Interstellar travel is the term used for travel between the stars. Interstellar travel is going to be much more difficult than interplanetary spaceflight since the distances are typically hundreds of thousands of AU, and usually expressed in light-years. You can read more about AU and Light Years in my distance guide.
Science fiction writers and TV shows have shown us countless visions of humanity spread out across the Universe, as well as countless space ships capable of faster than light speed. Let's have a look at some of them and the science behind the fiction.
In the classic 1979 film Alien, the crew aboard the deep space towing vessel, Nostromo is capable of travelling large distances through the use of hyper sleep. This allows them to survive travelling the long distances. The hypersleep chambers are capable of suspending the body's autonomic functions while maintaining the health of each individual cell during stasis. This is not the same as freezing, hypersleep is more like hibernation.
Could cryogenics be used for real-life interstellar travel? It's certainly a possibility and NASA has invested significant resources to this issue. In the not too distant future, interplanetary trips to the other planets would require months of travel through the vacuum of space. Maintaining the crew's health is a vital concern. If the crew could be induced to hibernate, the problems of survival become easier to solve. Hibernating astronauts would be immobilized in special capsules and would not need pressurized living space or artificially generated gravity.NASA News
For interstellar travel hypersleep would not be as effective, it would still take thousands of years to reach other solar systems. The key to interstellar travel is travelling very, very fast. In Star Trek, this is accomplished with the use of Warp Drive. Matter in the form of deuterium gas is combined with antimatter and annihilate each other producing vast quantities of energy. This energy is mediated by the dilithium crystals and is funnelled to the warp nacelles which in turn warp space-time allowing ships to travel faster than light. Travelling at warp speed means that solar systems can be reached in days rather than a millennium.
The bulk of scientific knowledge concludes that it's impossible, especially when considering Einstein's Theory of Relativity, however, a concept for a real-life warp drive was suggested in 1994 by Mexican physicist Miguel Alcubierre. Subsequent calculations found that such a device would require prohibitive amounts of energy, but recently physicists say that adjustments can be made to the proposed warp drive that would enable it to run on significantly less energy, potentially bringing the idea back from the realm of science fiction into science. Link.
In Gene Roddenberry's Andromeda, the Andromeda Ascendant is able to travel faster than light by navigating the Slipstream. Slipstream is a series of "strings" connected between planetary systems by gravity. A Gravity Field Generator drastically reduces the mass of the ship and then a slipstream drive opens a slippoint which the ship enters. The pilot then navigates the series of slipstream "tunnels" until they exit via the desired slip point. Usually one has to enter and exit slipstream several times before reaching their final destination.
While there is not any science behind the slipstream, there are several theories which suggest that it may be possible to "surf" gravitational waves which themselves travel at the speed of light. Surf's up!
In Stargate SG-1 our intrepid heroes explore the galaxy using a network of Stargates. Each Stargate can dial another gate, which establishes a wormhole through which matter can be transported. Wormholes are theoretical tunnels through the fabric of space-time that could potentially allow rapid travel between widely separated points.
While wormholes are possible according to Einstein's theory of general relativity, such exotic voyages will likely remain in the realm of science fiction. There are very strong indications that wormholes that a human could travel through are forbidden by the laws of physics. Also, traversable wormholes - if they can exist at all - almost certainly cannot occur naturally so we would need to devise some way of creating them artificially and that would take a tremendous amount of energy.
Spacecraft in Battlestar Galactica use an FTL Drive for faster than light travel. Prior to a jump, the drive is "spooled up" and then the ship "jumps" instantly to another point in space using a dimensional transportation effect. The series never explains how the theoretical jump drive works, only that after the drive spools up, presumably reaching an operating speed, building up momentum or charging the flux capacitor, the ship then teleports to another location.
I wouldn't get too excited about this being a possibility any time soon. Current theory indicates that teleportation is impossible. Quantum teleportation does not teleport matter, it's more of a communication of data between two previously quantum entangled particles.
Current and Emerging Technology
Science fiction writers have given us many images of interstellar travel, but travelling at the speed of light is simply imaginary at present, however several privately funded initiatives such as the Tau Zero Foundation, Project Icarus and Breakthrough Starshot have emerged in recent years, each hoping to bring us a little bit closer to reaching across the cosmos.
The powerhouses of the space age, chemical rockets have pretty much reached the end of their life. They require huge amounts of fuel and provide only enough thrust to escape orbit. Even a round trip to Mars isn't feasible since the fuel required would weigh so much, the space craft would never leave Earths atmosphere. Future refinements to the chemical rocket are not expected to produce any substantial gains in thrust or power to weight ratio.
These engines use electrical energy to create a super-heated plasma and fire it through a supersonic nozzle to generate thrust. While they are fuel efficient in terms of power to weight, the overall thrust is very small so their use has been limited to satellites orbiting the Earth. They were developed in the 1970's and continue to be used today.
The next generation space engines are looking like a contender for long range missions. The thrust produced is very low, but can be sustained for a long time, and is cumulative. Over the course of a long range mission, Ion drive engines can delivery 10 times as much thrust per kilogram of fuel than a chemical rocket can.
Ion drives work by ionising molecules of an unreactive fuel, such as xenon, and accelerate them by an electric field to be shot out the back.
The Dawn space probe used its ion drive to become the first spacecraft to enter and leave the orbits of multiple celestial bodies.
Particles of light (called photons) carry momentum, like ping pong balls bouncing off a wall. The solar sail idea is to catch enough of them to pick up significant thrust. Like Ion drives, the thrust is very small, but over a long period of time, the effect is cumulative. Unlike Ion drives, solar sails rely on the suns energy, so heading further out the solar system results in less solar pressure resulting in less thrust.
Plasma Propulsion Engine
These engines are like beefed up versions of the ion drive. Instead of using a non-reactive fuel, magnetic currents and electrical potentials accelerate ions in the plasma to generate thrust.
The most powerful plasma rocket in the world is currently the Variable Specific Impulse Magnetoplasma Rocket (VASIMR), being developed by the Ad Astra Rocket Company in Texas. Ad Astra calculates it could power a spacecraft to Mars in 39 days, however, the concept hasn't yet made it into space.
Back in the 1960s and 1970s, several designs for a nuclear-powered engine were built and tested on the ground in the US. These nuclear reactors would heat a propellant to extremely high temperatures and generate thrust. The project was about to be integrated into a spacecraft, but the Nixon administration shelved the idea of sending people to Mars and cut the project's funding, so the plans were shelved.