The energy efficient way to have a Bus to the Stars!

Imagine a bus timetable that arrived not every quarter hour, but every quarter century? The ‘bus’ is so big and the trip so long (maybe 20 to 30 years) that it’s less like buying a ticket, and more like immigrating to another country. Meet the Interstellar Cycler – a scaled up version of the Mars Cycler – but instead of being in an orbit out to Mars – this one is heading out to Alpha Centauri!

An O’Neil Cylinder is more like an island nation than a ship. Because it is so big, we usually think of it as a huge space colony in orbit the Earth or our sun. It’s 8km across and 32 km long, like a giant stainless steel beer can spun up so that people live on the inside. It’s so big that the atmosphere hugs the walls and the centre is vacuum like space. The image below seems to have the other end of the cylinder dark. (In night mode – but I cannot imagine why they would not all be on the same clock? Unless there turns out to be environmental, wildlife, or weather control reasons to do so? Or is it just temporary – like celebrating halloween or something down that end?)

Of course it would take phenomenal amounts of energy to accelerate something this big up to 20% light speed! But we’re often too linear in our thinking. Go exponential. Imagine a solar system with hundreds of trillions of people, most of whom are already living off earth in space-stations like this. A small fraction are in the energy business and operating huge Power Satellites or PowerSats that hover around our sun. It would only take a tiny fraction of that energy to fire on our space-station to use the gentle push of photons to accelerate the entire thing up to 20% light speed. See Interstellar Highways for a discussion about Stellar Lasers – below.


Let’s rename this thing. Interstellar Cycler is a six-syllable-salad, and I almost switched to using the blunter but far less elegant term Star Bus. But that’s too underwhelming for such a magnificent concept.

So why not just call it Rama, after Arthur C Clarke’s famous novel Rendezvous with Rama – a novel whose title itself beautifully expresses the concept anyway. SPOILER ALERT for the below!


In the novel, Rama is an alien probe exploring the galaxy that does a half year sweep of our solar system before shooting out the other side. Humanity manages to prioritise one ship – just one little ship – to fire out and meet this monster. But we’re talking about eventually setting up dozens or even hundreds of these as the Space Liners of the future – and having hundred of ‘ferries’ to rendezvous with it. All of this on a vast but predictable timetable of decades and centuries.

Being set in the future – it probably has fusion energy and an oil-tanker’s worth of hydrogen to power the thing for thousands of years. (If we can’t crack fusion, it will have a bunch of uranium breeder reactors to warm it for the long cold of interstellar flight.)

It uses ‘ferming’ instead of farming to feed the food needs of the passengers on board – along with some vertical salad and herb gardens for flavour.

That’s a huge amount of energy to feed and house and warm the hundreds of thousands or millions of people on board – but it’s nothing compared to the energy required to push Rama up to 20% C!

A Star Bus just keeps on going

The whole point of our Rama ‘Star Bus’ or Space Liner is that we don’t have to waste all that energy slowing something this big down again. After it is accelerated up to 20% C let’s just keep it that way! Why accelerate all this matter up and not just save all that energy to push smaller ships? In one word, redundancy. It has the resources for many hundreds or thousands of backup life support systems – maybe even (with fusion power) a natural ecology on board. It has huge thick walls to protect from cosmic radiation. It has the space and energy to not only take on board more passengers and cargo but even various ores to send to their manufacturing districts to smelt and build everything from the next spanner to the next computer chip. It’s a mobile civilisation – and all the multiple redundancies that implies. It’s a spacious and beautiful lifestyle choice for those passengers on board. It’s simply the safest, most beautiful way to travel between the stars that we know of. So we accelerate this monster up once, and then use ferries to load and unload it.

But how do we accelerate all those ferries up to 20% C? This is why Rama is most probably set up after target stars are already settled. It takes a settled system to have the huge PowerSat lasers to accelerate the smaller ferries up to meet Rama and decelerate disembarking ferries.

Isaac Arthur explains in the video below that a Rama can use long metal drag lines to tether to the magnetic field of the target star to steer. It can turn corners and even do a 180 degree turn – but at one gravity of turning stresses that’s still a 2500 AU turning circle. That’s 2,500 Astronomical Units – the distance from the Earth to our sun. That’s 370 billion km’s – 1.2 times the width of our solar system. That’s a big turning circle at 1g sideways thrust.

So it’s easier to tether in a much smaller angle change. Tether to the star’s magnetic field with those drag lines to make a tiny correction and our bus might visit say 26 star systems – one for each letter of the alphabet – before returning home. Then each course correction just drops the metal drag lines in system and Rama stays at 20% C.

How long would a round trip take? The average distance between stars in our galaxy is 5 years. That’s five light years, and as we are travelling at 1/5th light speed we need to multiply it by about 25 years or a generation per star system. Times 26 systems, and you’re looking at something like 700 to 800 years for a round trip. With life extension this may not matter so much. But even without it, if you want to spend a third of your life reaching the next star system, this is the best way to do it.

Remember, we’re talking about a future that might involve hundreds of trillions of people in our solar system alone. The further ahead we imagine, the more Rama routes would be established. Planning a trip really might start to look like a bus timetable. One that ran over decades. And 20% C is worth it! Alpha Centauri is 4 light years away. At 20% C, that’s ‘only’ a few decades to get to the next star – 20 to 25 years in a beautiful little man-made world.

It could be a colony ship, but hitting the breaks is tricky!

There are also tactics to use Rama to colonise a new star system in the first place. But it’s tricky – as that means hitting the breaks. How do we do that without already having a huge Interstellar Highway PowerSat Laser system set up to catch something the size of a mountain range hurtling in at 20% C? ‘Easy’. Push some ships ahead to deploy that system!

Rama is so big it can use its own lasers to push smaller ships ahead. Their job is to deploy the giant catching lasers a year or so before Rama even arrives. But how do these smaller ships slow themselves down? First they deploy solar sails, and use the target star’s solar wind to slow themselves down. A bit. There are also a mix of strategies where they bounce lasers between them to slow down the fleet. There might even be one or two sacrificial drone ships that go ahead to use the energy of the star to gather solar power and fire it back to slow down the fleet. Sacrificing one or two drone ships is a small price to pay to plant human civilisation in a new star system!

As the smaller ships slow down they scatter across the new system to set up giant lasers on various moons and planets and asteroids. They would all turn and target Rama, getting ready to catch her in an embrace of lasers as it came screaming in at 20% C. It takes a LOT of energy to decelerate something that big. So this is a risky strategy – as they need all those smaller craft to set up the huge Lasers ahead of schedule to catch Rama as she comes shrieking in.

But if the smaller craft had a bunch of bad luck, like freakish accidents or even some kind of rebellion – we have already said that Rama can turn corners. If something goes catastrophically wrong, she will use the target sun’s gravity and magnetic field to do an emergency turn and either head back home or at least angle backwards, even if it takes a few extra systems. (Rather than plough on in a straight line forever – a doomed ghost ship sailing on into the night – a Sci-Fi trope that gets dished out now and then.) It has some control and ability to turn – maybe even turn right around. Otherwise we wouldn’t be suggesting it as a Star Bus in the first place!

If it has fusion tech, it would even have the hydrogen on board to run its own lasers to slow itself down, even if that takes all the way to the next star system. Hey, a whole new star system. Something to go get! An O’Neil Cylinder is a mobile space-civilisation anyway. Once decelerated into an orbit around a nice planet or asteroid belt, it can start devouring the local resources to build more ships and one day even replicate itself into another Rama!

Economics – the travelling oil tanker

Rama is huge and charges to take passengers and cargo to the stars. But also think of it as a travelling oil tanker. Laser light is the ‘oil’ of the future. It has it’s own reactors and solar farms and mirrors and lasers, so Rama can easily push smaller ships around the target solar system as it takes a good part of a year to travel in system and then out the other side.

And in a fusion powered, AI driven robot post-scarcity future, there simply may not be economic concerns as such. If you’re paid a few hundred grand a year by the state simply for being a citizen of the earth, and the robots are doing all the work, and you have life extension out to a thousand years and beyond – why not catch a bus to Alpha Centauri?

And now, after that introduction, it’s time for me to hand you over to the man himself – Isaac Arthur.

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4 Responses to The energy efficient way to have a Bus to the Stars!

  1. Jim Baerg says:

    Thanks for the link to ‘ferming’.
    For quite a few years I wondered about the possibility of something like that, but I never had the right search term to find out if anyone was working on it.
    I thought in terms of making flour & sausage meat with such techniques, while using soil or hydroponics for growing fruit & vegetables.

  2. Eclipse Now says:

    Yes – ferming could be huge! It comes up in ABC’s Future Tense and programs like that. They’re still working on it, even during the pandemic.

  3. Jim Baerg says:

    BTW a bit of a quibble by a pedant:
    “It’s 8km across and 32 km long, like a giant stainless steel beer can spun up so that people live on the inside. It’s so big that the atmosphere hugs the walls and the centre is vacuum like space.”

    The centre is 4 km up from ground level. If the air thins out at the same rate as on earth that would make the air at the centre roughly the same as 4 km up on earth (Tibetan plateau, thin but tolerable). However the ‘gravity’ decreases so the ‘thinning’ is slower & air pressure at the center would be about what we get 2 km up on earth, not a problem for most people to breath.

  4. Eclipse Now says:

    Ah – that’s interesting! I’ve always just heard it’s vacuum – but given it’s only 4k from either ‘wall’ (ground, rim) and the whole thing is spinning – I guess your claim is as good as any. I can’t model what’s going on with the atmosphere and don’t pretend to be an expert in any of this stuff. I’m just reporting what other ‘experts’ have said about the stuff that excites me.
    The net impact if what you say works out to be true could be pretty cool. I wonder if hang-gliders could fly wall to wall challenges now that the middle is breathable! Thanks for commenting.

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