There's been another small spate of news articles in the past couple of weeks about Thorium, due to its safer and more environmentally friendly characteristics in comparison to Uranium. I'm wondering if there has been any fresh progress or advancements in the developmental research of Thorium as a nuclear fuel -- particularly Accelerator Driven sub-critical reaction systems. I'd always wondered if Thorium's potential for use as a controlled sub-critical nuclear power source might one day make it suitable for space propulsion. Perhaps a Thorium nuclear power source could be used to power a VASIMR or other type of rocket propulsion system, enabling a single-stage spacecraft to travel directly from the Earth to the Moon. I was also prompted by the latest news coverage of Blue Origin's private spacecraft test launch, which once again reminds of the challenges faced in using the limited energy in chemical fuels to attain orbit. If only we had much higher energy density fuels to work with, then there would be more margin for flexibility in the design of aerospace craft. (Not just spacecraft, but even large high-speed heavy-lift aerial intercontinental cargo transports to service our global economy.) I think humongous nuclear-powered launch vehicles would be great, but those old 1950s designs like Orion and NERVA are pretty scary, and no reasonable person would consider them safe. If Thorium could be harnessed sub-critically to power a rocket propulsion system, then what would the limit be on launch vehicle size and payload lifting capacity? I'm imagining you could build a launch vehicle the size of a Supertanker or the Empire State Building, if you wanted to. Comments?