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this is a useful summary
of current thoughts on the coal mining potential of
the Antarctic continent. The writers conclude that more
exploration is needed before 'a first approximation of the
potential of coal as a resource can be obtained', and
believe on balance that 'the many logistic, environmental,
political, sociological and marketing problems would
negate the economic mining of coal in Antarctica in the
forseeable future, certainly until well into the next century.'
Still, they provide a useful bibliography.
It is evident that the central capabilities that make
the Polar Platform of value to the Earth observations
community are the large and flexible capacity that is expected
to be present and the capability to do on-orbit servicing.
Capacity controls the amount of redundancy that can be
employed and the servicing approach and schedule dictate
the required design lifetime needed by a sensor and its
various subsystems. It is these parameters that will
determine the economics of future Earth observations.
The bounds within which favorable servicing costs must
lie are well established, because the annual investment
in operational environmental satellites and research
missions is well known. If servicing can be shown to
reduce those costs dramatically, or to allow greater
capability within those costs, an astronaut-serviced
Polar Platform will have the strongest justification
possible. It is the authors' belief that a demonstration
of this justification is within grasp of the Space Station program.
Communication missions benefit by lower transportation
costs using an orbital transfer vehicle (OTV) based on the
space station. In addition, communications satellites,
particularly those with large antennas, may be assembled
and checked out on the space station before committment to
GEO transfer. Communications activities include
1) component R&D, 2) q u a l i f i c a t i on of
large antenna satellites,
and 3) deployment of satellites to GEO.
Deployment of GEO satellites from the space station
could start in 1994, when a reusable, space-based OTV w i l l
most likely be a v a i l a b l e . Before then, it is assumed that
traffic to GEO w i l l go direct (using expendables) and bypass the station.
Satellites suitable for remote servicing
in GEO are expected to be launched in the mid-1990's.
For purposes of redundancy and absolute program continuity, two space geostationary platforms
could be colocated in orbit. Under nominal conditions, each would
carry approximately half of the traffic. While one of
these space platforms is being refurbished and updated
(through the automated replacement of some of its components from the Space Station) the traffic
would be transferred to the other satellite. Because the space platform
would be composed of many i n d i v i d u al modules, the traffic
diversion could be for a single network, whereas, the
remaining networks could continue to use the satellite
being upgraded. Figure 5 shows an early version of this
change out. The structure was hexagonal in cross-section
so that the maximum volume could be carried in the Shuttle
cargo bay. Semihexagonal sections could be removed and
replaced. In most cases the replacement is not for the
purpose of repair, but rather for inserting a more recent
module and upgrading the service quality or quantity.