Wikipedia:Reference desk/Archives/Science/2020 November 28

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November 28

Unicron on top of Cybertron

Here is a picture of Unicron standing on top of the planet Cybertron, as written by Simon Furman and drawn by Geoff Senior in issue #75 of the Transformers comic. I couldn't upload the image to Wikipedia or Commons as it's under copyright, so I'm linking to the image externally.

Notice how Unicron is pretty much as tall of the planet's diameter. Assuming he was able to land there without causing the entire planet and himself to explode, wouldn't this have affected the planet's gravitational forces and possibly thrown it off its orbit? JIP | Talk 01:54, 28 November 2020 (UTC)

The presence of narrativium dictates otherwise. Gråbergs Gråa Sång (talk) 10:06, 28 November 2020 (UTC)

This is not actually an answer. I know what happened in the comic. I want to know what would have happened according to real-life physics. JIP | Talk 13:34, 28 November 2020 (UTC)

The short answer is "yes, probably, given reasonable assumptions". The longer answer is to be found by studying Orbit#Newtonian_analysis_of_orbital_motion and deciding which assumptions you want to make. Let's assume that Cybertron is about as dense as an Earth-like planet (both are mainly composed of iron, oxygen and silicon). So this means that the total mass of the planet has roughly doubled BUT the very much larger (we assume) sun around which the planet was orbiting is of unchanged mass. So the force on the combined planet + Cybertron is just double what it was on the planet alone. A few other assumptions will need to be made such as whether Cybertron was already in orbit around that sun and "soft landed" on the planet or did he/it smash in at high speed: in which case their combined momentum complicates matters. You choose. Personally, I preferred Gråbergs Gråa Sång's answer. Mike Turnbull (talk) 14:06, 28 November 2020 (UTC)

The laws of physics in the universe of that comic could be different from the laws of physics in our own. ←Baseball Bugs ^{What's up, Doc?} carrots→ 16:36, 28 November 2020 (UTC)

See also Roche limit.  --Lambiam 00:06, 29 November 2020 (UTC)

That giant robot could be weightless. He could be made of Upsidaisium. ←Baseball Bugs ^{What's up, Doc?} carrots→ 00:19, 29 November 2020 (UTC)

Note that both Cybertron and Unicron are probably a lot smaller than you might think. (Either that, or Transformers artists have no sense of scale whatsoever). Iapetus (talk) 16:10, 30 November 2020 (UTC)

Actually, according to the WP article Unicron#Physical_dimensions the size varies a lot between editions of the comics. So the OP is perfectly entitled to ask that hypothetical question. Mike Turnbull (talk) 17:10, 30 November 2020 (UTC)

Tidally locked?

I am reading a science fiction book where I see this description of a planet and its moon:

It was a curious astronomical phenomenon - a planet tidally locked with its sun, a moon tidally locked with its planet, each with a day and night that never shifted across their respective surfaces.

Is this an error? I know that if the planet is tidally locked with the sun, then it has permanent "light" and "dark" sides. But if additionally the moon was tidally locked with the planet, this doesn't really tell us anything about light or dark sides on the moon, right? Is it even possible for the moon to have light and dark sides in this scenario? Or am I misunderstanding? Staecker (talk) 14:43, 28 November 2020 (UTC)

Take a look at the Tidal locking article. Most major moons are locked to the planet they orbit. Planets are more rarely locked to their stars but it certainly happens (Mercury in our case). Note that locking does not always mean that one face of the planet / moon always faces the object to which it is locked. The "odd" bit in the book you quote is perhaps the assertion that day and night never shifted. The moon, for example might have experienced some periods of daylight and others of darkness, since its light source is the star, not the close-by planet. Note that Gråbergs Gråa Sång's answer to the previous question also applies.... Mike Turnbull (talk) 15:09, 28 November 2020 (UTC)

Yes I agree that the tidal locking is possible- the day/night assertion is what I am questioning. Staecker (talk) 16:42, 28 November 2020 (UTC)

Yes, that is wrong. It's the moon that is locket to the planet, not vice versa. So as the moon circles the planet, it will show all sides to the sun (just like our moon does - if the Earth would stop spinning with respect to the sun, that would not change the fact that the moon goes around the Earth and rotates around its axis once per orbit). --Stephan Schulz (talk) 16:54, 28 November 2020 (UTC)

If the moon's orbit were at right-angles to the plane of the orbit of the planet, when it might (just!) be possible to get both to lock and have one side of the moon always face the sun while another always faced the planet. The nice thing about SF books is that they can describe a hypothetical situation and then explore the consequences. That's one aspect that attracts me to the genre. Mike Turnbull (talk) 17:27, 28 November 2020 (UTC)

OK yes I see that would be possible! (But I think one side of the moon will face the sun, and the side facing the planet will be half light, half dark, right?) Unfortunately this seems to be a throwaway line in the book- I don't think this special arrangement has anything to do with the plot. The action takes place on the moon, for which I guess the permanent day/night is important, but the whole scenario could've played out on a tidally locked planet, without the extra layer of the implausible arrangement that you suggest. But I haven't finished the book yet, so we'll see. Staecker (talk) 17:43, 28 November 2020 (UTC)

Actually, unless I misunderstand the claim, that would not be possible. If the moon's orbit is at right angles to the orbital plane of the planet, then it will be tangential a quarter year later. The plane of rotation of the moon would remain the same (conservation of angular momentum), not shift with the planets orbit. --Stephan Schulz (talk) 18:00, 28 November 2020 (UTC)

You are correct that the moon being tidally locked with its planet is not sufficient. But if the moon its planet were tidally locked to each other (with the planet also being tidally locked to its sun), then the description (of the unshifting day and night for both planet and moon) would be correct. -- ToE 19:31, 28 November 2020 (UTC)

[Edit Conflict] I think the book's scenario is geometrically possible, but quite probably not dynamically possible.

Consider: the planet is tidally locked to its sun, so it rotates on its axis once per year, always keeping one face towards the sun and one away, as Mercury was once believed to do. If planet and moon are also mutually tidally locked, each keeping one face towards the other (as the Earth and Moon might become in the very distant future), then the moon's month will be the same as the planet's year, and if the three bodies remain in syzygy, thus:

o0 >O< or thus:

0o >O<,

then the illumination would be as described. I doubt, however, if such an arrangement would be dynamically stable, even if the moon were at either of the two relevant sun–planet Lagrangian points, L₁ or L₂.

Purely for personal interest, Staecker, what is the book? {The poster formerly known as 87.81.230.195} 90.197.26.5 (talk) 19:39, 28 November 2020 (UTC)

Thanks for these answers- it sounds like the geometry is possible at an instant in time, but would not be stable. (I suppose this could indicate a plot point that the moon is being artificially manipulated by some super-technology, but I doubt it.) I had also considered some sort of syzygy arrangement, but assumed that would be impossible to persist- in that case both bodies are simply orbiting the sun, right? So it wouldn't really be a planet with a moon. Anyway that is almost certainly not what the author intended since this would not result in "light" and "dark" sides of the body furthest from the sun- it would be always fully dark (or have an annular light region if the other body was smaller, and then THAT would be a much more noteworthy feature to describe in the text).

The book is A Closed and Common Orbit which I quite like, though as you can see I'm hung up on what I think will end up being an unimportant detail. The quote is from the opening of a section early in "Part 1: Drift". Staecker (talk) 21:42, 28 November 2020 (UTC)

Ha! I read the book myself about 3 years ago but didn't recognise, and don't remember, that detail, which tends to confirm that it isn't particularly important. {The poster formerly known as 87.81.230.195} 90.197.26.5 (talk) 19:38, 29 November 2020 (UTC)

(ec) It is also dynamically possible, but highly improbable.

• First, there are only five theoretically possible positions (Lagrange point), which allow the effect of both bodies having constant orientation to the sun (day and night hemispheres) and to each other, but only two of them make the configuration stable (L4, L5).
• Second, it's almost impossible to have such configuration arise naturally. There is almost no friction in the space so the only force acting is gravity, which makes the whole system conservative. As a result if some body approaches a libration point at some velocity, there is no reason for it to stop there. I know the Lagrange point article says Several planets have trojan asteroids near their L4 and L5 points with respect to the Sun. Jupiter has more than a million of these trojans. I imagine in a big, dense cloud of small rocks it may happen some of them will get slowed down enough in collisions to get locked in a libration point – but objects of asteroid class would not be called 'moons'.
• Third, even if both bodies are big enough, we would hardly call bodies in a L4 Lagrange configuration 'the planet' and 'the moon'. A 'moon' is a relatively small body orbiting around a bigger 'planet' (which is not a star). In a case of Lagrange configuration neither body orbits around the other, but rather they co-orbit a common star, so they would be twin-planets or something, but surely none is a 'moon'.

So, the configuration is possible but would not make a 'planet and its moon' world. --CiaPan (talk) 22:25, 28 November 2020 (UTC)

There are some more problems. A moon is (by definition) orbiting a planet and must do so in its Hill sphere, or it wouldn't remain in orbit. There, tidal forces from the planet acting on the moon are stronger than those from the star, so the moon can only be tidally locked to the planet, not to the star (unless the month is as long as the year, allowing the moon to be locked to both at the same time, in which case the moon is in the L4 or L5 point, about which later more).

Further, if it is a significant moon (like our Moon, not like Mars' moon Deimos), the moon will exert significant tidal forces on the planet. If the planet is tidally locked (I mean, in a 1:1 spin:orbit resonance) to the star, the spin period of the planet matches its orbital period, the spin angular momentum and orbital angular momentum are constant, no angular momentum is transferred and the star exerts no torque on the planet. If the planet is orbited by a significant moon, the moon will exert a tidal torque on the planet, changing its spin period and breaking the tidal lock to the star. So a planet with a big moon cannot be 1:1 tidally locked to its star.

What happens next is that we get a quasi-equilibrium, where angular momentum is flowing from the moon's orbit into the planet's spin at the same rate as it's flowing from the planet's spin into the planet's orbit, with the planet's spin period somewhere between the moon's orbital period and the planet's orbital period. This is not a complete equilibrium, as angular momentum is draining from the moon's orbit (also into the moon's spin), so the moon will spiral in. Tidal forces between the planet and the moon get stronger as the separation decreases, until we've a very rapidly spinning planet as the moon disintegrates when it hits the Roche limit.

The only way you can have a sun and a moon in a fixed location in the sky, and the only way a moon can be tidally locked to the sun, is when the moon is in the L4 or L5 point of the sun-planet system. But that puts all three of them at the corners of an equilateral triangle, putting the moon so far away from the planet that it would be hardly visible. And it would be considered an asteroid, not a moon. It wouldn't be a twin planet, as the L4 or L5 configuration is only stable if one of them is gravitationally dominant over the other, and the one that's not dominant is by definition not a planet. PiusImpavidus (talk) 10:59, 29 November 2020 (UTC)