Someday, in exploring the outer planets of our solar system, humankind will want to do more than send small probes that merely fly rapidly by them. In time, we will want to dispatch spacecraft that will go into orbit around these alien planets, land robots on their moons, and even return rock and soil samples back to Earth. Eventually, we will want to send humans to their moons, on at least a couple of which liquid water — the fundamental requirement for life as we know it — is believed to be abundant. 
For missions such as these, we will need rockets powered by a nuclear reaction rather than igniting chemicals. Chemical rockets have served us well. But the relatively low amount of energy that they can deliver for a given mass of fuel is a severe drawback when dispatching spacecraft over long distances. To reach the outer planets, for example, a chemically-powered space vehicle must conserve fuel by having a very small mass and making extensive use of gravity "assists", in which the craft maneuvers close enough to a planet for the planet's gravity to propel the craft forward, boosting its speed. In technical terms, chemical rockets have low maximum velocity growth, which means that their exhaust velocities are not high enough to impart very high speeds to the rocket. The best chemical rockets, which are based on the reaction between hydrogen and oxygen, impart a maximum velocity of about 10 kilometers (six miles) a second to spacecraft departing from Earth orbit.
Nuclear rockets, in contrast to their chemical counterparts, could impart a maximum velocity of up to about 22 kilometers a second. Such a high value would make possible a direct path to, say Saturn, reducing travel time from about seven years to as little as three. A nuclear rocket such as this would be inherently safe and environmentally harmless: contrary to popular belief, a nuclear rocket need not be strongly radioactive when launched. The spacecraft, with its nuclear rockets, would be launched on top of a conventional chemical rocket. Then, once the vehicle had ascended to high-Earth orbit, above about 800 kilometers, the nuclear reactor module would detach from the chemical rocket and start up. The technology required to build a rocket motor powered by nuclear fuel is not far beyond current capabilities. In fact, my colleagues and I have designed a compact nuclear rocket engine, which we call Mitee (deriving the letters loosely from the words "miniature reactor engine"), that could be built in about six or seven years at a cost of $600 million to $800 million — actually quite modest in the context of space launches. In fact, the costs of developing the engine would be offset by savings in future launch costs. The reason is that a nuclear spacecraft powered by the engine would not need to haul along a large mass of chemical fuel, meaning that launching it would not require a gigantic rocket costing $250 million to $325 million. Instead, a lower-priced rocket, in the range of $50 million to $125 million, could be used.
In our design, the reactor's nuclear fuel would be in a configuration where layers of metallic sheets are penetrated by numerous small holes and then formed into a cylinder. Liquid hydrogen would flow from the outside of the cylinder inward, through the holes, quickly turning into a gas as it heated up and flowed toward the center. The superheated gas, at about 2,700 degrees Celsius, would flow at high velocity along a channel at the center axis of the cylinder and then out through a small opening at the end. 
A key attraction of nuclear rockets is that the fuel actually propelling the craft — hydrogen — is widely available in gases of the giant planets of the outer solar system and in the water ice of distant moons, planets, and comets. Thus, because the nuclear fuel would be relatively long lasting, it's possible to envisage a nuclear-powered craft touring the outer solar system for 10 or 15 years, collecting its hydrogen fuel as necessary. A vehicle could fly for months in the atmospheres of the giant gas planets collecting and purifying hydrogen for fuel, while also gathering detailed data on the composition of the atmosphere and weather patterns. Alternatively, a craft could fly to one of their moons to collect rock samples and also accumulate hydrogen, by separating water obtained from melted ice, for the trip back to Earth.
In the outer parts of the solar system, the sun's rays are too feeble to provide energy for spacecraft's instruments. So, they generally run on highly radioactive power sources, which are dangerous even during launch. In a probe with nuclear rockets, on the other hand, the instruments would be run off the same reactor that propels the spacecraft. Moreover, the amount of radioactive waste produced would be negligible — amounting to about a gram of highly radioactive material for a deep-space mission — and in any event the material would never come back to contaminate the Earth. Nuclear rockets are not new. Among the projects in this area was the US Space Nuclear Thermal Propulsion Program in the late 1980s. Its goal was to develop a compact, lightweight nuclear engine for defense applications, such as launching heavy satellites into high-Earth orbit. Although the work ended before a full-scale nuclear engine was built, engineers did successfully fabricate and operate low-power prototype reactors based on the concepts for such an engine and demonstrated that power outputs in the range of millions of watts could be achieved.
It is an easily provable fact that with only chemical rockets, our ability to explore the outer planets and their moons is hindered. In the near term, only nuclear rockets could give us the kind of power needed to allow us to significantly improve our understanding of the still largely mysterious worlds at the far edges of our solar system. Words: 1,005
