Something Is Wrong With Its Left Phalange

Ever since I first saw ‘Friends‘ back in the mid-1990s CE, I’ve wondered about the choice the writers made with Phœbe’s name. Is it connected with the moon’s name or not? If it is, it must be an extremely obscure in-joke because I imagine that most people had no idea what Phœbe was at the time, and of course the name is originally from Greek mythology, which raises the question of whether there was something about the titan herself which brought eccentricity or oddness to mind. Because Phœbe the moon is odd. It orbits the opposite way round to the majority of other moons in the Solar System, which is expressed in the stats as having a high orbital tilt, and Phœbe the ‘Friends’ character kind of does the same thing. She’s the odd one out and in the model of ‘Friends’ characters which approximates each to a personality disorder, she’s the schizotypal one. Not that I agree with that particular approach to personality disorders because they may be better characterised as combinations of unusually pronounced traits (which means that on the OCEAN model there could be thirty-two of them), but it’s been said a lot recently that there are various ways in which the sitcom has not aged well.

As is often so, Greek myths include several figures named Phœbe, but the moon is unequivocally named after the titan because it’s a satellite of Saturn and that’s how the naming scheme there went. That Phœbe is the grandmother of Artemis and Apollo, this last also known as “Phœbus” in Latinised form, who are respectively deities associated respectively with the lesser and greater luminaries. Hence it’s possible that naming a child Phœbe associates her with shining beauty, perhaps even a woman “with hair brighter than the Sun”. Phœbe’s daughter is Leto, alias Latona, goddess of night, chiefly known for being in labour for nine days owing to Hera keeping the midwife goddess Ilythia away from her when she birthed Apollo.

The question therefore arises as to why Phœbe the moon’s name was arbitrary beyond the order of its discovery leading to the need to seek decreasingly significant titans. This in itself raises an interesting question: does this mean that smaller Saturnian moons are more likely to have feminine names? If so, does that reflect a bias in classical times or more recently? This moon is the first to be discovered through photography alone, the second-largest retrograde satellite and as such is bumped down the scale of discovery, being likelier to be found later. It was first confirmed on photographic plates on 18th March 1899, the plates having been taken on the sixteenth of August the previous year. I find it a little surprising that it only took one night of plates to detect the moon’s movement and presume that it must’ve been quite far from maximum elongation at the time.

Just to return once more to the cultural aspects of this body, Phœbe is difficult to type on a computer. Although English, French and Latin all use the “œ” digraph, the letter isn’t present on the French AZERTY layout so far as I can tell, so on a typewriter it would have to be double-struck, and English lacks it entirely. I think of it as a letter in the French version of the Latin alphabet and it’s also used in the International Phonetic Alphabet with its French value. In Latin, the diphthong it represented shifted before the classical period into the sound /e/, and combined with our own vowel shift we now pronounce the moon’s name as “FEE-bee”, which incidentally also means we’ve conceded the shift from /ph/ to /f/, even though in classical Latin it would’ve been pronounced in the former way, at least for a while. In fact the only sound which has stayed the same in the name is /b/. I am, in any case, acutely aware of the fiddliness of typing the name as I’m writing this post

When I first heard about Phœbe in I think 1973, more than two decades before ‘Friends’ but millennia after the end of Dodekatheism, all that I and presumably most other people knew about it was that it was a small irregular moon, the outermost of Saturn’s, orbiting backwards compared to the other known moons, and was considered to be a captured asteroid. This last bit puzzled me because the asteroid belt is something like 750 million kilometres from Saturn. In the next few years, Chiron was discovered, and for a while this puzzled astronomers because it appeared to be an out-of-place asteroid. I will be talking about Chiron in a future post. Chiron, being named after a centaur, was just the first discovered minor planet of the “centaur” class, which I will eventually mention. There’s also Hidalgo, which is odd in that its aphelion is almost as far out as Saturn and its perihelion not so far from Mars’s, so it’s almost as if it belongs to both asteroid belts, as it were. But I’m getting ahead of myself.

Although Chiron was the first centaur to be discovered, in about 1977 I think, Phœbe seems to be a former centaur. This wasn’t picked up for getting on for a century since its discovery because they were otherwise unknown, but some of the characteristics of its orbit are highly compatible with this designation. Before I go any further, just as a centaur is a half-equine, half-human creature, astronomically centaurs are intermediate between comets and asteroids, which is also what Phœbe seems to be. The moon has an eccentricity of almost sixteen percent and averages almost thirteen million kilometres from Saturn. Since Saturn itself has an aphelion of 10.1238 AU, this means Phœbe reaches out to 10.2238 AU from the Sun. Chiron’s perihelion is actually inside Saturn’s orbit, so it’s entirely feasible to imagine Phœbe as a centaur.

While I’m at it, I may as well mention the other features of its orbit. It’s inclined to Saturn’s equatorial plane by 151° 47′, which actually just means it orbits backwards at a tilt of around thirty degrees, taking a year and a half to go all the way round. This distance also means it approaches Sinope, Jupiter’s outermost moon, to within about three and a half AU, which sounds like a lot, being slightly greater than the diameter of the orbit of Mars, but this is the outer Solar System where distances are increasingly larger than the inner. It’s about half a light hour. This is not a hugely consequential fact unless there were perhaps some kind of “moon-hopping” means of transport for getting between systems. There is of course The Solar Mass Transit System I mentioned a while back, but the gravity involved is insignificant. Nonetheless, out there somewhere is a neutral gravity point between the two, much closer to Sinope than Phœbe. That moon would also feel Jupiter’s magnetosphere most strongly out of any of the moon

I ask myself, is Phœbe genuinely the most distant of Saturn’s moons? Are there any bits and pieces in beyond it which still orbit it? Saturn’s Hill Sphere is bigger than Jupiter’s because it’s further from the Sun even though it’s also less massive, at sixty-one million kilometres in radius, which is almost five times the radius of Phœbe’s orbit. Nevertheless, matter is sparser out there than further in. And in fact there are fifty-five further moons, though some are extremely small. Some are only fourteen metres across, and it seems both hardly fair to include them as moons and also quite amazing that they’ve been detected at all. However, even the outermost moon is only half way to the surface of the Hill sphere, so it seems possible there will be even more. It’s thirty-four metres across and has no official name.

The moon is the largest of the so-called “Norse Group” of irregular satellites with retrograde motion. It’s over a thousand times the volume, ten times the diameter, of the next largest such moon, Ymir. Since it was discovered before the invention of this grouping, Phœbe has a Greco-Latin rather than a Norse name, Ymir being the frost giant nourished by the milk of the primordial cow and from whom the world was made in Norse mythology, thereby providing a possible link with Hinduism. There are probably a number of subgroups among the Norse moons. Among all of them, however, Phœbe is in a league of its own in terms of size, averaging about two hundred kilometres across, and as can be seen from the image at the top of this post, it somewhat approximates sphericality, more so in fact than the rather larger Hyperion. Other comparisons with Hyperion are worthwhile too. For instance, Phœbe lacks Hyperion’s spongy appearance and looks to me more like Deimos or a small asteroid. It’s also more massive than Hyperion, which is in fact connected to the appearance as it’s less porous too, and therefore denser. Phœbe is also as black as soot, reflecting only six percent of the light falling on it, which is darker than any other of Saturn’s largish or large moons. This would make it warmer than most of the other small moons which don’t experience substantial tidal forces, and certainly warmer than Hyperion, which is quite a bit paler and reflects a lot of light and therefore heat, being five times brighter than this moon.

Although there are maps of the place, it kind of makes more sense to label the “globe” because it’s too irregular to map without considerable distortion compared to a spheroidal object:

The craters are named after the story of ‘Jason And The Argonauts’, hence the very large crater called Jason at top left of this panel. This is more than eighty kilometres across and has walls sixteen kilometres high.

This moon is an exception to the exploration of the satellites undertaken by the Voyager probes. This is the best image taken at the time:

Hence research on the moon is rather behind that on the others. One thing which is noticeable about it is that it’s higher in dry ice than the others, which is one reason why it’s thought to be a centaur. It’s also the only such object which has been imaged as more than a dot, even by the Hubble Space Telescope. No space probe has been anywhere near any of the others, which basically means Chiron. It’s difficult, really, to talk about it without talking about the other, proper centaurs, which I want to leave until I get to Chiron.

Phœbe is unusual in having its own rotation period. Unlike Hyperion, whose rotation is chaotic, it does have a proper axis and takes nine and a quarter hours to rotate on it. This makes it the only sizeable moon of Saturn which has a proper day of its own, and Saturn will rise and set in its sky. Saturn is also usually in a position where its rings are fully visible, but unfortunately the planet is also very small and far-away.

Phœbe also has a ring, although unlike Rhea’s possible rings and the remnants of the one around Iapetus, if that’s what that is, it doesn’t encircle the moon but its orbit, through which the moon travels. It’s one of those irritating technical truths, like the fact that Alaska is the easternmost state in the US because of the Aleutian Islands crossing into the Eastern Hemisphere, that Saturn’s biggest ring is actually this one, which is so sparse as to be practically non-existent. It’s technically 23 million kilometres across, and may be the cause of the dark hemisphere on Iapetus, due to dark material leaving Phœbe’s surface and spreading inward as far as the two-faced moon. It’s probably caused by meteorites hitting Phœbe’s surface and the moon’s gravity not being strong enough to pull them back. It is, however, entirely within the moon’s orbit, suggesting that like the inner moonlets near Saturn’s more substantial and visible rings most of the way in, it also acts as a ring shepherd, although a particularly large one with a particularly diaphanous though large ring. Some of the larger impacts may also have caused bigger fragments to escape the moon’s pull and become other Norse moons in their own right, some of which have similar orbital characteristics.

That, then, is not only it for Phœbe but for the entire Saturnian system. Although most of the moons haven’t even been mentioned, these are all the moons discovered before the twentieth century. My impression of Saturn’s system is that it’s characterised by clutter. It has the rings, numerous small moons orbiting in unexpected places and a fair bit of matter exchange. It’s also quite light and of low density, with the exception of Titan. Due to being both quite massive, even given its low density, and far out in the system, it has a large sphere of influence and has managed to retain quite a lot of matter.

The next post on the Solar System will be about the initially mysterious and surprising object Chiron, discovered in 1977, and its “relatives”, the centaurs. These form a kind of second, outer asteroid belt. More on them in a couple of days.

Chiron And The Centaurs

‘Abigail’s Party’. ‘Calling Occupants Of Interplanetary Craft’. ‘Image Of The Fendahl’. The Southern TV broadcast interruption. The linking of three ARPANET nodes via TCP/IP. And Charles Kowal the astronomer discovers Chiron. That was November 1977 CE.

Partly due to ‘Star Wars’, 1977 was quite a “spacey” year. It was the year after the Viking lander and incidentally the US Bicentennial, and popular culture began to be invaded by SF and space opera themes, as characterised by space disco. A few weeks after the discovery of the planetoid, certain viewers of ITV Southern, but frustratingly not me because I was watching the ‘Horizon’ documentary on Von Däniken at the time, were informed by an apparent alien that their planet was in trouble and needed to throw away its nuclear weapons, but these two things happening at the same time is a telling example of how space-obsessed everyone was. I have visited this exact time period before on here.

Unlike the other objects orbiting the Sun I’ve mentioned on this blog, as opposed to some of the moons, Chiron was discovered within my lifetime. It was also discovered at a time when few new Solar System bodies were being found. A moon now called Themisto had been detected in 1975 but was lost before its orbit could be plotted, and Charles Kowal himself discovered the tiny Jovian moon Leda on 14th September 1974. Kowal worked at the Mount Palomar observatory in California with its famous 200″ reflector telescope, at the time the most powerful optical telescope humanity had ever built, but his prowess in astronomy at the time was legendary even if he had help from his hardware. Consequently, for me the discovery of Chiron was quite momentous, particularly as I was only ten at the time. It was the first time I experienced the discovery of a new object, and in fact a new kind of object, neither orbiting a planet Sun independently nor associated with the asteroid belt. For a while, before it was named, I called it “Lawok”, which is “Kowal” backwards, on the basis of the asteroid Ekard, which was discovered at Drake University. I think there may be other examples of reversing names for asteroids. Somehow I also expected people to understand what I meant by this name I’d just made up. I was a peculiar child, and am no doubt a peculiar adult.

Astrologers were quite taken with Chiron, perhaps because it was so novel, and rapidly compiled an ephemeris. The conjunction of the ’70s and the discovery of a new “planet” was bound to lead to this kind of thing. In fact there’s more information about the place astrologically than there is astronomically, so little known is it. Chiron has its own sigil:

⚷

This is based on the letters “OK”, for “Object Kowal”, but looks like a key to me. Maybe I should’ve talked about the other sigils before I reached this point, and in fact maybe I should just be covering every object which has one. Because of the fact that the centaur Chiron was a healer, Chiron the planet represents the wounded healer, something I identify quite closely with as a herbalist, and I suspect something which chimes with other herbalists, some of whom got into the profession in connection with their own chronic conditions, but this is not homeedandherbs and I won’t be digressing too far into that. Something that puzzles me about Chiron as a name is that I can’t tell if it was applied before it was realised that it was one of several such bodies and also that it was a kind of hybrid of two types of object, i.e. a centaur. I should also point out at this stage that Phœbe is also, physically speaking, a centaur and apparently also about the same size as Chiron, which right now I think is the largest centaur.

There is no good picture of Chiron. I very much doubt it was in a good position to be visited by the Voyager probes during the Grand Tour and in any case was discovered a few weeks after they were launched. The above, heavily pixelated image, was taken by the Hubble Space Telescope and is the best available. It’s still possible to extract some information from such an image because of fluctuations in brightness and colour as the object rotates, giving a vague impression of surface features. This was done with Pluto before New Horizons got there. Rotation period in particular is fairly straightforward provided the axis isn’t too inclined towards Earth. From this it can be gleaned that Chiron’s day lasts almost six hours. Its year is around fifty of ours. Its orbit is more eccentric than any planet’s, including Pluto, and it spends most of its time outside the orbit of Saturn, dipping inside it for a while. I say “a while” because I can’t do calculus and therefore can’t calculate how long that is. Its orbital inclination of six degrees is greater than that of any planet other than Pluto. At its greatest distance from the Sun it’s further out than the next planet (I will get to why I haven’t called that by its official name in a future post).

When I posted about Titan, I mentioned Chesley Bonestell and the difficulty of depicting worlds about which little was known other than their brightness from Earth and their orbital characteristics. In many cases in the Solar System, this is now resolved, as every planet has now been visited along with several dwarf planets, and also all of the larger moons except possibly Pluto’s. Before that happened, however, a large amount of guesswork was required. The largest body outside the asteroid belt this is still true for is Chiron. Incidentally, you may have noticed that I don’t number minor planets in what I’ve written, but on this occasion it would be ambiguous for me not to point out that the Chiron I’m referring to is not the hypothetical moon of Saturn but 2060 Chiron. Anyway, a lot of what can be said about Chiron is still in that vein, and in that respect the body in question is unique for anything large between the orbits of Jupiter and Neptune. Without a dedicated mission, nothing is likely to pass near the centaur, and only five probes have got beyond the orbit of Pluto. Since its discovery, most of a Chironian year has passed so it was in theory possible that one of them would have visited it, but the probability is very low.

This guessing game is not quite as bad as it used to be with the smaller worlds of the outer system before the space age because it so happens that Chiron (“2060 Chiron” if you insist) is not actually a typical centaur. It’s much closer to being a comet than average. During its summer it develops a coma. This, meaning “hair”, is the temporary slightly cloudy atmosphere which develops around a comet as it approaches the Sun, caused by some of its surface evaporating in the heat. This ultimately puts paid to a comet, a good example being Encke with its mere three year period, which is not doing well. Meteor showers are the remains of comets which have evaporated because of this. I would conclude on this basis that Chiron itself is a short-term member of the system, but this also seems to imply that there are meteor showers out there in deep space orbiting the Sun independently which used to be centaurs but have more circular orbits than the average comet, so it would take longer. In Chiron’s case, millions of years of evaporation may mean that it used to be a dwarf planet, although it’s now far too small. It also develops a tail like a comet.

One surprising thing about the place is that its spectrum shows no water ice at all, which is unusual for an object in this zone. It’s actually similar to a C-type, or carbonaceous, asteroid, a composition shared with Halley’s comet. You have probably gathered by now that the hybrid nature is between that of an asteroid and a comet. Chiron is not just referred to as 2060 Chiron, an asteroid designation, for this reason and is also considered a comet and named appropriately as 95P/Chiron. It can be compared and contrasted with a comet. In terms of composition, it’s very similar, but comets orbit in elongated elliptical orbits taking them into the inner system and out much further, on the whole, although there are also short-period comets like Encke which don’t get outside the asteroid belt. Centaurs, and apparently Chiron in particular, have less comet-like orbits which range between approximately Saturn and the next planet out (I’ll get there, don’t worry). Their orbits are more circular and this has consequences.

Chiron may also be ringed, and may also not be the only ringed centaur. It was initially thought that fluctuation in the brightness of stars in front of which Chiron passes could be explained by the venting of vapours from the surface as it heated up, so there were jets causing the stars to dim, but this hypothesis has now been rejected in favour of rings. They seem to be about seven hundred kilometres in diameter and to be quite sharp-edged. It shares this ringedness with at least one other centaur.

There are proposed missions to Chiron but so far as I can tell, nothing firmly planned, at least as of yet.

Chiron is not in fact the largest centaur. Chariklo, which is also probably ringed, is about three hundred kilometres across. It orbits quite a bit further out than Chiron. Both centaurs have a pair of rings, rather larger in Chariklo’s case.

Centaurs are defined as small solar system bodies orbiting between Jupiter and Neptune. Their orbits are generally unstable over a period of millions of years. I’m not sure why this is. On the one hand, cometary orbits tend to be unstable, so it sort of makes sense that centaurs would be as well, except that they don’t have elongated orbits. On the other, the contrast between the inner and outer systems is that the former is more crowded but has less massive planets whereas the latter is less crowded but has more massive ones. I’m not sure which would make more difference to the movement of objects between them, but it seems calculable. For instance, Jupiter is the most massive planet and is separated by a minimum of five AU from Saturn, the second most massive. Jupiter is 318 times as massive as Earth and Saturn ninety-five. Venus and Earth are the first and second most massive inner planets and the halfway point between them is 0.15 AU, roughly. This means that the scale of gravitational attraction is comparable in both situations. This line of thought doesn’t seem to lead to any firm conclusions.

Centaurs can be classified into three different groups by colour. They can be red, blue-grey or have unknown colour indices. The third category seems to be due to lack of information. Chiron is one of the bluest such objects, and is incidentally a similar colour to Neptune’s moon Triton. Pholus is a similar colour to Mars and Phœbe is one of the bluer ones. It isn’t known why these two groups exist, but possible explanations include the influence of cometary activity such as the development of comæ which lead to the loss of certain materials, the effect of space weathering, i.e. solar radiation causing chemical changes on the surfaces, or just being made ab initio of different substances. This vagueness reflects the lack of information available on them.

Another example of vagueness is the uncertainty about how many of them there are. Estimates vary between forty thousand and ten million objects over a kilometre in diameter ranging between Jupiter and Neptune. Bearing in mind that space is not two-dimensional, this region projects onto a flat ring on the ecliptic (the plane of Earth’s orbit, which is close to that of seven other major planets), an area of 2761 square astronomical units. If there are ten million large centaurs, the separation between them, ignoring inclination to the ecliptic, would average at around 2½ million kilometres. If it’s as low as forty-four thousand, this figure approaches forty million kilometres. Either way, this “second asteroid belt”, as it were, is far sparser than the inner one. Another big difference between centaurs and asteroids is that the latter are more stable and long-lasting. With one known exception, or two if Phœbe is counted, centaurs are not permanent residents of their region, and perhaps surprisingly they are more volatile than asteroids, gradually evaporating and outgassing depending on their locations, even though their regions are much cooler than the asteroid belt. This raises a question in my mind as to whether there are solar systems with much more powerful suns whose asteroids are like this rather than centaurs, so there are rocky bodies which are gradually vapourising and comets made of rock and metal rather than being icy.

The fact that they’re unstable suggests that there is a constant external supply of new centaurs. This is not really news of course. I’d assume that the cloud of planetoids outside the orbit of Pluto, either the Kuiper Belt or the Oort Cloud, is the source of these bodies and that they’re drawn into the planetary part of the system in the same way as comets are, i.e. by the gravitational influence of larger outer planets and to some extent nearby stars. At this point in the journey through the Solar System, the influence of other stars starts to become noticeable, even though they’re still astoundingly distant. It’s also felt through the appearance of comets in the inner system.

It’s possible that not all of the centaurs existed originally in their current form. Some of them may be fragments of larger bodies which have disintegrated through the gravitational influences of the gas giants, and at least one of them may have done the opposite of a Phœbe by escaping Saturn’s gravitational capture and taking on a solar orbit of its own. My impression of the different types, and there isn’t much information to go on, is that they arrive in their new orbits in forms less influenced by the higher temperatures and radiation from sunlight and proceed to be aged by the relatively warm environment of cis Neptunian space and possibly also by being yanked about by the giant planets, and eventually succumb to the ravages of the planetary part of the system, by which time interstellar perturbations have brought more centaurs into it.

Chiron is usually stated to be the first centaur discovered, but apparently this is not so. In 1927, a peculiar “comet” was found, now referred to as 29P/Schwassmann-Wachmann, or Schwassmann-Wachman 1, estimated to have a diameter of about sixty kilometres and an unusually broad and circular orbit for a detectable comet between Jupiter and Saturn. As such it would be unusually close to the Sun for a centaur. Around seven times a year, it suddenly becomes much brighter for about a week at a time. Nowadays it would’ve been considered a centaur, although it’s quite a peculiar one since it’s almost as close to the Sun as Jupiter. There is also a family of comets associated with Jupiter, whose periods are less than twenty years, and Schwassmann-Wachmann 1 can be considered a comet as well.

Quite a lot of centaurs seem to have a diameter of around sixty kilometres. Although none of them currently seem to be greater than about two hundred kilometres across, and there are only a couple of those, it’s been suggested that Ceres used to be one. If this is true, Ceres is far bigger than any current centaur, by a factor of about sixty, and prior to reaching its current position would’ve been even larger due to greater ice content. Nonetheless it is true that Ceres is not like other objects in the asteroid belt.

One asteroid I’m quite curious about in this respect is Hidalgo. This is more an “unusual object” than anything else. It could count both as an asteroid and a centaur, and as far as I know is unique in that respect. It commutes between the asteroid belt and the centaur region, near Saturn and within Jupiter’s orbit. Again, it’s around sixty kilometres in diameter, and is a carbonaceous asteroid (or centaur).

Due to the lack of information generally available on centaurs, that’s most of what I might say about them without venturing into the realm of tedium. I’ll just mention that tholins are what make them red, and the possible link with plutinos, which I will get to.

Next time: the planet with the silly name.