Interstellar propulsion

Most schemes that have been proposed to propel starships involve plasmas. Schemes differ both in the selection of matter for propulsion and the way it is energi2ed for ejection. Some proposals involve onboard storage of mass to be ejected, as in modem rockets, and others consider acquisition of matter from space or the picking up of pellets, and their momentum, which are accelerated from within the solar system (184,185). Energy acquisition from earth-based lasers also has been considered, but most interstellar propulsion ideas involve nuclear fusion energy both magnetic, ie, mirror and toroidal, and inertial, ie, laser and ion-beam, fusion schemes have been considered (186—190). 

The following section reviews the field emission process and various field emission applications in which CNTs have found particular recognition, including displays, microwave and X-ray sources, parallel electron beam lithography systems, gas ionization sensors, and interstellar propulsion. 

Spaceships capable of reaching stars other than the sun are expected to be more directly involved with plasmas than are contemporary spacecraft, in terms of their motion through the interstellar plasmas and their propulsion. Very high velocities are expected to be required for travel to other stars, eg, Proxima Centauri, which is 4.3 light years distant and would require 43 years at one-tenth the speed of light. 

The explosion of a supernova is a very happy event. It results in the spherical propulsion of matter into the interstellar medium, matter that has been simmering over millions of years, spiced up in the final moments by a little explosion and radioactivity. In the medium that lies between the stars, temperatures and densities are much lower than in stellar objects themselves. The supernova matter is diluted and cools down. Nuclei in the expelled material capture electrons to form various atoms and molecules. The cycle... 

There is considerable interest in microspacecraft to support interstellar robotic exploration and near-Earth environment characterization. The principle challenge here is to develop miniature electric propulsion systems capable of operating at much lower power levels than conventional momentum exchange electric propulsion hardware, and which meet the unique mass, power, and size requirements. The Busek Company, Inc. (Natick, MA) [338] has developed held emission cathodes based on CNTs for such an application. These devices have tum-on voltages about an order of magnitude lower than devices that rely on diamond or DEC hlms, and they have shown signihcant promise. 

---