View Full Version : Irregular Moon

Scott Kaelen
05-09-2013, 04:54 PM
I know that the larger planets have a number of irregular moons - large satellites that were pulled into planetary orbit long after the planet was formed - but here's my question:

Would it be possible for an Earth-type planet to have an irregular moon that was visible in the night sky by the naked eye? Or would Earth's comparatively small size make that an impossibility?

05-09-2013, 06:01 PM
Yes. The round shape is a byproduct of its mass. Smaller moons don't have that mass and so can be irregular, like Diemos and Phobos of Mars.

05-09-2013, 09:02 PM
Of course, the smaller size makes them harder to see. You could move them closer, but that would also increase their speed.

For example: The biggest irregular natural satelite is Proteus, with an average radius of 210 km. Our Moon has a radius of 1,710 km. So at a similar distance, Proteus would look around 1/81 the size of our Moon. (approx 1/9 radius, squared)

Scott Kaelen
05-09-2013, 11:17 PM
Thanks for the answers. I was a bit pushed for time earlier so I couldn't write much in my earlier post, but the reason I am asking is because in the story I am writing there are four moons. One is, to all intents and purposes, Earth's Moon. Another is about two thirds the size of the moon, with a distinct green colour. The smallest of the four is roughly twice the size and about the same distance from the planet as, say, the International Space Station. The fourth moon is on an erratic orbit and is only visible when it passes closer to the planet, once every couple of months (I'm riffing with that last). I'm hoping this sounds viable.

05-09-2013, 11:24 PM
An erratic orbit is not possible unless a force is acting on it. I'm not sure I'd believe a satellite, could have an orbit that is erratic without it crashing into something relatively quickly.

Michael Davis
05-10-2013, 12:51 AM
If a moon were hit by a massive object I'd think both the shape and orbit could be irregular if it were small enough.

05-10-2013, 01:35 AM
The orbit would stabilize or not very quickly. It would enter another stable orbit, or crash into the planet, or be ejected.

Here is a fun website that allows you to simulate the effects of a rogue star coming through our solar system, and what effects it would have on the planets. It will give you an idea of how orbital mechanics work.


05-10-2013, 07:30 AM
You need to define what you mean by "irregular". Some planetary moons in our solar system have highly elliptical orbits (Nereid, of planet Neptune, for instance):


One pretty large moon, Triton, also of planet Neptune, orbits in a retrograde manner:


Chances are that both these objects were captured by Neptune's gravitational interaction, rather than formed along with the planetary body.

In a sense, our own moon is pretty irregular, being the only one in the solar system believed to have formed via a major collision between the parent planet and another very large body.


Scott Kaelen
05-10-2013, 11:39 AM
Thanks for the link, Sarpedon :)

It may be that I'm thinking down the wrong track here. What I'm aiming for is this particular moon (one of four) to only be visible from a certain area of the planet several times a year. I considered making it a periodic comet instead, but again that doesn't sound feasible to have a comet, visible to the naked eye, pass several times a year.

This is the breakdown so far:
Moon 1: Almost exactly like Earth's Moon.
Moon 2: 2/3rds the size of Earth's Moon. Green to the naked eye.
Moon 3: Appears as a grey smudge in the sky. Roughly twice the size of the International Space Station. Seen only from a specific region, namely a continent roughly the size of Europe.
Moon 4: The problem moon! I want this to be somewhat larger to the naked eye than Moon 3, but smaller than Moon 2. I want it to be visible, let's say, five times a year, so about once every two and a half months. By 'visible', again I mean from a specific place the size of Eastern Europe or the whole of Europe. Any suggestions as to how I might best achieve this would be much appreciated. Moon, comet, or perhaps something else I haven't yet considered?

05-10-2013, 05:05 PM
I'd say moon 3 is also a problem moon, as it would have to have a geosyncrenous orbit/be tidally locked to the planet. Not impossible but a hell of a coincidence. Since it is such a small moon, it is unlikely to have tidally locked the planet, so it would have to have somehow fallen into a geosyncrenous orbit. (sp?)

Any object that is visible to anyone on earth will be visible to everyone in the same hemisphere (except in the above case). Everyone on Earth sees the moon, because the moon orbits. Certain stars are visible only in certain hemispheres. I think what you want is a satellite with a highly elliptical orbit. If you have the 75 day orbit, you could say that 7 out of those days is close enough to be visible. The problem would be that if it were small, then it would have to get close to the other moons, which would lead to collisions. It could be a much larger body, and orbit further out, but then the gravitational attraction likely wouldn't be sufficient.

Scott Kaelen
05-10-2013, 08:15 PM
Moon 3 is comparatively much closer to the planet, and much, much smaller both in actual size and visibility. I had meant for it to be a static object in the sky, wrongfully assuming that a geostationary orbit (yes, it's not really a moon) was possible from any position above the planet. If a geostationary orbit is only possible from directly over the equator, I can work with that.

Of course, to the planet's inhabitants, moon 3 is just a moon. They don't know why it doesn't move like the two largest moons, nor do they understand why moon 4 is only visible several times a year.

A highly elliptical 75-day orbit would work. For this fourth moon, I'm beginning to tilt more towards the option of a large periodic comet instead, one that perhaps passes annually. If this is a better idea then I'm ready to roll with it.

Essentially, what is going to happen is moon 4 (let's call it that for now) will collide with moon 3, sending moon 3 into a decaying orbit. The collision is enough to push moon 4 out of orbit, so bye-bye moon 4, your job is done. Now moon 3 is going to fall into an ocean, causing massive tidal waves. My hope is that the direction of impact is enough to lessen the tidal waves that occur in the opposite direction to the impact, and heighten the tidal waves that are pushed out from the front of the impact. (Remember this 'moon' is about twice the size of the ISS, with roughly twice the density. It's not going to have the devastating effect of a meteorite of the same size, since the density is much lower.)

Mr. GreyMan
05-11-2013, 02:25 AM
Our Moon is not 100% stable "The Moon is spiraling away from Earth at a rate of 3.8 cm per year."[1 (http://en.wikipedia.org/wiki/Lunar_Laser_Ranging_experiment)]

But, I think a small Moon with an elliptical orbit would be very plausible. However, that would mean it would grow and shrink in the sky. Not so much "disappearing" as growing smaller and smaller. Also having only seeable from one region would be hard to explain.

If Moon 3 is in geosynchronous orbit (https://en.wikipedia.org/wiki/Geosynchronous_orbit) (and small) it might be seeable from only one continent directly under it on the equator. But, Moon 4 with an elliptical orbit....

I think--were I you--I would make-up a handwaving ad hoc explanation about how the atmosphere over this one content has special properties. You could explain the wavelength reflected from the moons in question bounces off the atmosphere, except in that one place (and maybe only at certain times of year when the atmosphere is 'just right.')

"As a ray of white sunlight travels through the atmosphere to an observer, some of the colors are scattered out of the beam by air molecules and airborne particles, changing the final color of the beam the viewer sees. Because the shorter wavelength components, such as blue and green, scatter more strongly, these colors are preferentially removed from the beam."[2 (http://en.wikipedia.org/wiki/Sunset#Colors)]

So, the green moon's light could be scattered out of the atmosphere, except over this one content (special particles in the air from Applied Phlebotinum (http://tvtropes.org/pmwiki/pmwiki.php/Main/AppliedPhlebotinum)).

Depending on the nature of the setting, you could also have people living on this content have slightly better/different eyes. It would not take a huge change to make them see something other's could not. Like 20/19 on average, or having everyone else be slightly colorblind.