Why Interstellar Spaceflight is Hard

This was initially published in audio form as Burst 7 of the Wow! Signal. As part of the ongoing effort to tie this blog into that podcast and vice versa, we here present a slightly edited text version of the same.

This is a tutorial post, and is largely meant to bring some of our readers up to speed, since we are going to be talking about interstellar space flight more over the next months. If you have ever wondered by we can’t just get on a big rocket and fly to the stars, this is for you. If you already know why we can’t do that, then I think you might want to skip this one. 

OK, they’re gone, those nattering nabobs of negativity. Now let’s all just get on that giant rocket and go explore strange new worlds.

Although, maybe we should think it through just a bit before we finish packing our bags. It would be a shame if we left something off of our list, like say, a quadrillion solar masses of fuel. Oops.

Of course, we all want to go and colonize the galaxy (if you have to ask why that is, you may want to skip this as well) , and it seems like it ought not to be more than an engineering problem. OK, let’s assume that you and I are hardy pioneers up for anything (which of course, we are), and look at just the engineering problems.

And, if we’re going to do that, we have to use the laws of physics as they’re presently best understood. Engineers need laws of nature they can rely on, and we let the theoretical physicists worry about what might lie beyond that. The good news is, the physical laws that govern the kind of stuff we know how to build are very well understood.

So, let’s start with what the astronomers know. For well over a hundred years, the astronomers have been carefully surveying the stars around us in the galaxy, and even outside the galaxy. In the 1920s, Edwin Hubble, with lots of help from Milton Humason and building on the work of Henrietta Leavit, figured out about how far away the nearest galaxies were. The results stunned even professional astronomers.  The distances were enormous, and the other galaxies, Andromeda in particular, were huge. Even fifty years later, Douglas Adams was raving about it. “Space,” he said, “is big. Really big. You just won't believe how vastly, hugely, mind-bogglingly big it is.” He was right. You won’t believe it, and Hubble got a space telescope named after him for figuring it out.

Just four months ago, the New Horizons spacecraft flew past Pluto. With a gravity assist from the

New Horizons flew by Jupiter and picked up a gravity assist

giant planet Jupiter, it took New Horizons over 9 years to reach Pluto, and it zipped by it in just a few hours, not bothering to slow down. In rough numbers, Pluto is about 30 astronomical units from Earth, or equivalently,  about 30 times further from the Sun than the Earth. That is less than half of a thousandth of a light year. The nearest star to the Earth is a bit more than 4 light years away, so that the distance to Pluto is about one ten thousandth the distance to the nearest star, and it took us over 9 years with a powerful rocket, a small spacecraft and a gravity assist, to get there, and there was nowhere near enough fuel on board to slow down and orbit Pluto. In the time it takes New Horizons to cover an interstellar distance, it will be a dead, cold space artifact.

Daniel Cartin, whom we’ve had on the Wow! Signal before, did an analysis of the range a starship would need to really get colonization moving, and it’s around 10 light years for the local solar neighborhood. 10 light years is about one ten thousandth the size of our galaxy, so we are talking about a cosmically short haul. This number might be a bit less for a civilization closer to the center of our galaxy, where the stars are closer together. However, let’s assume you had an amazing rocket that could propel a starship on its journey at 1% of the speed of light. Now remember our ground rules - no unobtainiumm powered warp drives.  1% of the speed of light is very good, actually, as we’ll see.

Now, the best rockets on the drawing board right now might be able to propel a small payload to about 30 kilometers a second, give or take. One percent of the speed of light is 3,000 kilometers per second, and lets us get to the nearest star in a few hundred years. We would need 100 times as much velocity boost to get to that speed as our best current rockets. So, just build bigger rockets, then? The problem with this is something called the tyranny of the rocket equation.

The famous rocket equation, which can be derived from Newton’s laws of motion using high school math, says the amount of fuel we need goes up exponentially with the velocity boost we require. This is because a rocket gets heavier as it carries more fuel, and it has to use more fuel to boost that extra fuel, and then more fuel to boost that extra fuel...and the numbers just get completely out of control. A simple calculation shows that we need more than ten to the forty-third power as much fuel as we used to get to 30 kilometers per second. That’s just not going to happen. Sure, we could try to get clever about staging, or ion engines, reducing the mass of our payload, or what have you, but there’s only so much you can do, and we haven’t even talked about the propulsion we need to slow down as we arrive. Old school rockets as we know them just aren’t going to get us there.

We haven’t even talked about energy. As you probably remember from high school, the energy in a moving object is the total work required to get it moving that fast, and goes up as the square of the speed, until you get close to the speed of light, where it goes up faster than that. Now, the New Horizons spacecraft, which is small as spacecraft go, has a mass just under 500 kilograms. That same mass, at 1% of the speed of light, has an energy of more than 2000 Terajoules, and it will still take more than 1000 years (remember, it has to slow down at the end), to cover the required 10 light years. 2000 Terajoules is roughly the electrical generating capacity of a major developed nation for an hour, and New Horizons, as we have pointed out, would be a very tiny starship that wouldn’t survive the journey.  Although 1000 years is a cosmic twinkling of an eye, on a human scale, it seems too long. In reality, we are going to need hundreds of thousands or even millions of times more energy than that, and we have to generate it in space. The biggest power system now operating in space is the solar array on the International Space Station, generating up to 90 kiloWatts in orbit around the Earth. We are many orders of magnitude away from producing enough energy for a practical starship.

So, we need to either be willing to take a very, very long time to travel between stars, or come up with something else besides the technology we have now.

What that something else is will be the subject of future posts. We haven’t even gone into the problems of very long duration spaceflight, or how to slow down once we arrive, or the hazards of traveling through interstellar space at a significant fraction of the speed of light, how we’re going to phone home when we arrive, or even if there will be a home to phone to after all that time - but we should. We’re going to need engineering we don’t know how to do yet. As it is, we struggle with how we’re going to get humans to our near neighbor Mars - in such a way that they have a fighting chance to survive.