of thrusting may be required to achieve an earth escape velocity of 11.2 km/s if the vehicle is already in orbit at a velocity of 9,100 m/s. Upon escaping the Earth's gravitational field the rocket will have a heliocentric velocity of 30 km/s in interplanetary space. Eighty years of steady photonic thrusting would be then required to obtain a final velocity of 240 km/s in this hypothetical case. It is possible to obtain even higher specific impulse; that of some other photonic propulsion devices (e.g., solar sails) is effectively infinite because no carried fuel is required. Alternatively, such devices as ion thrusters, while having a notably lower specific impulse, give a much better thrust-to-power ratio; for photons, that ratio is , whereas for slow particles (that is, nonrelativistic; even the output from typical ion thrusters counts) the ratio is , which is much larger (since ). (This is in a sense an unfair comparison, since the photons must be created and other particles are merely accelerated, but nonetheless the impulses per carried mass and per applied energy—the practical quantities—are as given.) The photonic rocket is thus wasteful when power and not mass is at a premium, or when enough mass can be saved through the use of a weaker power source that reaction mass can be included without penalty. A laser could be used as a photon rocket engine, and would solve the reflection/collimation problem, but lasers are absolutely less efficient at converting energy into light than blackbody radiation is—though one should also note the benefits of lasers vs blackbody source, including unidirectional controllable beam and the mass and durability of the radiation source. The limitations posed by the rocket equation can be overcome, as long as the reaction mass is not carried by the spacecraft. In the Beamed Laser Propulsion (BLP) concept, the photons are beamed from the photon source to the spacecraft as coherent light. Robert L. Forward pioneered interstellar propulsion concepts including photon propulsion and antimatter rocket propulsion. However, BLP is limited because of the extremely low thrust generation efficiency of photon reflection. One of the best ways to overcome the inherent inefficiency in producing thrust of the photon thruster by amplifying the momentum transfer of photons by recycling photons between two high reflectance mirrors. Power sources Feasible current, or near-term fission reactor designs can generate up to 2.2 kW per kilogram of reactor mass. Without any