The Variable Specific Impulse Magnetoplasma Rocket (VASIMR) Mars engine would send astronauts to mars in 40 days. It uses radio waves to ionize and heat a propellant and magnetic fields to accelerate the resulting plasma to generate thrust.
Franklin Chang-Diaz set up the Ad Astra Rocket Company in January 2005 to begin development of the VASIMR engine. Later that year, the company signed a Space Act Agreement with NASA, and were granted control of the Advanced Space Propulsion Laboratory. In this lab, a 50 kW prototype was constructed, and underwent testing in a vacuum chamber. Later, a 100 kW version was developed, and this was followed by a 200 kW prototype. After a long period of rigorous testing in a 150 m3 vacuum chamber, the latest configuration was deemed space-worthy, and it was announced that the company had entered into an agreement to test the VF-200 engine on the International Space Station, in or before 2013.