Someday, in (62) the outer planets of our solar system, humankind will want to do (63) send small probes that merely fly rapidly by them. In time, we will want () spacecraft that will go (65) orbit around these alien planets, land robots on their moons, and even return rock and soil sample back to Earth. (66) , we will want to send humans to their moons, on at least (67) which liquid water—the fundamental requirement for life (68) we know it—is believed to be (69)
For missions such as these, we will need rockets powered by a nuclear reaction (70) igniting chemicals. Chemical rockets have served us well. But the (71) low amount of energy that they can deliver for a given mass of fuel is a severe (72) when dispatching spacecraft (73) long distances. To reach the outer planets, for example, a chemically-powered space vehicle must (74) fuel by having a very small mass and (75) use of gravity "assists", in which the craft maneuvers close enough to a planet for the planet’s gravity to propel the craft forward, (76) its speed.
In technical term, chemical rockets have low (77) velocity growth, which means that their exhaust velocities are not high enough to (78) very high speed to the rocket. Nuclear rockets, (79) their chemical counterparts, could impart a maximum velocity of (80) about 22 kilometers a second. Such a high velocity would make possible a direct path to Saturn (土星), reducing travel time from about seven years to as (81) as three.