“Janus, Mimas, Enceladus. . .”

When I was six, I set myself the task of memorising the then known moons of Saturn, and it stuck. Even today I can easily reel off “Janus, Mimas, Enceladus, Tethys, Dione, Rhea, Titan, Hyperion, Iapetus, Phoebe”. Several times that number of moons are known to orbit Saturn today, but even ten sounds like a lot. There are a couple of oddities on this list, but today I’m actually going to be talking about Janus. Sort of.

“Sort of” because Janus is not necessarily what we thought it was. It was sometimes, but at others it wasn’t. Janus was discovered in 1966 CE but although it had an unofficial name, it wasn’t officially called that until 1983. In the meantime, it had been discovered that “Janus” wasn’t what everyone thought it was. In the opposite situation to Venus, which had previously been called the morning and evening star (Phosphoros and Hesperos) and in ancient times was not recognised as the same thing, Janus turned out to be two separate moons. This led to confusion about the nature of its orbit, since it would appear to “jump around”. On arrival at Saturn, the Voyager probes were able to take a picture of two moons which seemed to be on a collision course with each other but were obviously still there in spite of previous apparent collisions, and it emerged that “Janus” was in fact two moons sharing the same orbit and swapping over when they got close to each other. Hence another name was needed, and one moon kept the name and the other was called Epimetheus. Epimetheus was in a sense the first Saturnian moon to be discovered by the Voyager missions and therefore has the number XI, but it had been seen before and just not recognised for what it was. Janus is considerably larger than Epimetheus, at three million cubic kilometres as opposed to 820 000, and since both are too small to be round it makes more sense to refer to their size by their volumes. Janus is in fact 203 by 185 by 152.6 kilometres, whereas Epimetheus is 129.8 by 114 by 106.2 kilometres. Neither are drastically far from being spherical and are, like a lot of other bodies of that size, potato-like in appearance, if potatoes have craters.

The situation with Janus and Epimetheus was the first time I realised that gravity doesn’t just attract. Janus and Epimetheus zoom around Saturn at around sixteen kilometres per second, kind of treating their common orbit like a race track. The inner moon catches up with the other, at which point they swing around each other and the inner becomes the outer. This works because the gravitational attraction between the moon in front and the one behind causes one to speed up and enter a higher orbit and the other to slow down and enter a lower one, after which they separate, i.e. move away from each other. In other words, the acceleration due to one moon “falling” towards the other leads to it being “pushed” away, so to speak. It would be interesting if some kind of jiggery-pokery from this happening could be harnessed to provide something which looks like anti-gravity, but it’s a very special case and I really don’t think it could be.

At their minimum distance, Janus and Epimetheus are only fifty kilometres apart. Since they are actually larger than that even in their minimum dimensions, each would practically fill the other’s sky at these times. Larger moons approaching at this sort of distance would smash each other to bits with their gravity, and it’s possible that this has already happened and caused the situation to arise in the first place. Maybe the two used to be a single dumb bell-shaped moon back in the day. The exchange occurs once every four years or so because at other times they aren’t close enough to have that influence on each other.

This is Janus itself:

Since the moon is only two hundred kilometres across, an individual pixel in this image would have a width of about two hundred metres. It isn’t minute, but it is fairly small. On the other hand, it’s also large enough to approach being round and doesn’t give the impression of being “cute” like some small moons and asteroids do because the features on its surface are not out of proportion. I only realised in the last couple of days that it was (kind of) discovered in 1966 because to me it’s always been there, which of course it sort of has, but it’s also a bit surprising that it was only discovered eight months before I was born, just after the Beach Boys’ ‘Good Vibrations’ had slipped off the number one spot (it was actually Tom Jones but I’ll breeze over that. He’s okay, but – well, you know).

There are four named features on Janus, named after characters from the legend of the twins Castor and Pollux, like other features on Epimetheus. These are Castor, Idas, Lynceus and Phoibe, all craters. There is a faint dust ring, about five thousand kilometres across, around the orbits, which isn’t surprising as they presumably claw at each other wildly every four years as they pass each other, which is bound to raise some dust, although it’s attributed to meteoroid impacts. They’re also shepherd moons, which isn’t just an album by Eithne but also refers to moons which keep rings in place and maintain their neat edges. Janus does a slightly better job than Epimetheus because it’s more massive, so the A Ring, which they shepherd, is neater when Janus is closer than when it’s the other way round. It’s also probably a rubble pile, hence the ring, and it’s quite icy. These two things together make it very light for its size, rather like Saturn, at sixty-three percent that of water, so it’s actually less dense than Saturn. It’s possible to measure this from the moons’ gravitational influence on each other. Surface gravity varies due to the irregular shape but is around a six hundredth of ours. It’s reddish-brown.

I might as well do Epimetheus while I’m at it. Epimetheus I would’ve expected to be paired with a moon called Prometheus as they were brothers, but apparently not. I also knew a cat called that so it’s a bit weird typing that name here. Here it is, seen from a pole:

It looks a lot more “moony” than Janus to me, because it has proper-looking craters. In fact I’m surprised how different they look. It was realised in about 1978 that astronomers were probably dealing with two different moons, and one of the Pioneer probes might have taken a picture of Epimetheus but it was too vague to enable it to have its orbit plotted. The craters are called Hilaeira and Pollux, which figures. There’s actually a photo of it with the shadow of the F Ring across it:

That’s it, more or less. Not a lot to say about such tiny moons. Oh, just that Janus used to be the god of doors and has a face on both sides of his head, which makes you think Janus the moon is special because it always has one face looking at Saturn and the other out into the rest of the system, but actually that’s normal for moons, in Saturn’s case all the way out to Titan.

Mimas next time.

Our Other Moons

Anyone who reads this blog regularly will know that I don’t call that luminary in the sky “the Moon”, but Cynthia. This is because I think it’s important to acknowledge its existence as a body in the Solar System in its own right rather than simply an adjunct to Earth, and because calling it “the Moon” is like calling Earth “the Planet” without having any other name for it. Also, Cynthia is arguably not actually a moon at all. Looked at from the Sun’s (yes I know) perspective, Earth and Cynthia weave in and out of each other’s paths as they orbit and if Pluto is excluded, Cynthia’s mass is a far greater fraction of Earth’s at 1/81 than the moon of any other major planet. The pull of solar gravity on Cynthia is greater than Earth’s.

This leads us into the “nut” situation, where the thing which we think of as the quintessential example of a category turns out not to be, such as peanuts, almonds and so forth, because maybe “the Moon” is not a moon at all. Further, we get to the predicament of claiming that Earth has no moon at all, and that “the Moon” is something else. This sounds absurd. However, the question arises of whether Earth has any moons now, or had any in the past, or perhaps had more moons which collided and became Cynthia, and again whether these “moons” counted as moons.

One thing which comes to my mind is the Chicxulub Impactor, which wiped out the non-avian dinosaurs sixty-six million years ago. Is it conceivable that that orbited Earth for a while before it crashed down onto it? There isn’t any scientific reason to suppose either that it did or didn’t, assuming it to be an asteroid rather than a comet. If it was a comet, it’s unlikely to have done so as most of its substance would’ve vaporised if it had orbited us for long. It may be worth considering the Chicxulub Impactor separately than just in this post, because the situation is complex and research has suggested different things. Also, in a sense there’s nothing special about it, as this planet has been repeatedly hit by massive bodies in the current Phanerozoic Eon (the time since hard-shelled animals evolved). It’s unlikely that the scientific method can be applied to the paths of any of these objects to determine whether or not they were previously in a long-term orbit about our planet. A side issue here, which I’ve mentioned previously, is the possibility that Earth has had rings at some point due to asteroids approaching this planet but not hitting, and breaking up close to the surface but still beyond the atmosphere. Again, all that can be said about this is that it’s plausible. Evidence might involve finding a higher incidence of meteorites around the equator or climatic differences, but those would both depend on the position of the continents at the time.

In fact it looks like rocky inner planets tend not to have moons if our system is anything to go by. Neither Mercury nor Venus have any, though in the past both were thought to have one at different times. Mariner 10 was briefly thought to have discovered a moon of Mercury in March 1974 but it was actually the star 31 Crateris. Venus was also once thought to have one, named Neith, repeatedly observed by astronomers from 1650 onwards but never detected during a transit. It is odd that it was supposèdly seen so many times even though it doesn’t exist. It was considered to be proportionately the same size as Cynthia and to orbit perpendicular to the ecliptic, which is in itself quite peculiar. It’s now thought that most of the apparent observations were merely stars near the line of sight. Inner planets in general have a bit of a problem keeping moons due to the fact that the Sun’s gravity is relatively greater and the radius in which a moon can exist is small. In fact Cynthia is a good example of this because it orbits separately from Earth.

Mars, of course, has two small moons, but its case is a little different. It orbits closest to the asteroid belt, enabling it to capture asteroids, and being further from the Sun gives it more opportunity to do so. However, its moons orbit unusually close to it and one is unstable and will be broken up by tidal forces in a few tens of millions of years, becoming a ring. I suspect Mars has had a series of moons due to its proximity to a large number of asteroids. If Earth were closer to the belt, it seems likely that it too could acquire at least temporary moons. As it stands, asteroids are mercifully sparser at our orbit and the “price” we pay for this is that we have no captured moons.

Another aspect of this, already noted in the case of Cynthia, is that orbits look different depending on where you see them from. As far as we’re concerned, Cynthia orbits us once a month and it’s very simple, but from a solar perspective the orbits of the two bodies are braided, somewhat like the coörbitals of Saturn. The same applies to some of the possible moons of Earth. The classic example right now is Cruithne (“kroo-ee-nyer”). This asteroid takes a year, actually 364 days, to orbit the Sun in a roughly similar looking orbit interlocking with Earth’s, but from Earth’s perspective it describes a centuries-long path consisting of various alembic and horseshoe shapes as it moves around us. It’s been described as our second moon, but this isn’t really true, and there are a number of other bodies with similar relationships to both Earth and the Sun. It has a diameter of around five kilometres and its orbit is not entirely stable.

In 1846 an astronomer called Frederic Petit, of Toulouse, reported the discovery of a moon which orbited this planet once every two and three-quarter hours with an apogee of 3 570 kilometres and a perigee of only 11.4! At the time, it wasn’t known how to account for air resistance but even back then scientists were sceptical of a moon which dipped thoroughly into what we’d now call the troposphere. As was fashionable at the time, Petit claimed this accounted for irregularities in Cynthia’s orbit around Earth. His results were never reproduced, but he did end up having his idea mentioned in Jules Verne’s 1865 novel «De la Terre à la Lune». This spurred a lot of people into looking for it, and notably William Henry Pickering, who predicted the position of Pluto and claimed to have detected plants growing on Cynthia, actually looked for a secondary moon of Cynthia itself, which he presumed would have to be a maximum of three metres in diameter.

In 1898, the Hamburger Dr Georg Waltemath claimed not just one moon but a whole string of them. One of them, he claimed, was approximately a million kilometres away, took almost six months to orbit and had a diameter of around seven hundred kilometres. He claimed it had been seen in Greenland during the night period of winter in 1881, and further that it would transit the Sun. He and some companions reported that an object about six arc minutes in diameter did indeed do so, but it so happened that some other astronomers were observing the Sun at the same time and only saw sunspots, so that was the end of that. It may be an illustration of how easily one can be drawn into perceiving something by another’s enthusiasm, conviction or charisma, or maybe just of the power of suggestion. The largest of these moons was named Lilith by an astrologer and an ephemeris was prepared.

Now there are thousands of artificial objects in orbit, to the extent that they threaten future space missions. These are in a sense moons in their own right, though artificial ones. These could also provide evidence for the presence of other moons because of their gravitational influence on their orbits. It has been claimed that this happens, but the data used, at the end of the 1960s CE, were insufficiently accurate to judge. Hence although it seemed that something was detected, it was within the margin of error in the measurements, and it can’t be concluded that there’s anything there.

One thing which definitely does happen is that small asteroids occasionally get temporarily captured by our gravity. Kamoʻoalewa is the name of an object which appears to be a small chunk of Cynthia which is temporarily orbiting Earth. Like many other small planetoids in the system, it’s quite red, but the particular shade of red is dissimilar to those of various asteroids so it’s likely to have come from our main satellite. It appears to be about forty metres across, although it may be very irregular, and actually does describe the kind of orbit attributed to Neith, perpendicular to Earth’s orbital plane. However, although it circles us, it’s also beyond the distance where Earth is the main gravitational influence on it. Like Cruithne, Kamoʻoalewa is what’s known as a quasi-satellite, taking almost exactly the same time to orbit the Sun as Earth does and therefore staying close to this planet, but from Earth’s perspective appearing to travel around us in the opposite direction to our orbit in a kind of bent closed curve. The phenomenon is a little like retrograde Mercury. Mercury occasionally appears to be moving backwards in a loop from our perspective, but it’s because of the relative speed and positions of the two orbits around the Sun, except that it’s exaggerated by the asteroid’s extreme proximity.

There are something like five other asteroids with this kind of relationship with Earth, and incidentally Earth is not unique in this respect. As mentioned previously, there are also the Lagrange points of both the Earth-Sun and terrestrial-lunar systems. Analogous positions associated with other bodies are common, particularly Neptune, as I’ve already been into. There are both clouds of dust occupying the terrestrial-lunar Lagrange points and Earth trojans 60° ahead of or behind Earth in its orbit. No trailing trojans have been detected so far but there are at least two leading ones, one of which has a diameter of three hundred metres. I covered much of this in Antichthon (apparently I called it “Counter-Earth”).

Many, perhaps most, NEOs are analogous to extra moons. A group I haven’t mentioned yet is the Amor asteroids, named after the asteroid Amor and also including Eros. These come within 0.3 AU of Earth, or 45 million kilometres, and approach the Sun closest outside our orbit with a period greater than a year. This means they always orbit outside our own path round the Sun and are therefore not Earth-crossers. Four dozen Amor asteroids come within seven and a half million kilometres of Earth’s average distance from the Sun. Of them, Eros has actually been visited by a spacecraft. Most of them cross Mars’s orbit, putting them in the asteroid belt proper at their greatest distance from the Sun.

To finish then, Earth currently has no permanent (other) moons, as might be expected given the status of the other inner planets, and in fact we arguably have no moons at all because of Cynthia’s peculiar nature. If we were closer to the asteroid belt we might acquire some. This raises the question of how many otherwise Earth-like planets have any moons and whether this is significant for the evolution of Homo sapiens, but as I’ve said before, this series is not going to focus on life because everything does that. Interestingly though, although it hasn’t been demonstrated scientifically, it’s quite plausible to suggest that we have had other moons in the past and just as a closing comment, some people believe Cynthia was originally two bodies which collided, partly explaining the difference between the near and far sides.