Environmental Engineering Reference
In-Depth Information
Asteroid mining has been seriously considered. A NASA design
study produced a 10,000-ton mining vehicle to be assembled in orbit that
would return a 500,000 ton asteroid fragment to geostationary orbit.
Currently the costs of solar panels are slowing their use, but the pro-
duction of solar panels needed to build a SPS system could reduce the
costs. The production of thin film solar panels called nanosolar could re-
duce production costs as well as weight.
The use of microwave transmission of power has been the most
controversial item in SPS development, but the safety of anything which
strays into the beam's path has been misrepresented. The beam's center
is the most intense region and it is far below dangerous levels of expo-
sure even if prolonged indefinitely. An airplane flying through the beam
protects its passengers with a layer of metal, which will intercept the mi-
crowaves. Over 95% of the beam will fall on the rectenna. The remaining
microwaves will be at low concentrations well within the standards for
microwave emissions.
The intensity of microwaves at ground level in the center of the beam
is likely to be comparable to that used by mobile phones. The microwaves
cannot be too intense in order to avoid injury to wildlife, especially birds.
Experiments with microwaves at reasonable levels have failed to show
any negative effects even over multiple generations. Some designs locate
rectennas offshore, but this presents other problems.
The proposed approach for fail-safe beam targeting uses a retrodi-
rective phased array antenna/rectenna. A pilot microwave beam is emit-
ted from the center of the rectenna on the ground to establish a phase
front at the transmitting antenna, where circuits in each of the antenna's
subarrays compare the pilot beam's phase front with an internal clock
phase to control the phase of the outgoing signal. This allows the trans-
mitted beam to be centered precisely on the rectenna and to have a high
degree of phase uniformity, but if the pilot beam is lost (the transmit-
ting antenna is turned away from the rectenna) the phase control sys-
tem fails and the microwave power beam is defocused. Outside of the
rectenna area the microwave levels rapidly drop off and nearby objects
should be completely unaffected. But the long-term effects of beaming
power through the ionosphere in the form of microwaves has yet to
be studied.
A potentially useful concept to contrast SPS with is the constructing
a ground-based solar power system that generates an equivalent amount
of power. Such a system would require a large solar array built in a well-
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