Major Areas of Opportunity - Implications of Emerging Micro-and Nanotechnologies - page 108

sources are simple and robust because they do not require a heating element or a
plasma discharge to function. Field emitters will find use in electric propulsion
systems as an electron source for neutralizing ion beams that generate thrust.
Microfabrication offers similar structures for the efficient production of ions.
Liquid metal ion sources exploit the instability of conducting liquid surfaces
when electric fields of 10 5 to 10 6 volts per centimeter are applied. 196 Electrody-
namic forces cause the liquid surface to form one or more Taylor 197 cones that
have sharp tips with radii of curvature between 5 and 50 nanometers, thus gener-
ating local electric fields in excess of 10 7 volts per centimeter through field
enhancement. Johannes Mitterauer proposed miniaturized liquid metal ion sources
that could be fabricated using microfabrication technology in the early
1990s. 198,199 These revolutionary devices were based on the Spindt microvolcano,
which is basically a reverse-polarity Spindt cathode with a hole drilled through
the emitter to feed gas or liquid to the high-field region near the gate. 200 The
advantage of this microfabrication approach is that gross electric fields of 10 6
volts per centimeter may be readily generated by applying 100 volts across a 1-
micron-wide gap. Since the field evaporation/ionization process is fairly efficient
(less than 10 eV loss per ion), ion engines with high thrust efficiency at low
specific impulse, e.g., 1,300 seconds for indium, become possible. A planar,
scalable electric propulsion system that can used for picosatellites through 5,000-
kilogram-mass satellites appears to be possible.

Tether Propulsion

A tether connecting two masses in orbit will always try to line up with the
radial gravity vector. This effect, called gravity-gradient stabilization, is used to
create passive Earth-pointing stabilization for many microsatellites. Electrody-
namic tethers exploit Earth's magnetic field and the local space plasma environ-
ment in LEO to generate either power or thrust; they can operate either as motors
or electric generators. In the power generation mode, the v B Lorentz force

drives a current in an Earth-pointing conducting tether when the tether velocity
has a component perpendicular to the local magnetic field. Electrons are emitted
at one end of the tether and collected at the other; the space plasma provides the
return current. Orbital kinetic energy is converted into electric power; this can be
used to provide emergency power, to provide primary power for short-duration
missions, to provide intermittent power to charge laser or microwave weapons, or
to provide active deorbit capability. In the thrust generating mode, a current is
forced through the tether to generate a j

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B force perpendicular to the local
magnetic field direction. This is an electric propulsion system with infinite spe-
cific impulse.
Tethers can also operate as momentum exchange systems to provide mo-
mentum and energy transfer between attached masses. Proposed systems essen-
tially catch a payload at one end and throw it away at higher velocity, as shown in

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