Asimov's New Guide to Science

by Isaac Asimov

  The most disturbing thing about Pluto, however, was its unexpected dimness, which indicated at once that it is no gas giant. If it were anywhere near the size of Uranus or Neptune, it would have been considerably brighter. The initial estimate was that it might be the size of Earth.

  Even this turned out to be an overestimate. In 1950, Kuiper managed to see Pluto as a tiny disk; and when he measured the width of the disk, he felt that it could only be 3,600 miles in diameter, rather less than the diameter of Mars. Some astronomers were reluctant to believe this estimate; but on 28 April 1965, Pluto passed very close to a faint star and did not get in front of it. If Pluto were larger than Kuiper had estimated, it would have obscured the star.

  Thus, it was clear that Pluto is too small to influence Uranus’s orbit in any perceptible way. If a distant planet accounted for the last bit of discrepancy in the Uranian orbit, Pluto was not it.

  In 1955, it was noted that Pluto’s brightness varied in a regular way that repeats itself every 6.4 days. It was assumed that Pluto rotates on its orbit in 6.4 days—an unusually long period of rotation. Mercury and Venus have still longer periods but are strongly affected by tidal influences of the nearby sun. What was Pluto’s excuse?

  Then, on 22 June 1978 came a discovery that seemed to provide it. On that day, the American astronomer James W. Christy, examining photographs of Pluto, noticed a distinct bump on one side. He examined other photographs and finally decided that Pluto has a satellite. It is quite close to Pluto, not more than 12,500 miles away, center to center. At the distance of Pluto, that is a very slight separation to detect; hence, the long delayed discovery. Christy named the satellite Charon, after the ferryman, in the Greek myths, who takes shades of the dead across the River Styx to Pluto’s underworld kingdom.

  Charon circles Pluto in 6.4 days, which is just the time it takes for Pluto to turn on its axis. This is not a coincidence. It must be that the two bodies, Pluto and Charon, have slowed each other by tidal action until each always faces the same side to the other. They now revolve about a common center of gravity, like the two halves of a dumbbell held together by gravitational pull.

  This is the only planet-satellite combination to revolve dumbbell-fashion. Thus in the case of the moon and Earth, the moon always faces one side to Earth, but Earth has not yet been slowed to the point of always facing one side to the moon, because the former is much larger and would take much more slowing. If Earth and the moon were more equal in size, the dumbbell fashion of revolution might have resulted.

  From the distance between them and the time of revolution, it is possible to work out the total mass of both bodies: it turns out to be only about one-eighth the mass of the moon. Pluto is far smaller than even the most pessimistic estimates.

  From the comparative brightness of the two, Pluto seems to be only 1,850 miles in diameter, almost the size of Europa, the smallest of the seven large satellites. Charon is 750 miles in diameter, about the size of Saturn’s satellite Dione.

  The two objects are not far apart in size. Pluto is probably only 10 times as massive as Charon; whereas Earth is 81 times as massive as the moon. That size differential accounts for why Pluto and Charon revolve about each other dumbbell-fashion, while Earth and the moon do not. Pluto/‌Charon is the closest thing in the solar system that we know of to a “double planet.” Until 1978, it had been thought that Earth/‌moon was.

  Asteroids

  ASTEROIDS BEYOND MARS’S ORBIT

  Each planet, with one exception, is somewhere between 1.3 and 2.0 times as far from the sun as the next nearer planet. The one exception is Jupiter, the fifth planet: it is 3.4 times as far from the sun as Mars, the fourth planet, is.

  This extraordinary gap puzzled astronomers after the discovery of Uranus (at which time, the possibility of new planets became exciting). Could there be a planet in the gap—a 4½th planet, so to speak, one that had evaded notice all this time? A German astronomer, Heinrich W. M. Olbers, led a group who planned to engage in a systematic search of the skies for such a planet.

  While they were making their preparations, an Italian astronomer, Giuseppe Piazzi, who was observing the heavens without any thought of new planets, came across an object that shifted position from day to day. From the speed of its movement, it seemed to lie somewhere between Mars and Jupiter; and from its dimness, it had to be very small. The discovery was made on 1 January 1801, the first day of the new century.

  From Piazzi’s observations, the German mathematician Johann K. F. Gauss was ablc to calculate the object’s orbit; and, indeed, it was a new planet with an orbit lying between that of Mars and Jupiter, exactly where it ought to have been to make the distribution of the planets even. Piazzi, who had been working in Sicily, named the new planet Ceres, after a Roman goddess of grain who had been particularly associated with the island.

  From its dimness and distance, it was calculated that Ceres had to be very small indeed, far smaller than any other planet. The latest figures show it to be about 620 miles in diameter. Ceres probably has a mass only about one-fiftieth that of our moon and is much smaller than the larger satellites.

  It did not seem possible that Ceres was all there was in the gap between Mars and Jupiter, so Olbers continued the search despite Piazzi’s discovery. By 1807, sure enough, three more planets were discovered in the gap. They were named Pallas, Juno, and Vesta; and each is even smaller than Ceres. Juno, the smallest, may be only 60 miles in diameter.

  These new planets are so small that in even the best telescope of the time they did not show a disk. They remained points of light, as the stars did. In fact, for this reason, Herschel suggested they be called asteroids (“starlike”), and the suggestion was adopted.

  It was not till 1845 that a German astronomer, Karl L. Hencke, discovered a fifth asteroid, which he named Astraea; but after that, further discoveries were made steadily. By now, over 1,600 asteroids have been detected, every one of them considerably smaller than Ceres, the first to be detected; and undoubtedly thousands more are as yet undetected. Almost all of them are in the gap between Mars and Jupiter, a gap now referred to as the asteroid belt.

  Why should the asteroids exist? Quite early, when only four asteroids were known, Olbers suggested that they were the remnants of an exploded planet. Astronomers are, however, dubious about this possibility. They consider it more likely that the planet never formed: whereas in other regions the matter of the original nebula gradually coalesced into planetesimals (equivalent to asteroids) and these into individual planets (with the last ones joining leaving their marks as craters), in the asteroid belt, coalescence never went past the planetesimal stage. The feeling is that the perturbing effect of giant Jupiter, nearby, was responsible.

  By 1866, enough asteroids had been discovered to show that they were not spread evenly through the gap. There were regions where asteroidal orbits were absent. There were no asteroids with an average distance from the sun of 230 million miles, or 275 million miles, or 305 million miles, or 340 million miles.

  An American astronomer, Daniel Kirkwood, suggested in 1866 that in these orbits, asteroids would circle the sun in a period that was a simple fraction of that of Jupiter. Under such conditions, Jupiter’s perturbing effect would be unusually large, and any asteroid circling there would be forced either closer to the sun or farther from it. These Kirkwood gaps made it clearer that Jupiter’s influence was pervasive and could prevent coalescence.

  A still closer connection between Jupiter and the asteroids became clear later. In 1906, a German astronomer, Max Wolf, discovered asteroid 588. It was unusual because it moved at a surprisingly slow speed and therefore had to be surprisingly far from the sun. It was, in fact, the farthest asteroid yet discovered. It was named Achilles after the Greek hero of the Trojan War. (Though asteroids are usually given feminine names, those with unusual orbits are given masculine names.)

  Careful observation showed Achilles to be moving in Jupiter’s orbit, 60 degrees ahead of Jupiter. Before the yea
r was over, asteroid 617 was discovered in Jupiter’s orbit, 60 degrees behind Jupiter, and was named Patroclus, after Achilles’ friend in Homer’s Iliad. Other asteroids were discovered clustering about each of these, and all were named after heroes of the Trojan War. This was the first case of the discovery of actual examples of stability when three bodies are found at the apices of an equilateral triangle. Hence, the situation came to be called Trojan positions, and the asteroids Trojan asteroids. Achilles and its group occupy the L-4 position, and Patroclus and its group the L-5 position.

  The outer satellites of Jupiter, which seem to be captured satellites, may once have been Trojan asteroids.

  Saturn’s outermost satellite, Phoebe, and Neptune’s outer satellite, Nereid, may conceivably also be captured satellites—an indication that at least a scattering of asteroids exist in the regions beyond Jupiter. Perhaps these originally existed in the asteroid belt and, through particular perturbations, were forced outward, where eventually they were captured by particular planets.

  In 1920, for instance, Baade discovered asteroid 944, which he called Hidalgo. When its orbit was calculated, this asteroid was found to move outward far beyond Jupiter and to have an orbital period of 13.7 years—three times that of the average asteroid and even longer than Jupiter’s.

  It has a high orbital eccentricity of 0.66. At perihelion it is only about 190 million miles from the sun, so that it is neatly within the asteroid belt at that time. At aphelion, however, it is 895 million miles from the sun—as far then from the sun as Saturn is. Hidalgo’s orbit is so tipped, however, that when it is at aphelion, it is far below Saturn and is in no danger of being captured; but another satellite on such a far-flung orbit might be closer to Saturn and eventually might be captured by it or by another of the outermost planets.

  Might not an asteroid be so affected by gravitational perturbation as to take up an orbit far beyond the asteroid belt at all times? In 1977, the American astronomer Charles Kowall detected a very dim speck of light that moved against the starry background, but at only one-third the speed of Jupiter. It had to be far outside Jupiter’s orbit.

  Kowall followed it for a period of days, worked out an approximate orbit, then started looking for it in older photographic plates. He located it on some thirty plates, one dating back to 1895, and had enough position to plot an accurate orbit.

  It is a sizable asteroid, perhaps 120 miles in diameter. When closest to the sun, it is as near to the sun as Saturn is. At the opposite end of its orbit, it is as far from the sun as Uranus is. It seems to shuttle between Saturn and Uranus, although, because its orbit is tipped, it does not approach very close to either.

  Kowall gave it the name Chiron, after one of the centaurs (half-man, half-horse) in Greek myth. Its period of revolution is 50.7 years and at the moment is close to its aphelion point. In a couple of decades, it will be at less than half the distance from us, and we may be able to see it more clearly.

  EARTH GRAZERS AND APOLLO OBJECTS

  If asteroids penetrate beyond Jupiter’s orbit, might there not be others that penetrate within Mars’s orbit, closer in to the sun?

  The first such case was discovered on 13 August 1898 by a German astronomer,

  Gustav Witt. He detected asteroid 433 and found that its period of revolution to be only 1.76 years—44 days less than that of Mars. Hence, its average distance from the sun has to be less than that of Mars. The new asteroid was named Eros.

  Eros, it turned out, has a fairly high orbital eccentricity. At aphelion, it is well within the asteroid belt; but at perihelion, it is only 105 million miles from the sun, not much more than the distance of Earth from the sun. Because its orbit is tipped to that of Earth, it does not approach the latter as closely as it would if both orbits were in the same plane.

  Still, if Eros and Earth are at the proper points in their orbits, the distance between them could be only 14 million miles. This is only a little over half the minimum distance of Venus from Earth and means that, if we do not count our own moon, Eros was, at the time of its discovery, our closest known neigh bor in space.

  It is not a large body. Judging from changes in its brightness, it is brickshaped, and its average diameter is ten miles across. Still, this is not to be sneezed at. If it were to collide with Earth, it would be an unimaginable catastrophe.

  In 1931, Eros approached a point only 16 million miles from Earth; and a vast astronomical project was set up to determine its parallax accurately, so that the distances of the solar system could be determined more accurately than ever. The project succeeded, and the results were not improved upon until radar beams were reflected from Venus.

  An asteroid that can approach Earth more closely than Venus can is called (with some exaggeration) an Earth grazer. Between 1898 and 1932, only three more Earth grazers were discovered, and each of those approached Earth less closely than Eros did.

  The record was broken, however, on 12 March 1932, when a Belgian astronomer, Eugene Delporte, discovered asteroid 1221 and found that though its orbit was similar to that of Eros, it managed to approach within 10 million miles of Earth’s orbit. He named the new asteroid Amor (the Latin equivalent of Eros).

  On 24 April 1932, just six weeks later, the German astronomer Karl Reinmuth discovered an asteroid he named Apollo, because it was another Earth grazer. It is an astonishing asteroid, for at perihelion, it is only 60 million miles from the sun. It moves not only inside Mars’s orbit but inside Earth’s as well, and even inside Venus’s. However, its eccentricity is so great that at aphelion it is 214,000,­000 miles from the sun, farther out than Eros ever goes. Apollo’s period of revolution is therefore 18 days longer than that of Eros. On 15 May 1932, Apollo approached within 6,800,­000 miles of Earth, less than 30 times the distance of the moon. Apollo is less than a mile across —large enough to make it none too great a “graze.” Since then, any object that approaches the sun more closely than Venus does has been called an Apollo object.

  In February 1936, Delporte, who had detected Amor four years earlier, detected another Earth grazer, which he named Adonis. Just a few days before its detection, Adonis had passed only 1,500,­000 miles from Earth, or just a little over 6.3 times the distance of the moon from us. What’s more, the new Earth grazer has a perihelion of 41 million miles and at that distance is close to the orbit of Mercury. It was the second Apollo object to be discovered.

  In November 1937, Reinmuth (the discoverer of Apollo) discovered a third, naming it Hermes. It had passed within 500,000 miles of Earth, only a little more than twice the distance of the moon. Reinmuth, on what data he had, calculated a rough orbit, from which it appeared that Hermes could pass within 190,000 miles of Earth (less than the distance of the moon) if both Hermes and Earth were at appropriate points in their orbit. However, Hermes has never been detected since.

  On 26 June 1949, Baade discovered an even more unusual Apollo object. Its period of revolution was 1.12 years, and its orbital eccentricity was the highest known for any asteroid—0.827. At aphelion, it is safely in the asteroid belt between Mars and Jupiter but, at perihelion, is only 17,700,­000 miles from the sun—closer than any planet, even Mercury, ever comes. Baade named this asteroid Icarus, after the young man in Greek myth, who, flying through the air on wings his father Daedalus has devised, approached the sun too closely; the sun melted the wax securing the feathers of the wings to his back, and he fell to his death.

  Since 1949, other Apollo objects have been discovered. Some have orbital periods of less than a year, and at least one is closer, at every point in its orbit, to the sun than Earth is. In 1983, one was discovered that approached the sun more closely than Icarus does.

  Some astronomers estimate that there are in space about 750 Apollo objects with diameters of half a mile and more. It is estimated that in the course of It THE SOLAR SYSTEM 135 a million years, four sizable Apollo objects strike Earth; three strike Venus; one strikes either Mercury, Mars, or the moon; and seven have their orbits altered in such a
way that they leave the solar system altogether. The number of Apollo objects does not, however, diminish with time; it is also likely that new ones are added from time to time by gravitational perturbations of objects in the asteroid belt.

  Comets

  Another class of members of the solar system can, on occasion, approach the sun closely. These appear to the eye as softly shining, hazy objects that stretch across the sky, as I mentioned in chapter 2, like fuzzy stars with long tails or streaming hair. Indeed, the ancient Greeks called them aster kometes (“hairy stars”), and we still call them comets today.

  Unlike the stars and the planets, the comets seem not to follow easily predictable paths but to come and go without order and regularity. Since people in pre-scientific days felt that the stars and the planets influenced human beings, the erratic comings and goings of comets seemed to be associated with erratic things in life—with unexpected disaster, for instance.

  It was not until 1473 that any European did more than shudder when a comet appeared in the sky. In that year, a German astronomer, Regiomontanus, observed a comet and put down its position against the stars night after night.

  In 1532, two astronomers—an Italian named Girolamo Fracastoro and a German named Peter Apian—studied a comet that appeared in that year, and pointed out that its tail always pointed away from the sun.

  Then, in 1577, another comet appeared, and Tycho Brahe, observing it, tried to determine its distance by parallax. If it were an atmospheric phenomenon, as Aristotle had thought, it should have a parallax larger than the moon. It did not! Its parallax was too small to be measured. The comet was beyond the moon and had to be an astronomical object.