When we think about future possibilities of intercontinental or interstellar travel we always face the same issue. Fuel tanks limit or range and fuel types, fossilized or any other kind, limit speed.
Theoretically a non stop flight around the world is possible for the average man to accomplish in 80 - 100 hours.
So what we are always looking for is energy sources that take a long time to deplete or are vastly available as for example hydrogen which can be easily extracted from water and much faster propulsion system
Nuclear power promised to be the technology of the future because of the extreme energy output. One nuclear power plant is said to generate as much energy per day as 50.000 wind power stations. The radioactive waste that is produced was simply ignored as a problem and claims were maid that we would be able to recycle radioactive waste and turn it into non dangerous substance within the next 15 years. That didn't sound bad at all, but commercial nuclear power plants have been built since the 1950's and now over 60 years later absolutely no progress has been made in solving the radioactive waste problems.
Today everyone knows about this deadly garbage we produce every single day but we seem not to be aware of what's happening with it. Huge nations Like the U.S.A. , Russia, Germany, China, Japan and so on simply buy the rights to dump their waste in third world countries. Wait, what? Take a sandbox and start dropping paint or dye in one corner. Just a drop a day and after a while the entire sandbox is going to be colored. Whats the difference to dropping nuclear waste in third world countries?
The persons in charge of dumping this waste have come up with the idea to simply seal the waste in boxes made of different metals but since they believe that the languages we speak may become extinct before this waste becomes safe again, they have painted warning signs depicting people wearing masks or otherwise dying in agony under the influence of radioactive poisoning. Seems like we need to put a lot more effort in making clean nuclear waste before we build more plants.
Back to the transport question the ion propulsion system seems very promising.
Rocket engines generally work by pushing propellant away from the craft. This pushes the craft in the opposite direction. It's just like what causes a balloon to rush around as the air is allowed to escape. In effect the air pushes the balloon as it leaves.
Ion engines use the same principle, only much more efficient. Xenon (which is a gas like helium or neon, but heavier) flows into the ion engine, where it is given an electrical charge.
Charged atoms are called ions. xenon ions, can be pushed around by an electrical voltage. A pair of grids in the ion engine, electrified to almost 1300 volts, accelerates the ions to very high speed and basically shoots them out of the engine. As the ions fly away from the engine, they push the spacecraft, propelling it in the opposite direction. Possible ways to ionize xenon in space are either the already in use solar energy systems or as on project prometheus a small nuclear reactor. The problem with the nuclear reactor is of course still that the craft is essentially filled with extremely harmful material.
Xenon ions travel at about 35 kilometers/second or 77,000 miles/hour. That's about 10 times the speed of the exhaust from conventional rocket engines. This means ion engines would be at least 10 times faster than the rockets we have today. It also means that it takes only one tenth as much fuel for an ion engine to work as it does for a chemical propulsion system. On order to launch successful missions outside of our solar-system we need to use fuel and propulsion systems that are much more efficient .