41 years ago, Neil Armstrong became the first man to set foot on the Moon and uttered his famous "It's one small step for [a] man, one giant leap for mankind". 518 years ago, Christopher Columbus set foot in the New World after his transatlantic journey, having thus undertaken an earlier "giant leap for mankind". History is filled with such milestones, motivated by political, military or technological reasons and driven by human ambition. Perhaps the next great milestone will be when humans first set foot on Mars, something which I hope will happen in my lifetime. But after we start expanding into the solar system the next giant leap will be to travel to the stars. Currently, we have no means of achieving interstellar travel in a reasonable amount of time, since the fastest man-made objects ever built would still take tens of thousands of years to reach the nearest star.
Interstellar travel can theoretically be of 2 types: slower than light or faster than light.
In this post I will only talk about slower than light interstellar travel.
Slower than light travel
One of the postulates of Einstein's theory of relativity is that the speed of light in vacuum is constant (exactly 299,792,458 m/s) and that nothing that carries mass, energy or information can travel faster locally. All experiments and observations carried out have obeyed this postulate. The theory of relativity has shown us this restriction, which is a major obstacle for interstellar travel. You might be thinking that the speed of light is absurdly great and is good enough for space travel. Assuming we could travel at that speed it would be convenient for travel inside our solar system, as it takes about 8 minutes for light from the Sun to reach Earth and depending on where you want to go it can take up to several hours to travel to other locations. But if we wanted to travel to our nearest neighbor, Proxima Centauri, it would take about 4.2 years, as it is located 4.2 light-years from our solar system. Travelling to other stars takes even longer, our galaxy, the Milky Way, is 100,000 light-years across. It would take more time to explore the galaxy, than the current age of the human species.
However, there is a loophole. I've been talking about time, but I haven't mentioned that in relativity time is ... relative. Different observers will measure different time intervals, and it can be shown that observers travelling at velocities close to the speed of light (starship reference frame) experience time dilation, with respect to the stationary observers (Earth reference frame). Time dilation means that less time passes inside the starship than on Earth. This is known as the twin paradox. I will not go into details as to why this happens (check out the links), the important thing is that it happens and it means that even if it takes one million years, in Earth's frame of reference, for the starship to explore the galaxy, in the ship itself only 50 years could have passed (the speed necessary to achieve this is 99.99999987% of the speed of light). In fact if you can go fast enough, you could explore the entire observable Universe within a lifetime, from your perspective, but it would also mean that everyone you knew back on Earth would have been dead for a very very long time. Travelling near the speed of light is basically time travel into the future. This isn't so much good news because it would still mean the people on Earth would have to wait years to receive information from the ship. Still, on short distances (relatively speaking this means under 40 light-years) it is practical to do this and explore many of our neighboring stars.
So what's keeping us from doing this?
The short answer is: lack of technology and financial support. We currently have no means to accelerate a ship anywhere near the speed of light but we do have some promising theories. To date, the fastest man-made objects, the Helios probes, achieved a speed of 252,792 km/h, which is just 0.023% light speed. All probes we've sent to explore the solar system have similar speeds and therefore it took years for them to reach their destinations. For example, the Voyager probes are the farthest of all probes, travelling on the outskirts of the solar system, but it took more than 3 decades for them go that far. Obviously we need to do better than that if we plan to send something light-years away, but we face many obstacles.
To begin with, the main problem is energy. An object travelling at relativistic speeds (close to the speed of light) will have a lot of kinetic energy and that energy has to come from somewhere. Let's say you wanted to accelerate a one hundred ton ship to one quarter light-speed. The energy required would be about 300 exajoules (10^18). Now take into account the fact that the total energy consumption in 2008 for the entire planet was 474 exajoules. As we can see, the energy requirements for achieving relativistic speeds are enormous. Even if we reduce the mass, and say one tenth light-speed is good enough, we still require tremendous amounts of energy. For this reason, no spacecraft can achieve those speeds through conventional means (that is through chemical engines, ion engines or any other propulsion method used by current spacecraft). Therefore, we must investigate the unconventional means of propulsion.
So far, the problem seems to be energy. Luckily we have an efficient means of releasing large amounts of energy, on the order of exajoules, in the blink of an eye: nuclear bombs. It's quite ironic that the weapons we feared for so long could destroy us, can actually save us by providing the means of accelerating ships to relativistic velocities, thus allowing us to reach other solar systems where potentially habitable planets exist. The project was called Orion and the idea is simple: the ship would be propelled by the shock waves produced by nuclear weapons, which would be detonated behind the ship. This is called nuclear pulse propulsion, and while it could solve the problem of achieving relativistic speeds (theoretically up to 10% light-speed could have been achieved) it is extremely problematic to implement. The problems that arise are of different natures: political problems since the Outer Space Treaty prohibits the placement of nuclear weapons or any other type of weapons of mass destruction, in space; environmental concerns due to the risk of nuclear fallout reaching Earth; financial problems, as the cost of constructing such a ship and transporting nuclear weapons (which are very heavy) in space is too great to make the project viable. The main advantage of project Orion is that it can be implemented with current technology.
Another idea was to use an inertial confinement fusion drive. The premise is to make a nuclear fusion reactor and propel the ship with the plasma and energy produced in the fusion reaction. This was known as project Daedalus, created by the British Interplanetary Society. The ship they designed could reach up to 12% light-speed and was supposed to travel to Barnard's star (almost 6 light-years away) which, at the time, was believed to have a planetary system (the initial claim proved to be false but there still may be a planetary system there and we just haven't detected it yet). The trip would have taken about 50 years, from Earth's perspective, which is reasonable as it is within a human being's lifetime. We can see that unlike project Orion, Daedalus can achieve a greater speed with no risk of nuclear fallout. However the problem with this idea, apart from the costs, is that fusion technology isn't advanced enough to construct the inertial confinement fusion drive, therefore this plan cannot be currently implemented and will only be possible in the near future.
As far as nuclear propulsion goes, all other ideas are variations of these 2 projects (you can also lookup project Longshot). So what other options are there?
Well, another method of solving the energy requirement problem is to use antimatter. Antimatter is the mirror "reflection" of matter and when they come into contact they annihilate each other releasing tremendous amounts of energy. Einstein's famous equation E=mc^2 (where c is the speed of light) shows the relationship between mass and energy as mass m can be converted into energy E and vice-versa. When matter of mass m encounters antimatter also of mass m the energy released from the reaction is exactly 2mc^2, as all matter annihilates all antimatter and everything is turned into energy. Comparatively, in nuclear fusion reactions only about 0.3% of matter is turned into energy (though it depends on the type of reaction), so matter-antimatter reactions are significantly more powerful. Where can we find antimatter? The apparent lack of antimatter in the observable Universe is a discussion for another post. For now what we need to know is that there are no known sources of antimatter in our solar system and while we can produce it in particle accelerators, by reversing the annihilation process and creating matter and antimatter from energy, the amounts produced are minuscule, on the order of hundreds of atoms. A CERN researcher said that all the antimatter produced at CERN would only be enough to power a light bulb for a few minutes.
We've explored techniques which rely on on-board fuel or energy source to propel the ship and while this is preferred since the ship has a larger autonomy we will also explore different propulsion methods. One such method is a solar sail. The idea is to attach the ship to a large, reflective sail. This sail would be propelled by radiation pressure and solar winds. We now know that energy and matter are different manifestations of the same thing and as such, photons or quanta of light have momentum and therefore light exerts pressure on matter. This would be the main driving force for the solar sail, radiation pressure coming either from the Sun or from high-powered lasers. Inside our solar system, the sail would also be pushed by solar winds and the combined action of these forces could accelerate it to a significant fraction of the speed of light though it would take a very long time to reach such a speed. During that time the sail could be damaged by dust and microasteroids. In fact another major problem for interstellar travel is that when travelling near the speed of light even a tiny grain of dust can destroy your ship if it hits you, since from your perspective dust is heading toward you at near light-speed and thus has an enormous kinetic energy.
A way to solve this problem is to use a Bussard ramjet, a ship with a giant scoop at its front to collect interstellar hydrogen, compress it until nuclear fusion is achieved and eject the exhaust from the back of the ship thus creating thrust. The scoop would also collect and eject interstellar dust in its path, however to gather sufficient quantities of hydrogen, it would have to be enormous (having a diameter of a few miles).
All theories presented here utilize only 2 of the 4 fundamental forces of nature to propel the ship: the strong nuclear force and the electromagnetic force. Why can't we use gravity or the weak nuclear force? Because they are the weakest interactions and using them in an intelligent manner to achieve our goal is extremely complicated. For example, a theory was proposed which makes use of gravity as the driving force: the idea is to create artificial black holes and propel the ship via Hawking radiation, though implementing this is impossible in the present and it's unlikely to be used even in the near future. As we will see in the next post, gravity plays an important role in faster-than-light travel.
Assuming a ship could be constructed that could travel to Proxima Centauri at sublight velocity, other problems that may arise are related to the passengers on board. They might not be able to endure such a long trip, even if from their perspective it's a shorter amount of time. Also, a large quantity of supplies would be required. Both of these problems could be solved if the people on board were put into cryogenic suspension for the entire trip. That means they would be put into a state similar to hibernation and not endure the effects of long-term space travel.
The conclusions we can draw are that there are many options for sublight interstellar travel as many solutions exists for the problems faced, but more research is needed before we can actually attempt a trip. As it stands no method for interstellar travel exists that is both technologically achievable in the present and economically feasible.