In one sense, this question is purely speculative and a matter of personal opinion because humankind is so far away from practically implementing interstellar travel. On the other hand, scientists from NASA and other cutting-edge facilities have been working on the problem for years, even decades, and they have been...
In one sense, this question is purely speculative and a matter of personal opinion because humankind is so far away from practically implementing interstellar travel. On the other hand, scientists from NASA and other cutting-edge facilities have been working on the problem for years, even decades, and they have been able to isolate some possibilities as more feasible than others.
Until now, rockets with some sort of chemical fuel have been used to take astronauts into space, land and take off from the moon, and ferry scientists back and forth from the space station. However, chemical rockets would be extremely inefficient for long distance space travel due to their low fuel efficiency. The engines would have to be enormous in comparison with their payloads. They are also slow; scientists estimate that a flight to a planet as close as Mars would take around nine months at the current level of chemical rocket technology.
An alternative that NASA and other agencies are looking into is plasma-powered rockets. NASA is now experimenting with a plasma-based propulsion system known as Variable Specific Impulse Magnetoplasma Rocket, or VASIMR. At present the experiments involve non-human exploration of space, but this could change in the future. According to NASA, VASIMR-powered vehicles could not only get astronauts to Mars much faster, but also work for interstellar travel. The principle involves charging fuels such as hydrogen, helium, or deuterium with electricity and then using magnetic fields to direct the plasma to provide thrust. An advantage of this propulsion system is that hydrogen is readily available elsewhere in the solar system, and presumably the galaxy, and so ships would be able to refuel. It is possible that chemical rockets would still be used to get astronauts and cargo into orbit, and then the much larger plasma-powered space vehicle would take them into the interstellar frontier.
Another possibility that has been much discussed involves vehicles powered by solar sails. Interstellar sail ships could be very light because they would not have to carry any fuel, being powered by particles of light known as photons. They would also be cheaper to construct than vessels with other sources of power. However, the farther a solar ship moves from the sun, the weaker its source of power becomes. Some scientists claim that beams from huge lasers might be able to focus on the sails and power them all the way to nearby star systems.
Other sources of power for interstellar vehicles have been proposed and discussed, including thermal fission, continuous fission, pulsed fission, and antimatter, but the three possibilities above currently receive the most serious scrutiny from scientists concerned with interstellar travel.