Oct 01 2011

Train tracks in space

Lunar Free Return Trajectory courtesy of Karen Wehrstein

Interesting articles about the scope of the challenges involved in interstellar travel:

To send spacecraft to other stars in the space of a human lifetime, new methods of propulsion are going to be needed to provide the necessary ‘oomph’ to break free of our Solar System. Currently the best bet is nuclear fusion power, but there’s a problem – it hasn’t even been shown to be a commercially viable source of energy on Earth yet.

There’s no harm in dreaming, but most people I talk to don’t even appreciate the difference between sending a sizable spacecraft to the moon and a survivable trip to Mars. The distance to Mars is so much greater than I can’t even the trajectories on this page without reducing the earth and moon to little bitty points. The moon is a quarter million miles distant. On average, Mars is roughly 100 million miles away. More over the moon is always the same distance away, we can get there and back with Apollo technology in about a week. Using traditional elliptical orbits it would take about two years to Mars and back.

But there may be another way, and the Apollo trips to the moon are a useful illustration: Cyclers.

The lunar free return trajectory (FRT) is a bit more complicated than the static diagram shown above. Since the moon circles the earth, the actual path traced out by a cycling spacecraft resembles petals built up by an old spirograph. What’s cool about the FRT is it takes very little fuel once the spacecraft is inserted into it. With a bit of tweaking the vehicle will keep cycling between the earth and the moon forever. The concept is highly scalable. Habs can be joined together into more and more elaborate structures, new vehicles can be added so that there’s a network of cyclers one whizzing by the earth and the moon every hour or two like trains.

A passenger leaving earth would first have to match vectors with the cycler as it whizzes by near Low Earth Orbit. That takes a lot of fuel; one reason the Saturn V was so large was because it had to put a departure stage bound for the moon with enough fuel on board to blast make translunar injection and/or the FRT. But once aboard the interior could be downright spacious compared to a cramped capsule. On the return trip, a tiny reentry vehicle could separate and use the earth’s upper atmosphere to slow down. 

An artist's concept of a cycler and a schematic showing how complex the orbits can be via Found Connections

The same idea might work for more ambitious voyages. There are cyclic orbits possible where an object could traverse the distance between Mars and Earth, either way, in a few weeks. Put enough cyclers in that orbit and there’s one coming and going at both planets every week or two. Add habs to the cycler and over time it can grown into a luxury space-hotel.

Cylers could be used to link up other planets and objects — the periods would be longer but the advantages still hold. Ceres, the moons of Jupiter and Saturn, even the inner planet Mercury (Which might be a big ball of valuable metals) could theoretically be part of the  network. And faster space craft using as yet non-existent engines could enhance the network greatly.

That’s a big project, comparable to colonizing the New World or the Polynesian expansion into the South Pacific. It would probably occupy us for a century or two, maybe millennia. But unless revolutionary new propulsion systems are developed, as far ahead of chemical rockets and elleiptical orbits as steam engines are ahead of sailing vessels and trade winds, it’s going to take a comprehensive approach like this to domesticate our solar system. Alas, given the vast distances of interstellar space, the stars will probably have to wait longer still.


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  1. 1

    Making guesses about future technology is often a fools errand, and I hope to be proven wrong, but I believe that no matter how long humans exist, we’ll never be able to travel to even the nearest stars.
    The distances are just so immense and traveling even a percentage of the speed of light is so far from how present capabilities, it’s hard to imagine us reaching a different star system. The nearest star, Proxima Centauri, is about four light years away, or 9,460,528,000,000 kilometers. It would take the fastest chemical rockets something like 90,000 years to reach it, but that’s besides the point, as even if we could convert ever bit of mass in the Visible Universe into rocket file, it wouldn’t be enough fuel to power a rocket to the nearest star.
    Obviously, if we are to reach Proxima Centauri, we would need to create alternative methods of space travel, like taking advantage of Wormholes or creating Warp Drives (neither of which are even known to even be theoretically possible).
    The engineering challenges seem to be even further away from our current capabilities than a chemical rocket is from a sailboat.
    Again, I hope I’m proven wrong!

  2. 2

    Interesting concept. But wouldn’t we already need some new propulsion technology to get those capsules up to the speeds of these orbits? Let alone getting large habitats into such orbits.

  3. 3
    Stephen "DarkSyde" Andrew

    Yes, the launch craft would have to match with the cycler. But it would only have to carry enough fuel to loop back around if it misses or wait for another cycler (Or maybe it’s just SOL in that case). It’s a tough call if that launch craft should have enough fuel and supplies on board to make the entire journey in a pinch. On the other end at both Mars and earth you can air brake. The advantage of cyclers is they can be inserted unmanned, scaled up, and multipe cyclers can be in play. All you have to do is get to them and your ride home is much safer. Actually it’s at least possible.

    To give an idea how dicey it would be in a capsule or a smallish hab like a shuttle, think of Apollo as an outrigger canoe and the earth-moon journey as a trip from Florida to Cuba. Using that same canoe for a voyage to Australia would be near suicidal. To stay in space for several months on end you need large facilities. Ideally, for trips to Mars you’d want almost ISS sized facilities. Or very small asteroids, i.e., or some combo; really big habs, lots of them, lots of shielding, and lots of supplies to make the trip safely and routinely.

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    bad Jim

    That is one hell of a cool idea, which I hadn’t heard of before. Still, I’m happy to leave planetary exploration to robots. We meat machines don’t add much value when you take maintenance and support into account.

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