The Bussard ramjet is a theoretical method of spacecraft propulsion proposed in 1960 by the physicist Robert W. Bussard, popularized by Poul Anderson's novel Tau Zero, Larry Niven in his Known Space series of books, Vernor Vinge in his Zones of Thought series, and referred to by Carl Sagan in the television series and book Cosmos. Bussard proposed a ramjet variant of a fusion rocket capable of reasonable interstellar travel, using enormous electromagnetic fields (ranging from kilometers to many thousands of kilometers in diameter) as a ram scoop to collect and compress hydrogen from the interstellar medium. High speeds force the reactive mass into a progressively constricted magnetic field, compressing it until thermonuclear fusion occurs. The magnetic field then directs the energy into rocket exhaust, providing thrust. Feasibility Since the time of Bussard's original proposal, it has been discovered that the region surrounding the Solar System has a much lower density of hydrogen than was believed at that time (see Local Interstellar Cloud). John Ford Fishback made an important contribution to the details for the Bussard ramjet in 1969. T. A. Heppenheimer analyzed Bussard's original suggestion of fusing protons, but found the Bremsstrahlung losses from compressing protons to fusion densities was greater than the power that could be produced by a factor of about 1 billion, thus indicating that the proposed version of the Bussard ramjet was infeasible. However, Daniel P. Whitmire's 1975 analysis indicates that a ramjet may achieve net power via the CNO cycle, which produces fusion at a much higher rate (~1016 times higher) than the proton–proton chain. Robert Zubrin and Dana Andrews analyzed one hypothetical version of the Bussard ramscoop and ramjet design in 1985. They determined that their version of the ramjet would be unable to accelerate into the solar wind. However, in their calculations they assumed that: The exhaust velocity of their interplanetary ion propulsion ramjet could not exceed 100,000 m/s (100 km/s); The largest available energy source could be a 500 kilowatt nuclear fusion reactor. In the Zubrin/Andrews interplanetary ramjet design, they calculated that the drag force d/dt(mv1) equals the mass of the scooped ions collected per second multiplied by the velocity of the scooped ions within the solar system relative to the ramscoop. The velocity of the (scooped) collected ions from the solar wind was assumed to be 500,000 m/s. The exhaust velocity of the ions when expelled by the ramjet was