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Tuesday, 10 March 2020

Answers for Authors: Are there any realistic stardrive ideas?

I just like this picture.
Interstellar travel is a sci-fi staple, but writing about it in a realistic way always hits the same problem: The distances involved, and speeds required, are mind numbing. Worse is the universal speed limit - lightspeed - which seems to dictate that interstellar journeys must take years or decades. Overcoming that isn't just a question of better power sources and engine design: As you approach lightspeed all sorts of weirdness breaks loose to slow an object down, and time itself slows, then virtually stops. Time running incredibly slowly prevents any engine or power source from operating (this is a huge oversimplification, but for a better explanation you'll have to follow the link), so on the face of it lightspeed can never be reached.

If you didn't follow the link, here's the video explanation.

Even so, for a writer looking for a touch of realism there are resources to draw upon: There have been a lot of interstellar drive ideas seriously proposed1, going back to designs from the 1950's. 2016 even saw the announcement of the privately funded 'Breakthrough Starshot' research project, which is an effort to solve the problems of sending a probe to the nearest star. So here are some of the ways it's been suggested we might tackle the problem of interstellar travel, from the 'with unlimited money we could actually build this today' type to the 'hey, someone's been feeding this physicist LSD2' type.
Let's start with sub-lightspeed drives, which are usually based on more realistic, near term, technology. For spacecraft that don't mind spending decades travelling the void, e.g. robot probes, sleeper ships or generation arks they're a perfectly serviceable option:

Orion drive:
Yes, nukes can be used for better things than genocide.

In the 1950's a bunch of the engineers who'd worked on America's nuclear program developed 'project Orion'. This was a plan for spaceships that would be propelled by throwing nukes out of a hatch in the back (with what was basically a huge air rifle) and setting them off.
Because... well, er... why not, I guess?
The interesting thing is that, according to modern computer modelling, with the right combination of low yield nukes and massive construction it probably would have worked. The ship would have consisted of a massively armoured pusher plate, topped with equally massive shock absorbers, topped in turn with a habitation module built like a nuclear submarine. For additional protection the pusher plater would have been enveloped in a magnetic shield to prevent plasma from the nuke actually contacting it. The nukes would have ranged from a little bigger than those developed for the 'Davey Crockett' nuclear grenade launcher to about 20 kilotons, and would have been surrounded by carefully shaped casings filled with beryllum oxide reaction mass, to direct the majority of the force towards the ship's pusher plate.

Above: The design for one of the Orion drive nukes.
The enormous amounts of energy available from riding nuclear explosions would allow an Orion ship to be both very long range, very fast (able to reach Mars in about a month), and absolutely huge by today's standards: The smallest complete designs were 40 meters in diameters and 80 meters from nosecone to pusher plate - that's nearly as long as the International Space Station.

Above: The design for the interplanetary Orion ship.
Two things closed the project down, and neither was engineering feasibility: The nuclear test ban treaty and one important piece of technology never happening - a fallout free nuke. Without that even starting an Orion drive up in orbit would have increased cancers worldwide. There're still people interested in the general idea today - as I said, it's very possible it could have been made to work. The original engineers even built a miniature Orion that used high explosive instead of nukes:

Above: The high explosive propelled Orion model. 

The biggest version of the Orion engine would have been a monstrous, 100,000 ton, nuke spitting behemoth capable of reaching 3% of lightspeed. It would have a long trip to the stars (around 150 years to the nearest), and a fairly radioactive one... but the obstacles are ones of engineering, political will, and money, not engineering plausibility.
Above: A video on Daedelus. From courtesy of Times Infinity, via youtube. 

The British Interplanetary Society developed this idea in the 1970s, as a cleaner update of the Orion concept. Their engine would use deuterium fusion explosions, heating coin sized pellets of deuterium fuel with an electron beam until they triggered fusion in the pellet's core - a process called 'inertial confinement fusion'.
The Daedelus design differed from Orion in other ways: The explosions would happen in a reaction chamber rather than against a pusher plate, and the plasma produced would get directed by powerful magnetic fields, generated by superconducting coils, out of a huge nozzle to produce thrust.

A breakdown of the two stages of the Daedelus design. Courtesy of the British Interplanetary Society.
Daedelus would use a much higher rate of detonations than Orion - 250 per second - to boost the ship to 12% of lightspeed. It would also be even bigger than most Orion designs, at 190 meters total lengths, and be divided into two sections: A 1700 ton 'boost' stage that would run for 24 months before being dropped, and a 1000 ton science stage, that would accelerate for another 21 months and carry the science and communications equipment (the reference design was unmanned) up to full speed.

A Daedelus ship compared to a Saturn 5 rocket. The Saturn 5 is about 100 meters tall. 

Daedelus would have been mother ship to a number of sub-probes, and a team of robotic 'wardens' that would maintain the ship during it's decades long mission - the reference plan called for a 50 year long cruise to Barnard's star, 6 light years away.
The Daedalus team weren't ever aiming to build a working starship, it was simply a theoretical exercise. But it established ICF engines as a potential interstellar drive.
Interestingly, ground based inertial confinement fusion reactors have continued to be tested and developed, becoming the first fusion reactor design to generate more energy than they used in 2015 - so this is another idea that's already gone beyond the theory stage.

Bussard Ramscoop:
Above: The starship Red Dwarf, probably the smeggiest ramscoop starship ever, and the best known. Courtesy of the BBC.
In a previous post I mentioned how, although it seems empty, the space between the stars is actually filled with a very thin soup of ionised gas and dust. That gas is mostly hydrogen, the main fuel for a fusion reactor, and in the 1960's an engineer called Robert Bussard came up with a remarkably simple idea: Build a ship that could collect and concentrate that diffuse fuel source - a ramscoop. It would have a huge electromagnetic scoop, which would funnel the interstellar hydrogen into the front of he ship, concentrate it, and use it as fuel.
It's a brilliant thought, but when engineers actually ran the maths they found problems: First, the interstellar medium is too thin in our part of the galaxy for the idea to work - though in other regions of the galaxy it might be more feasible. Second, collecting the gas in the magnetic scoop creates a huge amount of drag - in fact the scoop makes a more effective sail or brake than it does engine. Several variations seeking to overcome the originals problems have been proposed since:

This variation uses a fusion reactor with it's own fuel supply to generate power, and instead of collecting the hydrogen as fuel the scoop just redirects down the vessel's throat and accelerates it out the back. In effect the vessel is pulling and pushing against the gas without absorbing it, a little like a squid's jets propulsion. Because the scoop never has to bring the hydrogen up to the same speed as the ship the drag is greatly reduced.

Squid, doing their water-jet propulsion thing.

This design would use the same 'squid jet' idea as the RAIR engine, but instead of an onboard reactor energy would be beamed to it as laser light, from a beaming station on Earth. That would mean that, like the original ramjet design, the LAIR engine wouldn't need to carry fuel or propellant.

The original Bussard Ramscoop isn't an inherently flawed idea - it's just that the local interstellar hydrogen won't provide enough power to overcome it's own drag. But there's a way around that: Create a 'runway' of deuterium gas (a much more potent fuel than interstellar hydrogen) for the scoop to collect and use. That could be done by using something like a particle accelerator to fire ionised deuterium gas into space, creating a long 'tongue' of fuel. The speed at which the gas in the tongue was moving could be adjusted to match the expected speed of the vehicle as it reached it, eliminating drag from fuel collection. The idea could also be made to work using pellets of fuel, fired at such a speed that the ship would catch and collect each pellet just as it reached the right speed to overtake it.

Artists impression of a Ramjet starship, courtesy of Nick Stevens.

Laser boosted lightsail:
This is the idea being (tentatively) put forward by the Breakthrough Starshot initiative. Sunlight driven lightsails are a technology that is fairly well understood, and have already been flown in space. Both JAXA's IKAROS space craft and the Planetary Society's Lightsail satellite are propelled this way.

Above: JAXA's lightsail, the IKAROS.

The idea is very simple (to be fair the execution is fairly tricky): Although it's to subtle to influence much here on Earth, light produces a very tiny push on things it reflects from. In space, where there's no air friction, a lightweight mirror surfaced sail can collect enough of this push to slowly accelerate a spacecraft. Because it's 'wind' is travelling at lightspeed the sail can (in theory) keep accelerating up to just below lightspeed itself - it's the ultimate victory of fuel efficiency over raw thrust.

Above: The Planetary society give a run-down on their (now launched and successful) lightsail mission.

The downside is that light spreads out as you get further from its source, loosing intensity and reducing the push on a sail. So, to get a lightsail up to interstellar speeds, you'd need to replace the Sun with a laser beam - laser's don't spread out nearly as much. The approach of Breakthrough Starshot would be to have a square kilometer of Earth-based lasers, that focused their combined beams onto the sail, pushing up to between 5% and 20% of lightspeed.

Above: An animation showing the Breakthrough Starshot laser sail proposal. 

Like any idea it has it's own problems: An interstellar sail would need to be far higher quality reflector, not to mention more heatproof, than any sail flown to date. And the best sails we might build (in the near future at least) could only carry a few tens of grams at most. We can build micro-electronic sensors that small, but sending a signal back to Earth will be very difficult with so little room for a power source. But, all that said, this is probably the best developed potential starship engine.
For a manned mission, needing a ship massing many tons, the sail would need to be huge - at least a hundred of kilometers across. It would need to be driven by an equally huge array of lasers, which would probably be stationed near the orbit of Earth and powered by vast arrays of solar panels. The lasers would be focussed onto the sail by a gigantic Fresnel lens, floating in space in front of the lasers.

An alternative approach is to have the sail do a very close flyby of the Sun, and pick up enough speed from the intense light before it gets too far away. That eliminates the need for a laser beaming station, but has two drawbacks: The sail and probe need to be even more heatproof, and the maximum speed is limited to about 0.5% of lightspeed - which would get you to the nearest star in around a thousand years.

Above: An infographic on lightsails, from

Antimatter rocket:
Above: The rings of Saturn, the most abundant source of natural antimatter in our solar system. Courtesy of NASA/JPL.

Antimatter, made famous by Star Trek as the fuel of choice for starships, is very powerful stuff: Inert by itself, it reacts with regular matter to release astounding amounts of energy. A piece weighing only grams could produce a bigger explosion than the Hiroshima bomb. Various stardrive proposals over the years have been designed around various ways of harnessing the power of antimatter. Some are essentially a supercharged project Orion, others use the heat from an ongoing antimatter reaction to vaporize propellant, producing a continuous thrust like a regular rocket.

Four different types of continuous thrust antimatter engines. For more details see sections 6.2 and 6.3 of this paper from the University of Paris-Saclay plasma and gas laboratory.

They all run up against the same basic problem: Antimatter is fantastically rare in nature, and so hard to make that it's market price is 63 trillion dollars per gram. 
...that may not be as much of a showstopper as once thought, for two reasons:
1: There are a whole class of antimatter engines that use antimatter to enhance conventional fusion or fission reactions, and so need much smaller amounts.
2: There are small but significant sources of naturally occurring antimatter in our solar system, which it would be perfectly possible to mine. The biggest lies in the rings of the planet Saturn, but there are very small sources in near-Earth space too. An 'antimatter mine' could conceivable collect enough to make some of the lower end antimatter drives reality. 
A schematic of the ICAN 2 antimatter powered spacecraft, a sort of antimatter supercharged project Orion.

But even antimatter propulsion runs up against the lightspeed limit - so if we want to look beyond the very nearest stars we'll have to start investigating the more exotic options sooner or later.... 

Faster Than Light travel: Up to now we've been looking at engines we could build, or at least realistically research, today. They're great... for unmanned probes, generation ships, or sleeper ships to the nearest star systems.

Above: The nearest other solar systems to our own - it's lonely out here!

But they're limited by the 'speed of light' rule: Basically, anything approaching lightspeed begins to experience seriously weird effects: Time slowing down, mass increasing, and distances distorting - all stopping any further attempts to increase forward speed.

How quckly time moves, compared to how fast you travel- and if time slows down, so does whatever mechanism is running your engine, so your engines stop working.
Here's the thing: When you get into their details, Einstein's theories point to certain... loopholes. These point to ways a ship might reach it's destination sooner than the lightspeed limit strictly allows, without actually breaking the limit: Yes, we're into the realm of engines that warp space itself - although 'engines' might not be the right word. 'Bizzare theoretical phenomena that haven't yet been conclusively proven to be totally impossible' is a better way of describing them.
None of these are anywhere near even a laboratory demonstration3, but the theories are at least sound enough to be worth understanding better.


Above: The basic concept of a wormhole: If space is the sheet, and normally you'd need to travel all along the sheet to the destination, a wormhole lets you jump straight from one side to the other.

In many ways these are the grandaddy of FTL ideas, coming straight out of the equations of general relativity: Two distant points in the universe can be connected by a bridge made of space-time itself (called a wormhole) which stretches through a higher dimension than our usual three, along which the distance is much shorter. 
If the two points are very far apart then a wormhole can simply provide a shortcut, like in the picture above. The trick works because that light rays moving along the wormhole with you will still be moving faster than you - so the speed of light rule stays in one piece. But being a shortcut isn't the only way such a bridge can be used: The ends of the bridge could be made to connect to different times. Connect the other end of the wormhole to last Thursday and you could actually arrive before you set out. A wormhole sounds like just the ticket for reaching distant planets, but there are all sorts of problems with actually building and maintaining one:
  • It would be hugely unstable, prone to slamming shut and crushing travellers - in fact the vibrations of just one photon could cause it to collapse, making it useless even for communication. To counteract this it would need immense amounts of 'exotic matter', (matter with negative mass) pushing into it.
  • Bringing the mouths of the wormhole close to each other, if it could be kept open, would cause an endless loop of regurgitated photons to build up between them, resulting in a gigantic radiation pulse.
  • Building it in the first place would mean tailoring black holes to have specific properties, no small feat considering we don't even understand what goes on inside one.

All that said, there is some hope: A recent study found that, while a truly stable wormhole might be impossible, very small wormholes might be persuaded to collapse very slowly - slowly enough for communications at FTL rates through it, at least. 

Alcubierre (warp) drive:

Yes, warp drive inspired by Star Trek.
This is a (very slightly) less outlandish take on the trick wormholes pull. The Alcubierre 'engine' is inspired by the observation that the expansion of our universe is not simply due to galaxies flying apart through space; the space between them is actually expanding, like the skin of an inflating balloon. And, if space can expand, then in theory it can also  contract. 

Above: Space itself is expanding - courtesy of minutephysics, who do a lot of interesting videos!

So, in principle, if you could apply huge amounts energy to the space behind a ship, making it rapidly expand, and huge amounts of negative energy to the space in front of the ship, forcing it to rapidly contract. The ship itself is caught in a calm bubble in between these two zones, and that bubble is dragged across the universe at FTL speeds by the expansion behind and contraction in front, along with anything inside it.
A bit like the wormhole it works (or at least the equations don't totally rule out it working) because a ray of light, in the calm bubble with the ship, would still be moving quicker than the ship. So again the ship is not, technically, breaking the speed limit. It's like being in an airbus flying inside a fast wind: It's ground speed might actually exceed the speed of sound, but it's airspeed is always slower than sound because the air is flowing in the same direction as it - hence it encounters none of the shockwaves or extreme airframe stresses that would normally stop an airbus going so fast.

Above: The high speed jet stream created by storm Ciara, which allowed jets flying from New York to London to exceed the speed of sound with their groundspeed, without exceeding it with their airspeed, and make the trip in record time. Going the other way it made all the jets super slow. Courtesy of the New York Times.

To work the drive needs negative energy, which we can only make in minute amounts, and it would build up a gigantic 'radiation shockwave' as it travelled, probably vaporising the destination world. Even so NASA has seriously looked at what would be needed to apply the theory, albeit as a simulation only.
Do these seemingly impossible design problems mean we should give up on FTL? If you ask me... no. We may never succeed, but every time we've tried to look for a work around for lightspeed we've learnt new things about the laws of physics. And, who knows, maybe, one day, we'll learn enough...

Out-of-the-box ideas:

Let's say we'll always be limited to below lightspeed: It would take us decades just to reach the very nearest stars. That's no good for building an interstellar empire. There are, however, ways around the problem...

Make yourself immortal:

Tick tock, tick tock...
The longest lived humans make it to maybe 120 years. But there are creatures, like tortoises, that can easily clock up two centuries. There are simpler organisms, like jellyfish, that can live much longer: One species in particular can return to a larval state and regenerate, effectively living forever.

Above: Jonathan the 184 year old tortoise gets his first ever wash.

What if that's the real key to a species establishing itself in the galaxy - not high power stardrives but just having a long lifespan? Does that make us permanently excluded from the club?
Maybe not.
It shouldn't come as much of a surprise that life extending technologies are being investigated, with some promising results. We've only got lab only experiments so far, but fruit flies, mice, and other creatures have had their lifespans extended. Some insects have had their lifespan doubled by deliberate genetic mutation.

Above: Turritopsis dohrnii the tiny, immortal, jellyfish.

If we could count on a hale and hearty existence into our 230th year, would a wait of 100 years for a slow-boat mission to reach a nearby star system really seem so terrible?
And, if enough life extension to cross the galaxy seems too much to hope for, there're other ways of living to the end of a long mission...

Freeze yourself:

Space travel: A great excuse to get nude with everyone. Until the aliens star jumping out of people's chests...

People have been falling (and getting pushed) into freezing water since forever, and once in a while they would be found and revived - sometimes after hours. So what about freezing a person in a controlled way, and reviving them at a distant future date? It's a sci-fi staple and, although in the real word we're still a long way off, simple organisms have been revived and born after over ten years on ice.
Although freezing people for space travel is still impossible (don't believe people who claim they can freeze your body to be revived in the future - think about that promise for a second), research into it actually has more immediate life-and death applications. In hospitals suspended animation has been used to prevent gunshot and stabbing victims from deteriorating until they can be operated on.


Become an A.I.
Yup, we're into Matrix territory.

This is probably the most extreme of the 'live longer' options: Take the human mind, download it into a computer, and you can crew your ship without having actual bodies. The computerised minds won't age, and can run the spacecraft for centuries if needs be.
As bizarre and terrifying as this sounds, it's being investigated. 'Uploading' a human is a real goal for Ray Kurzweil, director of engineering at Google, who claims people will be able to "upload" their entire brains to computers by 2045. IF that were to happen then worries about lifespans on a space mission would be irrelevant: The uploaded people would be effectively immortal.
Although that immortality does have a weak spot.

Although uploading a full human mind is still far away (if it should be done at all) there are glimmers that it could eventually be done: The 'Openworm project' succeeded in copying the nerve patterns of a nematode worm (it's 'mind' as far as it has one) into an artificial body.  
Move Earth:
Earth. Ta-daa!

Yep, this has seriously been looked at and found, if not practical today, not impossible either. And, if you want to commute from Earth to another planet quickly, why not 'just' move Earth closer?
Well, OK, the obvious answer is because that's freaking insane. Except that the idea's obvious insanity haven't stopped astrophysicists making serious suggestions on how it might be possible to move the Earth. These include building a huge light sail (which we mentioned above), and towing Earth with it, using the planet's own gravity as a tether. Another involves slingshotting asteroids around Earth, gradually altering its orbit with the reverse of a spacecraft's gravitational slingshot manouvre.
Although these ideas show that moving a planet isn't as impossible as it sounds, it would take a huge amount of time to move Earth far. Even if you waited for a close pass by another star - as happened 70,000 years ago - you'd need a far more efficient method of propulsion to transplant Earth into another solar system. That said... it might be possible to engineer such a close encounter - and it might be desirable to do so, since one day our Sun will die.
Extreme as it is, in many way's it's more plausible than things like warp drives or wormholes - they need large amounts of negative energy, and precise control of black holes to be made to work - this would just need patience and precision.
And it's not the most audacious plan to improve interstellar commuting. Not by a long shot....
Move the solar system
If you're looking for an insane idea that would make any James Bond villain wet themselves in awe, look no further than the Shkadov thruster: A plan to turn the Sun into a gigantic engine and steer this entire solar system through the galaxy like... well I can't think of anything it'd be like. Well done Leonid Shkadov, you've broken my ability to metaphor.
To do this you'd have to take the planet Mercury, dismantle it and spin it out into thin, reflective sheets. Then wrap half the Sun in a gigantic structure called a Shkadov sail, made from those sheets. The pressure of sunlight would hold the sail up against gravity, much like today's solar sails, and the Sun's light and particle emissions would be diverted in one direction, generating a thrust.
Although such a drive would take millions of years to work, it would eventually allow a patient race to park their star system in easy commuting distance of any other star they liked the looks of - or several races together could park their stars into a miniature, artificial, cluster. In combination with the planet moving ideas mentioned above, that might allow them to make their homeworld immortal, hopping from solar system to solar system for ever.

Move to a star cluster...
Above: A globular cluster.

If a civilisation has the patience to consider Skadov's ultimate engine then they'd know there's a natural alternative: Around the edges of our galaxy lie collections of stars called 'globular clusters'. Although they sound like something you spit up when you've got a bad cold, they're actually huge complexes of long lived stars, all living within fractions of a light year of each other. Havard scientist Rosanne DiStefano has pointed out that, for the advanced alien who likes company, worlds inside such a cluster would be a prime location - species could live within easy travelling distance of each other, making something like Star Trek's Federation (Or Star Wars' Galactic Empire) practical, even with sublight drives.

Before anyone points out that at least three of those ideas are, well, insane.... yes I realise they are. But they make a great leaping off point for thinking and talking about our place in the universe, and just how big and grandiose engineering might get for an advanced star faring race. It's worth remembering too: There's a difference between an idea that's currently (utterly) impractical and one that's physically impossible.
If there's anything history on Earth has teaches it's that as long as something is possible it's not question of if it will be done, just when...

1: I use the word 'seriously' in the context of discussions with theoretical physicists here, so don't get too hopeful.

2: That sounds like hyperbole, but it's hard not to suspect that either the minds behind some of these ideas were either on something illegal, or should be put on something medicinal.

3:That would be a pretty astounding and terrifying lab, as it would need tame black holes and huge amounts of negative energy. Ever try house training a black hole?

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