by the Soviets to have gone through a series of tests at the nuclear test site near Semipalatinsk. The United States tested twenty different sizes and designs during Project Rover and NASA's NERVA program from 1959 through 1972 at the Nevada Test Site, designated Kiwi, Phoebus, NRX/EST, NRX/XE, Pewee, Pewee 2 and the Nuclear Furnace, with progressively higher power densities culminating in the Pewee (1970) and Pewee 2. Tests of the improved Pewee 2 design were cancelled in 1970 in favor of the lower-cost Nuclear Furnace (NF-1), and the U.S. nuclear rocket program officially ended in spring of 1973. Research into nuclear rockets has continued quietly since that time within NASA. Current (2010) 25,000 pound-thrust reference designs (NERVA-Derivative Rockets, or NDRs) are based on the Pewee, and have specific impulses of 925 seconds. Radioisotope Thermal Rocket A variant is the radioisotope thermal rocket, in which the reaction mass is heated by a radioisotope heat source instead of a nuclear reactor. Solar thermal rocket Solar thermal propulsion is a form of spacecraft propulsion that makes use of solar power to directly heat reaction mass, and therefore does not require an electrical generator as most other forms of solar-powered propulsion do. A solar thermal rocket only has to carry the means of capturing solar energy, such as concentrators and mirrors. The heated propellant is fed through a conventional rocket nozzle to produce thrust. The engine thrust is directly related to the surface area of the solar collector and to the local intensity of the solar radiation. In the shorter term, solar thermal propulsion has been proposed both for longer-life, lower-cost and more-flexible cryogenic upper stage launch vehicles and for orbiting propellant depots. Solar thermal propulsion is also a good candidate for use in reusable inter-orbital tugs, as it is a high-efficiency low-thrust system that can be refueled with relative ease. Laser thermal rocket A laser thermal rocket is both a type of beam-powered propulsion and a thermal rocket. The thermal energy source is a laser, which heats a working fluid in a heat exchanger. The working fluid is then expanded through a nozzle to produce thrust. Depending on the laser power, a laser thermal rocket can have a thrust-to-weight ratio similar to chemical rockets, while achieving a specific impulse similar to nuclear thermal rockets. For ground-to-orbit launches, the laser source for such a rocket would be a permanent installation capable of high-frequency