Interstellar Space travel is really difficult!
Substantial corrections were made to this text in June 2016: The fuel calculations were incorrect in the earlier version of the space travel calculator, and my discussion of acceleration at 1g was confused and wrong. I've also added the point about collisions at high speeds.
Dozens of interplanetary spacecraft have been launched. All of them are unmanned. The furthest travelled is Voyager 1. Although Voyager is now considered to be nearing the edge of our solar system, it is still less than 17 light hours from earth. That is less than one two thousandth of the way to our nearest stellar neighbour, Proxima Centauri. It has taken Voyager almost 35 years to get where it is. At that stately pace, it will need about 70,000 years to travel the equivalent distance to Proxima Centauri.
Interplanetary space travel is hard. Manned interplanetary space is very hard. Practical unmanned interstellar space travel is really, really difficult. And manned interstellar space travel is so difficult that it might never be achieved.
Here are some of the massive technological problems that have to be overcome if we are to send a manned spacecraft to another star, say Proxima Centauri which is a mere 4.2 light years from earth.
Doing a one-way trip in a lifetime
To make this as easy as possible, let us say at some point in the distant future, we will fly two young intrepid adventurers on a one-way trip to Proxima Centauri. They're prepared to spend a very long time doing it and they accept they will never return to earth.
To get there in a reasonable time, they are going to have to accelerate constantly for much, most or all of the first half of the journey. Then to ensure they don't overshoot Proxima Centauri when they get there, they have to decelerate during the second half of their journey, essentially mirroring the first half.
Ideally they would like to accelerate at 9.8m/s2, or 1g for the first of the journey. This exerts the same force on the astronauts that they experience from gravity on earth. Then they will decelerate at the same rate for the second half. The advantage of accelerating at exactly 1g is that if the spacecraft is designed so that our astronauts, when standing, are parallel to the direction the spacecraft is moving, they will experience earth-like gravity.
To see how this works out, open the space travel calculator. In the Distance field, enter "pr" to bring up Proxima Centauri and select it. The number 39734219300000000 is filled in for you in the distance. You will probably find the number displayed more intuitive if you change the units to light-years in the adjacent select box. Leave acceleration as is. Click Calculate. By selecting the appropriate unit measurement, you can now see that the maximum velocity, which our astronauts will achieve half way through their journey, is approximately 95% the speed of light. From an observer on earth's timeframe it takes our astronauts 5.8 years to reach Proxima Centauri. But Einstein's theory of relativity tells us that it will only be 3.5 years from the astronauts perspective, because the faster you go, the more time slows down for you. Run the animation to get a very simplistic view of what happens.
That all seems very promising. But sadly there's the problem of fuel. A great deal of it is needed to get to Proxima Centauri at a constant acceleration of 1g. To get the 22,000 kilogram payload of the space shuttle Endeavour to Proxima Centauri using hydrogen to helium nuclear fusion would require more than 100 million kilograms of fuel. By comparison, the weight of the space shuttle at take-off, with fuel, is about 2 million kilograms.
And nuclear fusion is orders of magnitude more efficient than any other rocket fuel in existence. Although the technology to make nuclear fusion bombs is available, controlled nuclear fusion for the purposes of powering a spacecraft is still science fiction.
It it possible to imagine a more efficient fuel than nuclear fusion. The E=mc2 equation tells us the maximum amount of energy you can get from a given mass. For nuclear fusion you get about 0.008 x mc2 joules, so there is significant room for improvement.
Antimatter rockets could in principle convert virtually all of their mass into energy. But these are not even on the technological horizon. Also, in practice 100% efficiency would not be achieved, as some of the thrust would be lost to heat.
Some means of space travel that don't use onboard fuel have been proposed, such as interstellar ramjets or beamed propulsion. The Wikipedia article on interstellar travel explains these in more detail. These proposed technologies are highly speculative.
One possibility is to reduce the spacecraft's acceleration, use much less energy, much less fuel and take a lot longer. At the stately acceleration of 0.5m/s2, they would eventually reach over 40% of the speed of light relative to earth, and take about 18 years for the trip.
There are further challenges:
Our travellers will have to spend their entire journey floating about in the weightlessness of space. This likely has adverse health effects over time. By accelerating the spacecraft at 1g, our travellers will feel the same weight they do on earth, but it's unlikely this will be achieved.
Our travellers would need enough food for 18 years (if it's a one way trip). Assume our travellers can live on the equivalent of six cans of 150g corned beef and one litre of water a day. Since there are two of them, and they need this for 18 years, the weight of their food will be in the region of 25,000kg. 2 And that's before we consider water for washing. Perhaps you could reduce this by recycling the water, but that's still a lot of weight, which means more fuel.
The smaller and therefore the lighter the spacecraft the less fuel it will need. But our astronauts are going to be spending their lives on this ship, so they need something that's big enough for recreation, thereby increasing the size and weight of the spacecraft and increasing the fuel needed. Though perhaps with nuclear fusion this will not be too large a challenge.
The psychological stresses of living in space for one's entire life are immense and possibly insurmountable. What would our astronauts do with their days and nights? Well there aren't days and nights; which is itself a massive psychological stress. As romantic as space travel might seem, on the vast majority of 24-hour time periods on a 100 year journey the view out of the space ship window would be the same and monotonous, no matter how beautiful our travellers find the stars. The boredom would be unimaginable, except perhaps to people who have spent years in solitary confinement. Perhaps, as in science fiction movies, we will invent a way of keeping the human body in stasis, in which case they could "sleep" most of the journey and not age. They might find however that when they get to Proxima Centauri, there isn't much to get excited about, or if there is, there is not enough fuel left in their spacecraft to explore.
The space shuttle has fantastically complicated systems to maintain its atmosphere. Yet the longest space shuttle journey has been just under 18 days. We are a long way from building systems that can reliably maintain artificial conditions for life for decades.
At the high speeds — close to the speed of light relative to everything around it — that our spacecraft will eventually travel, the risk of it having a catastrophic collision, e.g. with hydrogen gas, becomes a serious problem.
There are many other difficulties I haven't discussed. Manned interstellar travel seems a very distant dream at best. It might never be achieved. Unmanned interstellar travel on the other hand is a realistic future possibility, especially if we accept that the generation that launches the interstellar probe might not be the one that experiences the joy of that probe sending information on Proxima Centauri back to earth.
Another, (perhaps far-fetched?) possibility, is that one day it will be possible to download the human brain, to something as small as a microchip, with durability of hundreds if not thousands of years. Perhaps if this is the future evolution of our species, we will be able to travel in this form to distant stars without being too concerned about the time it takes. (A reader has expressed scepticism of this idea, and so have some philosophers and scientists.)
In the meanwhile, while we have our flesh and blood form, all is not doom and gloom. The solar system is a massive place with hundreds, perhaps thousands, of interesting objects to visit. We haven't fully explored the earth, so you can just imagine how much there is to explore on the nearest planets and moons. We're still a long way from being able to send manned vehicles to other planets, but it is feasible with enough investment.
Finally, when you consider how difficult manned interstellar space travel is and how inhospitable to sustained human life we know the planets and the moons in the solar system are, you realise that even the long term possibility of seeking permanent refuge for humanity in another part of space is minuscule. And then you realise how important it is that we don't mess up our current planet.
Footnotes
Even this is unlikely because it assumes all the fuel's energy conversion is directed to the rocket's thrust. ↩
Six cans of 150g corned beef provides over 8000kj per day, which is enough to live on comfortably. In addition 1l of water weighs 1kg. So 150g, 6 times daily for 365 days per year for 18 years for 2 people = (0.150kg x 6 + 1kg) x 365 x 18 x 2 = 24,966kg. ↩