Interstellar, Starshot The Challenges of Interstellar Travel , the Starshot Project
The interstellar travel episode was first released on October 29, 2018.
Hello, and welcome to Physical Attraction, the show that explains concepts in physics – one chat-up line at a time. This episode, we’re going romantic: “I know it’s a long shot, but together, we could reach the stars.” The subject is going to be the possibilities, the frustrations, and the general… wildness… of interstellar travel.
Just occasionally, you hear an idea for a scientific project so wild, so wonderful in ambition and exciting in scope, it fills you with a child-like sense of joy and wonder. Very often, these astounding ideas – antimatter drives, ‘wormholes’, ionic propulsion – come up when we try to figure out how we can move between the stars. Some of them are ridiculous and a little bit unworkable. For example, the idea of a traversable wormhole comes up a lot, but there’s always been… something of a concern about it.
So let’s talk about wormholes for a minute. The theory of general relativity is our best theory at present to explain the properties of gravity, and it does so incredibly well with a set of equations called the Einstein Field equations. One of the simplest solutions you can come up with for the Einstein Field equations is in fact what we know as a black hole. The guy who came up with this, Schwarzschild, did so only a few month after Einstein first published his theory – and he did so in 1916, while he was still at the Russian Front in the First World War, spending his day job calculating trajectories for shells. He would die shortly afterwards from a disease contracted at the front, but his famous black hole solution lives on, and the proper name for the event horizon of a black hole is the Schwarzschild radius.
So black holes for a long time were just solutions to this equation, and for a while people thought that they were really just a mathematical curiosity. There was a big debate amongst theorists about whether they could exist, and how they could form – all of this for our black holes episodes – but, in the 1970s, we started to observe them indirectly. Obviously the problem with trying to observe a black hole is that… since not even light can escape from it… you can’t “see” it in the traditional sense. So you have to infer that it’s there through its gravitational effects or other things. Yet eventually such a mass of evidence accrued that it was accepted that black holes were real.
The reason I spent all this time pointing out the origin story of black holes is that wormholes also exist, right now, as just a mathematical solution to Einstein’s Field equations. That doesn’t necessarily mean they exist, and none have been observed yet. So we don’t know if they are a real thing that actually exists in our universe, or just something that the laws of physics permit, but that can’t exist. Since a wormhole theoretically connects two different points in space and time, there’s been a lot of speculation that perhaps you could travel through one – and then instantaneously break the light speed barrier, travelling between two points in space and time. This is what you’d call a traversable wormhole.
There are reasons to believe that, bizarrely, any wormhole that did form would be so unstable that it would collapse almost instantaneously – and, if it connected two points in the same universe, there wouldn’t even be enough time for light (or anything else) to traverse the wormhole and reach the other point in the Universe. “Keeping the wormhole open” for long enough would require potentially infinite amounts of negative energy. In the case of black holes, scientists knew that there was a physical process – the collapse of stars – that could lead to the density required to make one. But we don’t know of any process that could produce a wormhole, and in all likelihood, it would collapse before we even realized that it was there. If no known natural process can make a wormhole, and humans would require some mythical infinitely energetic antigravitating substance to build one and keep its walls open for long enough to go through, it doesn’t seem likely that I can use one to cut down my commute any time soon.
We all know that being sucked into a black hole involves a rather unpleasant process called spaghettification, which effectively involves you being stretched out into a string of atoms. You die, obviously. So if you ever want to travel through interstellar distances, please, please, please – don’t jump into a black hole. It will not go well for you.
So much for science fiction: what about science fact? Interstellar travel is very difficult. Escaping the pull of Earth’s gravity is a difficult enough challenge, requiring huge rockets and tonnes of liquid fuel. You can calculate pretty easily the rough amount of energy you need to escape Earth’s gravitational field using Newton’s law of gravitation: it’s -GMm/R, where R is the radius of the Earth, M is the mass of the Earth, m is your mass, and G is Newton’s Universal constant. The earth’s radius is 6400km, the gravitational constant is 6.67 * 10 ^ -11 , the earth’s mass is around 6 x 10^23kg, so when I put in my mass, that means I’d need around 375 megajoules. (This is technically the energy to go to infinitely far away from the Earth. But since once you get a few thousand km away you’ve already done 99.999% of that work, and since I neglected air resistance and the energy you need to go at a certain speed in outer space, this is a big underestimate anyway.) So I’d need something like 10kg of petrol, perfectly converted into gravitational energy, to lift me into space. Then you need to take into account the weight of the fuel. This quickly becomes a problem. The actual maths requires you to solve a differential equation – maybe some other time – but the basic point here is that sending stuff into space is expensive and difficult, and you don’t want to waste any of that on heavy fuel. In fact, beyond a certain point, you won’t be able to launch, depending on the fuel you use. So the energy density of the fuel – how much kick you can get for a certain amount of mass – becomes crucially important for the launch.
But this is just the beginning of the challenge: the distances involved are simply stupendous; the nearest star to our solar system is around 4.3 light years from Earth. The fastest spaceship ever launched by humans was the New Horizons probe, headed to Pluto; it was launched with an escape velocity of 58,000 km/h. But this is tiny compared to the speed of light; light travels nearly six times as far in a second than New Horizons did in an hour. At the rate of our fastest-ever probe, it would take 80,000 years just to reach the nearest star. That’s just not going to cut it.
We clearly need to go at a considerable fraction of the speed of light for several reasons. If we want our probe to reach the nearest star within a human lifetime, so that investors don’t die before they see the fruits of their labours, then we need to travel at around 10% the speed of light or better. There is also a philosophical argument that sets a lower limit on the speeds we can use for interstellar travel; it’s called the Wait Calculation. The idea here is that, if you assume technology continues to grow back on Earth, it’s possible that your probe will simply be overtaken by a faster one that gets launched later on, with more developed technology. It will eventually become sensible to launch earlier, rather than wait for technology to develop and lose travel time, but this will likely only happen when we can travel at decent fractions of the speed of light. Obviously right now, we’re basically in the time when the wait calculation means it’s almost pointless trying to send something to the nearest star – at least, with technology that’s been used before. If we were going to take 80,000 years to get there with our fastest probe, we might as well wait 100 years for Ray Kurzweil’s singularity or the end of the world or better rocket-ships.
Once you leave the Earth’s atmosphere, continuing to accelerate gets even trickier. The more fuel you need to carry on-board your probe, the more expensive and difficult it becomes to escape Earth’s atmosphere in the first place. Worse, the faster you get, the more difficult it is to accelerate, thanks to special relativity. To accelerate 1kg from 0 to 10% the speed of light requires 125.8GWh of energy. That’s already the energy output from a big power plant for more than a day, for just a 1kg probe.1 To get that same 1kg from to 50% light speed takes 31 times more energy. This process eventually saturates, so that accelerating a mass to the speed of light is impossible, with the last fraction of a percentage requiring infinite energy to achieve. So I guess you’d probably need to make a calculation – is it worth supplying, say, ten times as much energy for the difference between 95% the speed of light and 99% (say?) probably not, because it’s a small difference in speeds, really. The problem that you can see here is that for interstellar travel, we’re not running up against ordinary stuff like air resistance – we’re running up against fundamental laws of physics, and they really don’t like to budge. That’s why you need so much energy – a day’s worth of power plant output, just to get 1kg to 10% the speed of light.
Worse, you can’t provide this energy from a power plant out in space. Liquid fuels like the kind that power rockets launching on Earth are completely out of the question: the best chemical reaction fuel can barely provide 40kWh/kg2 – it’s simply not worth taking it into deep space, without even considering the fact that you would struggle to burn it. Rockets powered by these liquid fuels just accelerate in Earth’s atmosphere, where they have something to push off, and then essentially coast through outer space. There’s no air resistance, so they go at more or less the same speed for a very long time, impeded only by space dust and debris, and occasionally swung around by gravitational fields. Some of the faster speeds that we have obtained with conventional rockets
Nuclear fuels can do better, with the nuclear decay of plutonium potentially providing 16,000x more energy3. Nuclear fusion is even more efficient than fission, but given that our best idea on how to do it will weigh thousands of tonnes and span many kilometers, it’s likely that the apparatus to harness energy from fusion will always be far heavier than the fuel, so the fact that the fuel has a greater energy density isn’t really relevant. You need the whole kit and caboodle to confine the nuclei close together, prevent them from escaping, produce the temperatures needed to ignite a sustaining fusion reaction… it would be very difficult.
The reason that fusion is more efficient than fission is because it converts a greater fraction of the mass into energy – around 0.002% for fission of plutonium, but 0.4% for fusion. Matter-antimatter annihilation releases 100% of its mass as energy – in terms of fuel, you can’t get more efficient, as long as the energy can be harnessed. Annihilation will still occur in outer space, releasing photons that could provide momentum to a spacecraft. But antimatter has its own problems for use as fuel. It’s highly unstable and difficult to contain; if the containment fails and the antimatter comes into contact with its matter counterpart, catastrophic destruction will follow. Then there’s the issue of directing this tremendous energy into useful thrust to drive the spaceship, rather than just irradiating the spacecraft with gamma rays. Also, although we can produce it in particle accelerators and even make some antimatter atoms for our experiments, it’s not exactly a cheap process. Current costs are often quoted as trillions of dollars a gram and making the antimatter requires astonishing amounts of energy. It’s also made in tiny amounts – basically as a byproduct of particle accelerator experiments, where a little bit of it gets captured and studied from time to time. I’m guessing that if you started actually trying to mass-produce the stuff, you could find better ways of making it, and you could make it cheaper than we do right now – but it would still probably cost billions of dollars. Since it instantly annihilates everything it touches, it’s kinda difficult to imagine that we’ll suddenly find a vast reserve of it that we can mine somewhere. To get our 1kg probe up to 10% light speed, we’d need around half a gram of antimatter, but only a few nanograms has been produced in the world so far. Worse, it’s difficult to imagine how you can contain the thing, practically. You have a substance that explodes whenever it touches matter. You CAN confine it – with a carefully shaped magnetic field – and as we discussed in our episode on the subject, the record for doing that is around 15 minutes. If some method is developed that can confine it indefinitely, you STILL need to solve the problem of how you don’t completely destroy your container, or even the whole ship, when you explode a little bit of the antimatter to give you some thrust. So there are lots of engineering challenges here, to say the least.
It’s for this reason that people are looking at more and more exotic forms of transport to truly cover these interstellar distances. The Breakthrough Starshot project, for example, envisions a fleet of tiny spacecraft that will be powered by a gigantic array of lasers. It was this mad project that made me want to write an episode about interstellar travel in the first place, because people are actually kinda working on it – it’s been endorsed by the usual cast of sci-fi dreamers that you find endorsing this kind of thing: Elon Musk, Stephen Hawking, that lot. So how does shooting a whole bunch of lasers at a fleet of tiny lightweight spacecraft work? The photons will transfer momentum – light can give a ‘push’ to lightweight wings, perhaps made of graphene or a similarly exotic material – and the spaceships could be accelerated to significant fractions of light-speed. This gets around the problem of having to have fuel on board, because what you actually do is just have a huge wingspan for the lasers to push on. And you can theoretically accelerate these lightweight probes to incredibly quick velocities, if you have the appropriate set of lasers. It tells you something about how ambitious your project is when you need a vast laser fleet to accelerate craft to tens of thousands of kilometres a second in just two minutes. I think theoretically they can get up to around a quarter of the speed of light – maybe more. Then you start beaming back data at the speed of light from the nearest star – so our probes could get information from the nearest star in, say, twenty years. Which means if this project is actually completed within the next few decades, there’s a chance that we might see the first data from a probe that visited a nearby star or planetary system within our lifetimes. Of course, our lifetimes might be extended in the future. Or the Starshot project may also fall victim to this wait calculation – while we’re developing it, maybe we figure out some way to get up to 99% the speed of light, and then we’d only have to weight eight years for the data. The Starshot is probably the best-thought out and most backed current project for interstellar travel. But it’s so much more difficult than going to Mars or other planets, which, while an incredible technical challenge, is something of an extension to what’s already been done. Many technological developments need to take place before the design can be feasible. You need to be able to keep the weight of the whole probe, including the wings, incredibly low – comparable to the weight of a smartphone nowadays. That requires huge scalings down of modern technology, although I guess not unfeasible given what we’ve seen with Moore’s Law and things like this. The fleet of lasers that could accelerate the craft would actually need to be built. You have to find a way to reliably beam all of that laser light onto a tiny sail which will be accelerating away from you quickly – and you have to keep it focused, presumably, on the dead centre of the probe. This is an incredible optical challenge. It’s even been mentioned that some unscrupulous person might launch a giant mirror into space to try and reflect the beams back down – and when your science project could be turned into a giant death ray, that’s when you know you’re really dreaming.
And once you get into space, you might even have to contend with space debris. If you’re travelling at 10% the speed of light, from your perspective, space debris is coming at you at 10% the speed of light and that’s enough to ruin anyone’s day (and potentially smash up your probe if you get unlucky.) That’s why most plans involve sending a huge fleet – you can afford to lose most of them, as long as a few get lucky and survive.
Yet the dream, and the lure of interstellar travel, remains strong – not least because we are discovering more and more exoplanets around nearby stars. The discovery of Proxima B, an earth-sized planet in orbit around our nearest star, led to considerable media hype. Scientists disagree, however, about whether it could be habitable or whether such a planet would lose its atmosphere too quickly for life to develop. Breakthrough Starshot is an exciting project to follow, especially because many of the technologies that it requires are likely being developed already for different purposes (miniaturisation of computers, better lasers); the same cannot be said for many of the more exotic fuels. Even so, it seems a very long time before we will be able to truly reach for the stars. We shall have to make do with looking at them for now.
Thanks for listening to this episode of physical attraction…
By the way, Centauri Dreams is a great website that I used a lot for this episode – they detail all of the different ways people have considered trying to get interstellar travel to work, and they have all KINDS of information about technical aspects of the starshot project, news, developments, and so on. Check it out.
1 Assumed 4GW is a big power plant – e.g. the UK’s largest https://www.theguardian.com/business/2010/aug/03/drax-coal-burning-carbon-emissions-biomass
2 https://www.engineeringtoolbox.com/energy-density-d_1362.html - compressed hydrogen converted into these units.
3 Plutonium-238, figures from here: https://en.wikipedia.org/wiki/Energy_density