‘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.