Off on a Comet

Chapter XXVII

Several variations on the old theme of comets, in the solar system and elsewhere

Professor Palmyrin Rosette now took it upon himself to lecture his audience on the subject of cometography. To begin with, and following the example of the best astronomers, he defined comets in the following manner:

“Stars, composed of a central point which we call the core, a nebulous surround which we call the nebulosity, and a luminous train we call the tail. These stars are not visible to the inhabitants of the Earth except during a short period of their courses, because of the very great eccentricity of the orbits that they describe around the sun.

Palmyrin Rosette added, to make certain his definition was rigorously precise, that it was possible for these stars to appear without core, without head and without tail and still be regarded as comets.

In this matter the Professor followed Arago in believing that, in order to deserve the fine name of comet, a deviating star:

First, must be endowed with its own motion, and

Second, must describe an elongated elliptical orbit, in consequence of which it travelled to such a distance as to be invisible from the sun and Earth.

The first condition was necessary so that comets could be distinguished from ordinary stars, and the second so that they could be distinguished from planets. In other words, if an observer were unable to decide the class of some asteroids, and they were neither planets nor stars, then they must be comets.

Professor Palmyrin Rosette, in the past when it had been his business to study these phenomena, had little suspected that the day would arrive when he himself would be carried by a comet throughout the solar system. He had always had a particular interest in these stars, hairy or not. Was this, in some way, a presentiment of what his future held for him? Certainly, given how learned he was in all matters cometographical, it was his particular regret, after the shock, to find himself in Formentera alone—without an audience to whom he could explicate the situation! To whom he could have explicated his detailed academic work on the subjects of comets, and with whom he could have discussed the following fascinating questions:

1° How many comets are there in space?
2° Which of these comets are periodic, which is to say, which returned within a definite timeframe; and which are non periodic?
3° What is the probability of a collision between the Earth and one of these comets?
4° What would be the consequences of such a collision, in the case either of a comet with a hard core or one without?

But now Palmyrin Rosette was able to address these four questions, and give satisfaction to the most demanding of his listeners. His answers form the basis for the present chapter.

Let us, then, take the first question first: how many comets are there in space?

Kepler claimed that there were as many comets in the sky as there are fish in the sea.

Arago, having established the number of these stars that orbit between Mercury and the Sun, estimated the total number of comets in the entire solar system to be in the region of seventeen million.

Lambert claimed that there were five hundred million in the region of Saturn alone—which is to say, at a distance of three hundred and sixty four million miles from the sun.

Other calculations suggested that there might be as many as seventyfour million billion comets.

The truth is that nobody can say for sure how many of these ‘hairy stars’ there are; they have never been counted and are unlikely ever to be counted. But they are certainly very numerous. To continue with the analogy suggested by Kepler: a fisherman located on the surface of the sun could cast his line anywhere and be sure of snagging one of these comets.

And this is not all. There are also a great many other such bodies, comets that have escaped the influence of solar gravitation. They are in a manner of speaking cosmic vagabonds, homeless sorts who have left the realm of one solar system altogether to wander capriciously into the sphere of another. These tramps come flying into our solar system in alarming numbers. Some appear in our terrestrial skies, as phenomena that nobody has seen before; others disappear and are never seen again.

But to stick to the subject of those comets that genuinely belong to our solar system: surely it is the case (you might say) that such asteroids remain in their fixed orbits, orbiting the sun in an unchanging manner, such that it would be quite impossible for them to collide with one another, or with the Earth? But, alas—not at all. These orbits are by no means free of the influence of other gravitational bodies. Since they are elliptical they can easily become parabolic, or even hyperbolic. Take, for example, the planet Jupiter—the largest “disturber” of cometary orbits in the solar system. Many astronomers have noticed how consistently Jupiter “gets in the way of” comets, and the way it exerts, by virtue of its enormous gravitational power, upon these weak bodies an influence that can be disastrous—for them.

Such, speaking broadly, is the world of comets, a world comprising many millions of stellar bodies.

Now to the second question: which of these comets are periodic and which are non periodic?

If we were to examine the astronomic records we would read about approximately five or six hundred comets which have been the object of serious observations at various times. Of this figure, barely forty have had their periods of revolution exactly calculated.

These forty stars we may divide into periodic comets and nonperiodic comets. The first reappear on the terrestrial horizon, after longer or shorter intervals, but with a dependable regularity. The second sort, whose return cannot with certainty be determined, can fly in their orbits to truly enormous distances from the sun. Among the periodic comets there are ten known as “short-period comets” whose movements have been calculated with extreme precision. They are: Halley’s Comet, Encke, Gambart, Faye, Brörsen, Arrest, Tuttle, Winecke, Vico and Tempel. It is worth saying a little more about these objects, because their relationship to the Earth is so similar to that exhibited by the Gallia comet.

Halley’s comet has been known since ancient times. It is likely that this object has been seen in the years 134 and 52 BC, and then again in the years 400, 1759, 400, 855, 930, 1006, 1230, 1305, 1380, 1456, 1531, 1607, 1682, 1759 and 1835. It travels from East to West, which is to say in the opposite manner to the planets that circle the sun. The interval between its appearance in the terrestrial sky is seventy-five or seventy-six years, from which it follows that its orbit is disturbed by its passage through the vicinity of Jupiter and Saturn—delays that can exceed six hundred days.

The illustrious Herschell noted the appearance of this comet in 1835 from his base at the Cape of Good Hope, which provided him with better observational conditions than were available to the astronomers of the northern hemisphere. He was able to observe its passage until the end of March 1836, after which its remoteness rendered it invisible. At its perihelion, Halley’s Comet passes within twenty-two million miles of the sun, which is to say, a solar distance less than that of Venus’s orbit (the case of Gallia presents a similar circumstance). At its aphelion it reaches a distance of some thirteen hundred million miles, i.e. beyond the sphere of Neptune.

Encke’s comet accomplishes its revolution in the shortest of periods, circling the sun on average in only twelve hundred and five days; that is to say less than three-and-a-half years. It moves from the west to the east. It was discovered on the 26th of November 1818, and, after careful calculation, was recognized as being identical with a comet observed in 1805. Since that date, and as contemporary astronomers predicted, it has returned in 1822, 1825, 1829, 1832, 1835, 1838, 1842, 1845, 1848, 1852, etc, and it continues to appear above our terrestrial horizon at the expected times. It orbits within the orbit of Jupiter, and thus moves no further away from the sun than a distance of one hundred and fifty-six million miles. Its closest approach is some thirteen million, i.e. closer to the sun than the planet Mercury. Here is one important note: it has been observed that the large diameter of the elliptic orbit of this comet is gradually decreasing, and that, consequently, the extent of its distance from the sun is increasingly diminishing. It is thus probable that the Encke’s comet will end up falling into the sun, which will absorb it, unless it does not first volatilise itself under the very great heat of its proximity to the solar body.

The comet first observed by Gambart, or perhaps by Biéla, was seen in 1772, 1789, 1795, 1805, but it was only on February 28th 1826 that these sightings were with any certainty assigned as belonging to the same body. Its orbit is relatively small, circling the sun in some two thousand four hundred and ten days, which is to say, in approximately seven years. At its perihelion it approaches to within thirty-two million seven hundred thousand miles of the sun, i.e. a little within the orbit of the Earth; and at its aphelion, it removes itself to two hundred and thirty five million three hundred thousand miles—beyond Jupiter’s orbit. A curious phenomenon pertaining to this body occurred in 1846: Biéla’s comet reappeared in two pieces on the terrestrial horizon. It had been sundered by some eventuality of its passage, undoubtedly under the action of an interior force. The two fragments were observed as travelling at a distance of sixty thousand miles from one to the other. In 1852, this distance was observed to have increased to some five hundred thousand miles.

Faye’s comet was discovered November 22, 1843. The elements of its orbit were calculated, and it was predicted to return in 1850 and 1851, after seven and a half years, or two thousand seven hundred and eighteen days. This prediction was fulfilled: the star reappeared at the time indicated and continues so to do, having travelled from a distance of sixty-four million six hundred and fifty thousand miles from the sun (further than Mars) to a distance of two hundred twenty-six million five hundred and sixty thousand miles (further than Jupiter).

Brörsen’s comet was discovered on February 26, 1846. It revolves about the sun in five and a half years, or two thousand forty-two days. Its perihelic distance is twenty-four million six hundred and fourteen thousand miles; its aphelic distance is two hundred and sixteen million miles.

As for other short-period comets, the one discovered by Arrest achieves its revolution in a little more than six and a half years, passing in 1862 to within only eleven million miles of Jupiter. Tuttle’s comet has a period of thirteen and two-third years; Winecke’s comet five and a half years, Tempel’s comet has a roughly similar period, and Vico’s comet seems to have been mislaid in celestial space. But these stars were not the subject of such careful observations as the five comets previously mentioned.

It remains to enumerate the principal comets with “long periods”, of which forty have been studied with greater or lesser precision.

The comet of 1556, known as “Charles Quint’s comet”, and expected to return in 1860, has not been seen.

The comet of 1680, studied by Newton, and which, according to Whiston, could have caused the Biblical deluge by approaching too close to the Earth, was seen in 619 BC and 43 BC, and then again in AD 531 and AD 1106. Its orbit, therefore, must be some six hundred and seventy-five years, and, at its perihelion, it passes so close to the sun that it endures a heat twenty-eight thousand times more intense than that which is received by the Earth—which is to say, a heat two thousand times the temperature of molten steel.

The comet of 1586 was comparable in brightness to a star of first magnitude.

The comet of 1744 trailed several tails after it, like those various pashas who revolve around the Grand Turk.

The comet of 1811, which gave its name to the year of its appearance, had about it a ring measuring a hundred and sixty and eleven miles diameter, possessed a nebulosity of four hundred and fifty thousand miles, and dragged behind it a tail of some forty-five million miles.

The comet of 1843 (that some believe to be the same object as was observed in 1668, 1494, 1317) was observed by Cassini. But astronomers do not at all agree over the duration of its orbit. It passes within a mere twelve thousand miles of the sun, at a speed of fifteen thousand miles a second. The heat which it receives then is equal to that which forty-seven thousand suns would send to the Earth. Its tail was visible even in full day, so greatly had this appalling temperature increased its density.

Donati’s comet, which shone with so fierce a splendour in the middle of the northern constellations, has been estimated to have possessed a mass seven per cent that of the Earth.

The comet of 1862, ornamented with luminous crests, resembled some fantastical shell.

Lastly, the comet of 1864, its solar revolution being achieved in no less than two thousand eight hundred centuries, will certainly be lost, so to speak, in infinite space.

We turn, now, to the third question: what is the probability of a collision between the Earth and one of these comets?

When we map out the planetary orbits and the cometary orbits we see that they intersect in many places. But a flat map is not adequately representative of the three dimensions of space. The planes which contain these orbits are tilted at various angles to the ecliptic (as the plane of the terrestrial orbit is called). So, and despite what might be called this “precaution” of the Creator’s, is it not the case that there are so many comets that one of them is bound to collide with the Earth?

To this we might answer as follows:

The Earth, as we know, never leaves the plane of the ecliptic, and the orbit which it describes around the sun is wholly contained within this plane. What, then, is required for the Earth and a comet to come into collision? The following:

1° That a comet encounter the Earth in the plane of the ecliptic;
2° That the point which the comet crosses exactly corresponds with the path the Earth takes in its solar orbit;
3° That the distance separating the Earth and the comet be less than their respective diameters.

So: can these three circumstances occur simultaneously, and, thereby, result in collision?

When Arago was asked this question, he replied:

“The theory of probability gives us the means of evaluating the chances of such a collision. It suggests that the odds are two hundred and eighty million to one against, which suggests that such a collision is highly unlikely.”

Laplace did not reject the possibility of a collision, and he described the consequences of such a catastrophe in his Exposition du système du monde.

Are these probabilities sufficiently reassuring? We must, each of us, come to a decision on this matter according to our various temperaments.

It might, however, be observed that the celebrated astronomer’s calculation relies upon two assumption which can, in fact, vary to an infinite degree. He assumes, firstly, that at its perihelion the comet is closer to the sun than the Earth; and secondly that the diameter of this comet is equal to one quarter of that of the Earth.

Even with such assumptions, it does not appear likely that a comet will come into collision with the Earth.

If we wished to quantify the chances of a meeting with the nebulous portion of any comet, our odds would have to be multiplied by ten—that is to say two hundred and eighty-one million to ten against, or twenty-eight million one hundred thousand to one.

But, whilst accepting the premises of the first problem, Arago adds:

“Let us admit for one moment that the comet which may perchance crash against the Earth would destroy the whole of mankind; then the danger of death resulting for each individual from the appearance of an unknown comet would be precisely equivalent to the following situation: imagine that there were an urn in which was deposited one single white ball and two hundred and eighty one million black balls; and that you would be sentenced to death if you pulled out the white ball on your first attempt!

It follows from this that the chance of the Earth being struck by a comet is virtually nil.

Has the Earth been struck in former times?

The astronomers tell us: no; “because the Earth turns invariably upon its axis,” says Arago, “we can be certain that it has never been struck by a comet. Had the Earth at some point been so struck, the impact would have resulted in an instantaneous replacement of the axis of rotation, which would have subjected all terrestrial latitudes to continual variations. Scientific observation has discovered no such terrestrial history. The constancy of the terrestrial latitudes thus proves that our Earth has never, in all its history, been hit by a comet… Therefore, according to some scientists, it must be an error to ascribe the fact that the Caspian Sea lies in a depression one hundred metres below sea-level to the impact of a comet.”

There has never been a cometary collision with the world, this much appears certain. But could one not occur?

Here we come naturally to the famous Gambart incident.

In 1832, the reappearance of Gambart’s comet spread terror throughout the world. A rather odd cosmographic coincidence means that the orbit of this comet cuts almost exactly across that of the Earth. On the 29th of October, just before midnight, the comet was observed to pass very close to the terrestrial orbit. Would the Earth intercept its passage? If it did then there would certainly be a calamitous celestial encounter, for, according to Olbers’s observations, the radius of the comet was equal to the radii of five Earths, and therefore a portion of the terrestrial orbit would necessarily be plunged into the comet’s nebulous sphere. Fortunately for us, the Earth arrived at this point on the ecliptic one month after the comet, on November 30th, and since that body was moving with a speed of six hundred and seventy-four thousand miles per day, when we arrived the comet had already moved more than twenty million miles away.

Very well; but if the Earth had arrived at this point on its orbital path one month earlier, or the comet one month later, a collision would have occurred. Could it be so? Clearly, yes; for whilst it cannot be shown that any disturbances modify the path of the terrestrial sphere in its orbit, no one can claim that the path of a comet may not be altered, these asteroids being subjected to so many gravitational influences in their travels.

Therefore, if the collision has not occurred in our past, it certainly could occur in our future.

Moreover, the aforementioned Gambart comet had, in 1805, actually passed ten times closer to the Earth than this—a mere two million miles! But, as we were unaware of this at the time, the passage did not cause any panic. The situation was not so tranquil in 1843, because it was feared that the terrestrial sphere would at least pass through the tail of the comet, a circumstance which could had had the most severe consequences for our atmosphere.

Which leaves us with the fourth question: assuming that there were a collision between the Earth and a comet, what would be its effects?

These effects would vary, depending on whether the comet possessed a hard core or not.

For, as far as these wandering stars are concerned, some possess cores, like the stones of certain fruits, and some do not.

Of the latter variety some are so diffuse, constituted of so thin a nebulosity, that we are able to see stars of the tenth order of luminosity through their bodies. Such comets undergo changes of form that can make it difficult to identify them. A material of a similar diffuseness also makes up their tails—in effect, these phenomena are an evaporation of the cometary body under the influence of solar heat. The proof of this can be seen in the fact that comets do not develop their tails until they come within thirty million miles of the sun. Moreover, it often happens that certain comets made from denser material, which are thereby more resistant to the effects of high temperatures, do not present any appendix or tail at all.

If a collision occurred between the Earth and a comet without a core, there would be no impact, in the true sense of the word. The astronomer Faye has said that a cobweb would present more obstacle to a rifle bullet than a cometary nebulosity would to the Earth. If the matter which composes the tail, or “hair”, is not in itself unhealthy, then there would be nothing to fear. But it is possible that the vapours of the tail might be, in themselves, incandescent, such that they would burn across the surface of the world, or might otherwise introduce poisonous gases into our atmosphere. However, it appears that this last eventuality is extremely unlikely. Indeed, according to Babinet, the Earth’s atmosphere, though thin in its upper reaches still possesses a very considerable density compared to that of the nebulous bodies or tails of comet, and it would not be easily penetrated by these latter. So tenuous, indeed, are these vapours, that Newton could affirm that were we to take a core-less comet of a radius of three hundred and sixty-five million miles, and condense it such that it achieved the density of the terrestrial atmosphere, then it would be constricted into a sphere no wider than twenty-five millimetres in diameter.

In other words it appears that, in the case of comets of simple nebulosity, there is little to be feared in the event of collision. But what would happen if the “hairy star” possessed a hard core?

To begin with, do such cores exist? One answer is that there must be at least some, formed when a comet reaches a sufficient degree of concentration to have passed from the gaseous into a solid state.

In such a case, the interposition of a comet between one of the fixed stars and an observer placed on the ground would result in a stellar eclipse. Now, four hundred and eighty years before Christ, at the time of the Persian Emperor Xerxes, Anaxagoras observed the sun being eclipsed by a comet. Similarly, a few days before the death of the Roman Emperor Augustus, Cassius Dio observed an eclipse of precisely this kind, one that could not be due to the interposition of the moon, since the moon, this day, was in a different portion of the sky.

It should be said however that scholars of cometary matters dispute both these examples of historical testimony, and perhaps they are correct to do so. But two more recent observations make it impossible to question the existence of cometary cores. The comets of 1774 and 1828 resulted in the eclipsing of stars of the eighth magnitude. It has also been determined, after direct observation, that the comets of 1402, 1532, and 1744, were of the “hard-core” variety. The case of the comet of 1843 is all the more certain, since the star could be observed at midday, close to the sun, and without the assistance of any instrument.

Not only do hard cores exist in certain comets, but they have even been measured. Thus we know some actual diameters: eleven and twelve miles for the comets of 1798 and 1805 (according to Gambart), and up to three thousand two hundred miles for that of 1845. The latter would thus have a core perhaps larger than the entire terrestrial sphere, such that any collision between us and the comet would necessarily favour the comet.

Some of the most remarkable nebulosities to have been measured vary in size from seven thousand two hundred to four hundred and fifty thousand miles.

In conclusion, we must agree with Arago when he says that there exist or may exist:

1° Comets without cores;
2° Comets whose cores are perhaps diaphanous;
3° Comets more brilliant than planets, possessing almost certainly a solid and opaque core.

And now, before examining the consequences of a collision between the Earth and one of these stars, it is important to note that, even if there were no direct impact, the consequences for the world would be extremely serious.

Indeed, a comet passing the Earth at a small distance, if its mass were great enough, would entail considerable danger. A comet of smaller mass would not be so dangerous. Thus the comet of 1770, which came within six hundred thousand miles of the Earth, did not alter the duration of the terrestrial year by so much as one second, while the action of the Earth added two days to the orbit of the comet.

But, if the masses of the two bodies had been equal and such a comet passed the Earth at a distance of fifty-five thousand miles, it would increase the terrestrial year by sixteen hours five minutes, and would change by two degrees the angle of the ecliptic. It might also, conceivably, snatch away our moon as it passed.

Finally what would be the consequences of a collision? We can guess.

If the comet were only to graze or skim the terrestrial sphere it would leave behind a piece of itself, and would tear off some fragments or splinters of the globe—as is the case with Gallia. Alternately, it might distribute itself across the world, so as to form a new continent. In all these cases, the tangential speed of the Earth’s rotation might be suddenly quelled. Then, people, trees, houses would all be projected into the air with the speed—eight miles a second—which they had previously possessed. The seas would leap from their natural basins and destroy everything before them. The central parts of our globe, which are still liquid, would flood out through the inevitable fractures of the outer layers. The axis of the Earth would be changed; a new equator would replace the old. Finally it is possible that the Earth’s rotation about the sun could be absolutely stopped, in consequence of which the Earth might tumble towards the Sun in a straight line, to drown in fire after a fall lasting sixty-four days.

Alternately, if we follow Tyndall’s theory that heat is merely a modality of motion, then the speed of the Earthly sphere, being so suddenly ceased, would be transformed directly into heat. The ground, suddenly heated to a temperature of a million degrees, would vaporise in a few seconds.

But to finish this very rapid summary, we must recall that the odds against this collision are two hundred and eighty one million to one.

“Perhaps so,” said Palmyrin Rossette, “perhaps so. But it seems that we have indeed pulled out the white ball.”

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Translation Copyright © 2007 Adam Roberts
Copyright © Zvi Har’El
$Date: 2007/12/23 17:44:44 $