Seeking help with a mainworld design effort...

[Flynn]

Good morning, All,

I want to design a world with rather specific features, and would like to request some scientific assistance on this, if you wish to contribute some of your knowledge this way.

I want a world that is roughly Earthlike, which will support humans and native lifeforms without a problem, with near to 1G of gravity at the surface.

I would like the world to have four satellites. (This is where it gets tricky.) One of the satellites needs to support life on its own, with an atmosphere breathable by humans. Of the three other moons, two revolve around one another as they chase across the sky, similar to some moons of Jupiter, and the last is somewhat Mars-like, in that it appears red to those on the surface of the planet.

Okay, the questions:

1. Can this be done in a scientifically plausible manner?

2. If so, what are the relative sizes of each of the satellites, assuming that the mainworld is near Earth-like?

3. What would be the revolution periods of these moons about the mainworld, based on whatever information is necessary to achieve the above effects?

Please let me know your thoughts,
Flynn

(If need be, I'll tackle this using First In or World Builder's when I get home, but I'm hoping there's some armchair astronomers with some thoughts on this matter before the day is out.)

 

[Andrew Boulton]

I don't think you can have moons with their own moons.

Breathable atmospheres are possible, for small values of "breathable".

 

[Anthony]

It's theoretically possible for a moon to have its own moons; it just needs to be a quite long distance from its primary, since tidal effects will eliminate the moon. Alternately, the moons can be recent phenomenon -- the earth's moon could theoretically capture an asteroid, and the asteroid might then stay in orbit for a few million years.

 

[Flynn]

Sorry, guys, I meant to say that all four moons orbit the mainworld. The smallest two are probably no more than large asteroids (such as Deimos and Phobos in size and/or shape) that orbit around a central gravitational point, and that point orbits the mainworld. Hence the appearance of two moons that orbit one another as they race across the sky (or creep, as the case may be...)

-Flynn

 

[Straybow]

Originally posted by Andrew Boulton:
I don't think you can have moons with their own moons.

Breathable atmospheres are possible, for small values of "breathable".

A Mars-sized planet with Earth-like density could have "small value" breathable atmosphere. Suppose N2/O2 atmosphere has partial pressures roughly 160/40 millibars. Inhabitants would need masks to concentrate the oxygen to a minimal 80 millibars. Slightly above 20th/21st century tech.

This mega-moon could orbit a planet with a diminutive heavy core and larger diameter than Earth, resulting in a higher total mass with roughly equal surface gravity.

The smaller red moon has just enough atmosphere to cause the iron oxide red coloration, and is in a coupled orbit with double the period of the habitable moon.

The tiny moonlet pair are coupled in a closer orbit with half the period of the habitable moon.

Could it be stable for tens of millions of years? Or only tens of thousands? Dunno.