edge of that darkened cigar-shaped region emanate two major bright jets of dust, the lower jet being brighter and wider. These two jets extend for at least 15 km. toward the Sun and represent the major apparent activity of the nucleus emanating from specific areas on the sunlit side.]

Comet chemistry

Comets are composed of a matrix of nonvolatile material, immune to the Sun's energy (except at very close range), similar in composition to the carbonaceous chondrite meteors. We know this because D. Brownlee recovered micrometeors in the stratosphere with a U-2 during several meteor showers, including the Aquarid, and analyzed them. The material is dark: it reflects, at best, only 5% of the light incident upon it. It consists essentially of silicate rock with carbon compounds.

The "volatiles" in this dark matrix are materials with low boiling points: water ice (H2O) predominates, followed by carbon dioxide (CO2), "dry" ice. There are other organic materials present, too, such as methyl cyanide (CH3CN), hydrogen cyanide (HCN), alcohols of various kinds, and hydrocarbons such as methane (CH4 and C4H8). When the comet approaches the Sun these volatiles absorb solar energy and go through the complex changes that produce the often spectacular appearance of comets.

The proportion of comet matrix to volatiles was originally considered to be in favor of the volatiles; astronomer F. Whipple has described a comet theoretically as a "dirty snowball". The actual images of Halley returned from the space-probes Giotta and Vega lead one, instead, to conclude that the nucleus more appropriately should be called an "icy dirtball". We must remember however that Halley has made many passages near the Sun (29 in recorded history) and so it is not a pristine comet. Perhaps the ratio of ice to dirt is higher in younger comets that have absorbed less solar energy.

When the "icy dirtball", 5 x 9 miles in dimension and resembling a peanut in shape, approaches the inner solar system the solid volatiles go directly to gas (sublime) because of the low pressure. When H2O, one of the more abundant gases, hits high fluxes of ultraviolet rays it dissociates into hydrogen and hydroxyl. The hydrogen in particular expands out to form a large envelope (0.1 AU) surrounding the comet's head. It is invisible except in the ultraviolet, where it absorbs and re-emits at 1216 Angstroms wavelength and so was detected.

Other materials evaporate, dissociate, and expand too, in a pecking order dependent on the ease in which each species is formed, the distribution of energy in the solar spectrum available to do the job, and, to a lesser extent, the shielding effects of other atoms. Methyl cyanide and hydrogen cyanide separate into methyl, hydrogen, and cyanogen (CN), and this cyanogen absorbs and re-emits in visual wavelengths, giving comets a pale blue tinge.

Hydrocarbons also crack, producing as one component carbon molecules, C2 and C3, which also can be detected with a small visual spectroscope attached to a moderate-sized telescope. Other materials expand also, and absorb, re-emit, and reflect sunlight. At this stage, the small nucleus develops a large reacting cloud; a mini-atmosphere called a coma. Sometimes, differing chemical species and reaction rates produce a two-level coma. The inner, more starlike coma is called a false nucleus.

[*!* Image: Halley's comet during its spectacular pass in 1910. People took "comet pills" to ward off the effects of the poisonous cyanogen gas that had been observed by astronomers in the spectrum of the comet's tail.]

Comet elecromagnetic interactions

As the comet nears the orbital radius of the planet Mars, the radiation flux from the Sun increases rapidly. The rotation of the comet (in Halley's case, 2.2 days) is no longer sufficiently rapid to warm the comet evenly. The Sun-facing side of the comet erupts jets and fissures, seen with Earth telescopes, and this even produces a "rocket effect' which perturbs the comet from its helpless trajectory in the solar gravity. In Halley, between 25 to 60 tons per second of volatiles are evolved out of the nucleus at this stage. The surface temperature of Halley's nucleus has increased to 135F., hardly an icy surface anymore. This prodigious efflux aids in the development of a new phenomenon: the tail.

When the volatiles escape, the pressure becomes low enough so they are no longer shielded, the cometary atoms and molecules ionize, that is, solar ultraviolet and X-rays knock electrons loose from the dissociated atoms. Species like Na+, H2O, CO+, OH+ begin to develop together with others, all absorbing and re-emitting light energy, and so become visible to the eye.

There are four equations governing electronic and magnetic phenomena: Maxwell's laws. One of these laws predicts the action of magnetism on moving charges, and ions are small moving charges. In interplanetary space, the Sun's magnetic field is carried by protons in the solar wind. It is feeble, 10-4 to 10-5 gauss, compared to the gauss of the Earth's surface magnetic field, so even a compass needle scarcely would be affected by it. But ions are not very massive at all (10-24 g. compared with 10-1 g. for a compass needle) and they are complete slaves of the solar wind's magnetic effects. They are accelerated by it to form a beautiful ion tail just according to the Lorenz force law, as they stream away from the comet perpendicular to the field lines.

Thus, large amounts of mass - tons per second - can be moved quite easily if it is first separated into molecules and then ionized.

There is, however, a dual role. Not only does the magnetic field accelerate the moving charges, but another Maxwellian law says that moving charges themselves create a magnetic field which, when combined with the ambient solar magnetic field, produces a magnetic field "draped over" the comet, as shown in Figure 1. One of the most outstanding discoveries of the ICE probe of Comet Giacobini-Zinner in 1985 was confirmation of this, including the discovery that the boundary between comet and solar wind is not sharp, but magnetically chaotic. Other interesting aspects of this dualism are kinks and breaks in the comet's ion tail, caused when the comet crosses the sector boundary where the solar wind magnetic field drastically changes in intensity or reverses altogether.

The ion tail was very bright for Comet Halley. But there was also a dust tail. Dust particles, from spaceprobe data, seem to range in mass from 10-2 to 10-17 grams. They break off from the nucleus with the jets and are the particles for the Aquarid and Orionid meteor showers. The dust can have a slight outside charge by means of the zeta potential, but it would also have displacement by sunlight pressure. The net effect is to displace the dust tail against the sun line in the shape of a large, flat fan.

[*!* Image: Figure 1: A diagram showing the basic features associated with a comet's interaction with the solar wind and the formation of the visible tail composed of magnetic lobes. See text for discussion. LABELS: Magnetic field lines organized into 2 lobes of N (north) and S (south) polarity. Electric current flows in plane separating lobes of opposite magnetic polarity. Visible tail. Bow Shock. Solar Wind]

Where do comets originate

This is an interesting subject, and one must be careful to distinguish fact from speculation. Jan Oort, and earlier T.J.J. See, recognized that even comets which frequent the inner solar system only one year for several millennia would not last throughout geological ages - they soon would be used up! To address this and other problems several thinkers addressed the accumulating evidence on comets with alternate theoretical ideas.

For example, S. K. Vselchsvyatskii proposed a variant version of an earlier general hypothesis that comets were thrown off by the giant planets, particularly Jupiter. Vselchsvyatskii presents evidence to show that comets are better understood as eruptively ejected material "From the surfaces of satellites and planets (the Moon, the satellites of Jupiter and Saturn, Venus, Mars, and the Earth)." [see KRONOS II:2] Vsekhsvyatskii marshaled a comprehensive argument for this mechanism but, because such an event has not actually been observed by astronomers, most opinion remains with the more popular Oort-cloud theory. Ironically, Oort's proposed mechanism for comet origins itself offers no chance at all, at least with present instruments, for confirmation by direct observation.

Jan Oort proposed that a giant swarm of comet nuclei surround the solar system at great distances (104 to 105 AU). They were assumed to have enough angular momentum to avoid falling toward the Sun until a passing star gravitationally perturbed an individual Oortid so that it fell into the solar system. Since stars pass near enough to the solar system only very rarely and since any individual Oortid's, chance of being deflected into the solar system generally is itself very low, the probability of any individual Oortid being nudged into a visible orbital path is tiny squared.

Another difficulty is that the composition of comets indicate an origin within the inner solar system. The theoretical Oort comet-cloud, to satisfy probabilities, must be immensely vast to supply the dozen new comets we see each year. Since their composition suggests comets evolved inside the solar system they must, then, have been ejected outward, according to the Oort model, by interactions with the primordial protoplanets. Only a small fraction of those produced formed the vast Oort comet cloud hypothesized to exist today. For such a result, the early solar system must have been seething with protoplaneary and protocometary activity, in a state familiar to the readers of Hans Hoerbiger, but billions of years ago.

Unfortunately, because of its great theoretical distance, nobody has devised an observational test for the Oort comet cloud. The evidence for its existence is entirely indirect, and its continued acceptance is, in part, only because of imperfections in other models. Yet, Oort's model, itself, makes a prediction not in accord with observations. If comets do in fact circle at 104 to 105 AU and are brought in by random stellar encounters, they should be random in their angular momentum. They aren't.

Some have emerged to compete and offer an alternative view. M.W. Ovenden and T.C. Van Flandern suggested comets are the high-energy residue of a planet that broke up to form the asteroids. This also requires large numbers of comets initially, but eliminates the two-step relocation of comets we see from their creation in the proto-solar system, ejection to the Oort cloud, and reperturbation back into the inner solar system.

In 1985, J. Heisler and S. Tremaine demonstrated the galactic tidal field can bring in comets to the inner solar system much more efficiently than passing stars, and would thus require a reservoir of comets less than 1/3 as large as by Oort's model. Nevertheless, the hypothetical Oort cloud remains beyond observational reach, whatever its proposed size. Still other theories about cometary origin no doubt will emerge as the observations and data base are refined with advancing technology. The conventional model already has been nudged by the evidence from its accustomed path and, like a comet perturbed by the great planets, may be ejected from the system altogether.

F. Glenn Graham holds a B.A. in Interdisciplinary Science and a M.S. in Astronomy from the University of Pittsburgh; he now is completing his doctorate in Planetary Science there. Mr. Graham also is a lecturer at the Buffi Planetarium in Pittsburgh. He has taught courses in Calculus, Physics, and Astronomy at the Community College of Allegheny County, Pittsburgh and is the Editor of the periodical, Selenology, the official publication of the American Lunar society.


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