The VASIMR Engine: How to Get to Mars in 40 Days
In theory, the VASIMR can also be used for interstellar travel, it can provide 1g constant acceleration (meaning it can provide artificial gravity). It's being developed since 1977. Thank you Franklin Chang Dรญaz for creating the ๐ต๐ ๐ฒ๐จ๐ฌ๐ฑ (Variable Specific Impulse Magnetoplasma Rocket) concept! Overall, it's near the speed of light propulsion.