In May 2015, the Planetary Society launched a CubeSat into orbit. Normally this is not very exciting, since CubeSats are launched on a regular basis. However, this satellite, called LightSail (see visual), uses a solar sail as its propulsion system. While this is only a flight to prove the concept, missions that are more ambitious are already being planned; all in order to make interplanetary travel more efficient.
One of new propulsion systems is very similar to something being used every day on Earth: a sail. Sailboats use the concept of wind hitting a large surface area to create a propulsive force, thereby moving the craft. Scientists are now testing that concept on spacecraft as well. Instead of using the normal wind present in the atmosphere, they use photons emitted by the sun. These photons hit a reflective sail on the spacecraft and thereby transfer their linear momentum to the spacecraft. One single photon obviously exerts a minute force on the spacecraft and a sail measuring 800x800m will only experience 5N of force at Earth’s distance from the sun, provided it is exactly perpendicular to the sun’s light rays. The main advantage of this system is that, unless it is in the direct shadow of a third body, the sun is always present. If this force is present over a long period of time the velocity change could be substantial. Therefore, the solar sail could be a feasible solution, especially for interplanetary missions.
In May 2010, IKAROS (Interplanetary Kite-craft Accelerated by Radiation of the Sun), was the first ever spacecraft to successfully demonstrate the principles of the solar sail . Launched together with a Venus explorer satellite, IKAROS deployed a solar sail measuring 14m by 14m using its own rotational velocity to pull open the sail. Although the sail measured 196m2, it only weighted 2kg, thanks to its 7.5µm thick sheet of polyamide. Scientists concluded that in the first six months of the mission, the spacecraft experienced a total velocity change of 100m/s. This might not seem much, it is certainly not enough for interplanetary missions, but it proved that the concept worked.
Currently more projects are in progress to demonstrate the abilities of a solar sail. LightSail is a CubeSat, measuring 10x10x30cm, made by a organisation called The Planetary Society and was launched in May 2015. It carried a 32m2 sail to demonstrate the deployment mechanism. In 2016, a second CubeSat will be launched in a 750km orbit to measure the effect of solar sailing .
NASA has worked on a solar sail as well, but scrapped it in 2014 due to lack of confidence on the delivery of the technology. The solar sail, however, still is a feasible way to travel vast distances in relatively short amount of time. A sail measuring 400x400m would provide enough force to accelerate a spacecraft to 66km/s in about a year.
Apart from solar sails, research is being conducted in many other types of advanced propulsion. Examples of these are ion propulsion and warp drive. Like solar sails, ion thrusters deal with very low thrust levels, however they have a high specific impulse. In contradiction to ion propulsion, warp drive achieves large accelerations by bending space-time. This is theoretically done by using negative energy.
Ion thrusters are already flying around in space. Their thrust levels are not spectacular; their specific impulse, however, is. The first spacecraft that used an ion propulsion system as main propulsion system was the Deep Space 1 (DS1) spacecraft, which was launched in 1998. The engine, developed in the NASA Solar Technology Application Readiness (NSTAR) project, propelled the DS1 at speeds up to 4.5km/s and over the entire mission operated for 16,246 hours. In the future, more missions will be launched which use ion thrusters as main propulsion system. Ion thrusters use inert gases as propellant, the NSTAR engine, for example, uses Xenon. This gas is injected in the discharge chamber. At the same time, electrons are generated by a hollow cathode. After they flow out of this discharge cathode, these electrons are attracted to the discharge chamber walls, because the walls are positively charged. Before the electrons reach the chamber’s walls high-strength, magnets redirect the electrons into the discharge chamber. While in transit, the electron bombardment ionizes the propellant. When the propellant is ionized, the ions are accelerated out of the discharge chamber by two electrodes, a positively charged one upstream and a negatively charged one downstream. An ion thruster expels a large amount of positive ions, which has to be neutralized. That’s why a second hollow cathode is placed on the outside of the engine, which expels the electrons that are needed for neutralization. (see Fig. 1) The force between the upstream ions and the accelerator causes the thrust and the exhaust velocity depends on the applied voltage.
The follow up to the NSTAR engine is the NASA Evolutionary Xenon Thruster (NEXT), which has even a better performance than the NSTAR. In 2013, it was reported that NASA finished a test in which the NEXT thruster had been operated for over 43,000 hours or more than 5 years. The thrust was under one newton but a total impulse of 30 million newton-seconds was reached, which shows the great potential of ion thrusters. The only big downside of ion thrusters is that they need a vacuum to work.
Because of the efficient use of fuel and power, Ion thrusters enable spacecraft to travel cheaper, farther, faster than more conventional propulsion systems, therefore ion propulsion might become the leading propulsion system for interplanetary travel.
Imagine a trip to the nearest star, Proxima Centauri, which is 4.22 light years away from the Earth. If one traveled from Earth to Proxima Centauri with a Space Shuttle, it would take around 160,000 years. With the Voyager 1, the first human made object that left the solar system, it would take more than 75,000 years.  It would be impossible to bring human beings to another star with conventional propulsion systems. If we wanted to fly to Proxima Centauri in a relatively short amount of time, we would have to achieve speeds higher than the speed of light. One way of making such travels possible comes from the TV show Star Trek. A Warp Drive makes interstellar travel possible, enabling a spacecraft to travel faster than the speed of light. However, special relativity theory dictates that nothing can travel faster than light. Therefore fast interstellar flight is impossible, or isn’t it?
In 1994, Miguel Alcubierre, a Mexican physicist, proposed a method of warping space-time in order to cross any given cosmic distance in an arbitrarily short period of time. The object that would bend space-time is named after him: the Alcubierre drive. By expanding space-time behind and contracting space-time in front of a spacecraft, it would locally not travel faster than the speed of light and thus it would not violate the principle axiom of Einstein’s relativity theory. Instead, the spacecraft pushes against space-time itself, making it effectively faster than light. For an observer outside the warp it would seem that the spacecraft is travelling faster than the speed of light, a person within the warp would feel no acceleration, because the speed inside the warp is negligibly small with respect to the warp velocity. But this is all in theory, is the warp drive practically possible?
There are some difficulties with warping space-time. In order for the warp drive to work, negative energy densities are needed. The existence of such negative densities is constrained by relativity theory; however, they are consistent with quantum field theory. For instance the Casimir effect, where two plates restrict the wavelengths of particles between them, can result in a negative energy density. It is however not clear whether this can be applied at a larger scale.  Finally, the needed amount of negative energy might be big. The energy needed to transport a small spaceship across the Milky Way Galaxy, might be equivalent to 1067 grams, which is bigger than the mass of the Universe. Counterarguments have been given, but it is not clear if the huge mass can be overcome. 
These three advanced propulsion systems are not the only ones currently being developed. New concepts will arise or are already being worked on. However, like the Alcubierre drive, they may not always seem achievable. However, when new innovations are made, some of them might prove to deliver substantial improvements for interplanetary travel. Since these projects always take years or even decades to develop there are still more than enough opportunities to join the research for interplanetary propulsion systems.