Stopping an Interstellar Freight Train

So you're travelling to Alpha Centauri at 10% the speed of light and the two stars are shining out of your viewscreen. Congratulations, you avoided life support failure, the crew going crazy, or the ship's computer going crazy and killing the crew in their hibernation pods. Now how do you stop? I put this blog post up because I was inspired by the Icarus Project's post on the same subject, and I've been giving this a great deal of thought over the years.

The bad news is that it's going to take all the energy you put into getting up to your velocity into slowing down. You could use your rocket engines (assuming they still work) to slow down, but that means your initial fuel load goes up exponentially and the spacecraft at launch would have been the size of a mountain. (Launching this is going to make the Apollo project look like the Bobsy Twins and their kindly uncle building ships in their back yard*). The alternative is to simply cut your initial velocity in half and use the other half of your delta V (total velocity capacity) to brake at the other end. But it means getting there slowly, and slow means that taxpayers and politicians are less likely to fund such a venture (and the mission is already going to take decades).

Obviously, you would ditch as much of your unneeded payload as possible (but, light years from anywhere, every scrap of material is valuable). Radiation shielding, habitats, soil, empty fuel tanks, all jettisoned except for landers, science payload and your colony equipment. Oh, and colonists, all crammed into a tiny habitat, or camping out in the landers. The high-efficiency motor itself used to get up to this speed is likely to be massive; if one could drop it overboard if there was some other way to break, one probably would. Still, perhaps a fuel reserve could be used to slow the spacecraft a little bit before saying goodbye to the motor. A 10 or 20% reduction in velocity might make all the difference - this must be weighed up against the risk of the motor not working after all these years in vacuum and hard radiation, and already thousands of hours of operation.

The good news is that there are ways to slow down, albeit each with their own limitations. The most viable way to slow down would be with a Magsail. This uses the interstellar medium, which although is a huge vacuum, actually has one or two ions per cubic centimetre. Travelling at such high velocities, this actually translates into a very, very thin mass flow. Running a current through a huge superconducting loop hundreds of kilometres in diameter would tug on these stray ions with a magnetic field, transferring the ship's momentum to the them and slowing the ship down. A very elegant solution which works better when decelerating from higher velocities. Perhaps it would be better to hang onto the engine and only fire it when the ship's velocity got down to ~0.001c, or in the region of a few hundred kilometres per second.

The second no-fuel alternative is to use a solar sail. A BIG solar sail. But to slow down from 0.1c, even a mere 50 tonne payload would require a sail 1000km in diameter, and using an efficient inflated sail made of beryllium diving right in. Making the sail out of super-high strength, high temperature carbon nanotube technology doesn't help that much. Where it does get better though is that when the slower clip you can come in at, the less of a problem it is. The diameter appears to scale with the incoming velocity; so at 0.01c the sail diameter is only 94km. And of course, the sail mass is proportional to the inverse square of the diameter, so a 94km sail only weighs 1% of what a 1000km sail would be. Interested readers should consult Pat Galea's article on this. Alpha Centauri, however, does offer the chance to use two suns to slow down - albeit not in line with one another, making it a lot more difficult to brake at one and then brake at the other. A gravitational slingshot might help, though - calculated to shave off velocity instead of boosting it.

Aerobraking is how planetary landers shed all their nasty orbital velocity, but could we do that to shave off a few percent off of our incoming velocity? Planetary atmospheric deceleration is basically impossible for even a tiny fraction of lightspeed; the probe would almost surely incinerate. At very high velocities, radiative flux heating rather than conductive heating dominates. The radiative flux from plasma sheath surrounding the probe would also incinerate the sidewalls. Also, since this is a huge spacecraft, aerobraking is problematic even for regular atmospheric entry, due to surface area to volume constraints. Heatshields are heavy. Aerobraking also requires a spacecraft to be moving at a speed that is still slow enough to allow the planet's gravity to pull it close in an arc, getting more braking effect. Decelerating from even ~0.001c would create something like a Tunguska explosion on the surface of the planet. Even then, a survivable deceleration spike (say a couple of hundred G's if everybody's vitrified) would only be for a few seconds and shave off a few dozen kps, because, travelling in a straight line at even low sublight means you only dip into the atmosphere for a very short distance. Space shuttles and capsules can afford to decelerate slowly because they travel thousands of kilometres because they are travelling slow enough that they are curving around the planet as they do so. At such high speeds, a planet's gravity wouldn't curve the spacecraft's trajectory at all.

So aerobraking is a no-go. Our spacecraft would disappear in a Tunguska like explosion. So what now? We've got all this mass sitting at (basically) zero velocity for us to slow down, but we're going too fast and planet atmospheres are too small. I considered trying to punch out a "corridor" of atmosphere with projectiles, but I think that would dissipate too fast. Plus, it would be rather catastrophic for the planet's environment, doing some rather drastic remodeling. Not what you want if you want to study it, especially if there's the possibility of life.

This is a somewhat insane proposal that involves using your environment and whatever you have lying about to your advantage. The idea is precisely that your spacecraft is a flying bomb; a snotty tissue tossed out the window at a passing planet would obliterate a city when travelling at even these "low" sublight speeds.

Towards the end of main engine deceleration, the spacecraft dispenses a number of small, independent solar sails. This zip on towards the target star while the main craft continues to large behind. Having no payload, they can decelerate to lower velocities by braking at the star. The main craft then arrives in the system, ditches its main engine and deploys its own solar sail. It brakes hard at the target star, bleeding off a large chunk of its remaining velocity and carries on past the star, heading towards the now near-stationary sails. The next phase is tricky and smacks of insanity. As it approaches each sail, a small railgun on board fires a pellet at each mini-sail in front, blowing it up. The explosion is like a small nuclear detonation, and the main craft's sail catches the debris, slowing it down almost like a parachute. Alternatively, the sails decelerate completely (if the closing velocity is slow enough) because they are so light, and then shoot back out towards the main craft at the same speed, getting a huge energy boost from the sun. If depending on thousands of mini-sails is too problematic, then one big sail could be released and then dole out its energy in the form of small pellets of ice, which the main craft could then ram into. The disadvantage is that not all of the available mass would be used up in the ramming. The mini-sails' total mass would be totally used up in the ramming explosions, whereas the mass of a larger sail wouldn't be used at all - it would just carry on out back towards Sol.

A brief understanding of this concept is that the energy for deceleration is only available at the target star for a short time, which gets even shorter the faster the probe is moving and thus requires much bigger sails. Using mini sails captures this energy and the main craft gathers it up in chunks (by basically ramming into them).


*I have no idea who these characters are, I only know Jerry Pournelle mentioned them. i.e. Something from before my time. I assume it's like the Hardy Brothers (which I also never read).

The Science of Mass Effect

What's a computer game and a propellantless drive doing in a blog about realistic interstellar flight with solar sails, fusion drives and the scuzzy facts of cosmic radiation? Well, I was thinking back about having played Mass Effect, and looking back at some discussion with some (smarter than me) folks who discuss and experiment with propellantless propulsion and advanced physics, I thought I'd try and show how some of this is cropping up in mainstream entertainment (what do you mean you never heard of Mass Effect?).



First off, Mass Effect, and its sequels. I'm sure a lot of you will have at least heard of it, and some played it. It's a combination of shooting game and role playing. While it's a fantastically well crafted and seriously fun game with some great acting, it's also really meaty in its science. Unlike Star Trek's "Particle of The Week" technobabble, the creators of Mass Effect carefully pondered what they could do with cutting-edge physics and technology. Ships, for example, have to have radiators or else they'll cook from their own internal heat in the vacuum of space. Weapons are variants on mass drivers and need to either cool down or deposit their waste heat into disposable heat sinks. Soldiers and everyday civilians are enhanced with cybernetic implants and nanotechnology.

Other topics are touched on, such as what happens when AI evolves and computing progress leads to Singularities. This is reminiscent of Sci-Fi authors like Peter F. Hamilton and Charles Stross. In fact, the game's fluff specifically mentions a couple of people I've had lively discussions with - space wargame designer Ken Burnside and the creator of the Atomic Rocket website, Winchell Chung. You can see a pair of recruits with their names being chewed out (with some mild profanity) here.

The most interesting aspect for me is the titular Mass Effect. In the game, the Mass Effect is generated by applying electrical charge to a chunk of Element Zero (just your average plot Unobtanium). There's a bit of waffle about how this Element Zero manipulates dark energy, the odd force causing the universe to expand. This Mass Effect is used to generate gravity, allow ships to accelerate at stupendous velocities, travel faster than light and to create force fields. In this video, everybody's favourite pop scientist, Michio Kaku, discusses the tech of Mass Effect.

What struck me is that the Mass Effect is an inertia-modifying effect. Which is exactly what the Mach Effect is, just without the Element Zero. It can be achieved by common or garden variety capacitors, so the theory goes, or anything else that fluctuates in internal energy quickly enough. I'll spare the long discussion, but it's really a logical outgrowth of Einstein's General Relativity to explain inertia - basically it's caused by the gravity of the rest of the universe pulling on an on object. Wiggle the object in the right way, and you can get those rubbery strings of gravity to work for you (note, nothing to do with String Theory). A bit like how a vibrating table can make an otherwise heavy object easy to push. Basically, the object's inertia is being lowered for a microsecond, and if you time the shove right, you can push it with less force. Interestingly, it appears that it would great gravity fields around it (because of all those stretched or relaxed gravity "strings"). Those gravity fields could give us artificial gravity generators, force fields, tractor beams and maybe even faster than light travel. Just like the Mass Effect universe.

The scientists (notably Dr. Woodward), engineers and Joe Averages (i.e. yours truly) who discuss the Mach Effect were talking about how to raise awareness of it, so instead of making a Doritos-and-Mountain Dew-fuelled looong email trying to explain all of this, I thought I'd put it here in the public eye, so to speak. A lot of promising physics concepts are familiar in Sci-Fi, or are otherwise making their way into the public consciousness thanks to the general curiosity of people surfing the net. Of course, Joe Average might look at you and go what? But those of us who watch Big Bang and have a vague clue about what Sheldon spouts may know. And maybe all that's needed to get promising revolutionary technologies off the experimental bench and into spacecraft...


When playing Mass Effect 2, I took a trip down to the engine room and saw the Mass Effect core vibrating... much like the way the Mach Effect devices would work (although you wouldn't necessarily *see* the vibrations, which would be in the mega to gigahertz range...). 3D artists, game programmers and designers are a smart bunch, and you often see unexpected references to some really intriguing ideas wrapped in a game or movie. I wonder if the inspiration for the Mass Effect was indeed the Mach Effect?

Interstellar Flight: The Generation Ship

I talked about hibernation previously - now let's talk about simply slogging it to the stars the hard way, no sleeping on the job. Many SF writers talk about generation ships as a way to get human colonists to a distant star. But there's a couple of problems with generation ships which don't make them so attractive.

First off is the sheer size of these things. In order to get a minimum viable population without the hazards of inbreeding, you'll need something like 180 individuals. Small groups have made it to colonise islands, perhaps 50 or less breeding members. A single female even could simply impregnate with frozen herself and raise her daughter to fly the ship (not that her daughter would likely be grateful for the thankless life she's born into).

The trouble is, we need a lot of people to cover all the scientific and engineering disciplines. Small armies of technicians, engineers and scientists service the space shuttle, for example. The 7 or so people on board really just push the buttons, as competent as they are. Fixing complex problems like those that would arise on a spacecraft require lots of smart brains. And there's no guarantee that subsequent generations would be as smart or motivated as their astronaut parents. The automated systems on the ship had better be pretty reliable or else extremely easy to fix. Possibly even to the point of having an AI or expert system and be almost self-repairing (can you say HAL 9000?) One might envision a generation ship with primitive humans, having lost the skills of their ancestors, worshipping the benevolent computer-deity literally controlling their world. This, plus the possibility of disease or accidents wiping out large chunks of crew, points to the need to have as large a crew as possible. And more humans equals more mass.

Mass requirements for keeping a human alive in space, and fed with soyburger and supplied with toilet paper, range from 100 tonnes to 1000s of tonnes. Biotechnology can really help here; improving crop yields and increasing efficiency of recycling systems. Certain tools, chemicals and medicines could also be grown in bioreactors. The entire ship (or at least the habitat) could be constructed of organic materials. This pretty good from a radiation shielding standpoint, the abundant hydrogen atoms in organics and plastics are great for stopping cosmic radiation and preventing the lethal backscatter of secondary radiation that occurs when a speeding iron ion smashes into structural aluminium.

In addition to the mass requirement, there's also volume. NASA studies estimate that 100 cubic metres is enough for a single human for an indefinite period. I rather think the ship's crew may grow up a bit nutty... anyway, with the life support requirements, rather more volume than that is likely. Inflatables seem the current best technology, but just how safe will they be after decades of hard radiation exposure? Perhaps they will need some extra reinforcement for a more permanent solution, but at the moment they look like a good bet.

SF is replete with stories of hollowed-out asteroids as generation ships, but the truth is that they're terrible spaceship hulls. For a start, they're weak - asteroids being composed of rubble, and would need the rock to be fused. The rock would still be fundamentally very weak for its mass. And that mass would weigh in the millions of tonnes for a generation ship a kilometre or so across. Furthermore, the rock is not such a good radiation shield. Plain old plastic, water or wood is better for cosmic radiation. And radiation shielding wants minimum volume to be used most effectively - generation ships are anything but minimum volume.

Hollowed-out comets or ice asteroids seem to be a somewhat better option - although frigging cold, they would provide water, oxygen and reaction mass. A layer of insulation could allow for an inner shell lined with water, and aquaculture. Everybody living in boats and stilt houses - how very appropriate for the island-in-space theme! Of course, the best option is still to purpose-build an actual hull for the job. And that's going to be heavy.

Assuming the minimum case of 180 people (at any one time), the ship needs to weigh at least 18 000 tonnes if 100 tonnes of life support infrastructure are necessary to keep things going - that's a WWII battlecruiser. Being more conservative, that could mean 500 000 tonnes of ship for 500 people and 1000 tonnes of life support. To match the current mass of the ISS at 400 tonnes, we would need something like 8 tonnes of life support for 50 people (and no idea of how they're going to go down to the surface). That's a pretty miserable cramped existence, eating algae glop for hundreds of years, living naked and escaping to VR all the time. But it might be possible.

The point is, at what point is it simpler to put more fuel up to go faster? With a solar sail, you've no choice - you're limited to 0.001c with a scorching approach. But fusion-powered craft might just prefer to burn more fuel and get there faster rather than build a big expensive habitat and risk the crew dancing around fires playing bongo drums when they should be getting in their landers to go the surface. Reducing the mass from 18 000 tonnes to just 180 yields a 100x jump in mass ratio, which can be cashed in for a 2.4x increase in speed over just having a 10x mass ratio (rough approximation). Instead of getting to Alpha Centauri in 200 years, a trip of 83 years becomes possible. The crew might be old duffers by the time they get there, but they can limit their numbers because they don't have to keep a society going on board. Or they could prolong their lives with life extension drugs.

Life extension also poses its own problems. While it's useful for long voyages where you want to actually live to see the target star and use your expertise instead of teaching it to your children and hoping they teach it to the next generation, it's a problem on a generation ship. Even with people sticking to two children per couple (or one child to one parent in polyamorous Heinlein-esque communal love-fests), room's going to run out real fast. Great-great-grandpa and grandma may have to be euthanised.... or their children only allowed to breed when their parents die. Which is a problem is females can only safely breed up to about 40. There better be some serious mojo in those pills if that's going to be the case.

Speaking of kids... can you imagine what a 2 year old would do in a delicate, tightly enclosed environment? Or a sulky teenager? Best just feed the crew contraceptives and boosterspice* til they get to their new Eden...



*anti-aging drug in Larry Niven's known space novels

Interstellar Exploration - Two Motivations

Chinese junks, similar to what Zheng He's fleet may have looked like. Credit: Wikimedia

Exploration is rarely about the pure, unfettered pursuit of knowledge. Scientists want to prove a theory. Politicians want to see high-tech industry stimulated and their national prestige elevated. Private citizens want to go "because it's there." History has shown that there are two broad kinds of exploration - with their own separate outcomes. The first is the show of force. The second is sustained interest.

1405 saw the launch of Zheng He's expedition of exploration from China. A massive flotilla of 317 ships and 27 000 crewmembers, with 44 huge treasure ships measuring 120-150 metres in length. Or, accounting for typical historical exaggeration, probably half that as 100 metres is the limits of what is possible with wooden ships (note that no remains of these craft have ever been found).

Christopher Columbus had 3 ships, 23 metres long, and 270 men (before the usual diseases started wiping them out). The only thing he had over Zhang He was the fact that he didn't sing soprano (Zhang He was a eunuch).

The Chinese weren't interested in colonisation or trade. They wanted to show off, impress people with their bling (hence the treasure ships) or else kick them around a bit. They wanted to remind people that China, just like every other Empire in the world, was the centre of the world. It was the Apollo project of the era, in more ways than one. Because after a flurry of these expeditions, the whole thing was called off due to escalating military conflicts and finally a big damn wall to keep the Mongols out.

Christopher Columbus of course, had different motivations. He wanted money by opening up an alternate trade route to Asia. And he didn't make such a big investment - three ships, perhaps a big thing in Medieval Europe. Although everybody thinks of him as a big success because he found this place called America, he failed to find the trade route. He wasn't even out to prove the world was round, everybody knew that.

Now, how did the New World wind up colonised? Why didn't the Chinese do it? Europe was a dirty little backwater, although the first glimmerings of scientific enquiry were beginning. Simple: China wasn't interested. China could have easily afforded to colonise and conquer these new lands. The Europeans were broke, and constantly fighting each other. But there was land to exploit, and savages to convert! There was also somewhere to run away to if you didn't fancy being oppressed!

So, we have two scenarios with which to place interstellar colonisation in. The first is the massive show of force, done mainly to impress and sustained by government interest. The second is a trickle which eventually becomes a flood, sustained by continued private interest (Hey Sven! Could I interest you in a place called Greenland!). This can tell us something about what kind of colonies we can expect, and who's going to found them.

Interstellar Flight: Hibernation

So let's say we've discovered a nice habitable planet around those nice, nearby sun-like and well-behaved stars, Alpha Centauri A or B. How are we going to get there? As it stands, there are three possible ways of getting there. Obviously it's going to take a long time.


The simplest in engineering terms is hibernation (aka "suspended animation"). This has the very important engineering advantage of not needing all the mass of a big habitat or a closed ecology. The ship can go faster or be smaller (and cheaper) as a result. Throw the humans in the hibernation chambers and let them sleep the journey away. Plus they won't go stir crazy or forget how to do their jobs. However, there are significant challenges: we don't really have the first clue as how to induce hibernation in humans.

Short-term hibernation is possible through induced (or accidental) hypothermia. Unfortunately it's dangerous - Swedish radiologist Anna Bågenholm was dunked under ice with a core body temp of 13.7°C. A drop below 28°C is often fatal. However - this hypothermia-induced hibernation can be extended for long periods. Mitsutaka Uchikoshi, a Japanese skiier, went missing and was recovered 24 days later with his organs shut down and his body temperature at 22°C. What was particularly amazing was how he survived for so long without any fluid intake.


Hypothermia can also be induced for medical reasons (with much better prospects of surival as it's controlled): English and Japanese doctors pioneered the technique of using deliberate hypothermia for heart surgery with packs of ice, and the Russians perfected the technique (the normal solution these days is to use a heart-lung machine). Basically the brain chilled to about 16°C (60°F) and the body to 24°C (75°F). This gives the doctors a 30 to 60 minute window where the heart stops and they can tinker with it, then whip the body back up to full temperature with no lasting brain damage. A recent discovery with mice showed that hydrogen sulfide combined with hypothermia could be used to induce hibernation in mice - but only mice. It probably won't be as simple with humans, but the US military is certainly interested in it for trauma applications. Extreme hypoxia might also be a trigger.

Unfortunately, humans just aren't built for hibernation; this technique must be done correctly otherwise there is severe risk of cardiac arrest. More so is the problem of simply lying inert for years on end. Wouldn't the body degenerate? What crucial processes might be impaired? It might be necessary to wake the sleepers up every year or so to let them recover, before climbing back in the freezer again - with significant impacts on life support requirements.

For really long term flights, it might be necessary to go all the way and use cryogenic preservation - ie freezing the astronaut solid and then thawing him/her out. Unfortunately, this is kind of problematic - small embryos and organs can be successfully quick-frozen, but damage from expanding ice crystals (especially during re-warming) causes all kinds of havoc. A way around this is vitrification - basically pumping the body with antifreeze, turning the body into a block of glass at -135°C. This unfortunately creates further problems in the form of the toxic antifreeze now flushing the body.

A vitrified rabbit kidney being thawed - notice the lack of frost and ice.

Although reviving the patients is tricky for both cryogenic preservation and hibernation, perhaps AI will have advanced enough for it to handle the process without having to have someone on hand. Continual waking and thawing may not be a good idea - so perhaps some brave souls will volunteer to spend a significant chunk of their lives in extreme boredom watching over their crewmates. It may also be possible to grow whole new bodies around the vitrified brains, with the ship becoming more like the mass (and cost) of an unmanned probe. Once it arrives in the system, it mines sufficient material to build a habitat and create nutrients to grow the crews' bodies. However, this is getting to the real bleeding edge of the possible - FTL may be possible before then.

The most likely scenario - hibernation through some combination of induced hypothermia, hypoxia and drugs, seems feasible enough that we could conceive of an interstellar mission if the propulsion technology is likewise developed (which seems to be the far more difficult problem). Since hibernation has many more applications than just long-duration spaceflight, it'll probably be developed sooner rather than later.

The hibernation "pods" would probably be some small climate-controlled chambers with a comfy bed, IV drip and some sensors, hardly weighing anything at all. More like the cheap-looking hibernation pods in the original Planet of the Apes than big bulky cryo-coffins. They'd be located right in the heart of the ship, surrounded by as much radiation shielding as possible, and with a bit of rotation to keep fluids behaving properly. The quarters for the awake crew would probably be right next to it, to take advantage of the proximity to the sleep chambers' radiation shielding. Upon arrival, the sleep chambers could be used as bunks for the crew.

Reliability of the hibernation mechanisms would be a big issue - springs, one of the most reliable components, rarely last beyond 60 years. New, highly fatigue tolerant materials need to be designed, along with fault tolerant systems - more so than with a ship where everybody's awake and able to fix things. This in itself is a significant challenge, but the aerospace industry tries to reduce maintenance and extend aircraft life to lower operating costs, so this is another area where industry may make the development anyway.

Appreciating What We Have Already

Sometimes we forget we're living in really amazing times. I saw this over on Wayne Hales' blog and shamelessly stole it. It looks like the cover of an SF book or a still from an SF movie, but it's not. It's astronaut Tracy Caldwell-Dyson looking out of the ISS cupola. Here's the full-sized original from wikimedia.

Wayne Hale talked about how images like this would inspire the public. I was personally wondering how she managed to look like she was lying down when she was in zero g, and whether her elbow was going to smudge the optically perfect glass.

For me, the constant SF eye candy can never dull the awesome reality of knowing that someone up there is seeing something like this right now. We have a 300 tonne space station, 3 different unmanned cargo vehicles, a space plane headed for retirement and the prospect of 3 or 4 new craft to replace it. Add to that the prospect of a private space station before the end of the decade, the Russians and Chinese possibly building their own and it gives you hope for the future. Yeah, space isn't happening as quickly as we would like but boy, what we have already boggles the mind.

If you've got the time (and bandwidth) why not watch a tour of the ISS from Youtube and get a taste of what it's like out there.