Astrophysics has to make sense, especially in a SF book. When designing your fictional worlds you need to make sure things can survive on their own, short of creating an alternate physical universe – and I won’t even start imagining how difficult this can possibly be. For example, a celestial body has to be put in the right place, if you want to avoid a premature demise in your galactic saga, and this can easily happen on unsuitable orbits (universe does it sometimes, though: look at poor Neptune’s moon Triton, and its fate. But this is another story).
Triton, the doomed moon
The good news: you can get some help, without even bothering with the maths behind. How? Just use wisely your Lagrangian points.
I have written two articles, “Lagrangian Points – In Reality and Fiction” (Part 1) & (Part 2), for Amazing Stories – in case you want the read the whole discussion about what they are, how they work, and their use in astronomy and fiction. Here I would just mention a few facts you may find interesting (and useful).
First of all, what are these points? Simply put, there are five special points where a body between two larger masses orbits in a constant pattern. It can happen because the gravitational pull of two large masses equals the centripetal force required for a small object to move along. Their name comes – unsurprisingly – from the mathematician Joseph-Louis Lagrange, who “discovered” their properties.
All bodies in orbits have Lagrangian points. While not all of them are stable in the same way, NASA and other space agencies regularly use the ones on Sun-Earth orbit. For example, L1 point allows an uninterrupted view of the sun and therefore the Solar and Heliospheric Observatory Satellite SOHO is set there. L2 is another quite busy point. In the words of NASA, it is “ideal for astronomy because a spacecraft is close enough to readily communicate with Earth, can keep Sun, Earth and Moon behind the spacecraft for solar power and (with appropriate shielding) provides a clear view of deep space for our telescopes.” It was the first home to the WMAP spacecraft, now to Planck, and it is going to host the James Webb Space Telescope in the near future.
Lagrangian points on the Earth-Moon orbit have also been considered as possible location for space colonies. As Gerald K. O’Neill, in his article “The Colonization of Space”, declared, “for the first colony it is probably best to choose a particular point on that sphere, within easy range of both Earth and Moon, not so close as to be eclipsed often, and preferably stable against displacements in all three coordinates. The L4 and L5 satisfy all these conditions. ” Sounds good, doesn’t it?
Unsurprisingly, Lagrangian points have been frequently used in SF – Rocheworld, by Robert Forward or Peter F. Hamilton’s The Reality Dysfunction are just two (good) examples. But they are also popular in the anime/manga universe. In Gundam this is where the colonies are based and many battles fought.
Gundam: colonies
Finally, impossible not to mention the infamous L3 of the Earth-Sun orbit. Differently from L1 and L2, L3 remains constantly hidden behind the Sun, and, while useless for practical applications, it has been since long a trope for SF writers and conspiracy theorists. It is said that’s where a hidden “Planet-X”, or a Counter-Earth, is based. Literary references abound, from The Illuminatus! Trilogy to Antigeos series of novel of Paul Capon, just to mention two (a good list can be retrieved here). Attractive as it might seem, Planet X is unlikely to exist: L3 is a point inherently unstable, by physics (L1 -L2 – L3 all are) and for the specific case of the Sun-Earth orbit: there are also other bodies in inner orbits, like Venus, that also would exert their gravitational pull. As a NASA paper on this topic explains in detail, a celestial body located there will only last 150 years.
If you want to get serious with your Lagrangian, a good explanation of the maths can be found here. And for an entertaining, yet accurate, explanation, have a look at this great animation, done by Artifexian.