Distance to the nearest star is vast, 4.2 light-years to Alpha Cen, 15,000 light-years to center of Galaxy.
So, we need to go fast, which means more fuel. Now we have a fuel mass problem, the amount of fuel needed to get to the nearest star exceeds our technology.
And, of course, to achieve these speeds, we need a rocket with maximum efficiency, one with a high impulse to thrust ratio. These types of engines do not currently exist.
Ultimately, relativity states that speed of light is upper limit for matter (186,000 mi/sec). While time runs slower as we approach the speed of light, this only helps the crew, the trip time from Earth's point of view is still large.
This rendition, by artist Les Bossinas, depicts a cockpit view of a hypothetical spacecraft traveling at eight-tenths the speed of light and shows the visual distortions that would be experienced at such high speeds. The star field is actually being wrapped toward the front of the craft in addition to being significantly blue-shifted.
No engine is likely to generate superluminal speeds; the laws of physics prevent us from doing that, but we will be able to go many times faster than our current propulsion methods allow. A matter-antimatter engine will take us far beyond our solar system and let us reach nearby stars in a fraction of the time it would take a spacecraft propelled by a liquid-hydrogen engine, like the one used in the space shuttle.
So, why haven't we built a matter-antimatter reaction engine? The problem with developing antimatter propulsion is that there is a lack of antimatter existing in the universe. If there were equal amounts of matter and antimatter, we would likely see these reactions around us. Since antimatter doesn't exist around us, we don't see the light that would result from it colliding with matter.
Matter-antimatter propulsion will be the most efficient propulsion ever developed, because 100 percent of the mass of the matter and antimatter is converted into energy. When matter and antimatter collide, the energy released by their annihilation releases about 10 billion times the energy that chemical energy such as hydrogen and oxygen combustion, the kind used by the space shuttle, releases. Matter-antimatter reactions are 1,000 times more powerful than the nuclear fission produced in nuclear power plants and 300 times more powerful than nuclear fusion energy. So, matter-antimatter engines have the potential to take us farther with less fuel. The problem is creating and storing the antimatter. There are three main components to a matter-antimatter engine:
Approximately 10 grams of antiprotons would be enough fuel to send a manned spacecraft to Mars in one month. Today, it takes nearly a year for an unmanned spacecraft to reach Mars. In 1996, the Mars Global Surveyor took 11 months to arrive at Mars. Scientists believe that the speed of an matter-antimatter powered spacecraft would allow man to go where no man has gone before in space. It would be possible to make trips to Jupiter and even beyond the heliopause, the point at which the sun's radiation ends. But it will still be a long time before astronauts are asking their starship's helmsman to take them to warp speed.
The Bussard Interstellar Ramjet engine concept uses interstellar hydrogen scooped up from its environment for propellant. In this image, an onboard laser heats the plasma, and the laser or electron beam triggers fusion pulses thereby creating propulsion.
Up to now most of the methods seen involve carrying your own fuel with you. This leads to requirements for very high fuel efficiencies. One concept that was proposed over 20 years ago was the Bussard ramjet. This concept relies on the fact that interstellar space isn't empty. The Bussard Ramjet collects and uses the hydrogen of interstellar space as a propellant.
Ahead of the star vehicle, a laser is fired to hit free-floating hydrogen and excite it so that an electron flies off giving the nucleus a positive charge. Then a huge electromagnetic field collects the charged hydrogen particles towards a funnel at the front of the vehicle. The funnel sucks up the hydrogen and uses it in a fusion engine (or even an antimatter catalyzed fusion engine).
The Bussard ramjet will only work when the vehicle is moving fast enough to collect enough interstellar mass. Since space is almost empty, The ship has to go faster than about 6% light speed for the ramjet to even work. Therefore a Bussard ramjet needs to carry sufficient fuel to get up to 6% light speed on its own.
Shown above is the typical flight profile of a Bussard ramjet vehicle. At the start of the flight (on the left) the vehicle is fueled and accelerates out of the system. Once at 6% light speed the ramjet kicks in accelerating the vehicle to cruising speed. After coasting the vehicle has to decelerate. Initially activating the ram scoop without activating the engines starts the deceleration. It also refills the propellant tanks while slowing the vehicle back down to 6% light speed. Then the vehicle turns around and uses the on board fuel to decelerate down to orbital speeds in the target system.
There are a few problems with the Bussard ramjet. One is the size of the magnetic field generated (shown above). The magnetic field require is HUGE, estimated at over 10 million tesla for an unmanned vehicle. Generating such a field will take a lot of power itself, not to mention the power to charge the atoms in the flight path. Also, the material captured in space would be mostly hydrogen, which is a lot harder to get a fusion reaction from than deuterium or tritium typically needed for fusion. The biggest problem is the electromagnetic ram scoop itself. As the field lines of the magnetic field contract at the inlet funnel, the charged particles will not want to go inward, but rather will bounce outward away from the vehicle. In effect the magnetic scoop will be a magnetic bottle trapping the mass in a wide cone in front of the vehicle, but never getting close enough to be injected for fuel. Perhaps there is a solution to this problem by pulsing the magnetic field, but such a solution wouldn't be easy to implement.
Current physics, called special relativity, does not allow for FTL travel for it leads to paradoxes of time travel and violations of causality. It is highly unlikely that relativity is wrong since it has a great deal of experimental support. The only ways to travel faster than the speed of light are: 1) make light go faster (this involves quantum mechanical manipulation of vacuum energy which is currently unproven), 2) give up causality, or 3) enter some realm where relativity does not apply (so-called hyperspace).
Thus, the only possibilty for FTL is that our current physics is incomplete, which it has been many times in the past. One area of interest is not to increase our speed but rather to decrease the distance, or what is called warped space.
This rendition, by artist Les Bossinas, depicts a hypothetical spacecraft with a "negative energy" induction ring, inspired by recent theories describing how space could be warped with negative energy to produce hyperfast transport to reach distant star systems.