THE PROBLEM of the origin of the solar system has not yet been satisfactorily explained for the relatively orderly planets (all revolving about the Sun in the same direction in almost the same plane) and the planetoids (which all revolve about the Sun in the same direction, and in a comparatively narrow plane). Comets remain haughtily aloof from the attempts at explanation. They don't revolve about the Sun in the same direction. Every single one of them is the complete individual. They have their own orbits, and every orbit has its own plane. The only laws they obey are the laws of motion and gravity. Any orbit those fundamentals allow, a comet finds, sooner or later. The rest of the solar system is pretty much of a pancake, but not the comets. They come slanting in at any angle, from any direction, and at almost any distance.
A planet is almost a point, for observational purposes, and therefore can be located accurately. Not so a comet; some of them have heads half the diameter of the Sun. The tail of a comet may stretch out across more than 100,000,000 miles of space, and be as much as 20,000,000 miles in diameter. That does not make for accuracy of observation, and to make the problem even more brutal, they cannot be followed, even in powerful telescopes, over any great arc of their orbits. Few comets have been followed out beyond the orbit of Mars. Therefore, in determining the orbit of a comet, we must work on (inaccurate) data gained while it traced a tiny fraction of its immense orbit near enough to the Sun to be illuminated brilliantly, and visibly.
But for all that, the comets represent the only observable bodies that plumb the outermost, black depths of the solar system. Those wanderers loop out in vast orbits to the far, raveling edges of the Sun's gravity, moving out there at those immense distances, 3,000,000,000,000 to 10,000,000,000,000 miles from the Sun, at creeping paces for perhaps millions of years before, at last, they sweep, very slowly at first, back toward the Sun.
But a comet that retired to 10,000,000,000,000 miles and a comet that retired to 100,000,000,000,000 miles would both fall back at last, in almost precisely straight lines. The tiny fraction of its orbit near the Sun, a month, perhaps out of a million-year orbit, would in each case be almost exactly a parabola. The 100,000,000,000,000-mile comet would probably retire -- and not fall back. With data made inaccurate because of the huge, blurred image of the comet's heard, we must draw a fine, thin distinction between the true retiring-to-infinity parabolic orbit, and the elliptical orbit that simply loops out for a near-infinite distance before returning.
We can't, as a matter of fact. About 400 comets have been observed, and orbits computed. Of those 400, 300 seem to have parabolic orbits. Astronomers have decided that they don't, not because of more careful observation, but because a parabola is the orbit a body must take under the following conditions; it must originally be at rest with respect to the solar system at a distance almost infinite. Such a body falling into and coasting back out of the Sun's gravitational field would follow a parabolic orbit, but none other could Those 300 out of 400, then, would have to represent 300 bodies not originally connected with the solar system, and yet, by pure chance, traveling along through space at exactly the same direction.
The chance is too remote. Those 300 are almost certainly true members of the system; comets, simply, that retire to such immense distances that they seem to go out forever.
IF A BODY roving through space, free entirely of any stellar system, should chance to stumble through our system, it would take up an hyperbolic orbit due to tits own original motion relative to the solar system. No such comet has been observed; the only hyperbolic orbits detected have been traceable to the perturbation of a parabolic orbit due to the influence of a near-by planet's mass.
All other observable bodies of the solar system lie in or near the plane of the planetary orbits; only the comets plunge wildly out in any direction, at any angle. Were they captured by the Sun as they wandered free through space? In many ways it seems unlikely, for an unattended star can capture a freely roving body only under very special circumstances.
If a body at rest outside the system were to fall in toward the Sun, it would gather speed with every mile of its immense drop. When it finally neared the Sun, it would circle it in a parabolic orbit with terrific velocity. And that terrific velocity would not be entirely dissipated by the dragging gravity of the Sun until the comet had at last returned whence it started -- outside the solar system, still free of the Sun.
If it were moving relative to the Sun, the fall into the system would again carry it out, this time in an immense hyperbolic orbit -- still free of the Sun.
Two special cases could change things: if the comet passed so immensely close to the Sun that it actually plowed through the resisting solar atmosphere, the slowing action would break the parabola or hyperbola to an ellipse, and the comet would be captured.
If, in the case of a body moving relative to the Sun originally, the direction and amount of relative motion were just right, the original energy of its motion might, for a time, struggle against the Sun's attraction, till it was overcome. The result then would be that the original energy of motion was lost, and, into the bargain, the comet would be at rest relative to the Sun, well within the Sun's gravitational field. The fall to the Sun, and the swooping dive on momentum outward again would find it at rest, once more, at the same distance from the Sun, captured.
This latter theory of nature will not account for the existent comets. Only one direction of motion would permit such capture, and hence all comets so captured would retire from the Sun in orbits in the same plane. Comets, conspicuously, don't.
But a stellar system attended by planets -- that's something else. Our solar system is a 9-jawed, cosmic trap for spatial wanderers, and Jupiter is the chief jaw. Imagine a comet falling in from space, diving down just in front of Jupiter as he swings round in his orbit. Say Jupiter is 100,000,000 miles from the point in his orbit the comet will pass; his gravitational mass tends to deflect the comet's path toward him. While it is outside his orbit, Jupiter, millions of miles distant still, hastens its fall by his attraction.
Then the comet passes the orbit, in front of the planet. Jupiter moves in behind. Now, Jupiter is not hundreds of millions of miles distant; he's moved in, say, within 100,000 miles instead of 100,000,000. Further, he's no longer speeding the fall, but dragging back with his mass, and the mass of 4 giant satellites. He's putting on the brakes a lot harder than he helped it along. The comet is not falling freely to the Sun; it cannot retire freely into infinite space; it has lost so much speed pulling past Jupiter that it retires in an ellipse, not an hyperbola. Captured!
SATURN, or any of the other planets can do the same; though naturally Jupiter, because of his far greater gravitational field, reaching out millions of miles, is the most serious obstacle. But even this will not account for the comets; they'd all have to pass near the plane of the planets if this were the secret. A remarkable number do, as a matter of fact, and they pass close to Jupiter. A comet doesn't have to come from infinity for Jupiter to make general alterations in its orbit; Jupiter severely works over anything that comes within a million miles of him, and, in the ages of time, a good many comets do. If they pass near him -- just once in time -- they pass near him from then on, if they stay in the system.
Because Jupiter works both ways; if a comet passes before Jupiter, it has its orbital period reduced. Brook's Comet 1889 V had a period of 29 years up to 1886. In that year, however, it passed within 56,000 miles of Jupiter. It's period was reduced to 7 years, and it is forced forevermore to return again to a point near Jupiter's orbit, where, in time to come, Jupiter will again go to work on it.
Lexell's Comet of 1770 passed close to Jupiter in 1779, but it passed behind the planet. Like a stone from a cosmic sling, Jupiter accelerated it violently, hurled it out from the system. It has never since been seen.
Jupiter has quite a family or comets -- bound by the titanic force of his far-reaching, gravitational arms. But there are, too, groups of comets of a different sort, a chain of comets that follow one immense orbit, one behind the other, like the links of a chain on an immense sprocket, with the Sun as its center. The great comets of 1668, 1843, 1880 and 1882 belong to one such group. They are not recurring appearances of one body -- though they look much alike, have the same general characteristics -- but separate individuals following one immense orbit. They cannot be the same, for all revolve in an orbit having a period of 600 to 800 years. (Indefinite, because we can observe them only during a few months, over a tiny portion of their immense arc.) These comets are interesting, further, in that they passed within 200,000 kilometers of the Sun's surface, actually within the vast, tenuous flames of its corona. Moving more than 500 kilometers a second at that short distance, they circle half round the Sun in a few hours.
They retire in the direction of Sirius; but they do not come anywhere near that star, for as they race from the Sun, their speed declines rapidly. Though Sirius is one of the nearest of the stars, they would not, even in 1,000,000 years, or 2,000,000, cover an appreciable fraction of the enormous distance to Sirius. When near the Sun, they were objects of dazzling splendor, immense tails flashing out from them for more than 100,000,000 miles, extending far out, beyond Earth's orbit, as a tongue of flame across all that space.
In addition to the unsolved problem of their origin, is the problem of their constitution. This, however, we have some means of understanding. To the eye, and even the telescope, the head of a comet appears as a solid thing of immense size. It is, however, no more than a loose collection of particles of matter, fragments of iron and rock held together by a slight mutual attraction. Since there is nothing in space to disturb them normally, those minute mutual attractions are sufficient to hold them together. However, if any considerable exterior force acts on them, the feeble attractions weaken drastically, and the entire comet may be shattered. In the far depths of space, there is little that can serve to wreck them, but the deeper heart of the system is a danger ground. Their approaches to the Sun mean inevitably that solar tides varying inversely as the cube of the distance will beset them. Planets that pass near, or even at considerable distances, will raise disrupting tides. The pressure of the Sun's light, the chance encounter with a sizable meteor, all may serve to bring about disintegration.
In 1846 Biela's Comet, which had been observed at intervals of 6.6 years since 1772, returned again. The comet was a member of Jupiter's captured family, but until December 20, 1846, it displayed no unusual appearance. On that night, however, it was considerably elongated. By January 1, 1847, it had become 2 separate bodies, traveling in parallel paths a quarter of a million kilometers apart, each having its own head and tail, but at times linked by a faint arc of light.
At their return in 1852, the 2 components were 2,00,000 miles apart. Several times since, when they were due, the area where they should have appeared has been carefully searched, but they have not been seen. They were due however, in 1872 and 1885, 2 years which furnished exceptionally fine showers of meteors, the Andromedes, which move in approximately the path of the lost comet.
The Great Comet of 1882, which passed so near the Sun, was broken up by it. So brilliant originally that it was visible in full daylight, after passing the Sun, as many as 8 separate fragments were detected near to, and traveling parallel with, the main body of the comet. In a year it had passed beyond the scope of the greatest telescopes. In approximately 1,000 years the comet should return. By then, so far separated will the parts have become, that, in all probability, the various fragments will appear at intervals of as long as 100 years.
THE STRENGTH of the gravitational bonds holding the comets together is, however, exceedingly hard to determine. That it is no feeble force is evidenced by the fact that, passing through the corona itself, the Comet of 1882 was not entirely destroyed, but just chipped slightly, so to speak. The mass of Haley's Comet has been estimated to be about one fifty-millionths that of Earth, an astronomically minute thing, but yet a mass of some 1,000,000,000,000 tons. In 1770, Lexell's Comet passed within 2,000,000 miles of Earth, and its orbit was greatly changed by Earth's pull. But Earth was not affected detectably in return. Had the mass of the comet been as much as one thirteen-thousandths of the Earth, the length of the Earth's year would have been permanently altered by about one second.
When Brook's Comet's orbit was changed by Jupiter's attraction from a 29-year period to a 7-year period, it spent several months within the limits of Jupiter's satellite system. Yet even the comparatively sensitive small masses of the satellites were not detectably altered. The gravitational method of comet analysis fails simply because the comets are too small gravitationally, certainly less than one one-millionth Earth's mass.
Yet a typical comet may be 100,000 kilometers in diameter. If such a comet had the evidently very high mass of one one-millionth that of Earth, its average density would be only one six-hundred-and-forty-thousandths that of ordinary air. It has been shown that the light of stars passing through 100,000 kilometers of cometary material is not appreciably refracted.
These figures deal with the heads of comets; the tails are even more tenuous. And they are even more puzzling. The tail usually curves out from the comet in a manner suggesting a repulsion from the Sun, and the general theory of this repulsion is that it is brought about by pressure of light.
Since light is energy, and physics has shown that energy has mass, it follows that mass moving through space at an immense velocity must exert pressure on anything it strikes. This pressure can readily be detected by direct measurement. It is typical of meteoric material (which seems to be about what comets are made of) that, when heated, large volumes of gas are released, gases absorbed in the solid particles as water is soaked up by a sponge.
Apparently, comets coming near the Sun are heated by radiation, and gaseous material driven out. The gas molecules, struck by the light of the Sun, are given considerable velocity away from the center of radiation, thus giving rise to a stream of gas trailing away from the Sun, frequently not behind the comet. When a comet is retreating from the Sun, in fact, the tail precedes it, to a certain extent.
But even this does not explain it perfectly; sometimes a comet moves so that, for example, the line joining it and the Sun sweeps one degree. Instances have been found where, in the same interval, the tail has moved through an angle of 16°!
The spectroscope can give no information as to the constitution of the swarm of solid particles that make up the head of the comet. However, the gaseous discharge is excellent material for the spectroscope, and the tail can be studied. As the comet retreats from the Sun, the tail continues to radiate for some time, as the molecules composing its gases have been greatly "excited" by the radiation they absorbed. Some of the energy has been stored. However, a large part of the light, which makes the tail visible, is sunlight reflected. The light available, furthermore, is not intense enough to make spectrum work easy.
To the extent they shine by their own light (reradiated after absorption) comets can be analyzed. The bands detected indicate the presence of nitrogen, carbon monoxide, cyanogen (a carbon-nitrogen molecule, CN), and various hydrocarbons, many of them highly unsaturated. Methane, CH4, is the lowest-saturated hydrocarbon, but molecules such as CH-, CH2- and CH3- have been detected, in which the carbon atoms lack one or more hydrogen atoms of satisfaction. Some sodium vapor has been detected as well.
IT HAS BEEN suggested that the comets originate in vapors ejected from the Sun at high speed, vapors such as those forming the enormous prominences frequently observed during total eclipses. These, it is believed, are supported by light pressure. If the pressure hurls the atoms of matter out from the Sun at high speed, and continues to accelerate it for a while, it may be driven out to immense distances. If it reaches a distance 32,000 times that of Earth, it does not reach its turning point for 2,000,000 years. For more that 1,000,000 years its velocity, with respect to the Sun, is less than one kilometer a second. During these ages, the matter condenses; its chrondules are drawn together by mutual attraction. Through long ages, the distant stars have an opportunity to work on them, giving cross pulls and slow components that, when they fall again toward the Sun, make them slat a bit, allow them to fall into orbits instead of directly back to the Sun.
If this proposed mechanism is correct, comets -- and meteors, which seem to represent a degradation product of comets -- unlike any other members of the solar system, are entirely cyclic, being produced at one end and destroyed at the other end of a never-ending cycle. Further, since the Sun radiates light in all directions, it is understandable that those wisps of light-driven matter should so radiate. The resultant comets, then, could logically be expected to return in orbits from any direction, in any plane, and rotate about the Sun in any direction, for the cross tugs of the almost infinitely remote stars alone may determine that.
But whatever their origin, comets appear to be a sort of cosmic Charlotte Russe: noble in proportions, brilliantly attractive, but consisting largely of a sort of whipped cream of tiny glassy or metallic particles in an almost non-existent, gaseous froth. Of grandiose proportions, hundreds of times larger than Jupiter in volume, their feeble bluff fails to disturb, in the slightest, Jupiter's smallest satellites, or, even, to refract the faint, far light of stars.