The Variable Specific Impulse Magnetoplasma Rocket (VASIMR) is a high-power propulsion concept using radio waves to create and accelerate plasma in a magnetic nozzle. Important features are its high and variable exhaust velocity, which greatly enhances performance. A NASA-led, research team is developing this technology in the U.S. Recent advances include demonstration of efficient propellant utilization in its helicon plasma source and experimental verification of high ion acceleration through ion cyclotron wave resonance absorption, as predicted by theory. This paper describes the physics and engineering of VASIMR, reviews recent progress and discusses its application in a piloted Mars mission architecture. (c) 2005 Published by Elsevier B.V.