Wednesday, January 9, 2013

Mini Solar Systems

Edit as of 6-23-2016. Many of our gas giant moons seem to have have internal liquid oceans. Liquid water  suggests regions with comfortable temperatures. These strata might also have human friendly pressures. There would certainly be lots of in situ water and organic compounds. I am becoming more interested in icey moons as potential homes for humans.

Originally I had pointed to Jupiter and Saturn suggesting similar moon systems in other star systems would make good science fiction settings. But perhaps the gas giant moons within our own system could provide such a setting. A moon need not reside within the "Goldilocks Zone" in order to accommodate humans.


Most pulp science fiction of yesteryear relies on fast paced story lines that take place over a short time. Not plausible in our solar system where Hohmann launch windows are years apart and trip times between planets are months to years.

A setting Retro Rockets suggests is a mini solar system where trip times and time between launch windows are on the order of days instead of months or years. The "mini solar system" proposed is a gas giant with a family of moons, all orbiting in a star's habitable zone.

This is a plausible setting in my opinion. This spreadsheet shows travel between the moons of Jupiter or Saturn can occur at a good pace. The interval between launch windows is called synodic period.

The gas giants in our solar system have respectable families of moons and many are a comparable size to Mars and Mercury. Here's a graphic comparing some gas giant moons to rocky bodies in our inner solar system:

Retrorockets notes that while mini-solar systems allow a story with an exciting tempo, delta v (needed change in velocity) is still high. But a setting with much less delta V is plausible.

Many of the gas giant moons  in our solar system are tidally locked with the planet they orbit. That is, they always present the same face to the orbiting planet. From the surface of a tide-locked moon, the planet-moon L1 and L2 regions remain in the same part of the sky, much like geosynchronous satellites appear to hover motionless when viewed from the earth's surface. For tide-locked moons, L1 and L2 are possible centers for a space elevator.

Between two moons there exists an elliptical transfer orbit whose apoapsis angular velocity (ω) matches that of the upper moon and whose periapsis ω matches the angular velocity of the lower moon. If the moons are nearly co-planar, trips can be made between the moon's elevators with very little delta V. Here's an illustration showing tide-locked moons Phobos and Deimos:

Expressions for transfer ellipse's eccentricity, apoapsis, periapsis are shown above. They can be generalized to any pair of tide-locked, coplanar moons.

Transfer ellipses between Saturn moon beanstalks:

Tranfer ellipses between Galilean Moon beanstalks:

Something to watch out for is the planet-moon L1 and L2 locations. If L1 and L2 aren't well below the departure arrival point on the beanstalk, the influence of the moon's gravity might substantially alter the shape of the transfer orbit. In the case of Jupiter's and Saturn's moons, the L1 & L2s are well below the tether tops.

Another thing to watch out for is gas giant rings. The chunks of ice in Saturn's rings might well be a debris field that would quickly cut some of these beanstalks.

It is a convention to label a tide-locked moons closest point as having 0 degrees latitude and 0 degrees longitude. For a civilization evolving on a tide-locked moon, I would predict religious significance being attached to 0º, 0º point. A viewer standing at this location will see the gas giant hovering in the sky's zenith. The far and near points will gain additional military and commercial importance when they anchor beanstalks going through L1 and L2.

Our earth globe has non-arbitrary features: the north pole, south pole, equator, tropic of Cancer and Capricorn and the arctic and antarctic circles. Cartographers of tidelocked moons will have additional non-arbitrary markings: A band separating the near side from the far side. I'd also expect a circle containing the near and far points as well as the north and south poles.  A simplified globe would look like a spherical octahedron:

Here is a painting I had done of Gielo (Giant In Earth Like Orbit) and Elm (Earth Like Moon):

A very interesting setting with lots of possibilities. I hope science fiction writers will do stories of habitable moons orbiting a gas giant.


Robert Clark said...

Thanks for that. Pretty cool. When I was younger I used to wonder how science fiction writers so often were able to predict what science was able to come with later, such as computers, robots, space travel, etc.
Come to find out, it was speculations of scientists that the writers had heard about that they based their writings on.
So these calculations of yours could be something some science fiction writers could base their stories on that will later be turned into reality.

Bob Clark

Hollister David said...

Bob, more SF stories are set in digital never-never lands. Less in outer space.

Some would say this is due to the dawning realization that space settlement is impractical. What some call realization I would call misperception.

Science fiction writers could help dispel this misperception and educate the public. They could show us ways to move beyond cradle earth.

But most present day writers will not take the time and effort to research possibilities. Do you think Brin has ever heard of EML2? Do you think Stross has a clue what extra-terrestrial propellant could do to Tsiolkovsky's rocket equation? Has Kim Stanley Robinson ever heard of momentum exchange tethers?

I couldn't even get through 2312. Inteplanetary Hohmann orbits that take weeks. "Aldrin Cyclers" departing from Mercury. A horrible, lazy, badly researched book. KSR wouldn't be fit to bus tables for Arthur C. Clarke.

The recent hard SF stories still rely on devices that were orginal decades ago: atomic rockets, Clarke elevators from earth's surface, lunar helium-3 mines, etc. Space fiction seems stuck in the 70s.

Print SF is dieing out. Not because of lack of possibilities. But due to mediocrity of writers.

Brian Mansur said...

Thanks for posting this. I've been writing a hard science fiction novel and spending prodigious amounts of time reading through the Atomic Rockets website among others (including yours!). No matter what I do, I keep running up against the problem that interesting space travel (for what I want to do in the story) requires truly astounding levels of energy. To help keep the story to something human-scale, I'm keeping most of the plot within the moon system of a gas giant.

Hollister David said...

Brian, I hope you will show me your efforts when you're finished.

Brian Mansur said...

Will do David!

Brian Mansur said...

David, my email is ecuasage at gmail. I have just polished up chapters 1-7 if you want a preview.

Ronald Stepp said...

Not sure it is entirely the "mediocrity of writers" as much as it is the intelligence and education of writers. You literally have to BE a rocket scientist these days to write sci-fi that doesn't get picked apart by everyone with access to google. We know so much more now about the science behind the science fiction that is is supremely difficult to write good sci-fi that aspires to be more than romance novel-like in its content.

Hollister David said...

Ronald, a writer can also use Google to educate himself. There are many forums they can participate. EML2 has been a recurring topic on the NASA spaceflight forum for years.

Are there any science fiction stories featuring a beanstalk reaching from a moon's surface through a planet/moon L1 or L2? I asked this a few years back in the science fiction stack exchange. The clueless mods opined the question was clueless and deleted it.

Information is easier to come by than ever. Yet we grow less informed.

Brian Mansur said...

HI Ronald Steep and Hollister David: There is some cause for hope. Andy Weir's "The Martian" was an intelligently written story with few scientific compromises. Alastair Reynolds and his Revelation Space series from the last decade is another great example. It is more speculative, but still underpinned by a respect for physics, time and distance. And more recently is Leviathan Wakes, which last fall (2015) got turned into a well-executed television series called "The Expanse."

Yes, the information we need is now more readily available than ever, but continues to be underutilized. Why? Because it takes a lot of effort. As I have continued to write my novel (putting the finishing touches on Chapter 13 and am up to 58,000 words!) I have been continually impressed by how much research and thought is required to get the science right and to think through the implications of things.

I have continued to find that the Atomic Rockets website is the single best resource out there. To be well-educated to write hard science fiction, the whole site really should be read through. I took the time to download every webpage it contained and it ran almost 1 GB and well over 1 million words.

And, again, excellent blogs like this one exist to help further the cause (keep it up!!!).