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.