by Isaac Asimov
The Saturn probes succeeded in finding eight small satellites that were too small to be detected from Earth, bringing the total number of Saturman satellites to seventeen. Of these eight new satellites, five are closer to Saturn than Mimas is. The closest of these satellites is only 85,000 miles from Saturn’s center (48,000 miles above the Saturnian cloud cover) and revolves about the planet in 14.43 hours.
Two satellites that are just inside Mimas’s orbit are unusual in being co-orbital: that is, they share the same orbit, chasing each other around Saturn endlessly. It was the first known example of such co-orbital satellites. They are at a distance of 94,000 miles from Saturn’s center and revolve about the planet in 16.68 hours. In 1967, a French astronomer, Audouin Dollfus, reported a satellite inside Mimas’s orbit and named it Janus. This, which was probably the result of sighting one or another of the intra-Mimas satellites, yielded erroneous orbital data because different ones may have been noted at different times. Janus is no longer included on the Saturnian satellite list.
The three remaining newly discovered satellites also represent unprecedented situations. The long-known satellite Dione, one of Cassini’s discoveries, was found to have a tiny co-orbital companion. Whereas Dione has a diameter of 700 miles, the companion (Dione-B) has a diameter of only about 20 miles. Dione-B, in circling Saturn, remains at a point 60 degrees ahead of Dione. As a result, Saturn, Dione, and Dione-B are always at the apices of an equilateral triangle. This is a Trojan situation, for reasons I shall explain later.
Such a situation, only possible when the third body is much smaller than the first two, can take place when the small body is 60 degrees ahead or behind the larger body. Ahead, it is in the L-4 position, behind, it is in the L-5 position. Dione-B is in the L-4 position. (L stands for the Italian-French astronomer Joseph Louis Lagrange, who, in 1772, worked out the fact that such a configuration was gravitationally stable.)
Then there is Tethys, still another of Cassini’s satellites. It has two co-orbital companions: Tethys-B in the L-4 position and Tethys-C in the L-5 position.
Clearly, the Saturnian satellite family is the richest and most complex in the solar system, as far as we now know.
The Saturnian rings are also far more complex than had been thought. From a close view, they consist of hundreds, perhaps even thousands of thin ringlets, looking like the grooves on a phonograph record. In places, dark streaks show up at right angles to the ringlets, like spokes on a wheel. Then, too, a faint outermost ring seems to consist of three intertwined ringlets. None of this can be explained so far, though the general feeling is that a straightforward gravitational explanation must be complicated by electrical effects.
The Outermost Planets
In the days before the telescope, Saturn was the farthest planet known and the one that moved most slowly. It was also the dimmest, but it was still a first-magnitude object. For thousands of years after the recognition that planets existed, there seems to have been no speculation about the possibility that there might be planets too distant, and therefore too dim, to be visible.
URANUS
Even after Galileo had demonstrated that there are myriads of stars too dim to be seen without a telescope, the possibility of dim planets does not seem to have made much of a stir.
And then, on 13 March 1781, William Herschel (not yet famous) was making measurements of star positions when, in the constellation of Gemini, he found himself staring at an object that was not a point of light but, instead, showed a small disk. At first, he assumed it was a distant comet, for comets were the only objects, other than planets, that showed up as disks under telescopic observation. However, comets are hazy, and this object showed sharp edges. Furthermore, it moved against the starry background more slowly than Saturn and, therefore, had to be farther away. It was a distant planet, much farther away than Saturn, and much dimmer. The planet was eventually named Uranus (Ouranos, in the Greek form) for the god of the sky and the father of Saturn (Crones) in the Greek myths.
Uranus is 1,783,000,000 miles from the sun on the average and is thus just about exactly twice as far from the sun as Saturn is. Uranus is, furthermore, smaller than Saturn, with a diameter of 32,200 miles. This is four times the diameter of Earth, and Uranus is still a gas giant like Jupiter and Saturn but is much smaller than those two. Its mass is 14.5 times that of Earth, but only 1/6.6 that of Saturn and 1/22 that of Jupiter.
Because of its distance and its relatively small size, Uranus is much dimmer in appearance than either Jupiter or Saturn. It is not, however, totally invisible to the unaided eye. If one looks in the right place on a dark night, Uranus is visible as a very faint star, even without a telescope.
Might not astronomers have detected it, then, even in ancient times? They undoubtedly did, but a very dim star did not attract attention when planets were assumed to be bright. And even if it were looked at in successive nights, its motion is so small that its change of position might not have been noticed. What’s more, early telescopes were not very good and, even when pointed in the right direction, did not show Uranus’s small disk clearly.
Still, in 1690, the English astronomer John Flamsteed listed a star in the constellation Taurus and even gave it the name of 34 Tauri. Later astronomers could not locate that star; but once Uranus was discovered and its orbit worked out, a backward calculation showed that it was in the place that Flamsteed had reported 34 Tauri to be. And half a century later, the French astronomer Pierre Charles Lemonnier saw Uranus on thirteen different occasions and recorded it in thirteen different places, imagining he had seen thirteen different stars.
There are conflicting reports on its period of rotation. The usual figure is 10.82 hours; but in 1977, the period was claimed to be 25 hours. We probably will not be certain until probe data is received.
One certainty about Uranus’s rotation rests with its axial tipping. The axis is tipped through an angle of 98 degrees, or just a little more than a right angle. Thus, Uranus, as it revolves about the sun once every eighty-four years, seems to be rolling on its side; and each pole is exposed to continuous sunlight for forty-two years and then to continuous night for forty-two years.
At Uranus’s distance from the sun, that makes very little difference. If Earth rotated in this fashion, however, the seasons would be so extreme that it is doubtful whether life would ever have developed upon it.
After Herschel discovered Uranus, he kept observing it at intervals and, in 1787, detected two satellites, which were eventually named Titania and Oberon. In 1851, the English astronomer William Lassell discovered two more satellites, closer to the planet, which were named Ariel and Umbriel. Finally, in 1948, Kuiper detected a fifth planet, closer still; it is Miranda.
All the Uranian satellites revolve about Uranus in the plane of its equator, so that not only the planet, but its satellite system, seem to be turned on its side. The satellites move north and south of the planet, rather than east and west as is usual.
The Uranian satellites are all fairly close to Uranus. There are no distant ones (at least, that we can see). The farthest of the five that are known is Oberon, which is 364,000 miles from Uranus’s center, only half again as far as the moon is from Earth. Miranda is only 80,800 miles from Uranus’s center.
None of the satellites are large ones after the fashion of the Galilean satellites, Titan, or the moon. The largest is Oberon, which is just about 1,000 miles across; while the smallest is Miranda, with a diameter of 150 miles.
For a long time, there seemed to be nothing particularly exciting about the Uranian satellite system; but then, in 1973, a British astronomer, Gordon Tayler, calculated that Uranus would move in front of a ninth-magnitude star, SA0158687. This event excited astronomers, for as Uranus passed in front of the star, there would come a period, just before the star was blanked out, when its light would pass through the upper atmosphere of the planet. Again, just as the star emerged from behind the planet, it would pass through its upper atmosphere. The fate of the starligh
t as it passed through the atmosphere might well tell astronomers something about the temperature, the pressure, and the composition of Uranus’s atmosphere. The occultation was scheduled to take place on 10 March 1977. In order to observe it, on that night an American astronomer, James L. Elliot, and several associates were in an airplane carrying them high above the distorting and obscuring effects of the lower atmosphere.
Before Uranus reached the star, the starlight suddenly dimmed for about 7 seconds and then brightened again. As Uranus continued to approach, there were four more brief episodes of dimming for I second each. When the star emerged on the other side, there were the same episodes of dimming in reverse order. The only way of explaining this phenomenon was to suppose that there were thin rings of matter about Uranus—rings not ordinarily visible from Earth because they were too thin, too sparsely filled, too dark.
Careful observation of Uranus during occultations of other stars, such as one on 10 April 1978, showed a total of nine rings. The innermost one is 25,200 miles from the center of Uranus, and the outermost one is 30,500 miles from the center. The entire ring system is well within Roche’s limit.
It can be calculated that the Uranian rings are so thin, so sparse, and so dark that they are only 1/3,000,000 as bright as Saturn’s rings. It is no surprise that the Uranian rings are hard to detect in any fashion but this indirect one.
Later, when Jupiter’s ring was detected, it began to seem that rings were not such an unusual phenomenon after all. Perhaps all gas giants have a ring system in addition to numerous satellites. The only thing that makes Saturn unique is not that it has rings, but that those rings are so extensive and bright.
NEPTUNE
Soon after Uranus was discovered, its orbit was worked out. However, as the years passed, it was found that Uranus was not following the orbit as calculated—not quite. In 1821, the French astronomer Alexis Bouvard recalculated Uranus’s orbit, taking into account early observations such as that of Flamsteed. Uranus did not quite follow the new orbit either.
The tiny pull on Uranus of the other planets (perturbations) slightly affected Uranus’s motion, causing it to lag behind, or pull ahead, of its theoretical position by a very small amount. These effects were recalculated carefully, but still Uranus did not behave correctly. The logical conclusion was that, beyond Uranus, there might be an unknown planet exerting a gravitational pull that was not being allowed for.
In 1841, a twenty-two-year-old mathematics student at Cambridge University in England tackled the problem and worked at it in his spare time. His name was John Couch Adams; and by September 1845, he had finished. He had calculated where an unknown planet ought to be located if it were to travel in such a way as to account for the missing factor in Uranus’s orbit. However, he could not get English astronomers interested in his project.
Meanwhile, a young French astronomer, Urbain Jean Joseph Leverrier, was also working on the problem quite independently. He completed his work about half a year after Adams and got just about the same answer that Adams did. Leverrier was fortunate enough to get a German astronomer, Johann Gottfried Galle, to check the indicated region of the sky for the presence of an unknown planet. Galle happened to have a new chart of the stars in that portion of the sky. He began his search on the night of 23 September 1846, and he and his assistant, Heinrich Ludwig D’Arrest, had barely been working an hour when they found an eighth-magnitude object that was not on the chart.
It was the planet! And it was nearly at the spot where the calculations had said it would be. It was eventually named after Neptune, the god of the sea, because of its greenish color. The credit for its discovery is nowadays divided equally between Adams and Leverrier.
Neptune travels about the sun in an orbit that places it about 2,800,000,000 miles away, so that it is more than half again as far from the sun as Uranus is (and 30 times as far from the sun as Earth is). It completes one revolution about the sun in 164.8 years.
Neptune is the twin of Uranus (much as Venus is the twin of Earth; at least in dimensions). Neptune’s diameter is 30,800 miles, just a bit smaller than that of Uranus, but the former is denser and is 18 percent more massive than Uranus. Neptune is 17.2 times as massive as Earth and is the fourth gas giant circling the sun.
On 10 October 1846, less than three weeks after Neptune was first sighted, a Neptunian satellite was detected and named Triton, after a son of Neptune (Poseidon) in the Greek myths. Triton turned out to be another of the large satellites, with a mass nearly equal to that of Titan. It was the seventh such satellite to be discovered, and the first since the discovery of Titan nearly two centuries before.
Its diameter is about 2,400 miles, making it a bit larger than our moon; and its distance from Neptune’s center is 221,000 miles, almost the distance of Earth from its moon. Because of Neptune’s greater gravitational pull, Triton completes one revolution in 5.88 days, or in about one-fifth the time our moon takes.
Triton revolves about Neptune in the retrograde direction. It is not the only satellite to revolve in this way. The others, however (Jupiter’s four outermost satellites, and Saturn’s outermost satellite), are all very small and very distant from the planet they circle. Triton is large and is close to its planet. Why it should follow a retrograde orbit remains a mystery.
For over a century, Triton remained Neptune’s only known satellite. Then, in 1949, Kuiper (who had discovered Miranda the year before) detected a small and very dim object in Neptune’s neighborhood. It was another satellite and was named Nereid (the sea nymphs of the Greek myths).
Nereid has a diameter of about 150 miles and travels about Neptune in direct fashion. It has, however, the most eccentric orbit of any known satellite. At its closest approach to Neptune, it is 864,000 miles away; but at the other end of its orbit, it is 6,050,000 miles away. It is, in other words, seven times as far away from Neptune at one end of its orbit as at the other end. Its period of revolution is 365.21 days, or 45 minutes less than 1 Earth-year.
Neptune has not yet been visited by a probe, so it is no surprise that we know of no other satellites or of a ring system. We do not even know whether Triton has an atmosphere, although since Titan does, Triton may well have one, too.
PLUTO
Neptune’s mass and position accounted for most of the discrepancy in Uranus’s orbit. Still, to account for the rest, some astronomers thought that an unknown planet even more distant than Neptune ought to be searched for. The astronomer most assiduous in his calculations and search was Lowell (who had become famous for his views on the Martian canals).
The search was not easy. Any planet beyond Neptune would be so dim that it would be lost in the crowds of equally dim ordinary stars. What’s more, such a planet would move so slowly that its change in position would not be easy to detect. By the time Lowell died in 1916, he had not found the planet.
Astronomers at the Lowell Observatory in Arizona, however, continued the search after Lowell’s death. In 1929, a young astronomer, Clyde William Tombaugh, took over the search, using a new telescope that could photograph a comparatively large section of the sky very sharply.
He also made use of a blink comparator, which could project light through one photographic plate taken on a certain day, and then through another plate of the same star region taken a few days later, and so on in rapid alternation.
The plates were adjusted so that the stars on each were focused on the same spot. The true stars would remain perfectly steady as the light flashed through first one plate, then the other. Any dim planet present, however, would have altered position so as to be present here, there, here, there, in rapid alternation. It would blink.
Discovery was not easy even so, for a particular plate would have many tens of thousands of stars on it and would have to be narrowly scanned at every part to see if one of those myriads was blinking.
But at 4 P.M. on 18 February 1930, Tombaugh was studying a region in the constellation of Gemini and found a blink. He followed that object f
or nearly a month and, on 13 March 1930, announced he had found the new planet. It was named Pluto, after the god of the underworld, since it was so far from the light of the sun. Besides, the first two letters of the name were the initials of Percival Lowell.
Pluto’s orbit was worked out and turned out to have numerous surprises. It was not as far from the sun, as Lowell and other astronomers had thought it would be. Its average distance from the sun turned out to be only 3,670,000,000 miles, only 30 percent farther than Neptune.
Furthermore, the orbit was more eccentric than that of any other planet. At its farthest point from the sun, Pluto was 4,600,000,000 miles away; but at the opposite end of its orbit, when nearest the sun, it was only 2,700,000,000 miles away.
At perihelion, when Pluto is nearest the sun, it is actually closer to the sun than Neptune is by about 100,000,000 miles. Pluto circles the sun in 247.7 years; but in each of these revolutions, there is a twenty-year period when it is closer than Neptune and is not the farthest planet. As it happens, one of those periods is the last two decades of the twentieth century, so that now, as I write, Pluto is closer than Neptune.
Pluto’s orbit does not really cross Neptune’s, however, but is strongly tilted compared with the other planets. It is inclined to Earth’s orbit by about 17.2 degrees, while Neptune’s orbit is inclined only slightly to Earth’s. When Pluto and Neptune’s orbits cross, therefore, and both are at the same distance from the sun, one is far below the other. As a consequence, the two planets never approach each other at a distance of less than 1,500,000,000 miles.