Compact Nuclear Rockets; The Future of Space Exploration; Scientific American Presents; by Powell; 2 Page(s)

Someday, in exploring the outer planets of our solar system, humankind will want to do more than send diminutive probes that merely fly rapidly by them. In time, we will want to send spacecraft that go into orbit around these gaseous giants, land robots on their moons and even return rock and soil samples back to Earth. Eventually, we will want to send astronauts to their intriguing 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 nuclear fission rather than chemical combustion. Chemical rockets have served us well. But the relatively low amount of energy that they can deliver for a given mass of fuel imposes severe restrictions on spacecraft. To reach the outer planets, for example, a chemically powered space vehicle must have very limited mass and make extensive use of planetary gravitational "assists," in which the craft maneuvers close enough to a planet for the planet's gravitational field to act like a slingshot, boosting the speed of the craft. To take advantage of these assists, mission planners must wait for "windows"-short periods within which a craft can be launched toward planets appropriately positioned to speed it on its way to more distant bodies.