Asteroid Research – Orbits

One of the fun things about writing science fiction is learning about science!  My current work in progress is set in the asteroid belt, so I’ve had fun studying it. 

In my story, the aliens travel from the asteroid they live on to visit several asteroids humans live on before returning to their own asteroid.  I wanted to find a set of real asteroids where this made sense.  

In order to find a real set of asteroids, I had to answer two questions: 1) which asteroids would it make sense for humans and aliens to live on?  And 2) how do asteroids move relative to one another and what would it take to travel between them?

For the first question, I thought that humans (and aliens) would establish bases on asteroids big enough to support them.  I thought the most important resources for an asteroid to have are water and organics.  I discovered that many asteroids are members of families, groups of asteroids that have similar orbits (semi-major axis [maximum distance from the sun], inclination [angle from the ecliptic plane], and eccentricity [a measure of how circular the orbit is]).  Asteroid families can be created by collisions, so most of the asteroids in the family were once part of the same parent body and would likely be composed of similar materials.  One such family is the Themis family.  I found multiple papers arguing there is evidence that the Themis asteroids contain water ice and organics (among them: 1, 2, 3, 4).  So, if I use members of the Themis family for the asteroids that my story humans and aliens live on, I can assume they have water, organics, and metal resources to be extracted.  

For the second question, I hoped that having asteroids in the same family might mean the asteroids travel together.  Over the long term (months and longer), orbital dynamics does not work that way (unless the asteroids happen to be at different points in the same orbit and one “follows” the other).  Asteroids closer to the sun have a shorter “year” than asteroids further away, so even with a small difference in semimajor axis, the closer-to-the-sun asteroids will eventually “lap” the further out ones.  So they will be close, and then far away, and then close again.  Plotting a course in the asteroid belt is going to be a challenge!  Happily for me, my story takes place within a short period of time, and I can put my asteroid bases wherever is convenient, so I just needed to find a handful of Themis-family asteroids that are “near” each other at a point in time.  

I found a really cool asteroid simulator on line (Catalina Sky Survey (CSS) Orbit View) where you can enter the asteroids and date of your choice and see where they are and how they move relative to one another. The pictures in this blog post were generated by this awesome tool. It’s really fun just to watch the asteroids move around!

I filtered the Minor Planet Center Orbit (MPCORB) database for Themis family asteroids, put the top 60 into the simulator, let it run starting at 2150, and followed 24 Themis.  In 2243, I found what I was looking for: 6 Themis family asteroids “reasonably” close to one another.  

Note: I am well aware that “reasonably” close together at 3.14 AU is still really far apart.  However, my alien spacecraft does not need to break the laws of physics and exceed the speed of light to get from one to another, which is enough for me.

My next step will be to determine what we know about these asteroids, so I can make these tiny worlds more realistic.