HALLEY'S COMET IS NOW PASSING among the inner planets and around the sun. The comet is appropriately named for the 17th-century English astronomer Edmund Halley; he was the first to discover that it periodically revisits the solar system. Employing laws of mechanics and procedures of observation previously outlined by his contemporary Isaac Newton, Halley found that comets recorded in 1531, 1607 and 1682 had similar orbits. Suspecting that all the sightings were of the same object, he concluded that the comet would return in 1758. It did, observed by an amateur astronomer on Christmas night. Actually recent work indicates that Halley's comet was seen as early as 240 B.C.- it has been observed approximately every 76 years since then. The present return is the fourth since Halley proposed his theory.

Halley's comet will be visible, albeit only faintly, to people living in the Northern Hemisphere, and so I shall discuss the observations that will be possible in those mid-latitudes. My information is derived from the International Halley Watch at the Jet Propulsion Laboratory of the California Institute of Technology and from books by Robert D. Chapman and John C. Brandt of the National Aeronautics and Space Administration's Goddard Space Flight Center [see "Books," page 34], and research by David W. Hughes of the University of Sheffield.

Because the comet will be faint, you will be able to see it only against a dark night sky. Because of the glare from artificial light scattered by the air, you will not be likely to observe it from residential areas. A bright moon will also make the comet invisible. A good test for an observation site is that the Milky Way should be visible. Allow at least 15 minutes for your eyes to adjust to the darkness.

You might try to see the comet with unaided eyes; binoculars will greatly improve the observation because they gather more light. Binoculars with a magnifying power of seven and a 50millimeter objective lens should be best; binoculars with an objective diameter of 35 millimeters will also serve. At times the comet will occupy a large angle in your field of view, so that you may have to scan with the binoculars to see all of it. A telescope with a large magnification is not suitable: the part of the comet visible through a telescope is too small to contrast strongly enough with the background darkness. You might try a wide-angle telescope at its lowest magnification.

Figure 8 indicates where the comet's head will be on various nights in the next few months for an observer at 40 degrees north latitude (approximately the latitude of San Francisco, Denver, St. Louis and Washington, D.C.). For observers at higher latitudes the comet will be lower in the sky and will be above the horizon for shorter periods. Each illustration is a plot of the altitude and azimuth of the comet in degrees. Altitude is measured from the horizon; azimuth is measured clockwise from due north.

The small circles on the graphs indicate the position of the comet's head at hourly intervals on various nights. Positions before midnight are connected with a broken line, those after midnight with a solid line. The first circle on each path shows the date and hour (in local standard time on the 24-hour clock) when the comet's head will be at that location. For example, the top line in the upper illustration indicates that the comet's head will be at an altitude of about 3 8 degrees and an azimuth of about 108 degrees at 17:00 (5:00 P.M.) on December 1. Later that night the head will rise to a peak altitude of about 60 degrees when it is due south. Then it will descend until it sets in the western sky at about 3:00 A.M.

As the nights go by in December and January the altitude of the comet's head at the earliest nightly sighting will decrease. By January 24 the comet will be only briefly visible before it sets in the early evening. During the latter half of both December and January the moon will be bright while the comet is above the horizon and so the viewing will be poor or impossible. The moon also will make viewing difficult toward the end of February, the last week of March and the first and last days of April.

During February and March the observation periods will be short. Since the comet will be low in the sky, you must find an observation point that offers a low and clear horizon. The comet will also be dim during those months; because of its low position, light from it reaches you only after a long trip through the earth's atmosphere. In some locations the scattering of the light by air molecules and aerosols during the long passage may make the comet so dim that it is unobservable. Many experts believe the best opportunity to see the comet will come on about April 15, when the head will be bright and will lie somewhat above the horizon while the sky is dark.

The positions of comets and other celestial bodies are often given as angles measured in relation to the equatorial plane, an imaginary reference object that extends indefinitely outward from the earth's Equator. The plane intercepts an imaginary celestial sphere, the reference object for locating celestial bodies. The equatorial plane divides the northern and southern hemispheres of the celestial sphere. The sun crosses the equatorial plane twice each year, on the vernal and autumnal equinoxes.

In Figure 5 an observer is at the center of the equatorial plane, which is tilted from a horizontal plane extending through his horizon. (The angle of the tilt is related to the latitude of the observer: the higher the latitude, the smaller the angle.) The horizon is indicated by the symbols N, S, E and W. The field of stars seen by the observer lies on the celestial sphere, which appears to rotate about the north celestial pole because of the rotation of the earth.

Two coordinates-declination and right ascension-serve to 1Dcate an object on the celestial sphere. Declination is the angle between the object and the nearest point of the equatorial plane. Right ascension is the angle between that point on the plane and the position of the vernal equinox on the celestial sphere. By convention the angle for right ascension is measured in units of time; one hour is equivalent to 15 degrees. This coordinate system is useful because it rotates about the north celestial pole along with the entire celestial sphere. Hence the coordinates for a star or a comet remain essentially constant overnight. Moreover, the coordinates are independent of the observer's geographic location.

Figure 6 demonstrates the motion of Halley's comet along the celestial sphere for the next few months. Until December 23 the declination is positive, that is, the comet is "above" the equatorial plane. The declination becomes negative as the comet descends "below" the plane.

The motion of the comet can also be described in terms of three angles measured with respect to the ecliptic plane: the plane of the earth's orbit around the sun [see Figure 9]. The orbital inclination of the comet is the angle through which the ecliptic plane must be rotated to coincide with the comet's orbital plane in such a way that the earth and the comet orbit the sun in the same direction. The orbital inclination of Halley's comet is 162 degrees.

The second angle of reference is the longitude of the ascending node: the point at which the comet passes upward through the ecliptic plane on its approach to the sun. The angle, 58 degrees for Halley's comet, lies between the direction of the vernal equinox and a line extending from the sun through the node.

The third angle has to do with the comet's perihelion (its closest point to the sun, which will be on February 9, 1986) and is called the argument of perihelion. It is the angle between two lines extending from the sun. One line passes through the perihelion, the other through the node. For Halley's comet this angle is about 112 degrees.

The illustration also includes approximate locations of the earth when it will be closest to the comet. Before perihelion the closest approach will be on November 27, 1985, when the separation will be about 93 million kilometers. After perihelion the closest approach will be about 48 million kilometers. It will take place on April 11, 1986. (In the comet's appearance in 1910 the separation was only 24 million kilometers, so that the comet looked much brighter than it will on its present visit.) When the comet reaches perihelion it will be unobservable because it will be near the side of the sun that faces the earth and so one's view will be obliterated by the sun's glare.

The full orbit of the comet in the solar system is shown in Figure 9, along with its position in various years. One full orbit takes about 76 years, making Halley a short-period comet. Long-period comets have orbital periods of 200 years or more. The two types differ in other ways. The orbital planes of the short-period comets generally lie close to the ecliptic plane, and the comets usually orbit the sun in the same direction as the planets (counterclockwise as seen from above the ecliptic plane). The orbits of the long-period comets can incline at any angle, and many of them are clockwise.

Comets come, it is thought, from a giant cloud of the objects that orbit the sun at a distance of between 20,000 and 100,000 astronomical units. (One astronomical unit,150 million kilometers, is the mean distance between the earth and the sun.) The cloud is named the Oort cloud after the Dutch astronomer Jan Oort, who proposed its existence in 1950.

At times the gravitational pull of a passing star draws comets out of the Oort cloud. Some go far afield and some enter the solar system. Many of the latter end up in large orbits with long periods. Others repeatedly pass close enough to Jupiter or another giant planet for the gravitational pull of the planet to shrink the orbit until the object becomes a short-period comet. Halley's comet is thought to be a member of this class.

The nucleus of a comet is a cohesive structure that ranges in width from one kilometer to several tens of kilometers. A comet may be irregular in shape rather than spherical; no observer has had a good look at a nucleus because the object is so small. Nevertheless, many of the characteristics can be inferred. About 25 percent of the mass is made up of dust particles similar in composition to carbonaceous-chondrite meteorites. The particles range in diameter from .1 micrometer to 10 micrometers. Larger dust particles and pebbles are also present. The other 75 percent of the mass consists of ice, mainly water ice and snow. Some comets may also have a rocky core. Pockets in the loose packing of ice crystals and snow contain an assortment of such molecules as ammonia, methane and carbon dioxide.

As a comet nears the sun the heat on the sunward surface transforms the ice directly into gas (a process called sublimation). Trapped molecules escape, and ultraviolet light from the sun dissociates and ionizes them to form simpler "daughter" molecules, atoms and ions. Spectroscopic examinations of comets reveal the presence of the cyanogen molecule (CN) when a comet is as much as three astronomical units from the sun.

As the comet moves closer to the sun spectroscopic records display more emission lines, showing that the number of daughter molecules is rising rapidly. They absorb energy of certain wavelengths and then emit energy as light either at the same wavelength (a process known as resonance fluorescence) or at a longer wavelength (fluorescence). In addition the released water molecules dissociate to form a huge hydrogen (H) cloud and a smaller hydroxyl (OH) cloud around the comet's nucleus. The diameter of the hydrogen cloud may be as much as .1 astronomical unit.

As the outer surface of the nucleus gets warm and loses ice it is transformed into a spongy dust layer that is from one centimeter to 10 centimeters deep. If the nucleus spins so that all the surface receives sunlight, the spongy layer forms over the entire nucleus. The layer insulates the deeper ice. The core of the nucleus probably remains at a temperature of approximately-150 degrees Celsius.

Dust particles on the surface gradually break off and are blown away by the pressure of gas molecules released by the sublimation of ice at the boundary of the spongy dust layer and the dusty ice. As the surface erodes, the boundary moves inward, maintaining the thickness of the layer.

The dust and neutral gas released by the nucleus form an elliptical, glowing cloud called the coma. It may have a diameter of as much as 100,000 kilometers. The coma increases in diameter until the comet is about 1.5 or two astronomical units from the sun. Thereafter the coma shrinks because the material expelled from the nucleus moves rapidly into two tails. At times the coma displays glowing streamers. They may result from uneven heating on the surface of the nucleus.

As the comet nears the sun it also begins to develop a plasma tail and a dust tail. The plasma tail is a straight stream of charged particles that points almost directly away from the sun. The dust tail forms a curve with its concave side facing toward the comet's previous locations. Both tails derive from the material expelled from the surface of the nucleus, and so they are more pronounced when the comet is near the sun.

The electrically charged atoms and molecules of the ejected material are forced into the plasma tail by the solar wind, which consists of a stream of protons and electrons moving outward from the sun. The collision between the solar wind and the ions emanating from the comet distort the interplanetary magnetic field so that the magnetic field lines wrap around the comet's head and extend along a radial line away from the sun. The cometary ions are forced to spiral around the radial lines. The spiraling ions constitute the plasma tail.

The tail is visible because the ions emit light. It is blue primarily because of the blue emissions from the positively ionized carbon monoxide molecule (CO+). Often the tail develops kinks and waves due to irregularities in the ejections from the nucleus or in the solar wind. At times the tail separates from the nucleus, after which a new tail forms. The separations seem to occur when the comet crosses a boundary in the magnetic field surrounding the sun. The arrangement of the field lines resembles a pinwheel consisting of spiraling sectors. In one sector the field lines extend toward the sun and in the adjacent sector they extend away from the sun. When a comet crosses the boundary between sectors, the change in the direction of the field lines releases the plasma tail. A new tail forms as the comet travels through the next sector.

The dust tail forms from the particles of dust released by the nucleus These grains, each approximately a micrometer in diameter, scatter the sunlight with a peak intensity in the yellow portion of the spectrum. The tail is therefore yellow.

The tail develops because dust in the coma is pushed away from the sun by the pressure of the sunlight. (Light has momentum and pushes on a surface on which it shines.) Each grain is pulled radially inward by the sun's gravitational field and pushed radially outward by the pressure of the sunlight. Grains larger than a micrometer are dominated by the gravitational pull and end up orbiting the sun in a belt along the orbital path of the comet. The smaller grains are dominated by the light pressure and move away from the sun.

It is these smaller dust particles that form the dust tail [see Figure 10]. I have assumed the particles are identical in size and so are pushed by the light at a uniform rate. Inasmuch as the grains are released with an initial momentum because the comet is moving, they move outward from the sun along a curved path. In the example given the particles released by the comet early in its orbit have moved outward to form the outer end of the tail. The particles released later in the orbit have not moved as far and so form the part of the tail closer to the nucleus. The composite tail gives rise to the illusion that dust particles ejected from the nucleus move along the tail.

In reality the particles composing the dust tail vary in size. The smaller ones are pushed outward from the sun faster than the larger ones, and so the tail is broader than it would be if the particles were of uniform size. Bursts of dust from the nucleus may further alter the apparent shape of the tail.

The dust tail normally becomes wider and longer after the comet has passed perihelion. By then the nucleus has swung around the sun, whereas the dust particles released previously have not. The apparent length of the tail depends on the observer's line of sight. If the tail is approximately along the line of sight, it occupies only a small angle in one's field of view. The best view comes when the line of sight is perpendicular to the tail.

At times a comet may have an antitail that seems to point toward the sun. The appearance is illusory. The antitail is actually an almost edge-on view of the larger dust particles strewn along the comet's orbit. They are visible if the observer is near the orbital plane of the comet and has a line of sight that virtually coincides with the path just taken by the comet. In these conditions the line of sight passes through enough dust to make the antitail visible.

The dust tail of Halley's comet will probably be short, narrow and straight before the comet goes behind the sun. If viewing conditions are good, you might see it extending nearly vertically from the comet's head. Just before perihelion the tail will begin to broaden, but the glare of the sun will mask it from observation. As the comet emerges from the glare later in February, the tail will be wider, longer and more pronounced. It probably will be most visible early in April, when it will extend generally westward. After mid-April its width will decrease. The antitail for Halley's comet will form when the comet reappears in late February. It will be faint and short, however, and you will be able to see it only if conditions are ideal.

The larger dust particles released by a comet lie in a belt roughly along the comet's orbital path. These particles continue to orbit the sun. If the earth passes through the residual cometary dust, the particles ablate and burn as they enter the atmosphere, giving rise to meteor showers. The earth passes through the orbit of Halley's comet twice a year. The Eta Aquarid meteor shower in May and the Orionid shower in October both result from previous passages of Halley's comet around the sun.

The nucleus of Halley's comet is believed to have a mass of 2 X 10¹⁴ kilograms; the comet's mean diameter is about 10 kilometers. The proportions of dust and solid matter to water ice are average for comets. On its most recent pass through the inner solar system the comet ejected about 2 X 10¹¹ kilograms of material, losing a surface layer about two meters thick. Even at this rate of loss of material Halley's comet is good for many more passes around the sun.

Bibliography

INTRODUCTION TO COMETS. John C. Brandt and Robert D. Chapman. Cambridge University Press, 1981.

COMETS. David W. Hughes in Contemporary Physics, Vol. 23, No. 3, pages, 257-283; May/June, 1982.

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