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The age of space exploration opened in the sixth decade of the
20th century.  In the few years since that time, robot probes
and then human beings first ventured beyond the limits of the
Earth's atmosphere and landed on another celestial object, the
Moon.  Probes have since gone on to explore the far realms of
the solar system and beyond.
Although the space age is still young, it is based on a long
history of theoretical and practical developments.  Long before 
the enabling technology for entering space was developed, a
theoretical basis had been laid by science (see ASTRONAUTICS).
The key to space exploration, however, lay in the production of 
the rocket engine (see ROCKETS AND MISSILES), which made
possible the lofting of objects beyond the Earth's atmosphere.
Once that was achieved, supporting technologies combined to
yield the broad range of activities now being pursued in the
realm of space.  Such technologies include the development of
scientific instruments to sense the conditions and processes
found in outer space and to observe the objects encountered
there, as well as the development of the transportation and
communications hardware to support these activities.  In order
for human beings to survive in space, the effects of the
vacuum, microgravity, and radiation conditions of that
environment had to be studied as well (see SPACE MEDICINE), and 
the appropriate LIFE SUPPORT systems developed to meet those
Space exploration today includes the investigation of celestial  
objects ranging in size from cosmic dust to the giant planets
of the solar system and the Sun itself.  The conditions
encountered in outer space also alter familiar terrestrial
processes, from simple chemical reactions to complex biological 
activities, and such effects are being explored to determine
how they might prove useful, as in crystallization and drug
purification processes.  In addition, objects placed in orbit
around the Earth provide platforms both for astronomical
studies and for a wide range of scientific and practical
activities relating to the Earth's surface, including surveys
of resources, studies of weather processes, and the relaying of 
communications and television images between distant points.
All of these subject areas lie within the field of space
exploration, and this article also observes the social and
military considerations that are necessarily involved. 


The basic principle of a rocket engine is that when fuel is
burned in the engine, the reaction mass is expelled at high
speed and pushes the engine in the opposite direction, in
accordance with Isaac Newton's law of action and reaction.  The 
energy to expel the reaction mass usually comes from some sort
of exothermic (heat-producing) chemical reaction that causes
the combustion products to expand violently and to stream out
of a nozzle.  In chemical reactions the actual reaction mass is 
usually the combustion products of the reaction.  A number of
other types of rocket engines are also possible, including ion
engines in which electrically charged ions form the reaction


The thrust, or "push," of a rocket engine is measured either in 
units of weight (kilograms or pounds)--where one unit of thrust 
gives to the equivalent unit of weight an acceleration of one
gravity, or "g" ((9.8 m/sec/sec, or 32.2 ft/sec/sec)--or, more
properly, in newtons.  A newton is a unit of force that gives
one kilogram an acceleration of one meter per second.  For any
rocket, thrust in kilograms can be converted to force in
newtons by multiplying thrust by g.  Large values are measured
in kilonewtons.


The efficiency of a rocket engine is a much more crucial
indicator of its performance.  Efficiency is measured by a
quantity called specific impulse (Isp), which is equivalent to
the propellant's exhaust velocity divided by g.  The resulting
unit of measure is seconds.  An equivalent concept is that the
Isp value is the duration of time for which one kilogram of
propellant can produce one kilogram of thrust.  The higher the
exhaust velocity and specific impulse of a rocket engine are,
the more efficient it is.  Solid-fuel rocket engines tend to
have Isp values of up to about 200.  Simple liquid-rocket
systems, such as those using kerosene and liquid oxygen, have
Isp values that measure in the mid-200s.  Hypergolic systems
such as those using hydrazine and nitrogen tetroxide, where the 
components ignite upon contact, have Isp values exceeding 300.
Cryogenic hydrogen fuel can deliver values in the mid-400s.  A
simple nuclear engine such as the NERVA project of the National 
Aeronautics and Space Administration (NASA) in 1970 can deliver 
800 to 900 seconds, but with significant complications in

Rocket staging is also required in order to create vehicles of
sufficient power for spaceflight.  When most of a rocket's fuel 
has been exhausted, the rocket is carrying a great deal of
empty-casing weight and is using a rocket engine whose thrust
has become too great for the remaining vehicle weight.  Schemes 
were therefore developed to discard empty tanks and large
engines during the course of a rocket's ascent.  The simplest
technique was to place an entire smaller rocket on top of a
larger one;  this is called tandem staging.  Other approaches
involve the use of side-mounted engines or even engine and tank 
assemblies that can be discarded in flight;  this is known as
parallel staging.

A typical space exploration mission requires, first of all,
that a vehicle be launched from the Earth's surface into outer
space.  The vehicle must then be able to survive and operate in 
space, after which it is sometimes returned to Earth.  Each of
these mission phases has special challenges that must be met by 
space scientists and engineers.

The initial phase of launch must use engines with high thrust
and compact fuel.  In practice, the launch engine involves
either kerosene/liquid oxygen or solid-fuel boosters.  As a
booster pushes against air resistance and the tug of gravity,
it loses much of its energy during the ascent.  By the time it
has achieved orbital altitude and velocity, more than 160 km
(100 miles) above the Earth's surface and moving horizontally
at about 7,600 m/sec (25,000 ft/sec), the booster must have
expended about 9,100 m/sec (30,000 ft/sec) of velocity gain.
This generally takes about eight or nine minutes, for an
average acceleration of 2 gs.
Satellites commonly enter orbit close to Earth, and this region 
of space is referred to as low Earth orbit, or LEO.  Propulsive 
stages can carry the payload higher, or into the 24-hour
geosynchronous orbit, or GEO, commonly used by communications
and weather satellites because they keep the satellites in
position above a selected point on the Earth's surface.

Alternatively, an upper stage on a rocket can fire to push a
payload to ESCAPE VELOCITY, or the velocity needed if an object 
is to totally escape the Earth's gravitational influence.
(This is speaking in practical terms since the gravitational
influence of any object in the universe is actually pervasive.) 
Escape velocity from Earth is about 10,800 m/sec (35,000
ft/sec).  Vehicles departing from earth are slowed by the tug
of gravity, but as they attain greater and greater distances
from the Earth, the gravitational pull decreases by the inverse 
square law (see GRAVITATION).  Ultimately, a probe launched
with the exact velocity for escape from Earth would reach an
infinite distance with no speed left.  In practice, vehicles
are effectively out of Earth's influence at a distance of about  
1,600,000 km (1,000,000 mi).  At this distance they would drift 
in orbit about the Sun near earth's own orbit.
In order to reach another planet, a vehicle must have a
velocity that exceeds escape velocity.  Added to or subtracted
from the Earth's own velocity around the Sun, depending on the
direction of aim, this excess velocity produces a new
interplanetary orbit that may intersect the orbit of the
intended target.  With proper timing of the launch--the
so-called launch "window"--a target planet will be at the point 
where such an interception can occur.
Inflight Operations

The inflight operations of a spacecraft involve guidance,
navigation, and control.  In space usage, these terms have
specific meanings.  Guidance refers to the determination of
which way a probe should go to achieve a desired end position,
such as a planetary intercept or a rendezvous with another
vehicle.  Navigation refers to the process of determining
exactly where a probe is at any given time, and where it will
be later along that same course.  Control refers to means of
altering the flight path of a probe, usually by means of small
Guidance is accomplished by computing a space vehicle's end
position compared to a desired condition, and then using the
differences to determine what changes in current motion would
result in smaller final differences.  The future position is
computed by propagating a vehicle's "state vector" (current
position and velocity) forward in time, taking all
gravitational influences into account as well as smaller
perturbations due to atmospheric drag, solar wind, spacecraft
venting, and similar disturbances.  Significant course
deviations can also be introduced by imprecise launch-vehicle
performance.  A certain amount of state-vector dispersion must
be expected due to imperfect executions of maneuvers.
Measuring a space vehicle's actual position is a complex task.
Powerful radar stations of Earth can track satellites out to
several thousand kilometers by bouncing radar beams off them.
Some satellites have transponders that automatically echo such
a transmitted pulse, which greatly facilitates trackings.
Satellites in GEO can be detected from Earth only by their own
radio transmissions, however, or by optical tracking through
powerful telescope cameras.  For deep-space vehicles, tracking
is accomplished by analysis of returned radio signals, in terms 
both of line-of-sight to the probe and of Doppler shifts (see
DOPPLER EFFECT) of the signal from the probe as it passes
through changing gravitational fields.  These measurements are
compared to computer models of where the probe would be
traveling if a certain initial position were assumed.  From
this comparison, a best estimate of position can then be

A space vehicle's own attitude, or pointing directions is
determined onboard.  Precise angles can be measured relative to 
an outside inertial frame of reference by means of periodic
star sightings, and GYROSCOPES are used to measure any
variations that occur subsequently.  A precise knowledge of
attitude is required to perform proper course changes.  Once
required velocity changes have been computed, the vehicle must
rotate itself in space in order to point its propulsion system
in the proper direction, and then perform the firing at the
precise moment for which it was computed.  The velocity change
actually executed can be observed by accelerometers on the
probe.  Following such correction maneuvers, additional periods 
of tracking may occur so that more navigation can refine the
knowledge of the probe's trajectory and end point, and
additional course corrections can then be performed as needed
Radio communications are required to command a probe and to
receive information about its status and about the findings of
its instruments.  Information received over a radio link is
called TELEMETRY.  Control is exercised by sending coded
instructions that are received by the spacecraft, interpreted
by a circuit called the command decoder, and then executed as
necessary by the probe's computer autopilot.  For deep-space
probes, the round-trip time of radio signals can become
excessive.  Round-trip time to the Moon, for example, is only a 
matter of seconds, but it can reach tens of minutes for Mars
probes and many hours for probes in the outer solar system.
Direct real-time commanding therefore cannot always be
accomplished, and a great deal of flexibility and anticipatory
programs are involved in preparing for such distant probes.
The use of ground TRACKING STATIONS is very different for LEO
satellites and for deep-space vehicles.  The latter move
through the Earth's sky very slowly and can remain in sight of
a single tracking site for up to 12 hours.  Because of the low
altitude and relatively great speed of LEO satellites, however, 
they quickly cross the sky of any ground site, moving from
horizon to horizon in five or six minutes.  This explains why
satellites near Earth spend most of their time out of radio
contact even though 10 to 20 tracking sites are available for
use, whereas vehicles millions of kilometers out in space can
be continuously monitored by only a handful of sites
strategically spread around the globe.  NASA's deep-space
network has three main sites, at Goldstone in California,
Madrid in Spain, and near Canberra in Australia.  In order to
overcome this geographical restriction and to reduce the
expenses associated with maintaining worldwide radio stations,
both NASA and the Soviet space program have been developing
geosynchronous relay satellites for use by other satellites.
Known in the United States as the Tracking and Data Relay
Satellite System (TDRSS), the network of two satellites and one 
spare is supposed to relay near-continuous data from manned and 
some unmanned satellite missions.  The equivalent Soviet system 
is called Luch ("ray").  Once operational, such relay
satellites could eliminate the need for the string of worldwide  
tracking sites that are now devoted to LEO payloads.
Electrical power is another feature common to all space
vehicles.  Probes on missions lasting only a few days may use
batteries or high-efficiency fuel cells that convert cryogenic
oxygen and hydrogen reactions into electricity, with water as a 
waste product.  Solar cells, arranged either in flat panels, or 
"wings," wrapped around the outer surface of a probe, are the
most common power source.  They must be supplemented by
batteries, however, for those times when spacecraft pass
through planetary shadows.  For missions to Jupiter and beyond, 
where sunlight is only a few percent as great as near Earth,
solar energy is too weak.  In such cases small nuclear devices
are used--thermoelectric systems in which the heat from
radioactive decay is converted by thermocouples into
electricity.  Full-fledged nuclear reactors have also been used 
in LEO, where large power output is required from compact
units.  The most notable example is the Soviet radar ocean
surveillance program, in which two satellites and their
reactors have fallen to Earth, one in 1978 and the other in
Return to Earth

For spacecraft that are to be returned to Earth, a controlled
descent is required.  This is initiated by a "deorbit" maneuver 
that uses onboard propulsion to slow the vehicle's speed by
about 1 percent.  This slight amount is sufficient to lower the 
orbital path into the upper atmosphere, where drag will slow it 
Entry into the atmosphere poses special craft-survival
problems.  Tremendous heat builds up, not on the skin of the
vehicle (friction is not involved) but just ahead of it, where
a shock wave creates severe air compression.  The resulting
PLASMA can reach temperatures as high as on the surface of the
Sun.  This heat will soak into the vehicle unless it is
shielded, either by an ablative covering that carries heat away 
as it boils off, by a very efficient insulator such as the
material used in the SPACE SHUTTLE tiles, or by an active
cooling system.  Atmospheric resistance slows the vehicle
sharply, creating deceleration forces of up to ten times the
force of gravity.  Superheated air becomes ionized and
surrounds the vehicle with a sheath that blocks all radio
communications.  These effects require a precisely guided
descent profile to enable a safe return.
One of the first discoveries made by early Earth satellites was 
that an unexpectedly high number of charged particles were
trapped in the Earth's magnetic field.  Soviet instruments had
earlier detected hints of this, but it was the U.S.  Explorer 1 
satellite that helped determine that the Earth is encircled by
what are now known as the VAN ALLEN RADIATION BELTS--named for
the scientist who designed the instruments aboard Explorer 1
and properly interpreted the readings.  Other characteristics
of the space environment, some anticipated and some unexpected, 
where also experienced by these early probes.  Such
characteristics include a very hard (that is, relatively very
pure) vacuum, so-called zero gravity, high solar illumination
levels, radiation, and micrometeorite hazards.
The vacuum conditions encountered in space required the
encapsulation of apparatuses and passengers in a space vehicle, 
or else the special and expensive design of equipment that
could work without an air environment.  The cooling of
electronics systems became a problem, and moving parts required 
special lubricating systems because they otherwise tended to
stick together when operating in space.
The free fall of satellites in low Earth orbits created the
condition commonly called zero gravity.  Technically, this term 
is a misnomer.  The force of gravity in low Earth orbits is
scarcely diminished from that experienced at the Earth's
surface;  it is the motion of the satellite that results in the 
effect of weightlessness.  The term "zero-G" has passed into
common usage, however, and is going to remain in writings about 
space.  Because slight accelerations actually do occur even on
a satellite--due mainly to air drag and satellite motion--the
more recent term microgravity has generally been adopted as
Unfiltered solar radiation can cause illuminated portions of a
spacecraft to rise to high temperatures.  Meanwhile, shaded
portions of the craft will radiate their warmth into space and
cool below the freezing point of common fluids such as water
and storable rocket fuels.  All such fluid containers and
lines are commonly equipped with electrical heaters, while
overall temperatures are moderated by rotating the spacecraft
along an axis perpendicular to the spacecraft-Sun line.  This
is known as passive thermal control, or, more colorfully,
"barbecue mode." Unmanned spacecraft to the inner planets must
be equipped with parasols to reflect away unwanted solar heat.
Those sent to the outer solar system--or to the the Moon's
surface, with its two-week long nights--often use radioisotope
heaters (see SNAP).
Radiation effects on spaceflights also took some time to
appreciate.  Satellites in LEO are protected by the
magnetosphere from solar charged particles and from a large
percentage of the cosmic rays arriving from outer space.

Vehicles operating at GEO or on interplanetary missions,
however, received the full force of these radiations.  Cosmic
rays have been known to penetrate integrated circuits in
spacecraft autopilots and to alter data and commands.  A space
version of static electricity has been built up on other space
vehicles during solar storms, resulting in electrical sparks
that caused severe problems in onboard electronics.
Experienced design of such systems has reduced the effects of
these influences.
The danger from micrometeorites, on the other hand, has proved
to be slight.  Although numerous impacts have been
recorded--and, on at least one occasion, actually heard by an
orbiting crew--no spacecraft is known to have been seriously
damaged by such particles.  Debris from other artificial
satellites appears to be increasing as a significant danger,
however, and one Soviet satellites may actually have been
destroyed by such a collision with "space junk."

The practical uses of satellites in orbit around the Earth are
described in many separate articles and in more general entries 
such as COMMUNICATIONS SATELLITES.  The scientific exploration
of space and the Earth has been advanced by many other orbital
systems.  NASA, for example, has successfully launched eight
Orbiting Solar Observatories (see OSO) and the Solar Maximum
Mission for studying the Sun.  To study regions beyond the
solar system, NASA launched three Small Astronomical Satellites 
(see UHURU), two Orbiting Astronomical Observatories (see OAO), 
European Space Agency (ESA) and the United Kingdom it has
launched the International Ultraviolet Explorer (see
ULTRAVIOLET ASTRONOMY), and, with the Netherlands, the Infrared 
Astronomy Satellite (see INFRARED ASTRONOMY).  ESA, for its
part, has launched the Exosat astronomy satellite for exploring 
the X-ray region of the electromagnetic spectrum (see X-RAY
ASTRONOMY).  Earth-observing satellites have included NASA's
six Orbiting Geophysical Observatories (see OGO) and many of
its EXPLORER series.  One of the most ambitious of all
scientific programs, NASA's SPACE TELESCOPE, is to be launched
by the Space Shuttle.
Geodetic surveys and accurate navigational data may be obtained 
from precise tracking of the positions of the U.S.  Navy's
TRANSIT satellites and the more recent NAVSTAR series.  The
most accurate geodetic data can be obtained from pulsed laser
beams.  The Laser Geodynamic Satellite, or LAGEOS, placed in
orbit in 1976, permits measurements of the movements of the
Earth's crust to an accuracy of within 2 cm (0.8 in), thereby
providing information vital to earthquake-prediction research.
Geosynchronous orbits provide ideal observational positions for 
many applications satellites, but another highly useful
location for Earth observation is a Sun-synchronous orbit,
which is slightly tilted off true polar orbit so that the
orbital plane shifts to keep pace with the Sun's annual motion
through the ecliptic.

The exploration of other bodies in the solar system began
within a few years of the first satellites.  Both U.S.  and
Soviet space engineers set their sights on the Moon.  Early
Soviet launches in 1958 all failed and were never announced.
Several U.S.  launches also failed, although two of them
(Pioneers 1 and 3) reached nearly 100,000 km into space before
falling back to Earth.  The first probe to escape Earth's
gravity was LUNA 1, launched on Jan.  2, 1959, which passed the 
Moon and continued into interplanetary space.  The U.S.  probe
Pioneer 4, launched two months later, followed the same path.
Later Soviet probes either hit the Moon or passed it and took
photographs of the hidden far side, relaying them back to
In the mid-1960s three NASA programs pursued the lunar
objective.  RANGER probes crashed into the Moon's surface but
succeeded in sending high-resolution photographs prior to
impact.  SURVEYOR probes soft-landed on the Moon and analyzed
its surface, while LUNAR ORBITER probes circled the Moon and
sent back pictures both of potential landing sites for
astronauts and of areas of general scientific interest.  Soviet 
efforts proceeded along similar lines and achieved limited
successes shortly before their U.S.  equivalents (Luna 9 made
the first successful soft landing on the moon in 1966, and Luna 
10 became the first probe to enter lunar orbit a few months
later).  The Soviets also operated a series of heavy automatic
probes that retrieved small amounts of lunar soil, deployed
wheeled rovers called LUNOKHODS, and made lunar-orbit surveys.

The USSR also aimed three Soviet rockets towards MARS in 1960,
but all failed;  the last caused loss of life when it exploded
during a launchpad checkout.  Other shots in the Soviet series
also failed, including later heavy-landing attempts.  The first 
successful Mars probe was the U.S.  MARINER 4, in 1964.  Two
more fly-by missions and an orbital photographic flight
followed before the sophisticated landings of VIKING spacecraft 
in 1976.  The USSR sent two probes to reach the Martian moon
Phobos in 1989, but one went out of control shortly after
launch, and contact was lost with the other soon after it began 
returning data form Mars orbit.

Soviet probes toward VENUS also failed in early attempts, and
the first spacecraft to reach the planet successfully was
NASA's Mariner 2, in 1962.  Later Soviet VENERA atmospheric
probes eventually returned some basic data, and Mariner 5
provided sophisticated measurements during a fly-by.  A Soviet
probe survived briefly on the surface of Venus in 1971, with
more advanced landers following in 1975.  Two NASA pioneer
missions in 1978 provided additional atmospheric and
topographical data, while a pair of Soviet probes conducted a
radar-mapping mission of the planet's northern hemisphere in
1983.  Two Soviet spacecraft dropped instrumented balloons into 
the upper atmosphere in 1985.
The only mission to Mercury was the triple fly-by performed by
Mariner 10 in 1974-75.  The craft returned detailed photographs 
and environmental measurements of the planet.
Among the giant outer planets, the first missions to Jupiter
were NASA's Pioneer 10 and Pioneer 11 probes, launched by
Atlas-Centaur boosters in 1972-73.  They each took more than
two years to reach their destination before continuing on
toward the outer solar system.  Their measurements of the
environment of Jupiter and particularly of its severe radiation 
belts paved the way for the VOYAGER probes launched in 1977.
Voyager 1 and Voyager 2 discovered new features of the Jupiter
system, such as new moonlets, another ring, and active
volcanism on Jupiter's satellite IO.
Saturn was first visited by the Pioneer 11 spacecraft in 1979,
and in the following year both Voyager probes passed the
planet, measuring its environment and observing its atmosphere, 
satellites, and impressive system of rings.  Voyager 2 went on
to become the first probe to reach the planets URANUS, in 1986, 
Neptune, in 1989. This left Pluto as the only planet unvisited
by investigatory probes.
The first probe to a comet was the International Sun-Earth
Explorer launched by NASA in 1978 into a high lunar orbit and
then diverted five years later for an interception of Comet
Giacobini-Zinner.  In 1986, four probes from Earth--two Soviet
Vega missions, ESA's Giotto, and a Japanese payload--passed

Many early Pioneer probes were launched to observe the Sun from 
different angles.  Two German spacecraft, Helios 1 and Helios
2, were sent well within the orbit of Mercury to measure the
Sun's magnetic field and other environmental features.  A later 
European probe, named Ulysses, is intended to explore the polar 
regions of the Sun.

Two further planetary probes got under way in 1989 after being
launched from the Space Shuttles.  One, Magellan, is to study
Venus, while GALILEO was sent toward Jupiter.

The most challenging and exciting aspect of space exploration
has been manned space flight.  Soon after the first satellites
were launched, both Soviet and American design teams began work 
on manned space vehicles.  The Soviet team was able to make use 
of a launch vehicle three times as powerful as the ones
available to NASA, so the USSR was able to choose familiar
systems known to be reliable.  NASA's need to venture into new
technological disciplines, on the other hand, later proved
crucial to the success of manned lunar flight--a challenge that 
the Soviet design teams were unable to meet.
After several unmanned test flights in 1960 and early 1961, the  
USSR launched the world's first manned spacecraft, VOSTOK, on
Apr.  12, 1961.  The pilot was a 26-year-old Russian named Yuri 
GAGARIN.  The basic spacecraft consisted of a three-ton sphere
and a two-ton service module.  The sphere was later rebuilt to
hold three crew members and was launched with the name VOSKHOD
in 1964.
In 1961, President John F. Kennedy had declared that the goal
of the U.S. manned space program was to land a man on the Moon
before the end of the decade.  The story of that endeavor is
recounted in the entries on the MERCURY PROGRAM, GEMINI
PROGRAM, and APOLLO PROGRAM.  The landing did take place on
schedule, on July 20, 1969.  Since those first years of flight, 
the Soviet and U.S. manned programs have taken divergent paths
Several major policy questions are involved in planning for the 
future in space exploration.  Such questions are concerned with 
the matter of international cooperation as opposed to
international competition, the desirability of manned versus
unmanned activities in space, the issue of long-range planning, 
the employment of reusable versus expendable systems, and
problems relating to space industrialization and the orbiting
of military systems.
Because most nations who fund space exploration do so in order
to accrue national benefits--whether technological military, or 
simply in national prestige--the development of international
cooperation has long been balanced against concerns for such
national benefits.  It should be observed that many
international space projects, such as Spacelab, Ariane, Vega,
and Kvant, have proved particularly successful because each
nation involved has contributed in its own areas of
specialization.  Such alliances have yielded large amounts of
knowledge that could then be shared satisfactorily--as could
the prestige involved, as well.
The costliness of manned space activities has fueled the
ongoing dispute over the desirability of involving astronauts
and cosmonauts in operations that can also be conducted
automatically by means of remote control.  Many tasks of space
exploration are in fact best performed by unmanned systems, but  
the most complex and unpredictable operations often exceed the
capacity of computers and telemetric systems and instead
require the presence of flexible, perceptive, imaginative
humans.  Experience on Earth has demonstrated that the most
valuable discoveries in exploration and research are often
those which were completely unexpected, and hence those which
unmanned systems might overlook.
Long-range planning for space exploration is bedeviled with
down-to-earth problems of political and budgetary support.
Despite the general desire for consistent levels of space
funding, nations customarily do not plan budgets for decades in 
advance.  National interest in space activities also waxes and
wanes with dramatic short-term variations, based on
opportunities that arise for the achievement of relatively
short-term goals.  Major space projects in the United States,
at least, have tended to take place in sprints, with long
pauses in between, and realistically speaking, they will
probably continue to do so.
The high cost of space exploration has led to major efforts to
economize, and this in turn has created the conflict between
reusable and expendable spacecraft systems, primarily in the
area of launch vehicles.  The issue is complicated by the fact
that the largest costs of space operations usually are not for
the hardware but instead for mission checkout, preparation, and 
management, and these costs may actually be increased if the
space vehicle being prepared has already been used several
times.  In addition, reusable space systems must sacrifice a
significant portion of their performance in order to include
recovery aids, such as wings or parachutes.  Thus, "throwaway"
systems may actually be cheaper to operate, pending the
development of advanced space vehicles that can be recycled
The promise of space industrialization remains thus far
unrealized, because of the continuing high cost of space
operations--primarily in the area of transportation--and
because ground industrial technologies also continue to
advance.  The high start-up costs for the space industry,
combined with the excessively long lead times between
investment and payoff, have been major factors in discouraging
the development of such projects by private Western
corporations.  Clearly, however, a number of high-value and
low-volume products such as crystals, drugs, and glass could be 
manufactured in space at an advantage, and the commercial sale
of such products has already begun.
Few space-exploration issues are more emotional or have more
diplomatic and ideological connotations than the conflict over
space militarization versus treaty restrictions.  Space is
already a transit region for military missiles, and it has
served as a vantage point for military reconnaissance since
soon after the first days of Sputnik.  Several limited
antisatellite (ASAT) systems have also been developed, and the
Soviet Union has an orbital "killer" satellite.  Nevertheless,
efforts to develop space-based weapons systems such as the U.S. 
STRATEGIC DEFENSE INITIATIVE program are considered by many to
be excessive and inflammatory.  Technical issues are poorly
defined in this dispute, which is grounded mainly in political

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