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 requirementfor 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.