Hello Halley; quite something for nearly nothing. (Special astronomy section) John J. Anderson.
Mark Twain in his 76 productive years missed out on at least one special treat: he never got to see Comet Halley. It appeared in 1835, the year he was born, and again in 1910, the year he died. His only solace was to have predicted it would be so--small solace though that might have been. We are luckier, for 1986 is another year for Comet Halley, constituting a true once-in-a-lifetime event.
Royal Astronomer Edmund Halley indeed observed the comet, though when he did so in 1682, it did not bear his, or any name at all. Its return was spotted 76 years later by amateur astronomer George Palitzch, Christmas night 1758, just as Halley had predicted. And so it was dubbed Halley's, perhaps to the chagrin of Mr. Palitzch. One can only speculate how Halley himself felt about this, having been dead for 17 years.
Since the beginning of recorded history, the comet that came to be called Halley's has been observed. The Chinese recorded its appearance in 1057 B.C. and were the first to note that the tail of such a body always points away from the sun. It has formed the basis of many a kingly vision and perhaps even of the Star of Bethlehem. The comet appeared in the auspicious year 1066 and is depicted alongside William the Conquerer in the Bayeux Tapestry. Comets have consistently raised more fear and superstition among skittish humans than any other type of celestial body. They have been thought to portend deadly events--even to spread poisonous vapors into the atmosphere. The latter has been scientifically disproved.
It was Halley who conceived that the bodies observed in 1531, 1607, and 1682, were indeed one and the same, a bright comet in an elliptical orbit with a period of approximately 76 years. He further theorized that the somewhat erratic timing of its flybys was due to perturbations among the Jovian planets. Although he contributed much more to the astronomical knowledge of his day, it is for these calculations that he is remembered today.
Edmund Halley was a most interesting gentleman. He published his first astronomical paper at the age of 20. More significantly, he happened to be close buddies with a certain Isaac Newton--and his greatest fan. It is probable that the publication of Newton's classic work Principia Mathematica was Halley's own idea, as he helped Newton collect the data, oversaw the printing, and in fact bankrolled the book. Much of the material within it concerning comets is almost certainly Halley's own, although that work is entirely derived from that of Newton.
Newton came up (or perhaps down) with the idea of gravitation and went as far as to say that the laws of motion caused heavenly bodies to orbit one another. At the time, this was indeed a radical assertion: Christiaan Huygens, master telescope craftsman and astronomer, dismissed Newton's work as "absurd." But Halley's faith in Newton was unflappable, and he extended the theory to explain the periodicity of comets.
Halley's friendship with Newton did not always serve him well. When, in 1720, he was named chief astronomer of the Royal Society, he arrived to find the Royal Observatory largely cleared of instruments. His predecessor, John Flamsteed, who had clashed with Newton more than once in the past, claimed most of it was his own property and had taken the best of the lab with him. Flamsteed's heirs and creditors also got an early shot at the remnants. Halley managed to obtain some new instruments, but never made another significant contribution to the field.
And this intermittent cosmic visitor itself--exactly what is it? Perhaps the greatest disservice done to this question can be pinned upon a modern astronomer by the innocuous name of Fred Whipple. Fred gave us the cometary term "dirty snowball." Accurate, yes, but somehow demeaning--it strips away all the mystery and beauty of the thing. It would be about as accurate to have called Mr. Whipple a "leaky bag of dirty water," but highly impolite at best.
And yet in the most basic sense, "dirty snowball" is right on the money, if we are willing to imagine a rather large and highly unusual snowball. Comets are thought to be made up of a mixture of rocky and metallic particles bound together by frozen carbon dioxide, methane, and water, as revealed by spectral study. Comets are probably among the earliest components of our solar system, and may tell us something about its origins. It should be remembered that like planets as opposed to stars, comets emit no light of their own. Rather they reflect the light of the sun.
Around the snowball "nucleus," which may or may not itself be visible, glows the coma of a comet. The coma is the pseudo-atmosphere of gas and dust pried loose from its surface by radiation and rotation. Together, the nucleus and coma form the head of the comet. Although the nucleus of a comet may be only a few kilometers across, its coma can be very large--perhaps upwards of 150,000 kilometers. The coma itself is very diffuse. Stars were observed shining brightly through the outer coma of Comet Halley during its visit in 1910. At that time, Comet Halley came between the earth and sun, yet no opaque nucleus was observable in silhouette. The resolution of instruments used indicates the nucleus of the comet itself to be less than 50 km across (astronomer Carl Sagan guesses it to be approximately 20 km). Whether comet nuclei are solid or a loose aggregation of particles remains to be discovered, but it has been recently established that they, like other heavenly bodies, rotate about a central axis.
Thereby Hangs a Tail
The most celebrated aspect of any comet, of course, is its tail. The tail is caused by solar radiation and solar wind. Most comets produce a tail only as they cross within 2 A.U. (astronomical units, one of which equals the distance to the sun from good old planet earth, or about 150 million km) of the sun. At that distance and closer, the radiation of the sun begins to shear the comet. Comets actually have two tails, though one is frequently consumed by the path of the other. Solar wind causes a gaseous tail, which always points directly perpendicular to the sun. Solar radiation causes a dust tail, made up of relatively large particles, which tends to lag behind the gaseous tail and curve in the direction of orbit. The tail of a comet may be huge--more than 160 million km, or greater than one A.U. in length.
And so, like a piece of chalk pulled across a celestial sidewalk, periodic comets expend themselves with each solar orbit. They suffer the erosion not only of solar radiation but of solar tidal forces. It is estimated that few periodic comets survive more than 100 or so orbits. In 1842, a comet known as Biela was observed to split in two as it rounded the sun. Comet Halley is quite hale, however, and is probably good for at least 60,000 more orbits, so don't fret.
After one has asked what, quite literally, in heaven a comet is, the next natural question is "Where does it come from?" The contemporary Dutch astronomer Jan Oort has posited that a "cloud" of cometary material surrounds the solar system at a distance of approximately 100,000 A.U. If this material actually comprises the basic goo of celestial matter, most of it congealed into the solar system around the gravitation of the sun, bringing us our own planet in the process of the creation of the universe. That which escaped such compression continues to halo the solar system, beyond the reach of its gravitational forces, save for the occasional straggler swung into periodic orbit. Oort estimates that there are more than 100 billion of them residing in the cloud, yet with a combined mass of less than 0.1 that of earth.
So it becomes clear why it has been said that comets are "the nearest thing to nothing that anything can be and still be something." In fact, the tail of a comet contains less matter than the best vacuum scientists can create on earth. Yet these wisps of near nothingness have captured man's imagination throughout history, and in 1986, the reappearance of Comet Halley has focused an international scientific effort.
No fewer than five space probes are slated to study Halley's Comet. The Soviets have already launched two craft bearing mass spectrometers, which will fly by at 10,000 km from the comet. A European probe is planned to pass within a mere several hundred km of it. The Japanese have designed two space vehicles of their own. Shamefully, the U.S. has planned no dedicated probe, as a result of the NASA budget crunch (see accompanying article). However a shuttle mission will carry special instruments for comet watching outside the obfuscating atmosphere of planet earth.
The Halley Search program (page 28) will help you locate Comet Halley in your local sky at your local time. Using a halfway decent telescope on a clear night, you will definitely be able to spot it. Don't be disappointed, however, if it is no more impressive than a star or a bright planet. This time around the comet will pass only within 39 million miles of the earth, which is farther away than it has been on previous trips (in the year 837, it passed by at a distance of only 3.7 million miles and was for a time visible in broad daylight). Depending on the date, time, and weather, you may or may not be able to spot the wisp of tail that makes Comet Halley so special. If you are in the Southern Hemisphere, you will have a better view.
In his book Cosmos, Sagan quotes from a work called "Theological Reminder of a New Comet," by Andreas Celichius: "[A comet is] the thick smoke of human sins, rising every day, every hour, every moment, full of stench and horror before the face of God, and becoming gradually so thick as to form a comet, with curled and plaited tresses, which at last is kindled by the hot and fiery anger of the Supreme Heavenly Judge." Sagan presents a piquant counter to this theory, positing that "if comets were the smoke of sin, the skies would be continually ablaze with them."
I'm not sure, but I think that may be a comforting thought.